Abstract Nanorods are a very effective pinning center in YBa₂Cu₃O₇ films, and the vortex pinning mechanism should be understood to further improve the critical current density (J_c) in the nanocomposite films. Matrix defects are naturally formed during nanocomposite growth, and in this study the effect of matrix defects on the J_c in the YBa₂Cu₃O₇ films containing nanorods is discussed. YBa₂Cu₃O₇ + BaHfO₃ and YBa₂Cu₃O₇ + BaSnO₃ films were prepared by varying the laser repetition frequency to control the matrix defects. All the films contained the nanorods. The YBa₂Cu₃O₇ + BaHfO₃ film of 2 Hz exhibits high J_c for magnetic field parallel to the c-axis (B//c) due to the nanorods and the isolated random point defects. In the YBa₂Cu₃O₇ + BaHfO₃ film of 10 Hz, while the short stacking faults and networked random point defects decreased the J_c for B//c, the short stacking faults improved the J_c for tilted magnetic field. In the YBa₂Cu₃O₇ + BaSnO₃ film of 10 Hz, the isolated random point defects improved J_c regardless of the magnetic field direction. While the ab-plane correlated defects are analyzed by the structural observation, the network or isolated nature of the random point defects can be discussed by the normal state resistivity. The matrix defects of YBa₂Cu₃O₇ nanocomposite films should be designed considering the correlated/network/isolated nature which is determined by the morphology and defect concentration.

The strength of the conventional Cu/Nb3Sn wires may be too low to be applied to much larger cable-in-conduit (CIC) conductors with the multi-stranded cable structure of short-pitch twisting and low-void ratio applied for ITER-CS conductors. In this study, the Nb-rod-method Cu-Nb reinforced ITER-type Nb3Sn wire was developed by using a bronze-process not only to realize superior superconducting properties under high electromagnetic stresses but also to mitigate damage to the superconducting wires during the cable manufacturing process. The critical current properties of the Cu-Nb/Nb3Sn wire were significantly higher performance under both tensile stress and transverse compression stress compared to a conventional non-reinforced Cu/Nb3Sn wire. The Cu-Nb reinforcement had no detrimental effect on the AC losses of the wire. The trial production of short-pitch sub-cables was conducted, and it was confirmed that the Cu-Nb/Nb3Sn wires can improve the manufacturability of multi-stage stranded cables co-twisted with copper wires.

For next generation of high field cryogen-free superconducting user magnets, we adopt a robust coil concept, which consists of two tapes co-wound in a face-to-back configuration, a fluorine-coated polyimide tape as the inter-bundle insulation and thin FRP plates glued on the coil edges. To validate our coil design, we investigated electromagnetic and current-voltage properties of 20-stacked REBCO pancake coils wound with Fujikura EuBCO tapes with the robust coil concept at 4.2 K under a background field of 14 T. The inner and outer diameters and total height of the coils are 68 mm, 266 mm, and 101 mm, respectively. The average of measured hoop and axial strain on the outermost winding at the mid plane of the 20-stacked coils were 0.23% and −0.15% respectively at a hoop stress of 370 MPa estimated by the BJR relation. The I - V properties of the coils showed no anomalous behavior up to 25 T totally generated by the coils and background field, suggesting that our coil structure has an advantage against large electromagnetic stress.

Using a low-cost fluorine-free metal organic decomposition (FF-MOD) method, REBCO films with BaMO 3 (BMO, M = Zr, Hf) artificial pinning centers were fabricated. When M elements are conventionally added to the solution, the size of BMO formation depends on the growth conditions. But by adding preformed BMO nanoparticles, the size can be maintained on the nanometer order, meaning the nanoparticles act as effective pinning centers at low temperatures and magnetic fields. Adopting a polycrystallization sintering process enables the BaCO 3 to decompose completely and polycrystalline REBCO films to form prior to REBCO orientation. After final sintering, REBCO orientation can be achieved over a wide temperature range even with the addition of BMO nanoparticles. By utilizing preformed BMO nanoparticles and a polycrystallization sintering process, BMO doped GdBCO over 100 m-long tapes which have a thickness of more than 3 μm can be achieved. This is the first report of 120-meter-long artificial pinned REBCO tapes with I c exceeding 200 A/4 mm width at 77 K, self-field made by the FF-MOD method.

$T_{\rm c}$=2.1Kのウラン2テルル化物単結晶における超伝導相図を磁気困難軸である結晶$b$軸方向に磁場をかけて調べた。電気抵抗と交流帯磁率を同時に測定した結果、低磁場超伝導(LFSC)相と高磁場超伝導(HFSC)相が、対照的な磁場-角度依存性をもつことがわかった。結晶の純良性が上がったことにより、LFSC相の上部臨界磁場は上昇したものの、HFSC相が現れる特徴磁場$H^{\ast}$は、単結晶の純良性に関わらず、15Tと一定であることがわかった。また、$H^{\ast}$近傍のLFSC相内に、弱い渦糸ピン留め力で特徴づけられる中間超伝導相を示唆する新たな相線の形跡を捉えた。

Superconducting properties were investigated in epitaxially layered films consisting of superconductor NbN and half-metallic Heusler alloy Co₂MnSi (CMS). Temperature dependence of the electrical resistivity ρ( T) was measured by applying perpendicular magnetic fields to the surface of NbN/CMS films. With the increase of the CMS thickness d_CMS, the upper critical field μ₀ H_c2 decreased monotonically, but the superconducting transition temperature T_c had the minimum of 10.1 K at d_CMS ≈ 5 nm. The T_c behavior was in qualitative agreement with the theory of the π-coupling. The pair-breaking parameter δ determined by the superconducting fluctuation theory took the maximum at d_CMS = 3 ∼ 5 nm, which would be related to the minimum of T_c. The experimental results reveal that the superconductivity of the NbN layer in NbN/CMS films is affected by the interplay between the superconducting NbN layer and the half-metallic CMS layer.

Neutron irradiation on Nb3Sn wire changes its superconducting properties. It is well known that the critical current increases by the irradiation once and then decreases by further irradiation. In this paper, the effective upper critical magnetic field and the critical temperature of the irradiated Nb3Sn wire were investigated, and the mechanism that causes such changes were discussed qualitatively by focusing on the role of the irradiation defects together with the normal metallurgical flaws. In addition, a critical surface was taken up and the general tendency of the change in the surface was discussed.

Characterizations of round wires of iron-based superconductors were performed by using two different X-ray techniques. The core of the round wire turns out to be concentrically textured. The texturing parameter (r) defined by r = I(002)/I(103) has a clear correlation with the value of the critical current density (J c) of the wire. Using ∼18 m-long (Ba,A)Fe2As2 (A: K, Na) wires, small magnets was constructed and successfully generated a magnetic field as large as 3.1 kOe at I = 53 A. The wire for the magnet was carefully cut and local variation of the critical current (I c) was evaluated. Variation of the core area was also measured by using X-ray tomography for the segments with the largest and smallest I c. Obtained images demonstrates that the core has a certain degree of sausaging effect and it has good correlation with I c of the wire. Using the largest value of measured I c and the minimum core area of that segment, record-high value of J c = 71 kA/cm2 at 100 kOe has been achieved.

We report the fabrication and systematic characterizations of multi-filament (Ba,A)Fe2As2 (A: Na, K) round wires processed under high pressure. The evaluated transport and magnetic critical current density show clear core-number dependence. This trend originates from degradation of the superconducting core during the wire-drawing process, degradation of the concentric texturing of the core, and the enhanced sausaging effect of the core (revealed by x-ray tomography) as the number of cores increases. Details of the characterizations and suggestions for the improvement of the fabrication process are discussed. We constructed a superconducting magnet using seven-filament (Ba,K)Fe2As2 hot-isostatic pressed round wires. The generated magnetic field and critical current (I (c)) of the coils at 4.2 K are 1.0 kOe and 18 A, respectively. Measurements of the I (c) of short segments picked up from the coil show that there is enough room for improvements.

We recently proposed a robust REBCO HTS (RareEarth BaCuO) insert coil concept in the framework of a project to upgrade the 25 T Cryogen-Free Superconducting Magnet at the High Field Laboratory For Superconducting Materials to reach 30 T. In this article, the conductor consists of a two-tape bundle to mitigate the risks posed by local tape degradation. So-called edge impregnation is used to enhance the mechanical stiffness while reducing delamination risk, providing conduction-cooling capability at the same time. This insert is to be protected against the risk of thermal runaway simply by using dump resistor, thanks to an early detection concept. The different ideas forming the robust REBCO insert coil concept were tested successfully on various small scale prototypes. We report here their first implementation on a large-size insert coil, consisting of 20 stacked pancakes from the 30 T CSM upgrade insert. The mechanical behavior of the coil is modelled and validated with experiments. The thermal runaway detection setup is introduced. Its sensitivity and selectivity is discussed based on previous works on thermal runaway scenarios.

Abstract We investigated the stress–strain relation and the applied strain dependence of critical current I_c(ε_app) of high-strength Bi₂Sr₂Ca₂Cu₃O_10+δ (Bi2223) R&D tapes reinforced by ~100-μm-thick Ni-based alloy sheets with various pre- tensions F_pre applied during a lamination process. We confirmed that the thickening of Ni-alloy laminations is useful to enhance Young’s modulus compared with commercial tapes (≈30-μm-thick laminations). I_c(ε_app) decreases gradually and reversibly with applied strains ε_app and then degrades irreversibly with larger tensile and compressive strains. The reversible strain regime tends to shift towards the tensile side with larger F_pre, indicating that application of large F_pre is effective to improve the reversible tensile strain limit. Observed I_c(ε_app) can be reproduced well by the model considering an ε_app-linear critical current density, a cross-sectional strain distribution, and a Weibull-type distribution of filament damages. We clarified that different irreversible I_c(ε_app) degradations with large tensile and compressive strains can be understood by differences in the dimension and number of cracks and ruptures (tensile strains) and bucklings (compressive strains).

Abstract The addition of artificial pinning centers has led to an impressive increase in the critical current density (J_c) of superconductors, enabling record-breaking all-superconducting magnets and other applications. The J_c of superconductors has reached ~0.2–0.3 J_d, where J_d is the depairing current density, and the numerical factor depends on the pinning optimization. By modifying λ and/or ξ, the penetration depth and coherence length, respectively, we can increase J_d. For (Y_0.77Gd_0.23)Ba₂Cu₃O_y ((Y,Gd)123), we can achieve this by controlling the carrier density, which is related to λ and ξ. We can also tune λ and ξ by controlling the chemical pressure in Fe-based superconductors, i.e., BaFe₂(As_1−xP_x)₂ films. The variation in λ and ξ leads to an intrinsic improvement in J_c via J_d, allowing extremely high values of J_c of 130 MA/cm² and 8.0 MA/cm² at 4.2 K, consistent with an enhancement in J_d of a factor of 2 for both incoherent nanoparticle-doped (Y,Gd)123 coated conductors (CCs) and BaFe₂(As_1−xP_x)₂ films, showing that this new material design is useful for achieving high critical current densities in a wide array of superconductors. The remarkably high vortex-pinning force in combination with this thermodynamic and pinning optimization route for the (Y,Gd)123 CCs reached ~3.17 TN/m³ at 4.2 K and 18 T (H||c), the highest values ever reported for any superconductor.

There is an ongoing plan to upgrade the 25 T cryogen-free superconducting magnet (25T-CSM), at the High Field Laboratory for Superconducting Materials (HFLSM), IMR, Tohoku University, to a 30T-CSM by replacing the existing Bi2223 insert coils with REBCO coils. For the REBCO coils of the 30T-CSM, we will adopt a robust coil concept, which consists of two tapes co-wound in a face-to-back configuration with thin FRP plates glued on the edges. To confirm the effectiveness of this concept, we performed hoop stress tests on full-scale-size four stacked pancake coils fabricated with Fujikura EuBCO tapes at conduction-cooled 20 K under a background field of 11 T. The inner and outer diameters of the pancake coils were 68 and 268 mm, respectively. The averages of the measured strains on the outermost winding were 0.17-0.24% at a hoop stress of 400 MPa estimated by the BJR relation. The I-V properties of the coils indicated no anomalous voltage, implying that our coil structure was robust against large electromagnetic stress.

We approached internal matrix reinforcement using Cu-Sn-Zn and Cu-Sn-In ternary bronze alloy matrices for enhancing the effective mechanical strength of Nb3Sn wire. In this study, we evaluated the critical current density (Jc) degradation due to transverse compressive stress on internal matrix reinforced Nb3Sn multifilamentary wires using these ternary bronze alloy matrices, and the effects of both Zn and In solute element on the changes of Jc property under compressive stress was investigated. In the cases of internal matrix reinforced Nb3Sn wires using the Cu-Sn-Zn and Cu-Sn-In ternary alloy matrices, no Jc degradation was observed within 100 MPa of the transverse compressive stress. The (Cu, Zn) and (Cu, In) solid solution matrices contributed to suppressing Jc degradation by the compressive stress. In addition, the In element was the more effective solute element in the ternary alloy matrix on the internal matrix reinforcement based on the irreversible compressive stress estimated by the loading-unloading test.

Nb3Sn superconducting wire will become the first candidate strand material for the high current capacity conductor on the magnet of a fusion prototype reactor (DEMO). Through the ITER procurement activities, however, the current sharing temperature (Tcs) and transport critical current (Ic) degradations by the Nb3Sn filament fracture due to the huge electromagnetic force was observed, and it was pointed out that the high mechanical strengthening of Nb3Sn superconducting wire is an important research issue for a future DEMO development. Therefore, we have been reconsidering on the Nb3Sn superconducting wire fabrication process in order to satisfy with the critical current density (Jc) and mechanical strength properties for DEMO design. In the past, various reinforcing materials such as the CuNb or oxide dispersed strengthening (ODS) Cu alloys have been added to Nb3Sn superconducting wires to increase their mechanical strength. In this paper, we reviewed the typical high mechanical strengthened Nb3Sn superconducting wires with various reinforcing materials and clarify the advantages and disadvantages of the high mechanical strength method using various reinforcing materials. In addition, we have proposed the “Internal Matrix Reinforcement” as the newly high mechanical strength method without the reinforcing material for Nb3Sn superconducting wires, which concept is clearly different from the conventional high mechanical strength method using reinforce material. The commercial-like internal matrix reinforced Nb3Sn superconducting multifilamentary wires were succeeded to fabricate and the present status of their superconducting properties and mechanical strength were discussed.

Abstract In order to improve the electrical transport properties of REBa2Cu3O7−δ nanocomposite films, SmBa2Cu3O7-δ films with and without BaHfO3 nanoparticles were grown by chemical solution deposition (CSD), and their microstructural and transport properties were investigated in a detailed study using transmission electron microscopy and transport measurements in magnetic fields up to 24 T. The optimization process of the crystallization step (temperature and oxygen partial pressure) as well as an aging effect, which is due to the release of trapped fluorine, are described. Critical temperature and critical current densities surprisingly improve initially during the aging. Due to the complex microstructure, the additional BaHfO3 nanoparticles have only a positive effect at low magnetic fields for our samples.

AC losses in a high temperature superconducting (HTS) coil are experimentally evaluated. Double-pancake (DP) coils with turn-to-turn electrical insulations are wound using bundle conductors formed with two pieces of rare-earth-based coated conductors located face-to-face without insulation to improve thermal stability. Five DP coils are stacked and subsequently four copper plates are soldered between the adjacent DP coils to form a small-size HTS coil for AC loss measurements. The fabricated HTS coil is immersed in liquid nitrogen. Before measuring the AC losses, the contact resistances between the DP coils are observed in DC operations of the HTS coil at first. After that, the total losses including the Joule losses in the joints between the DP coils are measured by integrating the products of almost resistive components extracted from the terminal voltages in the HTS coil and applied transport currents observed using a pickup coil over a cycle under AC operations. Net AC losses in the two-ply bundle conductor windings are obtained by subtracting the Joule losses from the measured ones. In order to understand the mechanism of AC losses in the two-ply bundle conductor windings, the influences of current amplitudes and frequencies are investigated experimentally and theoretically.

To understand the temperature, magnetic field, and angular dependence of the critical current density, J(c)(T, B, theta), of (Bi, Pb)(2)Sr2Ca2Cu3Oy (Bi2223) core composed of commercial Bi2223 tapes, we investigated J(c)(T, B, theta) of a bundle of Bi2223 filaments extracted from Ag-sheathed Bi2223 tapes under broad (T, B, theta) conditions. We found that flux pinning curves for B||c (theta = 0 degrees) exhibit a temperature-scaling relation at high T (50-77.3 K) but not at low T (30-40 K). We also found that J(c)(theta) exhibits a broad peak in the B||ab (theta = 90 degrees) direction and attributed it to the broadening of a sharp peak due to the intrinsic pin caused by a c-axis misalignment. We plotted J(c)(T, B, theta) as a function of the tape normal component of the magnetic field considering the c-axis misalignment, and it agrees well with the measured J(c)(T, B, 0 degrees). This indicates that the intrinsic pinning is strong in Bi2223 and J(c)(theta) of Bi2223 is dominated by the flux pinning for the B||c component. Based on these findings, we succeeded in reproducing J(c)(T, B, theta) at 50-77.3 K by a model taking account of the T-scaling law, c-axis misalignment, and intrinsic pinning.

We have successfully trapped a field of 17.89 T at 6.5 K at the center of a compact coated-conductor (CC) stacks (13 x x 11.7 mm(3)) within 75 min by suppressing flux jumps. The CC stack consists of 200 sheets of EuBa2Cu3O7 CCs with BaHfO3 nanorods to increase the critical current density at high fields and low temperatures. To enhance thermomagnetic stability, the central 50 CCs are coated with 1 mu m thick Pb with large specific heat at low temperatures. Numerical calculations based on the actual J(c)-H characteristics reproduces the trapped field quantitatively. New directions for achieving even higher trapped field at higher temperatures and making use of the trapped field are discussed.

<title>Abstract</title>The fusion power density produced in a tokamak is proportional to its magnetic field strength to the fourth power. Second-generation high temperature superconductor (2G HTS) wires demonstrate remarkable engineering current density (averaged over the full wire), <italic>J</italic>_{<italic>E</italic>}, at very high magnetic fields, driving progress in fusion and other applications. The key challenge for HTS wires has been to offer an acceptable combination of high and consistent superconducting performance in high magnetic fields, high volume supply, and low price. Here we report a very high and reproducible <italic>J</italic>_{<italic>E</italic>} in practical HTS wires based on a simple YBa₂Cu₃O₇ (YBCO) superconductor formulation with Y₂O₃ nanoparticles, which have been delivered in just nine months to a commercial fusion customer in the largest-volume order the HTS industry has seen to date. We demonstrate a novel YBCO superconductor formulation without the <italic>c</italic>-axis correlated nano-columnar defects that are widely believed to be prerequisite for high in-field performance. The simplicity of this new formulation allows robust and scalable manufacturing, providing, for the first time, large volumes of consistently high performance wire, and the economies of scale necessary to lower HTS wire prices to a level acceptable for fusion and ultimately for the widespread commercial adoption of HTS.

We report the fabrication of small (Ba,A)Fe2As2 (A: Na, K) coils using 10 m-class long round wires, fabricated by the powder-in-tube method. Coils are sintered using the hot-isostatic-press technique after glass-fiber insulations are installed. Critical current (I (c)) of the whole coil using (Ba,Na)Fe2As2 and (Ba,K)Fe2As2 are 60 A and 66 A under the self-field, and the generated magnetic fields at the center of the coil reach 2.6 kOe and 2.5 kOe, respectively. Furthermore, the largest transport critical current density (J (c)) and I (c) in (Ba,Na)Fe2As2 wires picked up from the coil reach 54 kA cm(-2) and 51.8 A at T = 4.2 K under a magnetic field of 100 kOe, respectively. This value exceeds transport J (c) of all previous iron-based superconducting round wires. Texturing of grains in the core of the wire due to the improvement of the wire drawing process plays a key role for the enhancement of J (c).

We attempted to control the volume fraction of the domains of REBa2Cu3O7-delta coated conductors (REBCO-CCs) by annealing under a bending strain applied at high temperature. We succeeded in changing the volume fraction of the region where the a axis of REBCO aligns along the longitudinal direction of REBCO-CC, f(A), up to similar or equal to 90% when a compressive bending strain of similar or equal to - 1.35% is applied along the longitudinal direction. We found that fA evaluated by repeated X-ray diffraction measurements decreases over time if the domain-controlled REBCO-CCs with a high f(A) value is kept flat, whereas f(A) remains high if the sample remains bent. In addition, we also observed that f(A) in the as-received sample increases over time if the sample is exposed to a compressive bending strain at room temperature. These results suggest that oxygen atoms can migrate in REBCO even at room temperature when a uniaxial strain is applied to the REBCO-CC. Such a temporal variation of f(A) probably leads to changes in the superconducting properties, and may become important for the design of REBCO pancake coils.

In order to enhance J(c) in (Ba,Na)Fe2As2 wires and tapes further by improving texturing, we examined the following approaches; (1) Ag-sheathed HIP tapes, (2) AgSn-sheathed cold-pressed tapes, and (3) multifilamentary HIP wires with each filament having tape-like cross section. In addition, we attempted to increase I-c by fabricating (Ba,Na)Fe2As2 wires with larger core areas, and constructed a small demo coil using one of these wires with length up to 9 m. J(c) characteristics of these new wires and the performance of the demo coil with and without an electrical insulation are reported.

The upgrade project of the 25T-CSM to 30 T is on-going at HFLSM, IMR, Tohoku University. The goal is to obtain 30 T in a 32 mm warm bore by replacing the present Bi2223 insert coil with one wound using REBCO. In order to develop a robust REBCO high field insert, we adopt a two-tape bundle conductor with a face-to-back configuration. We confirmed that this conductor configuration mitigates effectively the risk posed by local defects in an R&D coil: a stable operation was achieved with negligible degradation in coil Ic even with a severe local defect on one of the two tapes. Quench protection from possible hotspot in the REBCO insert using an early detection method is made possible by the two-tape bundle configuration and the thick Cu stabilizer. Other R&D coils using such conductor were tested with large hoop stresses up to 460 MPa with no degradation. On the basis of these R&D studies, a robust REBCO insert is designed for the upgrade. The designed REBCO insert has a capability to achieve 35 T with enough Ic margin if we accept a maximum hoop stress of 500 MPa.

Abstract We report the fabrication of 10 m-class (Ba, Na)Fe₂As₂ long round wires and coils using these round wires. Round wires are fabricated by powder-in-tube (PIT) method and hot-isostatic-press (HIP) technique. To increase I_c keeping the diameter of the wire constant, the cross section area of the superconducting core is enlarged by changing the dimensions of metal sheaths. At 4.2 K under a high field of 100 kOe, J_c of a short segment of HIP wire reaches 42 kAcm^-2, which is comparable to the highest J_c of IBS round wires of 44 kAcm^-2. Furthermore, I_c of the HIP wire at 4.2 K under 10 kOe and 100 kOe reaches 95 A and 54 A, respectively, which is about twice the value of the previous report. We also fabricated a small demonstration superconducting coil using 12.5 m long PIT wires by HIP method. I_c of the whole coil is ∼60 A under the self-field, and magnetic field at the center of the coil reaches 2.6 kOe. The coil generated the magnetic field of 43 Oe/A, which is almost consistent with the calculated field from the coil dimensions.

We fabricated a compact (13 x 12 x 12.5 mm(3)) trapped-field magnet by stacking 200 pieces of EuBa2Cu3O7 coated conductors (CCs) with BaHfO3 nanorods as artificial pinning centers. It was magnetized by field-cooling method using a 18 T superconducting magnet, and the maximum field of 14.8 T was trapped at 10 K at a field-ramp rate of 0.1 T min(-1). At higher temperatures, although the trapped field was decreased, we could trap the field with much faster ramp rate. In order to understand these results, we also performed calculations of the trapped field based on J(c) - H characteristics of the CC. The calculated trapped field value was larger than the experimental value at 10 K. This discrepancy can be understood by considering the reduction of the effective size of the stacked CCs due to the existence of extended regions of irregular flux penetration called 'flux jets'.

Abstract Critical current density (J_c) in YBa₂Cu₃O₇ films are improved by nanorods. J_c increases with increasing the nanorod content up to an optimum content, and decreases after exhibiting a maximum due to degradation of the nanorod structure. To overcome this optimization constraint, hybrid pinning consisting of multiple types of pinning centers was investigated. In the present study, Y₂O₃ nanoparticles were additionally introduced into the YBa₂Cu₃O₇ films containing high density BaHfO₃ nanorods to increase the effective matching field. As a result, the global pinning force maximum was shifted to high magnetic field, and the J_c was improved especially in high magnetic field. Thus, even in the case that the J_c improvement in the YBa₂Cu₃O₇ + BaHfO₃ was difficult by increasing the nanorod content, the Y₂O₃ addition successfully improved the J_c. This demonstrates that the hybrid pinning can overcome the optimization constraint for J_c in the YBa₂Cu₃O₇ films containing nanorods.

By considering the crystal symmetry of the Dirac semimetal candidate SrMnBi2, it is expected that a substrate based on a square lattice is preferred to form a single-domain growth of a thin film. In this study, however, we observed different schemes of interface formation of SrMnBi2 using molecular-beam epitaxy on two oxide substrates of SrTiO3(001) and LaAlO3(001), both of which are often applied to the growth of the films with square lattices. Although antiphase domains appear in the SrMnBi2 film on SrTiO3(001), a single domain develops on LaAlO3(001) with an abrupt interface. The distinct difference indicates that the surface of the LaAlO3(001) substrate plays a crucial role in the selection of the initial growth plane. Judging from the abrupt interface image in scanning transmission electron microscopy and the four-fold symmetric in-plane x-ray diffraction pattern representing the orientation relationship of SrMnBi2 film [110]//LaAlO3 [100], the polar surface termination with (AlO2)(-) or (LaO)(+) probably promotes the interface formation of the ionic Sr or Bi plane on the surface, respectively. According to the semimetallic electronic structure of SrMnBi2, the electrical transport properties of the films can be consistently evaluated by the two-carrier model with high-mobility electrons and low-mobility holes. Our demonstration of the single-domain growth of the Dirac semimetal provides a key technique toward the future engineering of heterostructures composed of topological materials.

We fabricated (Ba,Na)Fe2As2 superconducting wires using the powder-in-tube method and hot isostatic pressing. By improving powder synthesis processes compared with previous studies, highly pure raw materials for the wire fabrication were obtained. The largest transport critical current density (J(c)) reached 40 kA cm(-2) at T = 4.2 K under a magnetic field of 100 kOe. This value exceeds not only the value of transport J(c) of the previous (Ba,Na)Fe2As2 wire but also those of all iron-based superconducting round wires. Improvements of polycrystalline powder synthesis play a key role for the enhancement of J(c). Furthermore, it was clarified that higher densification by high-pressure sintering and the texturing of grains in the core of the wire due to drawing also increased J(c) effectively. Advantages of (Ba,Na)Fe2As2 wires compared with (Ba,K)Fe2As2 wires are also discussed.

We investigated the simple internal reinforcement method using special reinforcement ternary bronze alloy matrix on the bronze processed Nb₃Sn wire. The Cu & x2013;Sn & x2013;Zn ternary alloy matrix was transformed to the (Cu, Zn) solid solution after Nb₃Sn synthesis based on the solid solution strengthening mechanism. For the further mechanical strength improvement, we focused on the Indium (In) as the more effective solute element for the & x201C;internal matrix strengthening & x201D;, and succeeded to fabricate the bronze processed Nb₃Sn multifilamentary wire using various Cu & x2013;Sn & x2013;In-(Ti) ternary alloy matrices. Changes of the Vickers hardness before and after Nb₃Sn synthesis and the transport critical current (<italic>I_c</italic>) under the uniaxial tensile deformation were evaluated. Vickers hardness of the matrix after Nb₃Sn synthesis on the Cu & x2013;Sn & x2013;In ternary alloy matrix samples was higher compared with the conventional bronze and the Cu & x2013;Sn & x2013;Zn ternary matrix samples. On the other hand, the tensile stress obtained to the maximum peak <italic>I_c</italic> value on the Cu & x2013;Sn & x2013;In ternary alloy matrix sample was estimated to approximately 265 & x00A0;MPa, and this value was much higher than those of the Cu & x2013;Sn & x2013;Zn ternary matrix and conventional bronze processed samples. We found that the In element would become more attractive solute element than Zn element for the internal reinforcement ternary matrix.

Bronze-processed Cu-Nb/Nb3Sn wires, composed of theNb-rod-method Cu-Nb reinforcing stabilizer around the Nb3Sn filament bundle, have superior mechanical strength after Nb3Sn reaction heat-treatment at both low temperature and room temperature. The Cu-Nb/Nb3Sn wires are able to be applied to the react-and-wind (R&W) process. Moreover, the non-Cu- Jc values of Cu-Nb/Nb3Sn wires are remarkably increased by using both the high-tin-bronze and appropriate pre-bending treatments. In this study, we developed Cu-Nb/Nb3Sn wires with various crosssectional structures which were improved the Ic values under tensile stress and transvers compression stress. Also, we confirmed Ic improvements by pre-bending treatments of a Cu-Nb/Nb3Sn triplet cable and small size Rutherford cables. Cu-Nb/Nb3Sn wires and cables can be designed on requirements of Nb3Sn coils.

Magnesium diboride (MgB2) wires are highly attractive for various applications, given their high critical transition temperatures. The brittle compound filaments are protected by metallic sheaths that provide sufficiently strong reinforcement. The mechanical reinforcement provided to the brittle wires compensates for this weakness and enhances their electromechanical properties. Therefore, MgB2 wires can be considered a promising alternative to high-temperature superconducting wires in low magnetic fields such as those used in magnetic resonance imaging, wind power generators, and other applications. This study focused on the evaluation of the electromechanical behaviors of multifilamentary MgB2 wires of various configurations and with various sheaths. The irreversible strain/stress limits for I-c degradation were evaluated under practical environmental conditions at a magnetic field of 2 T and a temperature of 20 K. The influences of the reinforcing materials on the electromechanical properties were investigated as was the temperature dependence of the mechanical properties.

We have fabricated Ba0.6Na0.4Fe2As2 round wires using powder-in-tube method and hot isostatic pressing, and evaluated their critical current density (J(c)). Transport J(c) at 4.2 K of the wire reached 76 kA/cm(2) and 24 kA/cm(2) at self-field and 100 kOe, respectively. The weak magnetic field dependence of J(c) in Ba0.6Na0.4Fe2As2 demonstrates the good potential of this material as superconducting wires. Detailed characterizations of the wires using magneto-optical imaging, X-ray diffraction measurements, and compositional mapping by energy-dispersive X-ray spectroscopy analyses are also presented.

© 2002-2011 IEEE. We have developed a high-performance (high-JC) Nb3Sn wire via a distributed tin (DT) method. Non-Cu JC of 1100 A/mm2 at 16 T, 4.2 K has been achieved by improving the Sn diffusion and optimizing the Ti content. With the future circular collider magnet planned by European Organization for Nuclear Research (CERN), the target of non-Cu JC is set to 1500 A/mm2 at 4.2 K, 16 T. For this target, we have chosen the DT method, which is a type of internal Sn method, and because it has no limitation of Sn solubility, higher JC can be expected. This paper finds that further improvement of JC can be realized by controlling the Sn diffusion condition and the ternary additive elements. By setting the Sn diffusion distance to lower than 48 μm, the Nb3Sn composition in multi-Nb modules becomes uniform and fine. In addition, by controlling the ternary element content (Ti) for improving the characteristics of the middle magnetic field, it is possible to achieve high JC at 16 T.

The wave function of conduction electrons moving in the background of a noncoplanar spin structure can gain a quantal phase-the Berry phase-as if the electrons were moving in a strong fictitious magnetic field. Such an emergent magnetic field effect is approximately proportional to the solid angle subtended by the spin moments on three neighboring spin sites, termed the scalar spin chirality. The entire spin chirality of the crystal, unless macroscopically canceled, causes the geometrical Hall effect of real-space Berry-phase origin, whereas the intrinsic anomalous Hall effect in a conventional metallic ferromagnet is of momentum-space Berry-phase origin induced by relativistic spin-orbit coupling. Here, we report the ordering phenomena of the spin-trimer scalar spin chirality and the consequent large geometrical Hall effect in the breathing kagome lattice compound Dy3Ru4 Al-12, where the Dy3+ moments form noncoplanar spin trimers with local spin chirality. Using neutron diffraction, we show that the local spin chirality of the spin trimers as well as its ferroic/antiferroic orders can be switched by an external magnetic field, accompanying large changes in the geometrical Hall effect. Our finding reveals that systems composed of tunable spin trimers can be a fertile field to explore large emergent electromagnetic responses arising from real-space topological magnetic orders.

In this study, ultrasonic measurements were performed on a single crystal of cubic PrNi$_2$Cd$_{20}$, down to a temperature of 0.02 K, to investigate the crystalline electric field ground state and search for possible phase transitions at low temperatures. The elastic constant $(C_{11}-C_{12})/2$, which is related to the $\Gamma_3$-symmetry quadrupolar response, exhibits the Curie-type softening at temperatures below $\sim$30 K, which indicates that the present system has a $\Gamma_3$ non-Kramers doublet ground state. A leveling-off of the elastic response appears below $\sim$0.1 K toward the lowest temperatures, which implies the presence of level splitting owing to a long-range order in a finite-volume fraction associated with $\Gamma_3$-symmetry multipoles. A magnetic field-temperature phase diagram of the present compound is constructed up to 28 T for $H \parallel$ [110]. A clear acoustic de Haas-van Alphen signal and a possible magnetic-field-induced phase transition at $H \sim$26 T are also detected by high-magnetic-field measurements.

We report the magnetocapacitance effect and the magnetostriction in the spin-crossover (SCO) complex [MnIII(taa)] (H3taa = tris(1-(2-azolyl)-2-azabuten-4-yl)amine). It is shown that huge enhancements of dielectric constant and lattice volume are caused by the field-induced SCO transition from the low-spin to the high-spin states in [MnIII(taa)]. In addition, the temperature-field phase diagram of the field-induced SCO in thermal equilibrium state is clarified up to 28 T. Our analysis suggests that a large magnetocaloric effect can also be caused by the field-induced SCO. These results indicate the possibility of multiple control of various material properties by the field-induced SCO.

Magnetic field stability and hysteresis can be serious problems in a high-field high-temperature superconducting magnet, because of the large shielding current effect. Accordingly, the field stability of a 25 T cryogen-free superconducting magnet (25T-CSM) was investigated. This particular system can generate a central field of 24.6 T in a 52-mm room-temperature bore. It was found that the magnetic field relaxation rate of the 25T-CSM with a Bi2Sr2Ca2Cu3Oy (Bi2223) insert was due to the flux creep effect, and was small enough to allow nuclear magnetic resonance (NMR) experiments. Based on these experiences in the development of the 25T-CSM, primitive designs for a 30 T cryogen-free superconducting magnet (30T-CSM) were suggested based on upgrading the present Bi2223 insert to a REBa2Cu3Oy (RE123, RE: rare earth and Yttrium) insert incorporating two co-wound RE123 tapes. It was confirmed that the prototype test coil could be operated at 4.2 K under large electromagnetic stresses of up to 540 MPa without experiencing degradation.

A superconducting joint of REBa2Cu3Oy (REBCO) coated conductors (CCs) has been demanded strongly to fabricate long length CCs for high field magnet applications such as nuclear magnetic resonance and magnetic resonance imaging. In the previous reports of superconducting joint, specimens of REBCO CCs were jointed via melting REBCO phases or via solid state diffusion of REBCO phases. In our study, we propose a new method of joint for REBCO CCs. A precursor layer is additionally deposited on GdBCO CC by a metal organic deposition process, and then two pieces of them are stuck together face-to-face and crystallized the precursor to form 123 phase under mechanical pressure in an oxygen atmosphere. The microstructures and temperature dependence of resistance of the jointed sample are characterized by a cross-sectional transmission electron microscopy (TEM) and a four-probe method, respectively. As a result, TEM observation reveals that two CCs are jointed together without formation of secondary phases at the joint interface. Also, temperature dependence of resistance shows Tc onset and Tc zero of 93 K and 82 K, respectively. Consequently, a superconducting joint has been completed successfully. The concept of this method is combining film growth and solid-state diffusion for the additionally deposited precursor layers.

REBa2Cu3Ox (REBCO, RE = rare earth) coated conductors (CCs) are promising as superconducting wires for high-field magnets because of their high in-field critical current density (Jc) performance and high tensile tolerance. Fujikura, Ltd., has been developing BaHfO3 (BHO)-doped EuBa2Cu3Ox. (EuBCO, Eu = Europium) CCs using a hot-wall-type pulsed-laser deposition (PLD) in order to further improve the in-field performance. Although a high deposition rate in the PLD process is necessary for mass production, it has been found that the in-field performance greatly differ depending on the deposition rate. In this study, flux pinning properties of BHO-doped EuBCO CCs fabricated with different deposition rates by the hot-wall PLD on IBAD substrates were investigated in detail. From the scaling characteristics of the flux-pinning force density (F-p) curve, the BHO precipitates in a fast deposition rate REBCO film appeared to behave like random pinning centers, which was almost consistent with a result of transmission electron microscope observation. On the other hand, from a decrease of anisotropy in the magnetic field angle dependence of J(c), it was also confirmed that BHO precipitates were not completely random pinning centers, that is, it has some kind of anisotropy. Considering the critical current value per production time, it was also found that the fast deposition rate is advantageous since the REBCO layer could be thicker.

Na-substituted 122-type iron-based superconductors were synthesized and fabricated into superconducting tapes and wires. Good J(c) performance of Ag-sheathed Sr0.45Na0.55Fe2As2 pressed tapes was demonstrated, and it was further improved by using AgSn as a sheath material. The improved J(c) characteristics are due to the reduction of the optimum sintering temperature by similar to 100 degrees C, which suppressed the reaction between the sheath and the core. However, the fabrication of good Sr(0.4)5Na(0.55)Fe(2)As(2) round wire using the hot isostatic press method failed, due possibly to the segregation of Na in the core. To avoid the segregation of Na, we fabricated round wires employing (Ba,Na)Fe2As2 rather than underdoped (Sr,Na)Fe2As2. J(c) in Ba0.6Na0.4Fe2As2 round wire processed 700 degrees C under the pressure of 175 MPa reached 1.5 x 10(5) and 2.0 x 10(4) A/cm(2) at self-field and 100 kOe, respectively. These values are more than fifty percent of the largest J(c) in (Ba,K)Fe2As2 round wires, demonstrating the excellent performance of (Ba,Na)Fe2As2 wires.

© 2019 Published under licence by IOP Publishing Ltd. The degradations of superconducting properties by mechanical strain on actual Nb3Sn wire are a serious problem for the future fusion magnets operated under high electromagnetic force. Recently, we succeeded to fabricate Cu-Sn-Zn ternary alloy matrix, which is high strength alloy by the Zn solid solution strengthening, based on the internal reinforced bronze process in order to investigate the mechanical strength improvement. In this study, the upper critical magnetic field (Hc2) under uniaxial tensile deformation on several bronze-processed Nb3Sn multifilamentary wires were evaluated. We confirmed that the peak tensile strain values, which obtained maximum Hc2 under uniaxial tensile deformation, were shifted to higher tensile strain region with increasing amount of Zn composition in the Cu-Sn-Zn ternary alloy matrix. The internal reinforced bronze matrix via solid solution strengthening would become attractive and simple method without reinforcement material.

There has been demand to lengthen superconducting joints of REBa(2)Cu(3)Oy coated conductors to realize magnet applications. In this work, jointed samples of GdBa(2)Cu(3)Oy coated conductors were prepared by crystallizing precursor films deposited by a pulsed laser deposition method under mechanical pressures, and influence of the pressures on superconducting and mechanical properties were investigated. While areal fraction and shearing stress of the joints were increased by increasing the pressure, superconducting properties were degraded. X-ray diffraction analysis indicated that the degradation was due to the formation of a secondary phase and decrease of oxygen contents in GdBa(2)Cu(3)Oy. (C) 2019 The Japan Society of Applied Physics

There has been demand to lengthen superconducting joints of REBa Cu O coated conductors to realize magnet applications. In this work, jointed samples of GdBa Cu O coated conductors were prepared by crystallizing precursor films deposited by a pulsed laser deposition method under mechanical pressures, and influence of the pressures on superconducting and mechanical properties were investigated. While areal fraction and shearing stress of the joints were increased by increasing the pressure, superconducting properties were degraded. X-ray diffraction analysis indicated that the degradation was due to the formation of a secondary phase and decrease of oxygen contents in GdBa Cu O 2 3 y 2 3 y 2 3 y

© 2019 IOP Publishing Ltd. A Nb 3 Sn wire which was manufactured for the ITER toroidal field coil conductor by a bronze route process was prepared for this study to investigate the effect of neutron irradiation on the critical current in a high magnetic field. The critical current of the virgin wire was measured in liquid helium with a 28 T hybrid superconducting magnet at the High Field Laboratory for Superconducting Materials in Tohoku University. It was also measured in vacuum with a heat conduction type variable temperature insert (VTI) at the International Research Center for Nuclear Materials Science at Tohoku University. The wire was irradiated at below 100 °C by fission neutrons at up to 4.9 × 10 22 neutrons m -2 (>0.1 MeV) at BR2 in Belgium, and the critical current after the irradiation was evaluated with a VTI in the range of 8-15.5 T. The difference of the critical current measured with two facilities was discussed, focussing on Joule heating of the sample holder which was made of pure copper, and the neutron irradiation effect on the critical current was investigated in the range of up to 15.5 T. The results show that the critical current measured in vacuum becomes lower than that in liquid helium because of the temperature rise of the sample holder where the sample was soldered, the critical current was increased by the neutron irradiation, and the current ratio (I C /I C0 ) was almost constant of 1.75 in the range of 8-15.5 T at around 4 K.

Recent progress of round wires using iron-based superconductors is reviewed. Both transport and magnetic J(c) have been enhanced by several techniques such as purification of polycrystalline powders, high-pressure sintering (HIP), and control of drawing and sintering conditions. The present record of transport J(c) of round wires is realized when the wire with 122-type compounds is processed at 175 MPa with a value at 4.2 K under 100 kOe being 38 kA/cm(2) using (Ba,K)Fe2As2. We also discuss HIP round wires of (Sr,K)Fe2As2 and recently fabricated CaKFe4As4. Details of the optimization of round wires to achieve large J(c) values are described.

We fabricated CaKFe4As4 round wires through a powder-in-tube method followed by hot isostatic pressing up to 175 MPa. The transport critical current density, J(c), at 4.2K almost reached the level for practical applications (100 kA cm(-2)) under self-field and reached 7.6 kA cm(-2) at 100 kOe. Magneto-optical images confirmed that the grains were well-connected at low temperatures. Although high-pressure sintering at high temperatures enhanced the J(c) and density of the core of the wires, X-ray diffraction and scanning electron microscopy-energy-dispersive X-ray spectroscopy analyses indicated that impurity phases were present in both the polycrystalline powder and the core of the wire. (C) 2018 The Japan Society of Applied Physics

YBa2Cu3Oy (YBCO) coated conductors were prepared by a metal organic deposition method using a chemical solution with tri-fluoroacetate salts. A salt of Zr was also added into the solution to introduce BaZrO3 (BZO) flux pinning centers into the YBCO films. The solution was coated on CeO2/LaMnO3/MgO/Gd2Zr2O7/Hastelloy substrates, and the coated films were crystallized at 1053 K. In this work, an additional holding temperature step was introduced during the crystallization of the films at 873 K to miniaturize the size of BZO nanoparticles, and then the effect of the heat treatment on the electrical conductivity of the film during the crystallization was examined. It was found that the conductivity of the film with the additional holding temperature step was approximately 1.7 times higher than that of the film fabricated without the one when the film temperature reached the crystallization temperature of 1053 K. In addition, the period until the conductivity saturated was three times smaller for the film fabricated with the additional holding temperature step. These results indicate that the growth temperature of YBCO shifts down and the holding period for YBCO crystallization can be shorter by introducing an additional holding temperature step.

We report the fabrication of (Sr,Na)Fe2As2 superconducting tapes by the powder-in-tube technique and their characteristics, including the transport critical current density Jc at 4.2 K up to 140 kOe, the magnetic Jc estimated from magnetic hysteresis curves, magneto-optical (MO) images, and scanning electron microscopy images. In a tape sintered at 875 °C for 1 h, the transport Jc reaches 26 kA/cm2 at 4.2K and 100 kOe for a field perpendicular to the tape surface. When the field is parallel to the tape surface, the magnetic Jc exceeds the practical level of 100 kA/cm2 at 4.2K below 25 kOe. Analysis of the MO images reveals clear current discontinuity lines in the core, indicating that the current flows homogeneously and the connections between grains are strong in the core.

Anisotropy of superconductivity is one of the fundamental physical parameters for understanding layered iron-based superconductors (IBSs). Here we investigated the anisotropic response of resistive transition as a function of thickness (d) in iron selenide (FeSe) based electric-double-layer transistors (EDLTs) on SrTiO3, which exhibit superconducting transition temperatures Tc as high as 40 K below d=10nm. According to the analyses of the in-plane (Hc2//) and out-of-plane (Hc2) upper critical fields (Hc2) and the magnetic field angle dependence of the resistance (Rs-θ) in ultrathin condition, we found that the anisotropy factor 0=Hc2///Hc2 is 7.4 in the thin limit of d∼1nm, which is larger than that of bulk IBSs. In addition, we observed the shorter out-of-plane coherence length ξc of 0.19 nm compared to the c-axis lattice constant, which implies the confinement of the order parameter in the one unit cell FeSe. These findings suggest that high-Tc superconductivity in the ultrathin FeSe-EDLT exhibits an anisotropic three-dimensional (3D) or quasi-two-dimensional (2D) nature rather than the pure 2D one, leading to the robust superconductivity. Moreover, we carried out the systematic evaluation of the anisotropic Hc2 against thickness reduction in the FeSe channel. The in-plane Hc2 as a function of normalized temperature T/Tc is almost independent of d until the thin limit condition. On the other hand, the out-of-plane Hc2 near T/Tc∼1 decreases with increasing d, resulting in the increase of 0 at around Tc to 32.0 at the thick condition of d=9.3nm, which is also confirmed by Rs-θ measurements. The counterintuitive behavior can be attributed to the degree of coupling strength between two electron-rich layers possessing a high superconducting order parameter induced by electrostatic gating at the top interface and charge transfer from SrTiO3 substrates at the bottom interface. Besides a large Hc2 for d=9.3nm exceeding 20 T even at T=0.8Tc, we observe the decoupling crossover of the two superconducting layers at low temperature, which is a unique feature for the high-Tc FeSe-EDLT on SrTiO3.

Transport properties were investigated for epitaxial films of superconductor NbN and half-metallic Heusler alloy Co2MnSi under high magnetic fields up to 17 T. The superconducting transition temperature Tc of NbN/Co2MnSi/Au trilayer films was determined to be 16.1K in the absence of magnetic field. Temperature dependence of the resistivity ρ(T) was measured in both magnetic fields parallel and perpendicular to the surface of NbN/Co2MnSi/Au trilayer films. The activation energy U0(H) for vortex motion of the trilayer films in both magnetic fields was well fitted above 5 T by the similar model with the exponents in the field dependence of the pinning force density. From the resistivity ρ(T) measurements under high magnetic fields, the upper critical field Hc2(0) at 0 K was also deduced to be μ0Hc2 ∥(0)=23.2T for the parallel magnetic filed and μ0Hc2 ⊥(0)=15.8T for the perpendicular magnetic field, respectively. The experimental results under magnetic fields revealed the superconductivity of the NbN layer was affected by the interplay between the superconducting NbN layer and the half-metallic Co2MnSi layer.

We fabricated (Ba,K)Fe2As2 superconducting wires and tapes using the powder-in-tube method and hot isostatic pressing (HIP). HIP wires and tapes showed a high value of transport critical current density (J c) exceeding 100 kAcm-2 at T = 4.2 K and the self-field. Transport J c in the HIP wire reached 38 kAcm-2 in a high magnetic field of 100 kOe. This value is almost twice larger than the previous highest value of J c among round wires using iron-based superconductors. Enhancement of J c in the wires and tapes was caused by improvement of the drawing process, which caused degradation of the core, formation of microcracks, weak links between grains, and random orientation of grains. Details of the effect of the improved fabrication processes on the J c are discussed.

A superconducting joint and a persistent current switch are the key technologies in persistent current operation for MRI and NMR magnets to obtain higher resolution and sensitivity. We prepared a persistent current system including a superconducting joint, a persistent current switch, and a double pancake coil, which were fabricated with a REBCO tape. The magnetic field decay behavior in persistent current operation were observed in a self-field and the external magnetic field of 1 T at 20 K. In the self-field, the decay rate of persistent current excited up to170 A reached to 1.7 ppm/h after five days operation. This means that the practical persistent current operation has been achieved successfully using the REBCO superconducting joint. From the voltage-current characteristics estimated from the magnetic field decay, it was found that the decay behavior of persistent current is dominated by the joint resistance in 1 T, but by the shielding current in the self-field.

We demonstrate the fabrication of (Ba,K) Fe2As2 superconducting wires through a powder-in-tube (PIT) method and hot isostatic press (HIP) technique. Several fabrication processes with different sintering conditions of pressure and time were examined. The measurements of magnetization, I-V characteristics, filling factor, and Vickers hardness clarified that the improvement of fabrication process and sintering condition is effective for the enhancement of critical temperature T-c, which leads to the enhancement of critical current density J(c) in HIP round wire. Wires which are fabricated in different processes show significantly different T-c and J(c). The core density, which can be enhanced by sintering at higher pressures, is the main key factor for enhancement of J(c) in HIP wires, similar to pressed tapes. Changing the sintering time also slightly affect J(c).

Transport properties of coated conductor tapes can be affected by stress and strain induced during fabrication, cool-down, and operation. Therefore, it is important to understand the strain/stress dependence of the critical current of the coated conductor tapes in the reversible region under magnetic field considering practical applications such as magnets and coils. In this paper, the strain characteristics of critical current, Ic, in high temperature superconducting (HTS)-coated conductor tapes fabricated using reactive co-evaporation by deposition and reaction reinforced with brass-foil laminate were investigated under external magnetic field at various cryogenic temperatures while applying uniaxial tension. Results show that Ic dependences on magnetic field and temperature were similar between brass-laminated coated conductor and Cu-stabilized tape. The irreversible strain limit and the irreversible stress limit of brass-laminated coated conductor tape increased as temperature decreased from 77 to 20 K. This behavior resulted from thermal hardening of the coated conductor tape at low temperature.

Bronze-processed Nb3Sn-based wires reinforced by Nb-rod-method Cu–Nb composite (Cu-Nb/Nb3 Sn) are applicable to react-and-wind (R&amp W)-processed Nb3 Sn coils. The Cu–Nb composite material functions as a reinforcing stabilizer during the superconducting magnet operation. Moreover, it suppresses the damage of reacted Nb3 Sn filaments due to applied stress during the manufacturing process such as the prebending treatment, the insulating, and the winding. New Cu–Nb/Nb3 Sn wires with high tin bronze (Cu-15.7wt%Sn-0.3wt%Ti) were investigated in the superconducting properties. The round wire of 0.8-mm diameter with prebending strain εp b = ±0.5% demonstrated a non-Cu-Jc of 1150 A/mm2 at 12 T and 4.14 K. The rectangular wire of 1.13 × 1.7 mm with εp b = ±0.5% applied from both directions of flatwise and edgewise had a non-Cu-Jc of 355 A/mm2 at 17 T. In this paper, we discuss the cross-sectional designs of Cu–Nb/Nb3 Sn wires suitable for practical R&amp W Nb3 Sn applications.

Quantum states characterized by nontrivial topology produce interesting electrodynamics and versatile electronic functionalities. One source for such remarkable phenomena is emergent electromagnetic field, which is the outcome of interplay between topological spin structures with scalar spin chirality and conduction electrons. However, it has scarcely been exploited for emergent function related to heat-electricity conversion. Here we report an unusually enhanced thermopower by application of magnetic field in MnGe hosting topological spin textures. By considering all conceivable origins through quantitative investigations of electronic structures and properties, a possible origin of large magneto-Thermopower is assigned to the strong energy dependence of charge-Transport lifetime caused by unconventional carrier scattering via the dynamics of emergent magnetic field. Furthermore, highmagnetic- field measurements corroborate the presence of residual magnetic fluctuations even in the nominally ferromagnetic region, leading to a subsisting behavior of field-enhanced thermopower. The present finding may pave a way for thermoelectric function of topological magnets.

In coil and magnet manufacturing that requires long length 2G coated conductor (CC) tapes, the development of joining methods for CC tapes with zero or low resistance is necessary. Nowadays, although it is said that CC tapes with around 1-km length were produced, it is still inadequate to produce cables and coil windings. Therefore, the development of a stable and reliable joint for long CC tapes having a low joint resistance (Rj) and the understanding of the magnetic field and temperature dependence of Rj is essential in the design of high field magnets and coils. In this study, the characteristics of joint resistance in REBCO CC tapes jointed using ultrasonic welding (UW) method were experimentally evaluated in various temperatures and magnetic field. The CC joints were also fabricated by hybrid welding, which incorporates soldering to the UW joint. The magnetic field and temperature dependencies of joint resistance of both types CC joints were measured. The acquired results showed that the joint resistance decreased with a decrease of temperature, and increased slightly as magnetic field applied increased.

Large hoop stress causes the deterioration of the transport properties of an high-temperature superconducting (HTS) coil winding. A high-strength pancake coil structure called “Yoroi coil” has been suggested. Yoroi-coil structure is expected to decrease the electromagnetic force applied to the winding and the volume of the coil frame at the outside of the winding. The effect of Yoroi coil on the hoop stress reduction has been investigated in an ideal “Yoroi-coil” structure without bolts and electrodes. In a practical “Yoroi-coil” structure, however, there are electrodes and bolts for fixing frames and reinforcing outer plates to the coil. In this study, the effect of the stress reduction of an HTS coil winding using the practical “Yoroi-coil” structure with the bolts and the electrodes was numerically investigated by 3-D-finite element method (FEM) analysis. Numerical results showed that the circumferential strain and hoop stress generated in the winding depended on the bolt location. Local large stress was applied to the winding, especially around the electrode. In order not to apply the local large stress to the winding, the mechanical strength of the electrode was very important. Adding bolts to the electrode was effective as the counterplan of stress concentration at the electrode. The optimal number of bolts at the electrode, however, largely depended on the thermal contraction during the cooling process of the coil.

The critical current performance of REBa2Cu3Oy-coated conductors must be isotropically improved for use in high-field coil-based applications operated at low temperatures less than 50 K. The high and isotropic critical current densities (Jc) with respect to the directions of the magnetic field under these operating conditions were achieved by a 3.8 vol.% BaHfO3 (BHO)-doped SmBa2Cu3Oy (SmBCO) film on a metallic substrate deposited with low-temperature growth (L-BHO film). In particular, the L-BHO film attained a high flux pinning force density over 1.5 TN/m3 at a temperature of 4.2 K. From the anisotropic scaling approach based on the effective mass model, a high density of random pinning centers in the L-BHO film plays an important role in achieving the high Jc performance at 4.2 K. In order to reveal the reason why the L-BHO film includes high density of random pinning centers, we compared the flux pinning properties and the dominant flux pinning centers between the L-BHO film and the other two samples: A 3.8 vol.% BHO-doped SmBCO film fabricated with relatively high-temperature growth, and a nondoped SmBCO film fabricated with low-temperature growth. From the comparison, it is revealed that the random pinning centers are induced by BHO nanorods.

The high upper critical field and low anisotropy of iron-based superconductors (IBS) make them particularly attractive for high-field applications, especially for the construction of next-generation nuclear magnetic resonance spectrometers, particle accelerators and high-field magnets. However, for practical use it is essential to make IBS materials into wire and tape conductors with sufficient current carrying capability, which is limited by misaligned grains inside the conductors. Here, based on a simple and low-cost powder-in-tube (PIT) method, we demonstrate a high transport critical current density (J c) reaching 1.5 × 105 A cm-2 (I c = 437 A) at 4.2 K and 10 T in Ba0.6K0.4Fe2As2 (Ba-122) tapes by texturing the grain orientation with optimized hot-press technique. The transport J c measured at 4.2 K under high magnetic fields of 27 T is still on the level of 5.5 × 104 A cm-2. Moreover, at 20 K and 5 T the transport J c is also as high as 5.4 × 104 A cm-2, showing a promising application potential in moderate temperature range which can be reached by liquid hydrogen or cryogenic cooling. All these J c values are the highest ever reported for IBS wires and tapes. The high-performance PIT Ba-122 tapes in this work suggest IBS to be a strong potential competitor of cuprate superconductors for the race of high-field applications in the future.

We have developed a manufacturing method for superconducting joints for Bi-2223/Ag tapes (DI-BSCCO Type-H, Sumitomo Electric Industries Ltd., Yokosuka, Japan). The tapes' superconducting joints were fabricated under pressure and heat treatment processes. The critical currents of these joints under relevant temperature conditions were measured to investigate their applicability to superconducting devices such as magnetic shields and superconducting magnets using current transport tests. In addition, the critical currents were investigated in a parallel magnetic field to the tape surface up to 5 T at 4 and 20 K. We will report the results of these experiments.

We investigated the spin polarization P of Co-based Heusler alloy Co2FeSi by epitaxial contact Andreev reflection (ECAR) spectroscopy using epitaxially grown superconductor NbN and Heusler alloy Co2FeSi layered devices. Ferromagnetic Co2FeSi possesses the highest Curie temperature (TC ? 1100 K) and the largest spontaneous magnetic moment (ps ? 6 μB) in the class of Heusler alloys. The ECAR measurements revealed that the P value of Co2FeSi was 54 ± 2% with a finite barrier parameter Z, indicating that an intrinsic P value in ECAR spectroscopy would exceed reported values in point-contact Andreev reflection spectroscopy. We therefore established not only the epitaxial integration of ferromagnetic Co2FeSi with superconductor NbN on an MgO substrate but also the fabrication and evaluation techniques of their ECAR devices. This highly versatile superconducting spintronic system enables fundamental superconducting spintronic studies, and it is also a candidate for practical superconducting spintronic devices.

We have investigated the in-field critical current properties in BaZrO3-added (Y, Gd) Ba2Cu 3O7-δ (YGdBCO)-coated conductor fabricated by using a multi-coating trifluoroacetates metal-organic deposition (MOD) method. We compared these properties with EuBCO-coated conductor fabricated by the pulsed laser deposition (PLD) method. The sample using the 30-nm once-coat-layer-thickness donce shows superior in-field Jc down to 4.2 K than that of the previous standard coating using 170-nm-thick layer for each coating. From the analysis of E-J characteristics, an analytical expression for Jc as a function T and B has been derived. It was confirmed that the high Jc region in sample with donce = 30 nm is widely spread in high-temperature and high-magnetic-field region. By comparison with the Jc properties of the BaHfO3 -added PLD CCs, the minimum Jc, which is estimated from magnetic field angle dependence, shows even higher value up to 5 T of magnetic field at 65 K and up to 3 T at 77 K. From these results, the new MOD-YGdBCO process using the thin once-coat-layer-thickness is very promising for the practically important mid field region such as 3 to 5 T.

Fujikura Ltd., Chiba, Japan, has developed stable and high-throughput production techniques for REBCO coated conductors (CC) by using large-area ion-beam-assisted-deposition (IBAD), and hot-wall type pulsed-laser-deposition (PLD). Recently, we have been attempting to dope BaMO3 (BMO, M: Zr or Hf) to REBCO layer in order to enhance in-field critical current density (Jc). In the course of optimizing deposition conditions of PLD for BaHfO3 doped EuBCO films on IBAD templates, "high Jc condition" and "high growth rate condition" were found. For samples prepared under these conditions, the magnetic field angle dependence of Ic and REBCO thickness dependence were investigated. Finally, we succeeded in fabricating a 300-m-long CC with uniform Ic distribution at the high-growth-rate condition. The longitudinal Ic homogeneity (defined as standard deviation divided by average) at 77 K evaluated by magnetization method was 2.6%, which is comparable to nondoped conductors. Transport in-field Ic for the both end points were 1756 and 1786 A/cm at 30 K, 2 T, respectively.

Outstanding characteristics with high T c and H c2 and small anisotropy in iron-based superconductors (IBSs) have triggered the development of superconducting wires and tapes using these novel superconductors. In this short article, developments and present status of round wires of 122-type IBSs are reviewed. By introducing hot-isostatic pressing (HIP) technique, J c in round wires of 122-type IBSs has been improved significantly. Further improvements have been realized by refining the fabrication process of the core material and introducing partial texturing of the wire core. The largest transport J c for round wires at 4.2 K at self-field and 100 kOe are 2.0x105 A/cm2 and 3.8x104 A/cm2, respectively. We also compare the J c characteristics of wires and tapes processed by HIP.

High magnetic field was introduced to improve the saturation magnetostriction of cast Fe81Ga19 alloy during annealing process. The effects of high magnetic field on the microstructure, magnetic domain structure and magnetostrictive properties were investigated in this paper. Results indicated that high magnetic field could transfer Ga atom from precipitation to parent phase. High magnetic field also enhanced [100] direction orientation of FeGa alloy. The saturation magnetostriction for Fe81Ga19 alloy annealed at 10 T increased by about five times compared to one without high magnetic field. The improvement in saturation magnetostriction is attributed to the enhanced [100] orientation and Ga diffusion under high magnetic field during annealing process. (C) 2017 Elsevier B.V. All rights reserved.

Because of pressing global environmental challenges, focus has been placed on materials for efficient energy use, and this has triggered the search for nanostructural modification methods to improve performance. Achieving a high density of tunable-sized second-phase nanoparticles while ensuring the matrix remains intact is a long-sought goal. In this paper, we present an effective, scalable method to achieve this goal using metal organic deposition in a perovskite system REBa2Cu3O7 (rare earth (RE)) that enhances the superconducting properties to surpass that of previous achievements. We present two industrially compatible routes to tune the nanoparticle size by controlling diffusion during the nanoparticle formation stage by selecting the second-phase material and modulating the precursor composition spatially. Combining these routes leads to an extremely high density (8 x 10(22) m(-3)) of small nanoparticles (7 nm) that increase critical currents and reduce detrimental effects of thermal fluctuations at all magnetic field strengths and temperatures. This method can be directly applied to other perovskite materials where nanoparticle addition is beneficial.

The fabrication of iron-based superconductors with high transport critical current density (J(c)) and low cost is a crucial determinant of whether they can be used for practical applications. In this paper, Cu/Ag composite sheathed Sr0.6K0.4Fe2As2 (Sr122) tapes and Ba0.6K0.4Fe2As2 (Ba122) round wires were fabricated via an ex situ powder-in-tube method and heat-treated by the hot pressing and hot isostatic pressing process respectively. In order to thoroughly reveal the application potential of Cu/Ag composite sheathed '122' iron pnictide superconductors, transport J(c) of tapes and wires in high fields at temperatures up to 25 K was measured. High transport J(c) of 4.4 x 10(4) A cm(-2) at 4.2 K and 3.6 x 10(3) A cm(-2) at 20 K in 10 T was achieved in Cu/Ag composite sheathed Sr122 tapes. Transport J(c) of Ba122 wires is 9.4 x 10(3) A cm(-2) at 4.2 K and 1.9 x 10(3) A cm(-2) at 20 K in 10 T. These results demonstrate the great potential of Cu/Ag composite sheathed '122' iron pnictide superconducting tapes and wires for high-field applications at intermediate temperatures around 20 K, which can be easily obtained by using cryocoolers.

Nanorods, which are nano-scaled columnar-shape precipitates, have recently been used to improve critical current density J(c) in magnetic fields for REBa2Cu3Oy (RE123, RE: rare earth element) high temperature superconducting tapes/films. However, the flux pinning mechanism of the nanorod is not clear yet. We investigated the J(c) and resistivity rho properties in detail and discussed the flux pinning properties on the basis of the flux pinning state diagram for high-quality Sm123 films with well-aligned 5.6 vol% BaHfO3 nanorods. Plateaus were observed in the field dependence of J(c) and rho at high temperatures above the delocalization temperature. This suggests that nanorod pinning becomes effective in the vortex liquid phase and it grows up when the temperature decreases toward the delocalization temperature. In the 'many-nanorod' state in the high temperature region above the delocalization temperature, double peaks in the F-p curves appear due to the coexistence of nanorod pinning and random pinning. At low temperatures below 70 K, however, the well-scaled F-p curves at low fields and temperature dependent (non-scaled) normalized F-p curves are observed. From detailed analysis using the cooperation model of the random and the correlated pinning centers, we found that nanorod pinning is dominant below the matching field and the cooperation between nanorod pinning and random pinning determines the high field J(c) properties above the matching field.

A high flux pinning performance was obtained for a 3.8 vol% BaHfO3-doped SmBa(2)Cu(3)Oy superconducting film on a metallic substrate. At a temperature of 77.3 K, an irreversibility field of 16.8 T and the maximum flux pinning force density of 32.5GN/m(3) in fields applied parallel to the c-axis of the film were achieved, which are the highest values reported thus far for REBa(2)Cu(3)Oy films, to our knowledge. The introduction of wellaligned BaHfO3 nanorods with a high number density into REBa(2)Cu(3)Oy films with little or no degradation of the critical temperature is an effective method for improving the flux pinning performance. (C) 2017 The Japan Society of Applied Physics

REBa2Cu3Oy superconducting tapes are appropriate for high field magnet applications at low temperatures (i.e. below liquid nitrogen temperature). To clarify the morphology and the volume of the effective pinning center at low temperatures, we used a low-temperature growth technique to fabricate SmBa2Cu3Oy (SmBCO) films with various amounts of BaHfO3 (BHO) nanorods onto MgO-buffered metal substrates produced by ion-beam-assisted deposition; we investigated their flux pinning properties using a 25 T cryogen-free superconducting magnet that was recently developed at Tohoku University. According to the microstructural analysis using transmission electron microscopy, the BHO nanorods have a content-dependent morphology and are aligned for the higher content. The inclined and discontinuous BHO nanorods were observed in SmBCO films with BHO contents up to 3.8 vol%; they show an excellent flux pinning force density (1.5 TN m(-3) at 21 T and 4.2 K) even when the magnetic field is perpendicular to the films. Based on the effective mass model for the flux pinning, the random pinning centers are dominant at low temperatures. The correlated flux pinning is stronger for aligned nanorods; however, the random pinning center becomes weaker in the 4.5 vol% BHO-doped films. Therefore, the optimal BHO doping level is approximately 3.8 vol% in terms of the amplitude of the critical current density and the anisotropy from 4.2 K to 20 K because this provides the best mixture of correlated and random flux pinning centers.

Conventionally, delta T-c type (order parameter modulation) and delta l type (mean free path modulation) pinning mechanisms have been proposed to explain the temperature dependence of the flux pinning of superconducting materials. According to previous studies, it is assumed that the temperature dependence of J(c) of REBa2Cu3O7 (REBCO, RE = Y, Gd, Sm, etc) films without artificial pinning centers (APCs) is delta l type, but it is unidentified when APCs are introduced into the films. In this paper, GdBCO thin films doped with BaHfO3 (BHO) deposited on LaAlO3 substrates by pulsed laser deposition were studied. A target exchange method was used to alternately ablate two targets of pure GdBCO and BHO for introducing nanorods as APCs into GdBCO films. Since the insulative BHO acts as a strong pinning center, the delta T-c pinning mechanism is expected for the temperature dependence of J(c) of these thin films. However, the experimental results showed that the J(c) of the films with BHO nanorods was determined by the delta l pinning mechanism over a wide temperature range. In order to explain these unexpected results, we examined the pinning mechanism by nanorods based on a resultant pinning force model.

The effect of incorporation of nanoscale Y2BaCuO5 (Y211) inclusions on the vortex pinning properties of YBa2Cu3O7-delta (YBCO or Y123) superconducting thin films is investigated in detail on the basis of variation of critical current density (JC) with applied magnetic field and also with the orientation of the applied magnetic field at two different temperatures: 77K and 65K. Surface modified target approach is employed to incorporate nanoscale Y211 inclusions into the superconducting YBCO matrix. The efficiency of Y211 nanoinclusions in reducing the angular anisotropy of critical current density is found to be significant. The observed angular dependence of the critical current density is discussed on the basis of mutually occupied volume by a vortex and spherical and/or planar defect. A dip in J(C) near the ab-plane is also observed which has been analyzed on the basis of variation of pinning potential corresponding to a spherical (3-D) or planar (2-D) pinning center and has been attributed to a reduced interaction volume of the vortices with a pinning center and competing nature of the potentials due to spherical and planar defects. Published by AIP Publishing.

In order to apply REBa2Cu3Oy (REBCO, RE = rare earth elements or Y) coated conductors in high magnetic field, coil-based applications, the isotropic improvement of their critical current performance with respect to the directions of the magnetic field under these operating conditions is required. Most applications operate at temperatures lower than 50 K and magnetic fields over 2 T. In this study, the improvement of critical current density (J(c)) performance for various applied magnetic field directions was achieved by controlling the nanostructure of the BaHfO3 (BHO)-doped SmBa2Cu3Oy (SmBCO) films on metallic substrates. The corresponding minimum J(c) value of the films at 40 K under an applied 3 T field was 5.2 MA cm(-2), which is over ten times higher than that of a fully optimized Nb-Ti wire at 4.2 K. At 4.2 K, under a 17.5 T field, a flux pinning force density of 1.4 TN m(-3) for B//c was realized; this value is among the highest values reported for REBCO films to date. More importantly, the F-p for B//c corresponds to the minimum value for various applied magnetic field directions. We investigated the dominant flux pinning centers of films at 4.2 K using the anisotropic scaling approach based on the effective mass model. The dominant flux pinning centers are random pinning centers at 4.2 K, i.e., a high pinning performance was achieved by the high number density of random pins in the matrix of the BHO-doped SmBCO films.

We investigated the effect of Co doping on the superconducting properties and microstructure of ternary (Nd0.33Sm0.33Gd0.33)(1.18)Ba2.12Cu3.09O7-delta (NSG123) filaments. Filamentary NSG123 superconductors chemically doped with a nominal Co concentration between 0 and 0.5 at% were prepared by a solution spinning method. Filamentary samples were partially melted at 1050 degrees C for 30 min and rapidly cooled to 910 degrees C at a rate of 60 degrees C h(-1) under an atmosphere of flowing 0.1% O-2+Ar gas. The filamentary NSG123 superconductor with 0.05 at% Co exhibited a high critical current density (J(c)) of 2.0 x 10(4) A cm(-2), whereas the pure NSG123 sample exhibited a maximum transport Jc value of 3.0 x 10(3) A cm(-2). Furthermore, the sample doped with 0.05 at% Co maintained a high transport Jc value of 4.5 x 10(3) A cm(-2) under an applied magnetic field up to 17 T at 77K, whereas the superconductivity of the pure sample disappeared at approximately 11 T. The Co-doped samples also exhibited low electrical resistivity at normal state compared to that of the pure sample. These results confirm that Co doping effectively enhances the Jc and connectivity in the microstructure of filamentary NSG123 samples subjected to a heat treatment with rapid cooling.(C) 2017 Elsevier B.V. All rights reserved.

We have fabricated (Sr,Na)Fe2As2 tapes by optimizing the sintering temperature and time, and characterized their transport Jc at 4.2 K up to 14 T. The transport Jc for the (Sr,Na)Fe2As2 tape sintered at 875°C reaches 4.9 × 104 A/cm2 under self-field, and it is suppressed only down to 1.9 × 104 A/cm2 even at 14 T. We succeeded in fabricating very field-insensitive tapes, which are very promising for practical applications. We also report compositional distributions and magneto-optical images of current distribution in tapes. Magneto-optical imaging of the core of the tape clarified inhomogeneous intergranular current distributions.

High critical current density (J(c)) was achieved in low temperatures of 40 and 20 K in YBa2Cu3O7-delta films containing elongated BaHfO3 nanorods and stacking faults, which were prepared using pulsed laser deposition and post-oxygen-annealing. The YBa2Cu3O7-delta + BaHfO3 film exhibited global pinning force maximum (F-p,F-max) of 312 GN m(-3) at 40 K and 619 GN m(-3) at 20 K due to strong pinning by nanorods. These F-p,F-max values are comparable to those in the previously-reported high-Jc films with almost the same matching field. Thus, the present study demonstrated that YBa2Cu3O7-delta superconducting matrix realized high J(c) in low temperatures like other REBa2Cu3O7-delta (RE = Gd, Sm, (Gd, Y)). Furthermore, high J(c) was obtained also for magnetic field parallel to the ab-plane, indicating hybrid pinning of nanorods and stacking faults. Thus, the stacking faults significantly affected the J(c) properties in the YBa2Cu3O7-delta films containing nanorods. The present film structure with strong c-axis correlated pinning and hybrid pinning is effective in simultaneous control of J(c) values and J(c) anisotropy in YBa2Cu3O7-delta films.

We fabricated a 78 m long six-filament MgB2 wire using an internal magnesium diffusion (IMD) method. The microstructure and transport performance of this six-filament IMD wire were investigated. Layer J(c) values as high as 5 x 10(4) A cm(-2) at 4.2 K and 10 T are obtained for short samples cut from this long wire. In order to investigate the uniformity and application potential of this long wire produced by an IMD technique, we made two solenoid coils by a wind-andreact method using two wires of 26 m in length cut from this long wire. One coil exhibited Jc of 4.5 x 10(4) A cm(-2) at 10 T and 4.2 K, which is similar to the short samples. Another coil generated a central field of Bo = 1.67 T and a maximum magnetic field of Bm = 1.8 T at zero external field. These results demonstrate that the 78 m long multi-filamentary wire has sufficient longitudinal homogeneity for designing MgB2 coils.

A 25 T cryogen-free superconducting magnet (25T-CSM) was developed and installed at the High Field Laboratory for Superconducting Materials (HFLSM), IMR, Tohoku University. The 25T-CSM consists of a high-temperature superconducting (HTS) coil and a low-temperature superconducting (LTS) coil. A high-strength CuNb/Nb3Sn Rutherford cable with a reinforcing stabilizer CuNb composite is adopted for the middle LTS section coil. All the coils were impregnated using an epoxy resin for conduction cooling. Initially, a GdBa2Cu3Oy (Gd123) coil was designed as the HTS insert coil, and then a Bi(2)Sr(2)Ca(2)Cu(3)Oy (Bi2223) coil was also developed. The HTS insert and the LTS (CuNb/Nb3Sn and NbTi) outsert coils are cooled by two 4K GM and two GM/JT cryocoolers, respectively. The LTS coils successfully generated a central magnetic field of 14 T at an operating current of 854 A without any training quench. The Gd123 coil generated 10.15 T at an operating current of 132.6 A in the absence of a background field. Subsequently, the operating current of the Gd123 insert was increased in a step-by-step manner under a background field of 14 T. The Gd123 coil could be operated up to 124.0 A stably, which corresponds to 23.55 T, but quenched at around 124.6 A (23.61 T). The Bi2223 insert coil using a Ni-alloy reinforced Bi2223 tape successfully generated 11.48 T at an operation current of 204.7 A in a stand-alone test and 24.57 T in a background field of 14 T. The differences between the calculated and the measured values of the central magnetic fields are about 0.4 T for the Gd123 insert and 0.1 T for the Bi2223 insert around 24 T.

A cryogenic cooling system for a 25-T cryogen-free superconducting magnet (25T-CSM) was developed. The 25T-CSM consisted of 11-T high-Tc superconducting (HTS) coils and 14 T low-Tc superconducting (LTS) coils. The HTS coils, which had about 10-W AC-loss, were cooled to about 10 K by two GM cryocoolers and a helium gas circulation unit. In this system, the gas flow rate had an optimized value and was controlled by amass flow controller. The LTS coils were cooled to about 4 K by two GM/JT cryocoolers. Each GM/JT cryocooler had two sets of precooling gas lines. These pre-cooling lines were cooled by two single-stage GM cryocoolers and the HTS coil cooling system via a heat exchanger, between the HTS gas circulation line and the LTS cool-down line. As a result of cooling test of the 25T-CSM, the LTS coils and HTS coils were cooled from room temperature to 4.3 K and 4.6 K within 164 h (about seven days), respectively. During the excitation test of the HTS coils, their maximum temperature increased to 7.6 K, which was still sufficiently low.

We have developed and installed an 11 T Bi2Sr2Ca2Cu3Oy (BSCCO) insert coil for a 25 T cryogen-free superconducting magnet (25 T-CSM). The insert coil composed of a stack of 38 double pancakes was wound with DI-BSCCOTM Type HT-NX conductors which had high critical tensile strength of 400 MPa at 77 K. The insert coil is cooled by two two-stage Gifford-McMahon cryocoolers and the operation temperature of the insert coil is designed to be below 10 K. The insert coil has successfully generated 11.48 T without a background field and 24.57 T with a 14.0 T background field produced by outer low temperature superconducting coils in a 52 mm room temperature bore. Before the performance test, we fabricated and tested an R&amp D insert coil consisting of two double pancakes, which had the same configuration as the actual insert coil pancakes do, to confirm mechanical strength of the conductors.

Our project is to build a high-temperature superconductor (HTS) insert generating 10 T in its center and expected to work inside a 20 T background field. In the first step, the background field is provided by a resistive magnet to avoid major coupling issues. Nevertheless, the final goal is to have the insert operating inside an LTS magnet to yield a 30 T-class all-superconducting magnet. The insert is designed as a stack of pancakes and double pancakes wound with REBCO tapes. Preliminary tests are performed on a deposited polyimide insulated coil, although design has started with metallic-insulation coils. This paper presents results obtained on a double pancake insulated HTS coil that was tested inside both a resistive coil at LNCMI and a superconducting magnet at Tohoku University. The electrical and mechanical behaviors of the sample coil are discussed, and margins are estimated taking into account the field distribution on each point of the coil. Results are encouraging as the coil passes 370 A (82.9% lc) self-field and 260 A (68.6% lc) under 8 T. Moreover, the coil is still working even after the outer magnet quench.

The effect of the microstructure on the flux pinning state in BaHfO3(BHO) nanorods doped SmBa(2)Cu(3)Oy( SmBCO) films was investigated using high fieldmagnets with a rotator stage. Films with aligned nanorods showed the strong flux pining along c-axis, which disappears at high magnetic fields. This behavior corresponds to the existence of the Bose glass state in the film, which is also supported by the behaviors of the activation energy and the critical exponent of the liquid-glass transition estimated from the transport properties. Moreover, the field dependencies of the flux pinning force are explained by the flux pinning models for the correlated nanorods and the random pinning centers in the films with the aligned and the firework nanorods structures. The strong relationship is confirmed between the structure of the nanorods and the pinning properties in the BHO doped SmBCO films.

Iron-pnictides are hotly studied since 2008 in the superconducting materials research area, due to their special properties and unclear mechanism. Big achievement has been made in the pnictide research during the past years. For practical uses, pnictide superconductor should be fabricated in a form that can be used for different devices. In this paper, 100-m class 7-core Sr0.6K0.4Fe2As2 (Sr122) tapes have been made by using the powder-in-tube technique, which is reported for the first time. Clearly, an average J(c) of 1.3 x 10(4) A/cm(2) at 10 T was reached over the 115-m length, showing a high property and good uniformity of 100-m level Sr122 tapes. Using the 10-m long Sr122 tapes, two double-pancake coils were fabricated by a wind and reaction technique. No transport current could be measured for the coil made from the monofilamentary tape. However, transport I-c was obtained in the coil made from the 7-filamentary tape. The factors that affect the superconducting property of the coil were discussed in this paper.

Superior mechanical and electromechanical properties of the coated conductor (CC) tapes made them a viable option for coil and magnet applications. In this study, investigation on the characteristics of I-c under different B, T, and epsilon conditions were conducted. Tensile load was applied using the Katagiri type test rig under different high magnetic field and low-temperature conditions wherein irreversible strain, epsilon(irr) was determined. The I-c behavior with magnetic field of both reactive co-evaporation by deposition and reaction (RCE-DR) andmetal organic chemical vapor deposition (MOCVD) processedCCtapes at different T and B were obtained. The critical irreversible strain limit eirr of Cu-stabilized REBCO CC tape increased with decreasing temperature from 65 K down to 20 K. This behavior was resulted from the increased yield strength of the CC tape due to thermal hardening effect.

MgB2/Nb/Monel multifilamentary wires were fabricated using amorphous and crystalline boron powders by an internal Mg diffusion (IMD) process. The microstructure, morphology, and the critical current density (J(c)) of the reacted MgB2 layers in the multifilamentary wires made using different boron powders were analyzed and compared. The purity of boron powder influences greatly the transport properties of MgB2 wires. The Jc as high as 6.1 x 104 A cm(-2) at 4.2 K, 10 T is obtained in the seven-core MgB2 wire made using amorphous boron powder. The IMD-processed seven-core MgB2 wire fabricated using high-purity crystalline boron powder has also a high Jc of 4.9 x 104 A cm(-2) at 4.2 K and 10 T. However, the seven-core MgB2 wire prepared using low-purity crystalline boron powder has a relatively low Jc compared with the others, which is expected to allow the fabrication of large-scale and low-cost superconducting wires for practical application. The further improvement of layer J(c), nonbarrier Jc and engineering critical current density J(e) is necessary because of the irregular subfilaments and the inhomogeneous thickness of the reacted MgB2 layer in multifilamentary wires.

The effect of high magnetic field on microwave absorption of cobalt particles, dispersed uniformly in a paraffin matrix, was investigated in microwave range of 2-18 GHz. The high magnetic field during annealing process could significantly improve the dielectric loss of cobalt particles. The maximum reflection loss of the cobalt particles annealed in high magnetic field is -25.00 dB while the coating thickness is 2.5 mm, which is raised by 62.87 % compared with the ones annealed without high magnetic field. The enhancement in microwave absorption may be ascribed to the improved dielectric loss of cobalt particles induced by high magnetic field.

The critical current density (Jc) of YBa2Cu3Oy (YBCO)-coated conductors in a magnetic field can be enhanced by the. doping of flux-pinning centers in a metal organic deposition (MOD) process with trifluoroacetates (TFA). The size of these flux-pinning centers should be less than 10 nm to achieve commercial use due to the coherence length of YBCO at 77 K. In this paper, BaHfO3 (BHO) nanoparticles were introduced into YBCO films using the TFA-MOD method. Microstructures and the. Jc properties of the films prepared using. a two-step heating process at crystallization were compared with film prepared using. a conventional one-step heating process. The two-step heating process produced 15 nm average-sized BHO nanoparticles in the film compared to 19 nm nanoparticles in a film prepared using. a. one-step process. It was revealed that the size of nanoparticles in the films could be miniaturized by improving the. heating processes in the MOD method, and the miniaturized nanoparticles could contribute to increased Jc in magnetic fields. The mechanism of miniaturization is. also discussed based on microstructure observations of quenched films.

前回の(Sr,Na)Fe_2_As_2_テープの高磁場測定において、磁場依存性の低いテープの作製に成功した。しかし、その臨界電流密度は実用化レベルである10^5 A/cm^2には届いていない。そこで今回は線材コアの高密度化を図ることで、臨界電流密度のさらなる向上を目指す。また、完成した線材において、磁気光学イメージングによる評価も報告する予定である。

We report structural, magnetic, and transport properties of epitaxial films of superconductor NbN and highly spin- polarized Heusler alloy CO2Fe0.4Mn0.6Si (CFMS). The saturation magnetization Ms of the film was 870 emu/cm(3), which is slightly smaller than values reported earlier. With an increase of CFMS thickness d(CFMS), the sheet resistance R-sq decreased and the upper critical field mu H-0(c2)(0) decreased monotonically, while the superconducting transition temperature Tc exhibited a minimum around d(CFMS) = 3 nm. A theoretical analysis of superconducting fluctuations revealed that the high spin polarization of the CFMS layer affected the pair breaking parameter d, the superconducting coherence length. (0), and the diffusion coefficient D.

This paper reviews the progress of studies to determine optimum shapes of the artificial pinning center (APC) of REBa2Cu3Oy thin films and coated conductors towards superconducting magnets operating at temperatures of 77 K or less. Superconducting properties vary depending on the kind and quantity of BaMO3 materials. Therefore, we study changes in the shapes of nanorods that are due to the difference in the quality of additives and growth temperature. In addition, we aim to control the APC using an optimum shape that matches the operating temperature. In particular, we describe the shape control of nanorods in SmBCO thin films and coated conductors by employing lower temperature growth (LTG) technology using seed layers. From the cross-sectional transmission electron microscopy observations, we confirmed that using the LTG method, the BaHfO3 (BHO) nanorods, which were comparatively thin and short in length, formed a firework structure in the case of SmBCO films with coated conductors. The superconducting properties in the magnetic field of the SmBCO-coated conductor with the optimum amount of BHO showed that F-p(max) = 1.6 TN m(-3) on a single crystalline substrate and 1.5 TN m(-3) on metallic substrate with a biaxially textured MgO layer fabricated by ion-beam assisted deposition method tape 4.2 K.

The pin size effect and mixed pinning of nanorods and matrix defects are discussed for YBa2Cu3O7-delta films containing nanorods. BaSnO3 nanorods with a diameter of 11 nm and BaHfO3 nanorods with a diameter of 7 nm were prepared, and critical current density (Jc) and resistivity were measured in the films. When the coherence length was larger than the nanorod size at high temperatures near the critical temperature, the trapping angle and activation energy of the vortex flow depended on the nanorod diameter. At a moderate temperature of 65-77 K, the pin size effect on Jc disappeared since the coherence length became smaller than the nanorod size. At a low temperature of 20K, the contribution from matrix pinning became comparable to that of nanorods in a high magnetic field due to the small coherence length. Thus, the temperature-dependent coherence length caused the pin potential situation to vary significantly, namely, the pin size effect and mixed pinning, which strongly affected vortex pinning in YBa2Cu3O7-delta containing nanorods.

We measured critical currents and irreversibility fields of Ag-sheath (Sr, K) Fe2As2 tapes at various temperatures, magnetic fields and magnetic field directions. The sample tapes were made by a powder-in-tube with a hot-press process in the final heat treatments. The inverse anisotropy in J(c) in comparison with the effective mass anisotropy can be found in the hot-pressed (Sr, K) Fe2As2 tapes. We found that the broad c-axis peak in angular dependence of J(c) appears in a low field region and it decreases with increasing field and temperature. In addition, the crossover of J(c) anisotropy appears around 10 T at 4.2 K and the crossover field decreases with increasing temperature. It is considered that these unusual anisotropic J(c) behaviors may be originated by the strong random pinning center which is larger than the coherence length.

BaMO3 (BMO, M: Zr or Hf)-doped REBCO-coated conductors were prepared by Hot-Wall PLD method on IBAD template. The highest J(c) was obtained for BaHfO3- doped EuBCO. Quite strong pinning force density ( F-p) of 1.6 TN/ m(3) was observed at 4.2 K, 15 T, for a sample deposited with growth rate of similar to 5 nm/s. It had strong c-axis correlated pinning at the temperature above 20 K with the matching field of similar to 5 T. A 45-m- long sample was formed by productive high growth rate over 20 nm/s, being consistent with commercial nondoped conductors. It had also large I-c of 1687 A/cm and Fp of 0.96 7 TN/ m3 was obtained at 4.2 K, 15 T. The end-to-end longitudinal Ic homogeneity ( sigma) at 77 K in low field was evaluated to be 2.2% by magnetization. Transport in-fields Ic were also measured for the both end points at 30 K, 2 T, resulting in 1863 and 1883A/cm, respectively

AC losses in a high temperature superconducting (HTS) insert coil for 25-T cryogen-free superconducting magnet during its initial energization are numerically calculated under the assumption of slab approximation. The HTS insert consisting of 68 single pancakes wound using coated conductors generates a central magnetic field of 11.5 T in addition to the contribution of 14.0 T from a set of low temperature superconducting (LTS) outsert coils. Both the HTS insert and the LTS coils are cooled using cryocoolers, and energized simultaneously up to the central field of 25.5 T with a constant ramp rate for 60 min. The influences of the magnitudes and orientations of locally applied magnetic fields, magnetic interactions between turns and transport currents flowing in the windings are taken into account in the calculations of AC losses. The locally applied fields are separated into axial and radial components, and the individual contributions of these field components to the AC losses are simply summed up to obtain the total losses. The AC losses due to the axial fields become major in the beginning of energization, whereas the total losses monotonically increase with time after the AC losses due to the radial fields become major. (C) 2016 Elsevier Ltd. All rights reserved.

Effects of high magnetic field annealing on microstructure and multiferroic properties of Bi1-xLaxFeO3 have been investigated. It is found that the grain size of these materials prepared by annealing under a magnetic field becomes small and uniform. The magnetic coercive field of the samples synthesized under a magnetic field is higher than that of the samples prepared in the absence of magnetic field. It is also found that application of a magnetic field during annealing process can suppress the leakage behavior in Bi1-xLaxFeO3. As a result, ferroelectric properties are significantly improved by magnetic field annealing and the Bi0.85La0.15FeO3 specimens heated at 5 T had the best remnant polarization value (2P(r)) of 46.8 mu C/cm(2), around 8 times higher than that of the samples fabricated at 0 T. (C) 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Effects of densification of precursor disks on the density of residual voids and critical current properties for YBCO melt-textured bulk superconductors were systematically investigated. Six YBCO bulks were prepared from precursor pellets with different initial particle sizes of YBa2 Cu3Oy (Y123) powder and applied pressures for pelletization. It was revealed that use of finer Y123 powder and consolidation using cold-isostatic-pressing (CIP) with higher pressures result in reduction of residual voids at inner regions of bulks and enhance J(c) especially under low fields below the second peak. (C) 2016 Elsevier B.V. All rights reserved.

(Ba,K) Fe2As2 superconducting wires and tapes are fabricated by using hot isostatic pressing (HIP) technique, and their superconducting properties are studied. In the HIP round wire, transport critical current density (J(c)) at 4.2 K has achieved record-high value of 175 kA cm(-2) at zero field, and exceeds 20 kA cm(-2) even at 100 kOe. Improvement of polycrystalline powder synthesis may play a key role for the enhancement of J(c). In the HIP tape, even larger transport Jc of 380 kA cm(-2) is realized at zero field. Based on magnetization and magneto-optical measurements, possible further enhancement of J(c) is discussed.

Monofilament MgB2/Nb/Monel wires were fabricated using three different MgB4 precursors by an internal Mg diffusion (IMD) process. The wire geometry and heat-treatment conditions were optimized in order to improve the critical current density (J(c)) of the MgB2 wire. The influences of the quality of MgB4 powders, such as the particle size and MgO impurity, on the microstructure and superconducting properties of the wires were discussed. Although there were small amounts of voids, unreacted MgB4 particles and MgO impurity existed in the superconducting layers, and the transport layer J(c) of the wire with the MgB4 precursor reached 3.0 x 10(4) A cm(-2) at 4.2 K and 10 T, which was comparable to that of IMD-processed wires fabricated using boron precursors. Both the non-barrier J(c) and engineering J(c) of MgB2 wire made using a MgB4 precursor were enhanced due to the improved grain connectivity and the enlarged fill factor.

Cu is an attractive metal to use as the sheath material for Sr1-xKxFe2As2 superconducting wires. However, unfavorable reactions between the Cu sheath and the superconducting core severely affect the superconducting performance. In this work, Cu-sheathed Sr1-xKxFe2As2 tapes are fabricated at a low temperature of 740 degrees C using a hot pressing method. A systematic correlation between the sintering holding time, reaction layer, microstructure, and superconducting properties is investigated. It is found that the samples that are hot pressed for 60 min exhibit the highest transport critical current density (J(c)), i.e. 3.5 x 10(4) A cm(-2) in 10 T and 1.6 x 10(4) A cm(-2) in 26 T at 4.2 K, respectively. Furthermore, the engineering critical current density (J(e)) value of the samples is over 1.0 x 10(4) A cm(-2) in 10 T at 4.2 K. This is by far the highest J(c) reported for Cu-sheathed iron-based superconducting tapes.

Transport measurements revealed flux pinning properties and vortex phases in SmBa2Cu3O7-delta (Sm123) superconducting films with BaHfO3 nano-rods on LaAlO3 substrates. The films have large matching fields B-Phi up to 9.9 T, nano-rod diameters of similar to 6 nm, and a slight T-c degradation with T-c similar to 91.8K by using the low temperature growth technique. According to the transport results, a small critical exponent similar to 4 indicates the presence of a Bose-glass phase in the films. Double peaks of the flux pinning force density are unexpectedly observed at high temperatures over 80 K, which is accompanied by steep drops of the crossover magnetic fields between the single vortex pinning and the collective pinning states. The drops are explained by the delocalization of the vortex where the vortex is pinned by many nano-rods in the single vortex pinning state. From the viewpoint of the vortex delocalization, we conclude that B-Phi should be less than 11 T for applications at liquid nitrogen temperature. Published by AIP Publishing.

A large-bore 14-T CuNb/Nb3Sn Rutherford coil was developed for a 25 T cryogen-free superconducting magnet. The magnet consisted of a low-temperature superconducting (LTS) magnet of NbTi and Nb3Sn Rutherford coils, and a high-temperature superconducting magnet. The Nb3Sn Rutherford coil was fabricated by the react-and-wind method for the first time. The LTS magnet reached the designed operation current of 854 A without a training quench at a 1 h ramp rate. The central magnetic field generated by the LTS magnet was measured by a Hall sensor to be 14.0 T at 854 A in a 300 mm cold bore.

We have developed a CuNb-reinforced Nb3Sn (CuNb/Nb3Sn) wire. Critical current (I-c) of prebent CuNb/Nb3Sn coils with various diameters was investigated in order to develop a Nb3Sn superconducting magnet by a react-and-wind (R&W) method. It was found that the prebending treatment enhances I-c of the small pure bending coil. In the small pure bending strain region, I-c of the prebent coil with over 96-mm outer diameter is larger than that of the as-reacted coil. However, the as-reacted coils exhibited large I-c in the large pure bending strain region. Calculated results on the basis of the perfect current transfer model with a neutral axis shift successfully indicated the enhancement behavior of I-c by pure bending strains. Hence, we can understand the relationship between coil diameter and prebending treatment for the R&W method using CuNb/Nb3Sn wires.

The coated conductor (CC) tapes exhibit greater potential in magnet and coil applications with high critical current density, J(c) and good mechanical properties along longitudinal direction as compared with the low temperature superconductors (LTSs). Differences in fabrication processes, architecture, and layer materials in REBCO CC tapes produce different mechanical and electromechanical behaviors under magnetic field B, temperature T, and strain epsilon. For practical applications of HTS CC tapes such as magnets, the understanding of critical current, I-c behavior under B, T, and epsilon is necessary. In this paper, the I-c (B, T, epsilon) characteristics of REBCO CC tapes have been evaluated. Using a Katagiri-type tension rig, the strain effect on I-c in CC tapes with varying magnetic field and temperature was determined. In addition, the magnetic field dependence of I-c with temperature have also been measured. CC tapes with artificial pinning centers (APC) showed better electrical performance under magnetic field. Finally, the strain sensitivity and magnetic field dependence of I-c decreased with decreasing temperature from 77 up to 20 K.

The powder-in-tube method is widely used in fabricating iron-based superconducting wires and tapes. To make tapes, a multi-pass rolling process is usually adopted. However, the multi-pass rolling process limits the efficiency of tapes. In this work, rolling deformation technique was studied systematically by fabricating Sr1-xKxFe2As2 superconducting tapes. The total rolling reduction ratio is about 80% and the difference of superconducting performance of tapes rolled by 2, 3, 5 and 7 passes has been investigated. The critical current density J(c), Vickers micro-hardness and microstructure of the superconducting core indicate that tapes after 2, 3, 5 and 7 rolling passes exhibit a similar trend. The width of the tapes and the area of superconducting cores increase with decreasing the number of rolling passes, but the transport J(c) of tapes after different rolling passes seems to be the same, except for the tape rolled by 2 passes, whose transport J(c) is lower than the other tapes. Concerning the geometry uniformity for the superconducting cores, the sausaging phenomenon was not observed from the photograph of longitudinal cross-section of all the samples. "Lobes" phenomenon on transverse cross-section can be suppressed through decreasing the rolling passes. Therefore, we can obtain uniform and high-performance Ag-sheathed iron-based superconducting tapes by cutting the number of rolling passes down to 3, which is more advantageous to the large-scale producing in the future. (C) 2016 Published by Elsevier B.V.

Columnar defects (CDs) were installed into YBa2Cu3Oy thin films at the crossing angle +/-theta(i) relative to the c-axis using 200 MeV Xe ions, in order to clarify the influence of +/-theta(i) on the angular behaviours of critical current density J(c). The width of a J(c) peak occuring at B parallel to c linearly increases with broadening CD-crossing angle, where its shape maintains a single peak in the range of theta(i) &lt;= +/- 25 degrees. The CDs crossing at +/- 45 degrees, on the other hand, drastically change the angular behaviour of J(c) around B parallel to c: the J(c) is equally elevated over the angular region from -45 degrees to 45 degrees in the lower magnetic field, whereas the rapid reduction of the J(c) occurs at B parallel to c with increasing magnetic field, leading to the appearance of double peaks near the irradiation angles, +/- 45 degrees. This crossover phenomenon is attributed to the difference in the pinned states of flux lines between B parallel to c and at the irradiation angles: the CDs crossing at +/- 45 degrees cannot provide the correlated flux pinning along the c-axis, resulting in the uncorrelated pinned state at B parallel to c.

A 100 m long six-filament MgB2 wire was successfully fabricated using an internal magnesium diffusion (IMD) process. We investigated the transport properties and the uniformity of this long multifilament IMD wire. The MgB2 layer and the sub-filament region are regular, and the J(c) values have a fairly homogenous distribution throughout the wire, suggesting that there were no obvious defects along the length of the wire. The uniformity problem of long multifilament IMD MgB2 wires can be mitigated by optimizing the starting composite parameters, multifilament geometry, fabricating process and annealing conditions. A layer J(c) as high as 1.2. x. 10(5) A cm(-2) at 4.2 K and 8 T was obtained, which was comparable with the highest reported value for a short multifilament IMD wire. The transport layer J(c), non-barrier J(c) and J(e) values are independent of the wire diameter. In addition, the analysis of the stress-strain characteristics and the n value of the IMD wire is also presented. These results indicate that the long multifilament IMD-processed MgB2 superconducting wire is suitable for practical applications.

Our group has been developing an Nb-rod-processed CuNb-reinforced Nb3Sn Rutherford cable for developments in high-field superconducting magnets, such as a 25-T cryogen-free superconducting magnet. The Rutherford cable has good mechanical, electrical, and superconducting properties in high fields. In this paper, we investigated the influence of winding coil diameters on the transport properties of the Rutherford cable for the coil application wound with a react-and-wind method. We prepared two Rutherford coils with diameters of 110 mm (0.2% pure bending strain) and 70 mm (0.6% pure bending strain) with 0.7% prebending treatment in order to measure the critical current at various temperatures in fields up to 18 T. We found that the Rutherford coil with a diameter of 110 mm has good transport properties. The critical current of the Rutherford coil with a diameter of 70 mm exhibited a deterioration of about 14% in comparison to the coil having a diameter of 110 mm. Although the tensile strain dependence of the critical current (I-c) suggests a large deterioration for the 0.6% pure bent coil, the Rutherford coil can keep the high Ic under a large pure bending strain.

In this paper, normal zone in a REBCO single-pancake coil induced by quenches in a low-temperature superconducting outsert coil is investigated in both experimental and numerical methods to discuss the quench protection of REBCO coils. The normal zone is experimentally clarified by measuring a partial voltage in the REBCO coil at temperatures from 30 K to 50 K during quenches of the outsert coil charging 5 T. Using practical fittings for the physical properties of the components of the REBCO coil in ranges of 0-5 T and 10-77 K, the numerical results have good agreement with the experimental results. The simulation reveals the heating distribution with initial temperatures from 10 K to 65 K, which indicates that the maximum temperature in a hot spot becomes higher for the lower initial temperature. Unexpectedly, the temperature distribution becomes rather homogeneous with an initial temperature of 65 K. It suggests that the hot spot at higher temperature has a lower risk of burning out. Finally, possible coil protections are discussed as the operation at the high temperature or the quench heater method to keep the REBCO coil at high temperature when a hot spot appears.

The magnetic field stability such as relaxation and a magnetic field hysteresis of a practical 20-T cryogen-free superconducting magnet (20 T-CSM) with a Bi2Sr2Ca2Cu3Oy (Bi2223) insert coil was investigated. The relaxation of a central magnetic field is basically related to the flux creep phenomena in the magnetization of the Bi2223 tapes. We found that the relaxation rate S of the magnet is more than two orders smaller than that of the Bi2223 tapes. The magnetic field stability of the 20 T-CSM becomes less than 2G/h even at 19 T after a lapse of half a day from the charging. It is small enough for the solid-state NMR studies. However, the repeating excitation increases the central magnetic field beyond the saturated values under the relaxation.

A cryogen-free 25-T superconducting magnet with a 52-mm room temperature bore consists of an 11-T REBa2Cu3O7-delta (REBCO) insert coil and outer 14-T low-temperature superconducting (LTS) coils. The REBCO insert coil is composed of a stack of 56 single pancake coils. The inner and outer diameters of the REBCO insert coil are 102 and 263 mm, respectively, and the total conductor length is about 14 km. The REBCO insert coil is cooled by circulating helium gas using both single-stage and two-stage Gifford-McMahon cryocoolers, and the LTS coils are cooled by two Gifford-McMahon/Joule-Thomson cryocoolers. The maximum hoop stress of the REBCO insert coil was estimated to be 387 MPa when the central magnetic field was 25 T. If thermal runaway occurs in a conduction-cooled system, the REBCO coils will almost certainly be burned out. Therefore, the coil should have no damaged area in the winding because a damaged area would generate heat locally, eventually resulting in thermal runaway. In order to avoid the degradation of the coil due to thermal stress, the coil was divided into all winding parts in the radial direction. In practice, the coil was divided by inserting a polyimide tape coated with a fluorine resin, which has low adhesive strength, into the epoxy resin at every turn. Before fabricating and testing the actual REBCO insert coil, two kinds of coils were fabricated and tested in order to evaluate the validity of the winding method. Two stacked pancake coils wound with REBCO-coated conductors were fabricated and tested in liquid helium with a 4-T background magnetic field in order to evaluate whether the coil could withstand the hoop stress of over 387 MPa. Moreover, we fabricated and tested a demonstration coil which was composed of a stack of four single pancake coils with the same size as the pancake coils used for the REBCO insert coil in order to evaluate whether the coil could withstand the thermal stress and the electromagnetic force.

A GdBa2Cu3Oy (Gd123) insert coil for the 25 T cryogen-free superconducting magnet was constructed, installed and tested. We succeeded in the generation of 10 T using a Gd123 insert coil without a background field. The temperature of the Gd123 coil increased from 4.5 K gradually and reached about 5.5 K, when the magnet was energized with 0.036 A/s, which corresponds to a 1 hour energizing mode. The calculated and measured central magnetic fields are 10.61 T and 10.15 T, respectively, because of the magnetization current effect in RE123 tape. The maximum heat load by the AC-losses estimated from the temperature rise is about 3 W, which is consistent with the slab model combined with tape stacking effect.

Nb-rod-method Cu-Nb reinforced Nb3Sn (Cu-Nb/Nb3Sn) wires have been successfully developed for commercialization of the react-and-wind (R&W) processed Nb3Sn coils. Superconducting performance of the Cu-Nb/Nb3(S)n wires is improved by applying the prebending treatment at room temperature, because of releasing the residual stress of Nb3Sn filaments (prebending strain effects). The purpose of this study is to confirm the efficient use of the prebending strain effects for optimizing critical current (I-c) characteristics under pure bending strain on the R&W coil design. The Cu-Nb/Nb3Sn strands of 0.8-mm diameter were heat treated at 670 degrees C for 96 h on stainless steel grooved cylindrical holders of different radii of 10.1-32.1 mm. After half of the reacted strands were applied repeating prebending strain of +/- 0.5%, each of the prebent strands and the as-reacted (without prebending) strands was transferred to a Ti-6Al-4V alloy grooved cylindrical holder of 15.6-mm radius. Then, those Ic measurements were carried out under applied pure bending strains ranging from -0.9% to +0.84% at 11-17 T, 4.2 K. As a result, the prebent strand showed no deterioration of I-c in the applied pure bending strain ranging from -0.5% to +0.5% and had larger I-c than that of the as-reacted strand at no pure bending strain. In this paper, the I-c enhancements of the prebent Cu-Nb/Nb3Sn strands under pure bending strains are analyze d by using the I-c characteristics under axial strains.

Cu-sheathed SmFeAsO1-xFx tapes were fabricated by the ex situ powder-in-tube method. A series of low-temperature (400 degrees C) hot-pressing process and ordinary sintering process was applied on the tapes, respectively. A transport critical current density (J(c)) value of 2.37 x 10(4) A cm(-2) (at 4.2 K and self-field) was achieved in the Cu-sheathed SmFeAsO1-xFx hot-pressed tapes. This is the first time that a large transport Jc is obtained in Cu-sheathed 1111-type wires. The scanning electron microscopy also revealed that the low-temperature hot-pressing technique is effective in preventing the reaction layer between Cu-sheath and superconducting core.

YBCO films with BaHfO3 (BHO) nanoparticles were fabricated by a metal organic deposition method using trifluoroacetate (TFA) solution with Hf salt. The diameter of BHO nanoparticles was minimized by the additional holding temperature during the crystallization process. Critical current density (J(C)) in magnetic fields of YBCO films with BHO nanoparticles was higher than that of the film with BaZrO3 (BZO). Accordingly, it was revealed that BHO nanoparticles act as very effective artificial pinning centers.

BaSnO3 (BSO) nanocolumns and Y2BaCuO5 (Y211) nanoparticles have been successfully incorporated into YBCO thin films by premixed and surface-modified target approach. The effect of these artificially incorporated nanostructures on the vortex pinning properties of YBCO films is investigated based on the variation of critical current density (J(C)) with applied magnetic field and its variation with respect to the orientation of the applied magnetic field at two different temperatures: 77 and 65 K. The incorporation of Y211 nanoparticles into both YBCO and YBCO + BSO2% films results in improved J(C) - H characteristics, which is reflected in the enhanced pinning force density (F-p) values. The angular dependent J(C) measurement reveals that the YBCO film containing BSO nanocolumns exhibits J(C) peak along the c-axis, whereas the film containing Y211 nanoparticles exhibits isotropic enhancement in the J(C) values along the entire investigated angular regime. The YBCO film consisting of both kinds of nanostructures (BSO nanocolumns together with Y211 nanoparticles), on the other hand, exhibits mixed characteristics of different kinds of pinning: strong c-axis J(C) peak together with isotropically enhanced J(C) in the intermediate angular regime. A possible vortex pinning mechanism due to different kinds of artificially incorporated nanostructures is also discussed.

MgB2/Nb/Monel monofilament wires were fabricated using four different boron powders by an internal Mg diffusion (IMD) process. The microstructure, morphology and the critical current density (J(c)) of the used boron powders and the formative MgB2 layers were analyzed and compared. It was found that the purity and particle size of the boron powder influence the superconducting properties of MgB2 wires; further that the optimized heat-treatment condition also depends on the quality of the boron powder. The highest Jc was obtained in the MgB2 layer made using amorphous boron (AB) powder, although a certain amount of voids existed in the superconducting layer. The IMD-processed MgB2 layer fabricated using high-purity boron (HB) powder had also a high Jc compared with the powder-in-tube (PIT) process and a few unreacted boron particles remained in it. MgB2 wire fabricated using low-purity boron (LB) powder had a high cost-performance ratio compared with the others, which is expected to allow the fabrication of large-scale and low-cost superconducting wires for practical application. However, the enhancement of the Jc was not found in the MgB2 layer manufactured using the ball-milled LB (MLB) powder as expected due to the increased percentage of impurity.

A 360-mm room-temperature bore cryogen-free superconducting magnet (CSM), consisting of Nb3Sn coils and NbTi coils, for a hybridmagnet (HM) has generated the maximum magnetic field of 9.5 T. However, the magnetic field of the CSM has been limited to 8.5 T in the hybrid magnet mode because of a cooling problem. As a result, the hybrid magnet composed of a 19-T water-cooled resistive magnet (WM) had the utmost field generation of 27.5 T. Therefore, we improved the CSM to generate higher magnetic fields. For the improvement of the cooling problem, Nb3Sn coils were replaced, and thermal conduction was improved between coils and a 4K-GM cryocooler. Furthermore, support structures with a tensile strength over 80 kN and a spring support were adopted against the magnetic force to support the self-weight of coils and to absorb stress caused by thermal contraction difference between each coil. After the improvement, the CSM generated 9.5 T within 1 h and the maximum magnetic field of 9.7 T in a 360-mm room-temperature bore. The HM succeeded in generating 28 T in a 32-mm room-temperature bore with the CSM operated at 9.0 T.

We have studied vortex pinning and dynamics in a Sr0.6K0.4Fe2As2 superconducting tape with critical current density J(c) similar to 0.1 MA/cm(2) at 4.2K and 10 T. It is found that grain boundary pinning is dominant in the vortex pinning mechanism. Furthermore, we observe large density of dislocations which can also serve as effective pinning centers. We find that the temperature dependence of critical current density is in agreement with the model of vortices pinned via spatial fluctuation of charge carrier mean free path. Magnetic relaxation measurement indicates that the magnetization depends on time in a logarithmic way. The relaxation rate in the low and intermediate temperature region is small, and it exhibits a weak temperature and field dependence. A crossover from elastic creep to plastic creep regime is observed. Finally, we conclude a vortex phase diagram for the high performance Sr0.6K0.4Fe2As2 superconducting tape. Published by AIP Publishing.

For the innovation of spintronic technologies, Dirac materials, in which low-energy excitation is described as relativistic Dirac fermions, are one of the most promising systems because of the fascinating magnetotransport associated with extremely high mobility. To incorporate Dirac fermions into spintronic applications, their quantum transport phenomena are desired to be manipulated to a large extent by magnetic order in a solid. We report a bulk half-integer quantum Hall effect in a layered antiferromagnet EuMnBi2, in which field-controllable Eu magnetic order significantly suppresses the interlayer coupling between the Bi layers with Dirac fermions. In addition to the high mobility of more than 10,000 cm2/V s, Landau level splittings presumably due to the lifting of spin and valley degeneracy are noticeable even in a bulk magnet. These results will pave a route to the engineering of magnetically functionalized Dirac materials.

For use in high-magnetic-field coil-based applications, the critical current density (J(c)) of REBa2Cu3Oy (REBCO, where RE = rare earth) coated conductors must be isotropically improved, with respect to the direction of the magnetic field; these improvements must be realized at the operating conditions of these applications. In this study, improvement of the J(c) for various applied directions of magnetic field was achieved by controlling the morphology of the BaHfO3 (BHO) nano-rods in a SmBCO film. We fabricated the 3.0 vol. % BHO-doped SmBCO film at a low growth temperature of 720 degrees C, by using a seed layer technique (T-s = 720 degrees C film). The low-temperature growth resulted in a morphological change in the BHO nanorods. In fact, a high number density of (3.1 +/- 0.1) x 10(3) mu m(-2) of small (diameter: 4 +/- 1 nm), discontinuous nano-rods that grewin various directions, was obtained. In J(c) measurements, the Jc of the T-s = 720 degrees C film in all directions of the applied magnetic field was higher than that of the non-doped SmBCO film. The Jc min (6.4 MA/cm(2)) of the former was more than 6 times higher than that (1.0 MA/cm(2)) of the latter at 40 K, under 3 T. The aforementioned results indicated that the discontinuous BHO nano-rods, which occurred with a high number density, exerted a 3D-like flux pinning at the measurement conditions considered. Moreover, at 4.2 K and under 17 T, a flux pinning force density of 1.6 TN/m(3) was realized; this value was comparable to the highest value recorded, to date. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

We have developed an apparatus that allows the observation of the transient rotational motion of fine particles under a high magnetic field in order to determine anisotropic magnetic susceptibility. The anisotropic susceptibilities of spherical nanoparticles of bismuth and commercially available carbon nanofibers were determined. The estimated Delta(X) = 3.9 x 10(-5) of spherical bismuth nanoparticles with a diameter of 370nm was fairly consistent with the value determined previously by the magnetic field dependence of diffraction peak intensity in the X-ray diffraction (XRD) pattern, but was slightly smaller than the value for the bulk crystal. In contrast, the transient behavior of carbon nanofibers did not obey the theoretical motion of a single crystal. The wide distribution of fiber lengths, the irregularity of the structure in the fiber, and the connections between the fibers are suggested for the anomalous behavior. (C) 2016 The Japan Society of Applied Physics

The evolution of the superconducting properties of round wires of (Ba,K)Fe2As2 fabricated by the powder-in-tube (PIT) method is systematically studied. After establishing the method to obtain the largest transport critical current density (J(c)) in round wires using the hot isostatic press technique, we investigated how the transition temperature (T-c), J(c), and microstructures change at each step of the wire fabrication. Unexpectedly, we find that superconducting properties of the wire core are significantly damaged by the drawing process. Systematic measurements of J(c) and T-c of the core superconductor after each drawing and sintering process clarified the evolution of degradation by the drawing process and recovery by heat treatment.

We have fabricated 5.6 vol.% BaHfO3 (BHO)-doped SmBa2Cu3Oy (SmBCO) films at a low substrate temperature of 750 degrees C by using the low temperature growth (LTG) technique with a seed layer. Using the LTG technique, we can obtain a purely c-axis oriented SmBCO film even at low growth temperatures. The LTG technique also greatly increases the number density of BHO nanorods. The size and matching field of the BHO nanorods in the films were 5.4 nm and 9.9 T. Transport measurements were carried out, and a high irreversibility field (B-irr) and strong flux pinning force density (F-p) were obtained. The B-irr was 15.1 T at 77 K for B//c. The maximum F-p of 407 GN m(-3) in 10 T at 40 K and 770 GN m(-3) from 9 to 17 T at 20 K have been measured. We believe that the LTG technique can be considered as important for improving of flux pinning performance in BaMO3 (BMO: M = Zr, Sn and Hf)-doped SmBCO coated conductors.

A low-temperature (300-500 degrees C) heat treatment process under ambient pressure or uniaxial pressure was performed on Sn-added SmFeAsO1-xFx superconducting tapes fabricated by the ex situ powder-in-tube method. A highest transport critical current density (J(c)) of 3.95 x 10(4) A cm(-2) (at 4.2 K and self-field) was achieved by this process. The low-temperature process allows tapes to endure much longer heat treatment without J(c) degradation than the high-temperature method. Microscopic analysis also revealed that this method could obtain a clear boundary without a reaction layer or interdiffusion between a superconducting core and sheath metal.

Monofilament MgB2/Fe wires or tapes were fabricated with two different purity crystalline boron powders, using internal magnesium diffusion (IMD) and in situ powder-in-tube (PIT) processes. To evaluate which method was more insensitive to the purity of the boron powders, a sensitivity factor was used. It was found that the IMD process was less sensitive to the purity of the boron powders, compared to the PIT method. Furthermore, J(c) values of the IMD-processed wires were higher than those of the PIT-processed samples. The reduced porosity and hence the increased density in the IMD-processed samples was thought to be the main reason. Although a number of B-rich particles remained in the MgB2 region, a layer J(c) as high as 4.8 x 10(4) A cm(-2) at 4.2 K and 10 T was still obtained in the IMD-processed wire, which was comparable to that of IMD-processed wires fabricated using amorphous boron powders.

We investigate magnetotransport properties in a single crystal of pyrochore-type Nd2Ir2O7. The metallic conduction is observed on the antiferromagnetic domain walls of the all-in-all-out-type Ir 5d moment ordered insulating bulk state that can be finely controlled by an external magnetic field along [111]. On the other hand, an applied field along [001] induces the bulk phase transition from insulator to semimetal as a consequence of the field-induced modification of the Nd 4f and Ir 5d moment configurations. A theoretical calculation consistently describing the experimentally observed features suggests a variety of exotic topological states as functions of electron correlation and Ir 5d moment orders, which can be finely tuned by the choice of rare-earth ion and magnetic field, respectively.

A cryogen-free 20 T superconducting magnet with a 52 mm room-temperature bore was developed and installed in Tohoku University in 2013. This magnet consists of a Bi2223 insert coil, which generates 4.5 T, and outer low-temperature superconducting (LTS) coils. Both coils were cooled by a GM/JT cryocooler with 4.2 W-class cooling capacity at 4.3 K. To generate a higher magnetic field, a new cryogen-free 25 T superconducting magnet using a REBCO insert coil, which generates 11.5 T, and new outer LTS coils is now under development. The magnetic field contribution of this REBCO insert coil is considerably higher than that of the previous Bi2223 insert coil, and the ac-loss of the insert coil during field ramping rises to approximately 9.7 W. The LTS coils have to operate at about 4 K, but the REBCO coil can operate at various temperatures above 4K. In addition, the cooling capacity of a GM cryocooler is greater than that of a GM/JT cryocooler around 10 K. Thus, the REBCO insert coil is cooled to about 10 K by using two GM cryocoolers, independently of the LTS coils, which are cooled by two GM/JT cryocoolers. To protect the cryocoolers from the leakage field of the magnet, the two GM cryocoolers cool circulating helium gas through heat exchangers, and the gas is transported over a long distance to another heat exchanger provided for the REBCO insert coil. The maximum temperature of the REBCO insert coil was calculated under the most severe condition where an insert coil ac-loss of 9.7 W was generated continuously. And it was confirmed to be less than the target maximum temperature of 12 K.

A 25-T cryogen-free superconducting magnet (25T-CSM) is being developed at the High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University. In the case of a cryogen-free superconducting magnet, the coil temperature rises during a sweep of an operating current due to ac losses. Hence, the ac loss estimation is very important for the cooling design. The critical current density and the magnetization of Gd123 tapes at 4.2 K were measured in order to estimate the ac loss of a Gd123 insert of the 25T-CSM. The ac loss is actually the hysteresis loss, which are calculated from the J(c) properties taking the magnetic field distribution in the Gd123 coil into account. Regarding the effect of tape stacking (or winding) in a pancake coil, the slab approximation can be used for hysteresis loss calculation. In the case of slab model, the full penetration field at the center of the tape becomes higher than the maximum applied magnetic field in the most part of the coil. As a result, the hysteresis loss increases with increasing a magnetic field when the magnet energizes. The hysteresis losses assuming the slab model, however, show an opposite field dependence to those calculated from the strip model without the stacking effect. Hence, the ac loss of 5 W is estimated when the magnet energizes to 25.5 T within 60 min.

Nb3Sn Rutherford cables have been investigated for the development of a 25-T cryogen-free superconducting magnet. We developed Rutherford cables composed of 16 Nb-rodprocessed CuNb-reinforced Nb3Sn (CuNb/Nb3Sn) strands with an 0.8-mm diameter. In this paper, the transport and mechanical properties of the epoxy impregnated multilayer coil of the (CuNb/Nb3Sn) Rutherford cable were investigated in high magnetic fields. The coil reached the maximum transport current of 1655 A at 4.2 K and 13.5 T after four times of training quenches. The electromagnetic stresses at the maximum transport current were 240 MPa at an inner surface of the coil and 210 MPa at an outer surface of the coil. The hoop strain of 0.2% for the coil was measured, which was applied at the outer surface of the coil. The strain value was the same as the calculated result using Wilson's equation and stress-strain curve of the strand.

Ag-sheathed Sr0.6K0.4Fe2As2 superconducting tapes with various thicknesses were produced by flat rolling and cold uniaxial pressing based on an ex-situ powder-in-tube method. The transport critical current density (J(c)) for both rolled and pressed tapes was found to increase with the decrease of tape thickness. Compared with flat rolling, cold pressing can further enhance the transport Jc of Sr0.6K0.4Fe2As2 tapes. A transport Jc as high as 4.8 x 10(4) A/cm(2) at 4.2 K and 10 T was achieved by pressing Sr0.6K0.4Fe2As2 tapes at about 1.0 GPa. This superior transport Jc in the pressed tapes can be ascribed to improved mass density and grain texture of Sr0.6K0.4Fe2As2 superconducting cores.

A 25 T cryogen-free superconducting magnet is under development at the High Field Laboratory for Superconducting Materials (HFLSM), Institute for Materials Research (IMR), Tohoku University. Fujikura has manufactured and supplied REBCO coated conductors for the 25 T cryogen-free superconducting magnet. In-field critical current (I-c) properties in high magnetic fields at lower temperatures below 20 K and mechanical properties are important for design of these superconducting magnets. However, there is insufficient data of I-c properties in high magnetic fields above 15 T. In this work, in-field I-c properties at 20 K, 15 T of several samples cut from commercial coated conductors are evaluated at HFLSM, IMR, Tohoku University. The correlations between I-c at 20 K, 15 T and I-c at 77.3 K are compared. As a result, it is confirmed that I-c at 20 K, 15 T have a stronger correlation with in-field I-c at 77.3 K rather than I-c at 77.3 K, self-field. In addition, tensile strengths at 77.3 K of more than 30 samples of coated conductors are also evaluated.

A cryogen-free 25-T superconducting magnet with a 52-mm room-temperature bore consisting of an 11.5-T REBa2Cu(3)O(7)-delta (REBCO) insert coil and outer 14-T lowtemperature superconducting (LTS) coils is currently under development. The REBCO insert coil is composed of a stack of 68 single pancake coils. The inner and outer diameters of the REBCO insert coil are 102 and 276 mm, respectively; and the total conductor length is 17.6 km. The maximum hoop stress of the REBCO insert coil was estimated to be 455 MPa when the central magnetic field was 25.5 T. The REBCO insert coil is cooled by circulating helium gas using both single-and two-stage GM cryocoolers, and the LTS coils are cooled by Gifford-McMahon/ Joule-Thomson cryocoolers. If thermal runaway occurs in a conduction-cooled system, the REBCO coil will almost certainly be burned out. Therefore, it is important to avoid thermal runaway of the REBCO insert coil under all operating conditions and protected operations. As operating conditions of the REBCO insert coil, quenching of the LTS coils should be assumed in the severest situation, since the current of the REBCO insert coil will increase after the LTS coils quench. From the results of quench analysis of the LTS coils and circuit simulations, it was confirmed that thermal runaway can be avoided by setting the intermediate potential of the REBCO coil to the ground potential and by using an external dump resistance of 11 Omega(.)

A Rutherford-type cable is one of candidate conductors for a large-scale or high-field superconducting magnet. It is important to evaluate the internal strain state of strand consisting of a cabling conductor to understand the strain dependence of superconducting properties. Internal lattice strains under a tensile load for Nb-rod-processed Nb3Sn Rutherford cable with and without the prebending treatment were measured directly by neutron diffraction at room and low temperatures. Stress-lattice strain curves of Nb3Sn were obtained, and it was found that the internal lattice strain is smaller than the strain measured on the cable surface by an extensometer above 100 to 150 MPa. It is important to evaluate not only the strain on the cable surface but also the internal strain of a strand in cabling conductors.

The effect of hybrid (columnar and spherical) artificial pinning centers (APCs) on the vortex pinning properties of YBCO thin films is studied in this paper on the basis of variation of critical current density J(C) with applied magnetic field and also with the orientation of the applied magnetic field. YBCO+BSO3% composite target is used for preparing film with 1-D (columnar) APCs, whereas the same composite target is modified by putting two differently sized (2.2 area% and 3 area%) Y2O3 sectored pieces for preparing films having different concentrations of 3-D (spherical) APCs along with 1-D APCs. Film consisting of only 1-D APCs exhibits enhanced in-field J(C) values as compared to the one without any APC, and the ones consisting of hybrid APCs exhibit even better J(C)-B characteristics with increase in the fraction of 3-D APCs. F-pmax. values increase systematically with incorporation of 1-D and 1-D + 3-D APCs, and it also shifts toward higher applied magnetic fields. Film with 1-D APCs exhibits strong J(C) peak at Theta = 0 degrees (H//c-axis), whereas films consisting of hybrid APCs exhibit enhanced J(C) at all the investigated angular regime. A possible mechanism of vortex pinning in samples with hybrid APCs is also discussed, suggesting the role of 1-D and 3-D APCs.

We succeeded in the complete detwin of (Y, Gd) BCO coated conductors using a newly developed in-situ tensile apparatus at high temperature. The intensities of (020) and (200) peaks in a longitudinal and a transverse directions of the tape measured by XRD were similar to each other for usual (Y, Gd) BCO coated conductors before annealing. However, the (200) peak disappears for the longitudinal direction after annealing under the tensile stress. It means that the coated conductors are detwinned by annealing under the tensile stress. We found that T-c linearly decreases with increasing strain for the B-domain (b-axis). This behavior is similar with a uniaxial-pressure effect in (Y, Gd) BCO single crystals. However, the strain dependence of T-c for the A-domain (a-axis) behaves as a power-law function, whereas the uniaxial-pressure dependency of T-c for the a-axis is reported to be linear. In addition, the strain dependencies of J(c) are similar with those of T-c

The vortex state under the magnetic field around vortex glass (VG)-Bose glass (BG) transition field was investigated in the SmBa2Cu3Oy (SmBCO) films with BaHfO3 (BHO) nanorods. We prepared two films with different forms of the nanorods (thick-straight and thin-tilted), and we used the magnetic field angular dependence of flux flow resistivity (rho - theta) to identify a vortex state. From a comparison between the rho - theta curves in completely BG and VG phases, it was revealed that the existence of deep and sharp dip structure along the c-axis direction of the SmBCO matrix (B//c) was an evidence of the BG state in both the films. In the magnetic field region around VG-BG transition, shallower and broader dips than the dips in the BG state were observed at B//c in both the films. In addition, the dips became deeper with increase of the magnetic field in this region in each film. Therefore, we considered the vortex state of both the films was an intermediate and/or a transient state of BG and VG. Under all magnetic fields, the film with thick and straight BHO nanorods showed stronger c-axis correlated flux pinning force than the film with thin and tilted nanorods. We suggested the intermediate state of the film with thicker and straighter nanorods was similar to the BG state compared with another film from the both of experimental results and reported theory.

A high density of BaMO3 (BMO) nanorods within a REBa2Cu3Oy (REBCO) film can be generated by heavy doping of the BMO. This is very effective in improving superconducting properties in high magnetic fields. However, heavy doping causes a significant T-c reduction. Introduction of a high density of BMO nanorods into REBCO films without reduction in T-c is required. To this end, we deposited SmBa2Cu3Oy (SmBCO) films with various amount of BaHfO3 (BHO) by a pulsed laser deposition (PLD) method using a seed-layer technique and a low-temperature growth (LTG) technique. Using this technique, we can fabricate a purely c-axis-oriented SmBCO film even at low substrate temperatures T-s. The SmBCO films with BHO were fabricated at low Ts of 750 degrees C without heavy doping of BHO. As a result, the BHO nanorods became thinner and denser than those within SmBCO films fabricated by conventional PLD methods at high T-s. The maximum density was 4780/mu m(2), and the B-phi of the film reached 9.9 T. These samples showed high T-c (&gt; 90 K). We have therefore successfully fabricated high-T-c SmBCO films with high densities of BHO nanorods using the LTG technique.

We have investigated the current transport properties of batch furnace processed TFA-MOD YxGd1-xBa2Cu3Oy (YGdBCO) coated conductor (CC) doped with nanoscale BaZrO3 artificial pinning centers (APCs). As the comparison with those of previous processed YGdBCO CC, it has been confirmed that the present process is very effective to improve current carrying performance of the CC at not only high-temperature but also low-temperature region, for example, the I-c at 4.2 K and 17 T is 450 A/cm-w, i.e., the engineering J(c) is 4.3 x 10(8) A/m(2), which is comparable value to that of Nb3Sn wire at 4.2 K. In addition, angular dependences of the Ic clearly show the effectiveness of the APCs. Namely, the minimum Ic in the whole angle region at 77 K, 3 T increase from 7 to 35 A/cm-w, and the anisotropy of Ic is reduced. We will also discuss the analytical expression of in-field current transport property based on the percolation transition model and the scaling law of the flux pinning. By using this analysis, electric field versus current density characteristics in arbitrary conditions of temperature and magnetic field can be described, and also, the statistical distribution of Jc can be estimated. These results are important not only for the design of superconducting devices using CCs but also for understanding the pinning properties.

Practically, coated conductor (CC) tapes are apt to be exposed to external magnetic fields, hoop stress, and other inevitable stress-and strain-producing factors during fabrication and cool-down and under operating conditions. In addition, the issue on the strain effect on the I-c degradation of REBCO CC tapes under magnetic field is not yet completely understood. In this paper, further investigation on the electromechanical response of CC tapes under magnetic field conditions was conducted. Differently stabilized CC tapes by Cu lamination and by addition of external brass lamination with different substrate materials were investigated. The I-c degradation behaviors at respective magnetic field intensity and against uniaxial tensile strain induced were reported. As observed, magnetic field dependence of I-c is not affected by macroscopic strain within the reversible strain region. I-c degradation behavior with strain was almost constant for all samples from self-field up to 0.5 T. In addition, the macroscopic pinning force and the irreversibility field were calculated and extrapolated, respectively, to investigate the effect of strain.

A cryogen-free 25-T superconducting magnet with a 52-mm room-temperature bore consisting of an 11.5-T REBa2Cu(3)O(7)-delta (REBCO) insert coil and outer 14-T lowtemperature superconducting (LTS) coils is currently under development. The REBCO insert coil is composed of a stack of 68 single pancake coils. The inner and outer diameters of the REBCO insert coil are 102 and 276 mm, respectively; and the total conductor length is 17.6 km. The maximum hoop stress of the REBCO insert coil was estimated to be 455 MPa when the central magnetic field was 25.5 T. The REBCO insert coil is cooled by circulating helium gas using both single-and two-stage GM cryocoolers, and the LTS coils are cooled by Gifford-McMahon/ Joule-Thomson cryocoolers. If thermal runaway occurs in a conduction-cooled system, the REBCO coil will almost certainly be burned out. Therefore, it is important to avoid thermal runaway of the REBCO insert coil under all operating conditions and protected operations. As operating conditions of the REBCO insert coil, quenching of the LTS coils should be assumed in the severest situation, since the current of the REBCO insert coil will increase after the LTS coils quench. From the results of quench analysis of the LTS coils and circuit simulations, it was confirmed that thermal runaway can be avoided by setting the intermediate potential of the REBCO coil to the ground potential and by using an external dump resistance of 11 Omega(.)

BaHfO3(BHO)-doped SmBa2Cu3Oy (SmBCO) superconducting films were fabricated with a seed layer and low-temperature growth (LTG technique) via pulsed laser deposition. The LTG technique enables the fabrication of high-quality SmBCO films even at a low substrate temperature during the deposition, and also greatly increases the number density of BHO nanorods. Transmission electron microscopy observations showed that the 5.6 vol% BHO-doped sample had a high number density [(4.8 +/- 0.3) x 10(3) mu m(-2)] of small (5.4 +/- 0.7 nm) BHO nanorods. This sample exhibits maximum flux pinning forces (F-p) of 405 GN m(-3) in 9.0 T at 40 K and 105 GN m(-3) in 9.0 T at 65 K. The high density and small size of the nano-rods in a REBa2Cu3Oy matrix (where RE is a rare earth element) is considered to be very effective for enhancing Fp at low temperatures under a high field.

We propose a microscopic approach to derive angular dependent critical current density J(c)(theta) of YBCO films with c-axis correlated nanorods, based on the energy gain by pinning. Results are in good agreement with experimental ones. Irreversibility fields and J(c) in YBa2Cu3O7-x thin films containing segmented BaSnO3 nanorods are systematically changed by selection of nanorod length and its spacing, and the variations are explained by the "harmonic oscillator" approach based on the Bose glass state. Analysis of pinning energies and J(c) indicates that stacked BSO nanorod segments in line behave as single columnar with a smaller diameter pin for B parallel to c, in contrast, as nanoparticle pins for B parallel to ab.

Recently, we succeeded in detwinning REBa2Cu3O7 (RE123, RE = rare-earth elements)-coated conductors by annealing under an external uniaxial strain. Using the untwinned RE123 tapes, the uniaxial-strain dependencies of the critical temperature T-c along the a and b crystal axes were investigated over a wide strain region from compression to tension. We found that the strain dependencies of T-c for the a and b axes obey a power law but exhibit opposite slopes. In particular, the maximum value of T-c is obtained when the CuO2 plane becomes a square, and its lattice constant is close to 0.385 nm. It is suggested that a tetragonal structure with a approximate to 0.385 nm is the optimum condition for a high critical temperature in high-T-c cuprates.

Ag-sheathed Ba0.6K0.4Fe2As2 superconducting tapes were fabricated by a scalable rolling process with high quality precursor. At 4.2 K and 10 T, the critical current density J(c) is 5.4 x 10(4) A cm(-2). We ascribe the excellent performance to the high purity and homogeneity of the superconducting phase, and the high density and high c-axis texture of the superconducting core. Our method provides a simple way to scale up the production of long-length iron-based superconducting tapes with high J(c). (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

We report the effect of BaHfO3 (BHO) introduction into SmBa2Cu3Oy (SmBCO) films on the transport properties at artificial grain boundaries (AGBs) by comparing pure SmBCO and BHO-doped SmBCO films deposited on bicrystal substrates. Improvement of the critical current density (J(c)) at the AGB of BHO-doped SmBCO films was confirmed under the magnetic fields at which Jc was restricted at the AGB. By investigating the superconducting properties and from TEM observations, we found that the in-field J(c) value for BHO-doped SmBCO films was improved through flux pinning of the Abrikosov-Josephson vortices by BHO nanorods. (C) 2015 The Japan Society of Applied Physics.

We report the effect of BaHfO<inf>3</inf>(BHO) introduction into SmBa<inf>2</inf>Cu<inf>3</inf>O<inf>y</inf>(SmBCO) films on the transport properties at artificial grain boundaries (AGBs) by comparing pure SmBCO and BHO-doped SmBCO films deposited on bicrystal substrates. Improvement of the critical current density (J<inf>c</inf>) at the AGB of BHO-doped SmBCO films was confirmed under the magnetic fields at which J<inf>c</inf>was restricted at the AGB. By investigating the superconducting properties and from TEM observations, we found that the in-field J<inf>c</inf>value for BHO-doped SmBCO films was improved through flux pinning of the Abrikosov–Josephson vortices by BHO nanorods.

Flux pinning properties of a BaHfO3 (BHO)-doped SmBa2Cu3O (y) (SmBCO) film, which was fabricated by pulsed laser deposition method adopting a seed layer technique, have been investigated. Nano-rods of the film were small in a diameter of 5.4 nm and had high number density (high-n) of 4,775/mu m(2). The flux pinning properties of the film were compared with a SmBCO film including BHO nano-rods with a larger diameter and a lower number density (low-n). At 65 K, the flux pinning force density of the high-n sample (105.0 GN/m(3)) showed much higher than that of the low-n sample (70.6 GN/m(3)).

The high irreversibility field B-1 and strong flux pinning force were obtained for BaHfO3-doped GdBa2Cu3Oy tapes on a metallic Hastelloy substrate with an ion beam-assisted deposition MgO buffer layer. At 77.3K, B-1 of 15.8 T and the maximum flux pinning force density of 23.5 GN/m(3) for B parallel to c were achieved for the 1.5 vol % BaHfO3-doped GdBa2Cu3Oy tape, which are the world's highest recorded values for REBa2Cu3Oy-coated conductors on metallic substrates (RE = rare earth). From the flux pinning analysis, the c-axis correlated pinning by well-aligned BaHfO3 nanorods plays an important role in achieving the high irreversibility field and flux pinning force density performance. (C) 2015 The Japan Society of Applied Physics

The dependence of BaMO₃ (BMO: M = Zr, Sn, Hf) materials on reducing critical temperature (<i>T</i>_c) and lattice stress in BMO-doped SmBa₂Cu₃O<i>_y</i> (SmBCO) films is investigated by comparing the <i>T</i>_c and <i>c</i>-axis length of BaZrO₃ (BZO)-, BaSnO₃ (BSO)- and BaHfO₃ (BHO)-doped SmBCO films with various BMO contents in this report. The <i>T</i>_c reduction rate against <i>c</i>-axis expansion was identical in all BMO-doped SmBCO films. It was revealed that the lattice stress on the SmBCO matrix in the BHO-doped SmBCO films was weaker than that in the BZO- and BSO-doped SmBCO films when comparing the SmBCO-BMO interfacial area dependence of the <i>c</i>-axis length of the SmBCO matrix. In addition, we confirmed that BHO was strongly strained by the SmBCO matrix as compared to BZO and BSO in SmBCO films. This is because the Youngs modulus of BHO was smaller than that of BZO and BSO. We conclude that restraining <i>c</i>-axis expansion is effective for keeping <i>T</i>_c high, and the BMO material with a small Youngs modulus can inhibit <i>c</i>-axis expansion.<tt> </tt>

A cryogen-free 25 T superconducting magnet using a ReBCO insert coil that generates 11.5 T in a 14 T background field of outer low-temperature superconducting (LTS) coils is currently under development. The AC loss of the insert coil during field ramping is approximately 8.8 W, which is difficult to dissipate at the operating temperature of the LTS coils (4 K). However, since a ReBCO coil can operate at a temperature above 4 K, the ReBCO insert coil is cooled to about 10 K by two GM cryocoolers, and the LTS coils are independently cooled by two GM/JT cryocoolers. Two GM cryocoolers cool a circulating helium gas through heat exchangers, and the gas is transported over a long distance to the cold stage located on the ReBCO insert coil, in order to protect the cryocoolers from the leakage field of high magnetic fields. The temperature difference of the 2nd cold stage of the GM cryocoolers and the insert coil can be reduced by increasing the gas flow rate. However, at the same time, the heat loss of the heat exchangers increases, and the temperature of the second cold stage is raised. Therefore, the gas flow rate is optimized to minimize the operating temperature of the ReBCO insert coil by using a flow controller and a bypass circuit connected to a buffer tank. (C) 2015 The Authors. Published by Elsevier B.V.

We have investigated current transport properties in two G(1)dBa(2)Cu(3)O(7-delta) (GdBCO) coated conductors deposited on different quality of IBAD substrate, i.e., in-plane alignments of the substrate are 1.98 degrees and 2.78 degrees, respectively. From the comparison, J(c) enhancement has been confirmed in wide temperature and magnetic field region in the sample using highly oriented IBAD substrate, whereas the irreversibility field and shape of pinning force density ( F-p) does not change much. Furthermore, higher in-filed J(c) than that of BaHfO3 doped GdBCO coated conductor deposited on standard IBAD substrate has also been observed in practical high field region at low temperature. These results indicate that the improvement of in-plane texturing in the substrate is still very effective even in such range of 2 to 3 degrees to improve in-field J(c). (C) 2015 The Authors. Published by Elsevier B.V.

A cryogen-free 25 T superconducting magnet using a ReBCO insert coil that generates 11.5 T in a 14 T background field of outer low-temperature superconducting (LTS) coils is currently under development. The AC loss of the insert coil during field ramping is approximately 8.8 W, which is difficult to dissipate at the operating temperature of the LTS coils (4 K). However, since a ReBCO coil can operate at a temperature above 4 K, the ReBCO insert coil is cooled to about 10 K by two GM cryocoolers, and the LTS coils are independently cooled by two GM/JT cryocoolers. Two GM cryocoolers cool a circulating helium gas through heat exchangers, and the gas is transported over a long distance to the cold stage located on the ReBCO insert coil, in order to protect the cryocoolers from the leakage field of high magnetic fields. The temperature difference of the 2nd cold stage of the GM cryocoolers and the insert coil can be reduced by increasing the gas flow rate. However, at the same time, the heat loss of the heat exchangers increases, and the temperature of the second cold stage is raised. Therefore, the gas flow rate is optimized to minimize the operating temperature of the ReBCO insert coil by using a flow controller and a bypass circuit connected to a buffer tank.

RE-mixing is one of the most effective ways to enhance critical current density J(c) of RE123 melt-solidified bulks; however, the intrinsic effects of RE-mixing on J(c) have not yet been well clarified. Through synthesis and analysis of eight types of melt-solidified bulks containing one or two RE elements, the effects of RE-mixing on J(c)-H characteristics were systematically investigated. It was revealed that RE-mixed bulks show higher J(c) than RE123 bulks accompanied by the secondary peaks of J(c) in a high field region. Analyses on the relationship between J(c) and orthorhombicity suggested that the RE substitution level for Ba sites can be controlled by RE-mixing to enhance J(c) in magnetic fields. Combined with the results of magnetic measurements at 40 and 60 K up to 15 T, RE-mixed bulks originating from Y123 powders were found to be high potential materials for high field generating applications.

The doping effects of three different homemade nanocarbons on microstructure and superconducting properties of in situ powder-in-tube ( PIT) processed MgB2/Fe tapes were studied. Compared with pure tapes, all homemade nano-carbon additions led to an order of magnitude enhancement of the supercurrent at 4.2 K, 10 T. Among them, hollow carbon nanoboxes doped samples exhibited the highest transport J(c)-B properties, about 3.4 x 10(4) A cm(-2) at 4.2 K, 10 T (J(e) similar to 10(4) A cm(-2)). The large J(c) improvements of doped samples are thought to be attributed to the enhancement of flux pinning force. It is also interesting that the change of microstructure and critical field of three nanocarbons doped MgB2 samples are not obvious, which mainly because that the performance parameters of these nanocarbons with different morphologies are almost same. (C) 2014 Elsevier B.V. All rights reserved.

Graphene is a special form of carbon which can effectively enhance the critical current density Jc of MgB2. In this work, a systematic investigation on the impact of sintering conditions and doping level was carried out for graphene-doped MgB2 tapes. It is found that an appropriate addition level, i. e., 8 at% in this work, is very critical to obtain a high Jc in graphene-doped samples. The critical field and pinning force are improved obviously due to the graphene doping. The magnetic Jc of samples sintered at 800 degrees C with 8 at% graphene doping reached 1.78 x 10(4) A/cm(2), at 5 T, 20 K. At the same time, the transport Jc was up to 2.38 x10(4) A/cm(2) at 10 T, 4.2 K. The lattice distortion caused by C substitution and residual C at the grain boundaries were thought to be the major factors affecting the Jc of graphene-doped MgB2 samples.

High-performance Sr0.6K0.4Fe2As2 (Sr-122) tapes have been successfully fabricated using hot pressing (HP) process. The effect of HP temperatures (850-925 degrees C) on the c-axis texture, resistivity, Vickers micro-hardness, microstructure and critical current properties has been systematically studied. Taking advantage of high degree of c-axis texture, well grain connectivity and large concentration of strong-pinning defects, we are able to obtain an excellent J(c) of 1.2 x 10(5) A/cm(2) at 4.2 K and 10 T for Sr-122 tapes. More importantly, the field dependence of Jc turns out to be very weak, such that in 14 T the J(c) still remains, similar to 1.0 x 10(5) A/cm(2). These J(c) values are the highest ever reported so far for iron-pnictide wires and tapes, achieving the level desired for practical applications. Our results clearly strengthen the position of iron-pnictide conductors as a competitor to the conventional and MgB2 superconductors for high field applications.

We report irreversibility fields B-irr and critical current densities J(c) in YBa2Cu3O7-x thin films containing segmented BaSnO3 nanorods. B-irr and J(c) are systematically changed by selection of nanorod length and its spacing, and the variations are explained by the "harmonic oscillator" approach based on the Bose glass state. Analysis of pinning energies and J(c) indicates that segmented nanorods behave as single columnar pin for B parallel to c, in contrast, as nanoparticle pins for B parallel to ab. The present results may help to further investigate anisotropy tuning of pinning structures. (C) 2014 AIP Publishing LLC.

To enhance J(c) in high magnetic fields, we fabricated a SmBa2Cu3Oy (SmBCO) film with a high number density (2830/mu m(2)) of BaHfO3 (BHO) nanorods by pulsed laser deposition (PLO) adopting a low-temperature growth (LTG) technique (LTG-SmBCO + BHO). The LTG technique consists of seed layer deposition and subsequent film deposition at a lower temperature. The number density of BHO nanorods in the film was approximately 4 times higher than that in a BHO-doped SmBCO film fabricated by conventional PLO without a seed layer. The flux pinning performance of the LTG-SmBCO + BHO film (F-p = 23.8 GN/m(3) at 77 K under 4.5 T) was comparable to the reported record high value of 28.3 GN/m(3) at 77 K. (C) 2014 The Japan Society of Applied Physics

We have fabricated (Sr,K)Fe2As2 superconducting wire through a powder-in-tube method that is combined with a hot isostatic pressing technique. The transport J(c) in the wire at 4.2 K has reached 1.0 x 10(5) A cm(-2) under self-field, and 9.4 x 10(3) A cm(-2) at 100 kOe. This is the first report that the transport Jc reaches 1 x 10(5) A cm(-2) in a (Sr,K) Fe2As2 round wire. Magneto-optical imaging of the wire has confirmed the large intergranular critical current density in the wire core. The microstructure and compositional analyses imply that there is room for improvement of the J(c) value through the optimization of the wire fabrication process.

Recently, the BaHfO3 (BHO) nanorod has attracted attention as a new c-axis-correlated pinning center in REBa2Cu3Oy films. We fabricated SmBa2Cu3Oy (SmBCO) films with BHO nanorods using an alternating-targets technique with pulsed laser deposition on single-crystal LaAlO3(100) substrates, and then compared the microstructure and flux pinning properties with those of BaSnO3(BSO)-doped SmBCO films. Transmission electron microscopy observations indicated that the BHO and BSO nanorods both grew straight, but the inclination of the BHO nanorods from the c-axis of the SmBCO was less than that of the BSO nanorods. The inclination had a strong influence on the flux pinning property. The flux pinning force of the BHO-doped SmBCO film (F-P(MAX) = 28: 0 GN m(-3) with J(c) = 2: 0 MA cm(-2) at 77 K under 1.4 T) became stronger than that of the BSO-doped SmBCO film (F-P(MAX) = 24: 5 GN m(-3) with J(c) = 1: 4 MA cm 2 at 77 K under 1.8 T) due to the shape of the BHO nanorods without inclining.

The new high magnetic field research laboratory network is recognized as one of the Japanese Master Plans of Large Research Project by the Science Council of Japan. Recently, the project of the 25 T cryogen-free superconducting magnet (25 T-CSM), which is operated under a conductive cooling condition by cryocooler, was approved under the high magnetic field research laboratory network. We adopted a high strength CuNb/Nb3Sn Rutherford cable with a prebending treatment for the middle section coils of the 25 T-CSM. The central magnetic field of 14 T is generated by the operational current of 851 A by the Nb3Sn middle section and NbTi outer section coils in a 300 mm bore. The induced maximum hoop stress in the CuNb/Nb3Sn section is about 250 MPa. In addition, the 11.5 T high temperature superconducting insert coil is also designed using Gd123 tapes. Therefore, a total central magnetic field of 25.5 T can be achieved.

A YBCO insert coil has been developed for upgrading a cryogen-free 18 T superconducting magnet installed in the High Field Laboratory for Superconducting Materials (HFLSM) at Tohoku University to a 22 T superconducting magnet. The YBCO insert coil is designed to generate 6.5 T at 200 A in 15.5 T outer LTS coils. The YBCO insert coil is composed of a stack of 50 single pancake coils wound with YBCO-coated conductors (0.23 mm X 4 mm). The inner and outer diameters of the YBCO insert coil are 96 mm and 178 mm, respectively, and the total conductor length is about 3 km. The maximum hoop stress of the YBCO insert coil was estimated to be 310 MPa when the central magnetic field was 22 T. The magnet system is cooled by a GM/JT cryocooler and two single-stage GM cryocoolers. Thermal runaway may cause burnout of the YBCO insert coil, and therefore, it is important to calculate the voltage-current characteristics of the coil from the superconducting properties of the YBCO-coated conductors. The coil should have no damaged area in the windings because a damaged area would generate heat locally, eventually resulting in thermal runaway. Therefore, a demonstration coil with almost the same size as the pancake used for the YBCO insert coil was fabricated and tested in conduction cooling conditions in order to evaluate whether the coil could withstand the thermal stresses and electromagnetic force.

We have constructed an 18 T superconducting magnet conductively cooled by a GM/JT (Gifford-McMahon/Joule-Thomson) cryocooler. A double-pancake stacked insert coil using Ag-sheathed Bi2Sr2Ca2Cu3O10 (Bi2223) tape with stainless steel reinforcement has generated 2.5 T in a 15.5 T outer LTS coil. After we constructed the cryogen-free 18 T superconducting magnet (18 T-CSM), Bi2223 tape conductors have been improved in both critical current and strength. The critical current of the conductor doubled and a tape conductor with copper alloy reinforcement which has a strength of 250 MPa at 77 K was developed. So we developed a new Bi2223 insert coil for the cryogen-free superconducting magnet which can generate 4.5 T in a 15.5 T outer LTS coil by exchanging the insert coil. The new 20 T cryogen-free superconducting magnet (20 T-CSM) can generate constant magnetic fields up to 20 T in a 52-mm room-temperature bore.

122 type pnictide superconductors are of particular interest for high-field applications because of their large upper critical fields H-c2 (&gt; 100 T) and low anisotropy gamma (&lt; 2). Successful magnet applications require fabrication of polycrystalline superconducting wires that exhibit large critical current density J(c), which is limited by poor grain coupling and weak-link behavior at grain boundaries. Here, we report our recent achievement in the developing Sr0.6K0.4Fe2As2 tapes with transport J(c) up to 0.1 MA/cm(2) at 10 T and 4.2K. This value is by far the highest ever recorded for iron based superconducting wires and has surpassed the threshold for practical application. The synergy effects of enhanced grain connectivity, alleviation of the weak-link behavior at grain boundaries, and the strong intrinsic pinning characteristics led to the superior J(c) performance exhibited in our samples. This advanced J(c) result opens up the possibility for iron-pnictide superconducting wires to win the race in high-field magnet applications. (C) 2014 AIP Publishing LLC.

SmFeAsO1-xFx (Sm1111) tapes were prepared by an ex-situ powder-in-tube method with a Sn-presintering process. Scanning electron microscopy revealed apparent difference in microstructure between Sn-presintered tapes and the previously reported polycrystalline Sm1111 bulk, since Sn has reduced FeAs wetting phase and filled the voids between Sm1111 grains. The Sn-presintered tapes showed significant enhanced field dependences of transport J(c) compared with Sn-added tapes. A highest transport critical current density (J(c)) of 3.45 x 10(4) A cm(-2) at 4.2 K and self-field is achieved. Magneto-optical imaging further confirmed large and well-distributed global and intergranular Jc in Sn-presintered Sm1111 tapes. (C) 2014 AIP Publishing LLC.

Improving transport current has been the primary topic for practical application of superconducting wires and tapes. However, the porous nature of powder-in-tube (PIT) processed iron-based tapes is one of the important reasons for low critical current density (J(c)) values. In this work, the superconducting core density of ex-situ Sr0.6K0.4Fe2As2 + Sn tapes, prepared from optimized precursors, was significantly improved by employing a simple hot pressing as an alternative route for final sintering. The resulting samples exhibited optimal critical temperature (T-c), sharp resistive transition, small resistivity and high Vickers hardness (Hv) value. Consequently, the transport J(c) reached excellent values of 5.1 x 10(4) A/cm(2) in 10 T and 4.3 x 10(4) A/cm(2) in 14 T at 4.2 K, respectively. Our tapes also exhibited high upper critical field H-c2 and almost field-independent J(c). These results clearly demonstrate that PIT pnictide wire conductors are very promising for high-field magnet applications.

We report the controlled incorporation of perovskite, BaSnO3 (BSO), and double-perovskite, YBa2NbO6 (YBNO), nanocolumnar structures into a YBa2Cu3O7-delta (YBCO) film matrix by controlling the target rotation speed. A surface modified target approach has been employed to deposit YBCO + BSO and YBCO + YBNO nanocomposite films using a laser ablation technique. The effect of target rotation speed on the microstructure and subsequently on the superconducting properties has been studied in detail. The density of BSO and YBNO nanocolumnar structures is found to depend on the target rotation speed, which subsequently affects the vortex pinning properties of the superconducting films in the absence and presence of applied magnetic fields. Three rotation speeds, 3, 2 and 1 s/rot., have been attempted in this study. Compared to pure YBCO, the YBCO C BSO and YBCO + YBNO nanocomposite films exhibit superior in-field critical current density (J(C)) and also exhibit a strong J(C) peak for H parallel to c-axis, indicating strong c-axis pinning. The irreversibility line has also been found to improve significantly in the nanocomposite films. For both the target combinations (YBCO + BSO and YBCO + YBNO), the target rotation speed of 2 s/rot. has been found to give the optimum superconducting properties.

The basic physical reasons rooted in the current instability phenomenon of AC regimes are discussed. To explain their stable formation, the conduction-cooled high-temperature superconductors (HTS) are investigated studying mutually dependent thermal and electrodynamics states of a tape. It is proved that there exist characteristic times defining time windows of stable development of overloaded AC regimes. They lead to the states for which the limiting peak values of charged current and induced electric field before instability onset much more than the corresponding critical values of a superconductor. Moreover, the limiting stable peak values of the temperature, electric field and current are higher than the corresponding quench ones calculated at DC modes. It is shown that the stability boundary of overloaded AC modes slowly depends on the current frequency. As a result, the high-T-c superconducting magnets may stably operate under very high AC losses even at the conduction-cooled conditions. The obtained results should be taken into consideration when the range of stable losses is defined in HTS during AC regimes.

We have prepared spherical and plate-like bismuth nano-particles by the polyol process. The sphere size was varied from 200 to 400 nm by the addition of polyvinylpyrrolidone to inhibit the aggregation of atoms. Nano-plates 800 nm wide and 50 nm thick were formed by adding a probe-level ferric chloride. The crystal axis of the nano-spheres (diameter: 350 +/- 50 nm) dispersed in polysiloxane was oriented under high magnetic fields because of the anisotropic susceptibility of bismuth. The anisotropic susceptibility was estimated as 3.6 x 10(-5) from a numerical simulation of the field dependence of the (110) peak in the XRD pattern. The value was slightly smaller than that for a bulk single crystal. Composites of bismuth nano-spheres and plates dispersed in poly(methyl methacrylate) were prepared under high magnetic fields. The composites showed different transmittance depending on whether the direction of the propagation of light was parallel or perpendicular to the applied magnetic field, because of the anisotropy of the dielectric constants and the shape of the nano-plates.

Understanding the characteristic response of critical current, I-c with stress/strain under magnetic field is important for the application of coated conductor (CC) tapes to large scale superconducting coils or magnets. In this study, the strain and magnetic field effects on I-c in brass laminated reactive co-evaporation by deposition and reaction (RCE-DR) processed GdBCO CC tapes at 77 K were investigated. In the case of RCE-DR GdBCO CC tape samples with Hastelloy substrate, the additional lamination of brass foils to the Cu-stabilized only resulted in an increased irreversible strain limit of 0.50 % up to 0.80 %. But, not as much of these improvements were observed in CC samples having stainless steel substrate. Also, the additional brass laminate to all GdBCO CC tape samples did not affect the magnetic field response of I-c at 0 % strain. Under magnetic field, the strain response and tolerance of I-c was improved with additional brass laminates showing a much lower I-c reduction as compared with that at the irreversible strain limit of the Cu-stabilized CC samples. The strain sensitivity behavior of I-c under magnetic field (I-c-epsilon-B relation at 77 K) in RCE-DR GdBCO CC tapes was discussed.

A Rutherford flat cable that is applicable for operational currents up to 1000 A at 13-14 T was developed using a bronze-processed high-strength Nb3Sn strand with CuNb reinforcement (CuNb/Nb3Sn). The critical current for a 0.8-mm diameter CuNb/Nb3Sn strand is 98 A at 14 T and 4.2 K in the residual strain state. A test coil using the CuNb/Nb3Sn Rutherford flat cable composed of 16 CuNb-reinforced Nb3Sn strands was fabricated. We measured the critical current properties of the Rutherford test coil and obtained an excellent critical current of 1840 A at 14 T and 4.2 K. By using the strain gauges attached onto the stainless-steel reinforcement tape that was co-wounded with the Rutherford flat cable, it was found that a 300-MPa hoop stress at 4.2 K was applied to the CuNb/Nb3Sn strand. This implies that the critical current for a CuNb/Nb3Sn strand is enhanced to be 115 A at 14 T and 4.2 K through the stress-strain effect of the critical current at 300 MPa.

In order to clarify the influence of RE/Ba substitution on superconducting properties on REBCO films in magnetic fields, we fabricated BHO-doped Nd1+xBa2-xCu3Oy films in which Nd composition were controlled by PLD method using alternating-targets technique. The T-c of NdBCO(x = 0.01, 0.08) films were about 92 K. For the field applied parallel to the c-axis, a J(c) and a characteristic magnetic field of the maximum pinning force of the NdBCO(x = 0.08) film increased compared with those of the NdBCO(x = 0.01) film. Additionally, a J(c) of a BHO-doped NdBCO(x = 0.08) film improved for all applied angles of magnetic field in 1 T. The forms of J(c)-theta curves for the BHO-NdBCO films with x = 0.01 and 0.08 were similar form. Therefore, it is considered that the reason of J(c) increase in the BHO-doped NdBCO(x = 0.08) film is introduction of three-dimensional pinning centers of fine Nd/ Ba solid solutions and the solid solution do not influence on the BHO growth. (C) 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license.

We studied growth substrate temperature dependent morphologies of BaHfO3 (BHO) nano-rods for SmBa(2)Cu(3)Oy (SmBCO) films with BHO fabricated by pulsed laser deposition (PLD) method. The morphology of BHO nano-rods within the SmBCO matrix could be controlled by changing substrate temperatures adoping a Low Temperature Growth (LTG) technique. Using the LTG technique, we could fabricate SmBCO films showed c-axis orientation even at low substrate temperature during deposition without a degradation of superconducting properties, which is one of the most advantages of the LTG technique. In this study, we found an optimal morphology of the BHO nano-rods and the morphology was fine, straight and various growth directions. The BHO nano-rods were more effective to improve the Jc in a magnetic field than other morphologies of the BHO nano-rods.

For the improvement of superconducting properties of BaMO3 (M=Zr, Sn, Hf)doped REBa2Cu3Oy films, we focused on the Tc reduction caused by lattice misfit between BaMO3 and REBa2Cu3Oy. We compared the influence on the T-c reduction of BaZrO3, BaSnO3 and BaHfO3-doped SmBa2Cu3Oy (SmBCO) films with various BMO contents. From the correlation between configuration of nanorods and c-axis expansion, it was suggested that the lattice strain working on the SmBCO matrix became stronger in the order of BaHfO3, BaZrO3 and BaSnO3. Therefore, the combination of SmBCO and BaHfO3 is better regarding the suppression of the Tc reduction compared with other combinations of SmBCO and BaMO3.

Practical superconducting wires are designed with a composite structure to meet the desired engineering characteristics by expert selection of materials and design of the architecture. In practice, the local strain exerted on the superconducting component influences the electromagnetic properties. Here, recent progress in methods used to measure the local strain in practical superconducting wires and conductors using quantum beam techniques is introduced. Recent topics on the strain dependence of critical current are reviewed for three major practical wires: ITER-Nb&lt;sub&gt;3&lt;/sub&gt;Sn strand, DI-BSCCO wires and REBCO tapes.

The limiting current-carrying capacity of high-T-c superconductor and superconducting tape has been studied in the alternating current states. The features that are responsible for their stable formation have been investigated under the conduction-cooled conditions when the operating peak values of the electric field and the current may essentially exceed the corresponding critical values of superconductor. Besides, it has been proved that these peak values are higher than the values of the electric field and the current, which lead to the thermal runaway phenomenon when the current instability onset occurs in the operating modes with direct current. As a result, the stable extremely high heat generation exists in these operating states, which can be called as overloaded states. The limiting stable peak values of charged currents and stability conditions have been determined taking into account the flux creep states of superconductors. The analysis performed has revealed that there exist characteristic times defining the corresponding time windows in the stable development of overloaded states of the alternating current. In order to explain their existence, the basic thermo-electrodynamics mechanisms have been formulated, which have allowed to explain the high stable values of the temperature and the induced electric field before the onset of alternating current instability. In general, it has been shown that the high-T-c superconductors may stably operate in the overloaded alternating current states even under the not intensive cooling conditions at a very high level of heat generation, which is not considered in the existing theory of losses.

The construction of a 25-T cryogen-free superconducting magnet (25T-CSM) has started in 2013 at the High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University. The 25T-CSM consists of a low-T-c superconducting (LTS) coil and a high-T-c superconducting (HTS) coil. A high-strength CuNb/Nb3Sn Rutherford cable with the reinforcing stabilizer CuNb composite is adopted for the middle LTS section coil. The characteristic feature of the new technology using a CuNb/Nb3Sn Rutherford cable is a react-and-wind method for the coil-winding process. The LTS coil of 300-mm winding inner diameter is fabricated, and a central magnetic field of 14 T is generated at an operation current of 851 A. The HTS insert coil wound with GdBa(2)Cu(3)Oy (Gd123) tape has a 52-mm experimental room temperature bore, and a central magnetic field of 25.5 T will be generated at an operation current of 150 A in a background field of 14 T.

Since the development of the 18T cryogen-free superconducting magnet (18T-CSM) at the High Field Laboratory for Superconducting Materials (HFLSM), IMR, Tohoku University, it has been routinely utilized for the various high field researches such as the in-field materials processing and so on. After the Great East Japan Disaster, however, the 18T-CSM is under the repairing due to the serious damages of the system. At this time, a part of the middle Nb3Sn section coil and the Bi2223 inner most coil were renewed. The bronze-route Nb3Sn coil with a low ac-loss was adopted instead of the previous internal-Sn Nb3Sn coil for the reduction of the heat-generation during a field ramping. The new Bi2223 coil will generate the central field of 4.5 T under the background field of 15.5 T by the LTS outer coils, although the previous Bi2223 coil generated 2.5 T. Hence the 18T-CSM was successfully upgraded to the 20T-CSM. In addition to the Bi2223 insert, the Gd123 insert coil is being constructed as well. When the Gd123 insert coil is installed instead of the Bi2223 coil, it will be upgraded to the 22T-CSM from the 20T-CSM.

New Nb-rod-processed CuNb reinforced Nb3Sn superconducting wires have been developed for large high field magnet applications. The critical current, I-c, mechanical properties and resistivity are investigated. The residual strain and irreversible strain are found to be 0.35% and 0.77%, respectively. We found that the Nb-rod-processed wire has a large strain sensitivity to I-c. However, the wire has good mechanical properties, having a 0.2% proof stress of 280 MPa and a Young's modulus of 150 GPa. This means that the stress sensitivity of the I-c for the wire is small. These results show that the Nb-rod-processed wire is suitable as a strand of Rutherford cable for a high field superconducting magnet.

In YBa2Cu3O7 (YBCO) + BaSnO3 (BSO)/PrBa2Cu3O7 (PrBCO) multilayer films, behavior of vortices pinned by BSO nanorods was evaluated to discuss influence of ab-plane non-superconducting layers on c-axis correlated vortex pinning. Critical current density (J(c)) and activation energy for thermally activated flux flow in magnetic field parallel to the c-axis were reduced by the PrBCO layers and a c-axis peak in an angular dependence of J(c) became small, as YBCO + BSO layer thickness decreased or PrBCO layer thickness increased. Irreversibility temperature and trapping angle were not changed by the PrBCO layers for thick YBCO + BSO layers and thin PrBCO layers. This indicates that the ab-plane non-superconducting layers accelerated the vortex motion along the ab-plane without changing the vortex structure. Thin YBCO + BSO layers or thick PrBCO layers resulted in much more drastic degradation of the c-axis correlated vortex pinning due to two dimensional vortex nature or partially prevailed vortex structure, which decreased not only J(c) and activation energy, but also irreversibility temperature and trapping angle. Thus, the c-axis correlation of vortex pinning is varied by addition of the ab-plane non-superconducting layers. (C) 2013 AIP Publishing LLC.

Fe-clad Sr0.6K0.4Fe2As2 (Sr-122) superconducting tapes were fabricated by cold uniaxial pressing. It is found that the pressing can improve the mass density and induce c-axis texture in the Sr-122 core. With an increase of pressure from 0.6 to 1.8 GPa, the degree of texture increases monotonically. The transport current density (Jc) is correspondingly improved until the applied pressure comes up to 1.4 GPa, and finally becomes saturated at 1.8 GPa. A transport Jc as high as 16.8 kA cm-2 (at 4.2 K in self-field) is achieved at 1.4 GPa. The saturation effect of transport Jc can be ascribed to the microcracks in the grains induced by the strong deformation force when the applied pressure is high. © 2013 IOP Publishing Ltd.

High strength against electromagnetic force is required for high magnetic field and large capacity coil in order to develop large-capacity superconducting magnetic energy storage (SMES) systems for electric power system control. Also, suppression of delaminating of Yttrium (Y) based coated conductor in a coil is required to manufacture the highly reliable and durable superconducting coil. Insulating coating, using liquid resin of low-temperature-curable-polyamide, was developed and showed durability at very low temperature without deterioration of transport properties of superconducting wire in a coil. Combining paraffin molding, delaminating, and deterioration of transport properties of superconducting wire were not also observed in a coil. These insulating techniques were applied to the pancake coil, in which superconducting wire and the reinforcing outer plates of the coil withstand electromagnetic force. The double pancake coil of this coil structure, called "Yoroi-coil; Y-based oxide superconductor and reinforcing outer integrated coil" was prepared and the durability against electromagnetic force by hoop stress test was verified. The coil achieved 1.5 kA transporting at 4.2 K in 8 T backup magnetic field without the degradation of transport properties. Maximum hoop stress at the hoop stress test reached 1.7 GPa, based on the calculations. This result confirmed that Yoroi-coil structure has a capability to withstand the large hoop stress, which exceeded the tensile strength of Y-based coated conductors, and bring out highly durable and reliable superconducting coils.

The high temperature superconductor (HTS) coil development, especially in the interest of high field insert coil, requires characterizations of a prototype coil in high magnetic field. Our test probe fits into the bore of circle divide 170mm20 T dc magnet, and the sample space circle divide 130 mm allows test coils with similar to circle divide 110 mm and 140 mm height. A fast data acquisition method at sampling period of 0.1 ms was developed for coil quench test program, and a working principle of the method is demonstrated for a small test coil. To accommodate high current required in tests such as I-c and hoop stress tests, the new current leads were manufactured with an optimal current measured in the range of 600 similar to 700 A, which was sufficient for the envisaged coil tests. For short duration, the current lead could carry more than 1000 A without any influence of overheating in short sample I-c test. A demonstration of short sample I-c test is also presented.

A Rutherford flat cable composed of sixteen 0.8-mm-diameter Nb3Sn strands with CuNb-reinforced stabilizers (CuNb/Nb3Sn) has been developed for use as an outsert for a 47-T hybrid magnet. To guarantee the safety of the magnet in the event of a quench, the low residual resistance ratio characteristic of arc-melted-in-situ CuNb composite had to be improved upon, and to obtain high electric conductivity, a new CuNb fabrication method using a Nb rod was attempted. The thermal runaway characteristics of chemical vapor deposition-YBa2Cu3O7-delta (Y123) coated-conductor tapes with varying Cu stabilizer thicknesses were measured in magnetic fields of up to 10 T at temperatures ranging from 17 to 60 K. It was found that the ratio of the thermal runaway over-current to the critical current decreases as temperature decreases and that thermal runaway occurs just above the critical current level at 5 K. The properties of a cryogen-free, 20-T superconducting outsert for a 47-T hybrid magnet utilizing CuNb/Nb3Sn Rutherford flat cables and Y123 tapes and having a room temperature bore of 400 mm were investigated.

In this study, the reversible response of I-c with uniaxial strain in differently processed GdBCO coated conductor tapes with different substrates has been investigated under self field and magnetic field conditions at 77 K. Higher irreversible strain limit, epsilon(irr) was observed in those samples with stainless steel substrate. Additional brass lamination to the Cu-stabilized samples produced enhancement of both epsilon(irr) and endurable load limit. However, no significant widening of the I-c/I-c0-tensile strain window with brass addition was observed in the case of samples having stainless steel substrate. Furthermore, under magnetic field, the I-c degradation behavior was independent on the kind of substrate material, but varied with the kind of manufacturing process adopted. The GdBCO samples showed different I-c peak positions and degradation behavior depending on the level of magnetic field applied.

BaHfO3 (BHO) is attracting attention as a new material forming nanorod as well as BaMO3 (BMO: M = Zr, Sn), and it was reported that the flux pinning force of BHO nanorod is effective even at low temperatures. It is essential to understand the flux pinning properties of an APC-doped REBCO film at low temperatures in order to achieve the high-performance superconducting applications. We fabricated 8.4 mol. % BHO-doped SmBa2Cu3Oy (SmBCO) film on LaAlO3 (LAO) substrates by pulsed laser deposition method and measured the superconducting properties at 77, 65, 40, and 20 K. As for the magnetic field dependence of J(c), the J(c) of the BHO-doped SmBCO film was significantly higher than that of the pure SmBCO film at all temperatures. Furthermore, in the magnetic field angular dependence of J(c), the BHO-doped SmBCO film exhibited the peaks of J(c) around B//c at 77, 65, and 40 K in 3 T. These results suggested that the flux pinning force of BHO nanorod was effective even at 65 and 40 K.

GdBa2Cu3Oy coated conductors were annealed under external strain (strain-annealing). The residual strain after annealing was evaluated using synchrotron radiation. We found that the residual strain is changed by the strain-annealing. The volume fraction of the a-axis domains along the longitudinal direction is increased by compression-annealing and decreased by tension-annealing. In addition, the J(c) value of the tension-annealed sample increases drastically below 70 K, although the J(c) value is smaller than that of the as-received sample at 77.3 K for B//c. The peak of the angular dependent J(c) at theta = 0 degrees, which originates from the c-axis correlated pinning, for the strain-annealed sample is much smaller than that of the as-received sample. It is considered that the shrinking of the c-axis correlated pinning because of the reduction of the twin structure and the increase of carrier doping may be related to the change in J(c) properties.

We have investigated the in-field current transport property in BaHfO3 doped GdBa2Cu3O7-delta coated conductors in a wide range of temperatures and magnetic fields. Significant improvement of in-field critical current I-c was observed, e. g., I-c@77 K, 3 T = 93 A/cm-w, I-c@20 K, 17 T = 700 A/cm-w, which is a comparable value to that of Nb3Sn wire at 4.2 K. Enhancement of the irreversibility field was also observed. These results suggest that BaHfO3 is one of the most promising materials as effective artificial pinning centers and leads to the enhancement of in-field I-c. Furthermore, we have also shown that our analytical expression of electric field versus current density characteristics based on the percolation transition model [1-3] agrees well with the experimental results over a wide range of magnetic fields and temperatures. This analytical expression is useful for the design of superconducting devices because this allows us to predict the current carrying capability of coated conductors not only J(c) but also n-value at arbitrary operating conditions of temperature and magnetic field.

In REBa2Cu3Oy (RE123, RE=rare earth element) coated conductors, the coexistence of a- and b-axes domains related to a twin structure is one of the origins of the complicated critical current density property and its strain dependence. Hence, control of the twin structure is a very important issue. In the case of Y123 single crystals, detwinning is widely carried out by oxygen annealing under uniaxial stress along the a/b axis. In this study, commercial RE123 superconducting tapes were oxygen-annealed under external strains, similar to the detwinning process of Y123 single crystals. We found that the volume fraction ratio of the a- and b-axis domains changes with the applied external strains during annealing, indicating a change of twin structure. In addition, the residual strains, which are strains induced from the thermal contraction difference between RE123 and the substrate, increase with compression annealing and decrease with tension annealing for RE123 tapes with [100]/[010] orientation along the tape axis. However, those phenomena by strain annealing are not obtained for the [110] oriented RE123 tapes. The strain annealing method that we propose here is very useful to control the internal residual strain and twin structures in an RE123 tape with a [100]/[010] orientation along the tape axis. © 2013 IOP Publishing Ltd.

High strength against electromagnetic force is required for high magnetic field and large capacity coil in order to develop large-capacity superconducting magnetic energy storage (SMES) systems for electric power system control. Also, suppression of delaminating of Yttrium (Y) based coated conductor in a coil is required to manufacture the highly reliable and durable superconducting coil. Insulating coating, using liquid resin of low-temperature-curable-polyamide, was developed and showed durability at very low temperature without deterioration of transport properties of superconducting wire in a coil. Combining paraffin molding, delaminating, and deterioration of transport properties of superconducting wire were not also observed in a coil. These insulating techniques were applied to the pancake coil, in which superconducting wire and the reinforcing outer plates of the coil withstand electromagnetic force. The double pancake coil of this coil structure, called "Yoroi-coil; Y-based oxide superconductor and reinforcing outer integrated coil" was prepared and the durability against electromagnetic force by hoop stress test was verified. The coil achieved 1.5 kA transporting at 4.2 K in 8 T backup magnetic field without the degradation of transport properties. Maximum hoop stress at the hoop stress test reached 1.7 GPa, based on the calculations. This result confirmed that Yoroi-coil structure has a capability to withstand the large hoop stress, which exceeded the tensile strength of Y-based coated conductors, and bring out highly durable and reliable superconducting coils.

The high temperature superconductor (HTS) coil development, especially in the interest of high field insert coil, requires characterizations of a prototype coil in high magnetic field. Our test probe fits into the bore of circle divide 170mm20 T dc magnet, and the sample space circle divide 130 mm allows test coils with similar to circle divide 110 mm and 140 mm height. A fast data acquisition method at sampling period of 0.1 ms was developed for coil quench test program, and a working principle of the method is demonstrated for a small test coil. To accommodate high current required in tests such as I-c and hoop stress tests, the new current leads were manufactured with an optimal current measured in the range of 600 similar to 700 A, which was sufficient for the envisaged coil tests. For short duration, the current lead could carry more than 1000 A without any influence of overheating in short sample I-c test. A demonstration of short sample I-c test is also presented.

A Rutherford flat cable composed of sixteen 0.8-mm-diameter Nb3Sn strands with CuNb-reinforced stabilizers (CuNb/Nb3Sn) has been developed for use as an outsert for a 47-T hybrid magnet. To guarantee the safety of the magnet in the event of a quench, the low residual resistance ratio characteristic of arc-melted-in-situ CuNb composite had to be improved upon, and to obtain high electric conductivity, a new CuNb fabrication method using a Nb rod was attempted. The thermal runaway characteristics of chemical vapor deposition-YBa2Cu3O7-delta (Y123) coated-conductor tapes with varying Cu stabilizer thicknesses were measured in magnetic fields of up to 10 T at temperatures ranging from 17 to 60 K. It was found that the ratio of the thermal runaway over-current to the critical current decreases as temperature decreases and that thermal runaway occurs just above the critical current level at 5 K. The properties of a cryogen-free, 20-T superconducting outsert for a 47-T hybrid magnet utilizing CuNb/Nb3Sn Rutherford flat cables and Y123 tapes and having a room temperature bore of 400 mm were investigated.

In this study, the reversible response of I-c with uniaxial strain in differently processed GdBCO coated conductor tapes with different substrates has been investigated under self field and magnetic field conditions at 77 K. Higher irreversible strain limit, epsilon(irr) was observed in those samples with stainless steel substrate. Additional brass lamination to the Cu-stabilized samples produced enhancement of both epsilon(irr) and endurable load limit. However, no significant widening of the I-c/I-c0-tensile strain window with brass addition was observed in the case of samples having stainless steel substrate. Furthermore, under magnetic field, the I-c degradation behavior was independent on the kind of substrate material, but varied with the kind of manufacturing process adopted. The GdBCO samples showed different I-c peak positions and degradation behavior depending on the level of magnetic field applied.

BaHfO3 (BHO) is attracting attention as a new material forming nanorod as well as BaMO3 (BMO: M = Zr, Sn), and it was reported that the flux pinning force of BHO nanorod is effective even at low temperatures. It is essential to understand the flux pinning properties of an APC-doped REBCO film at low temperatures in order to achieve the high-performance superconducting applications. We fabricated 8.4 mol. % BHO-doped SmBa2Cu3Oy (SmBCO) film on LaAlO3 (LAO) substrates by pulsed laser deposition method and measured the superconducting properties at 77, 65, 40, and 20 K. As for the magnetic field dependence of J(c), the J(c) of the BHO-doped SmBCO film was significantly higher than that of the pure SmBCO film at all temperatures. Furthermore, in the magnetic field angular dependence of J(c), the BHO-doped SmBCO film exhibited the peaks of J(c) around B//c at 77, 65, and 40 K in 3 T. These results suggested that the flux pinning force of BHO nanorod was effective even at 65 and 40 K.

GdBa2Cu3Oy coated conductors were annealed under external strain (strain-annealing). The residual strain after annealing was evaluated using synchrotron radiation. We found that the residual strain is changed by the strain-annealing. The volume fraction of the a-axis domains along the longitudinal direction is increased by compression-annealing and decreased by tension-annealing. In addition, the J(c) value of the tension-annealed sample increases drastically below 70 K, although the J(c) value is smaller than that of the as-received sample at 77.3 K for B//c. The peak of the angular dependent J(c) at theta = 0 degrees, which originates from the c-axis correlated pinning, for the strain-annealed sample is much smaller than that of the as-received sample. It is considered that the shrinking of the c-axis correlated pinning because of the reduction of the twin structure and the increase of carrier doping may be related to the change in J(c) properties.

We performed hoop stress tests of REBCO multilayer pancake coils impregnated by epoxy resin. The mechanical deformation and electric field-current properties were measured under the large hoop stress. The maximum hoop stress of about 530 MPa per Hastelloy substrate, calculated from the BJR relation, was applied in the background magnetic field of 8 T. Because the hoop stress level is much smaller than the mechanical tolerance of the GdBCO tape, the coil performance was limited by the angular dependence of critical current in this test. Furthermore, the hoop stress test under the large electromagnetic stresses was also carried out for the other (Y, Gd)BCO coil. The (Y, Gd)BCO epoxy impregnated coil was operated without any degradation even in the huge hoop stress over 1300 MPa. The mechanical deformation of the coil is analysed on the basis of the measured strains. We confirmed the large strain about 0.3%-0.5% because of the hoop stress over 1300 MPa at I-op = 460 A and B = 13.5 T but no degradation of the coils. However, it is suggested that the coil deformation is very complicated under the large electromagnetic stress.

A high-strength pancake-coil structure called "Yoroi-coil" (Y-based oxide superconductor and reinforcing outer integrated coil), in which superconducting wire and the reinforcing outer plates of the coil withstand electromagnetic force, was developed for high magnetic field applications. The double-pancake (DP) coil of this coil structure was prepared and verified for durability against electromagnetic forces using the hoop stress test. The coil achieved 1.5 kA transporting at 4.2 K in an 8 T backup magnetic field without degradation of transport properties. Maximum hoop stress at the hoop stress test reached 1.74 GPa, based on the calculations. This result confirmed that the "Yoroi-coil" structure has the capability to withstand large hoop stress, which exceeds the tensile strength of Y-based coated conductors, not only owing to the strength of superconducting wire but also owing to the integration of the whole coil structure itself. Additionally, the strengthening effect of the structure was verified by analysis.

Atom probe tomography was used to characterize the A15 phase in a bronze-route Nb3Sn superconducting wire with a bronze matrix composition of Cu-8Sn-0.3Ti (in at.%). We observed depletion of niobium and segregation of Cu and Ti atoms at Nb3Sn grain boundaries. While the Nb depletion is about 15% relative to the grain interior, the average ratio between Cu and Ti excess values is 9 to 2. Segregation extends to a distance d similar to 9 angstrom from the point of maximum Cu and Ti concentrations. Such local variation in the stoichiometry at the grain boundary region can be an additional source of flux-pinning in the Nb3Sn phase. Other microstructural parameters, such as the grain size and chemical composition of the Nb3Sn layer, were investigated by electron backscatter diffraction and transmission electron microscopy.

The high-magnetic-field magnetization measurements of Heusler alloys, Fe2MnSi and Fe2Mn0.8V0.2Si, were performed up to 27 T, and the magnetization M vs T measurements were carried out up to 12 T. We observed a magnetic anomaly at 64 K, which persisted even for high magnetic fields at 12 T. We investigated the field dependence of the magnetic anomaly at 64K on the basis of magnetization measurements and discuss this anomaly with respect to other measurement results. An Arrott plot for x = 0 at 4.2K indicates that the critical field is approximately 70 T, at which the magnetic moment is 3.0 mu(B)/f.u. It can be inferred that the magnetic transition from the antiferromagnetic state to the ferromagnetic state with a half-metallic band structure occurs, when the magnetization reaches 3.0 mu(B)/f.u. at a critical field of B-s = 70 T.

Polycrystalline Sn-added SmFeAsO1-xFx tapes were prepared by an ex-situ powder-in-tube method. It is found that the transport critical current density (Jc) of the SmFeAsO1-xF x tapes can be significantly improved with 10-30 wt. Sn addition. A transport Jc of 2.11 × 104 A cm-2 at 4.2 K in self-field was obtained in SmFeAsO1-xFx 30 wt. Sn sample. Most interestingly, Sn-added SmFeAsO1-xFx tapes without any sintering also showed substantial transport Jc of 9.81 × 103 A cm-2, which is reported for the first time. X-ray diffraction and scanning electron microscopy characterization revealed that the elemental Sn dispersed in the sample transformed into FeSn2 during the post-rolling sintering (PRS). The reaction between Sn and FeAs helps to eliminate the detrimental FeAs wetting phase, which blocks the supercurrent path between SmFeAsO1-xFx grains. The combined impact of fluorine loss and Sn reaction was discussed in this work by varying the Sn addition quantity and PRS condition. © 2013 American Institute of Physics.

Melt textured bulks of (Sm0.33Eu0.33Gd 0.33)Ba2Cu3Oy (SEG123) show high Jc compared with that of YBa2Cu3Oy (Y123), because of nanoscopic network consisting of crossing rare earth (RE) compositional stripes. In this study, we fabricated SEG123 films by using pulsed laser deposition (PLD) method on LaAlO3 (LAO) substrates and evaluated the Jc and Tc in magnetic field. Jc of the SEG123 film was typically 1.4 MA/cm2 at 77 K in self-field. Jc in self-field of SEG123, Y123 and Sm123 films were almost the same value, however, Jc in magnetic fields showed a notable difference. The Jc of SEG123 film were 62.7 kA/cm2 at 5 T. We conclude that the Jc in magnetic fields were improved by RE-rich particle made of RE/Ba substitution acting as 3-dimensional pinning centers. Moreover we have confirmed that the superconducting properties changes by varying Sm, Eu and Gd composition ratio.

In order to improve superconducting properties of REBCO films under magnetic fields, we fabricated BHO-doped SmBCO films on IBAD-MgO substrates by PLD method using alternating-targets technique which can change the content of the BHO in the SmBCO films. Consequently, T(c)s and J(c)s at self-field of the BHO-doped SmBCO films were decreased from 92 K to 89 K and 3.0 MA/cm(2) to 1.2 MA/cm(2) at 77 K in self-field with increasing the BHO content. However, J(c) of the BHO-doped SmBCO films in high magnetic fields exceeded that of a pure-SmBCO film. (C) 2013 The Authors. Published by Elsevier B.V.

Bismuth is a half metal having anisotropic susceptibility and optical constants. Therefore, the crystal axis of bismuth particle can be oriented by magnetic torque under magnetic fields. Recently, bismuth nano-particles, which have spherical, plate-like and rod-like shapes, have been prepared by some chemical methods. Orientation of bismuth nano-spheres in a viscous liquid was demonstrated by using in-situ observation system of x-ray diffraction under high magnetic field in this study. The anisotropic susceptibility determined from the orientation behavior was slightly smaller than that of the bulk bismuth crystal. We have successfully introduced oriented-bismuth nano-spheres into the transparent polymethyl metacrylate under high magnetic fields up to 8 T. The composite showed a difference in transmittance depending on the direction of propagation of light being parallel or perpendicular to the applied field, indicating an anisotropic nature of optical constants of the nano-particles. Transmission spectra, which were simulated on the nano-spheres oriented composite by rigorously coupled wave analysis (RCWA), indicated that the nano-sphere has larger anisotropy in optical constants than bulk bismuth.

The strain and the critical current of the GdBCO coated conductor were investigated in order to understand the mechanism of the strain effect for a coated conductor. The internal strain behaviors of the longitudinal, width, and thickness directions were measured by the X-rays of the synchrotron radiation. From these results, the residual strains of a, b, and c-axes for the GdBCO crystal were estimated. In addition, the strain dependence of the critical current and the n-value at 4.2 and 77 K were measured under the tensile strain. The relationship between superconducting properties and the triaxial strain in the GdBCO crystal of the coated conductors is discussed. © 2012 IEEE.

In order to enhance J(c) in magnetic fields for all applied angles, we fabricated multilayered films consisting of Sm1.04Ba1.96Cu3Oy (SmBCO) layers and BaSnO3 (BSO)-doped SmBCO layers on MgO substrates (multilayered SmBCO) by the pulsed laser deposition (PLD) method. We varied the numbers of layers (8, 16, 32, and 64). In the magnetic field angle dependence of J(c) (B 1 T, 77 K), only in the 8-layer sample was J(c) higher than that of pure SmBCO thin films for all applied angles. The other sample showed different J(c) characteristics. (C) 2013 The Japan Society of Applied Physics

The macroscopic thermal and electrodynamical phenomena occurring in high-Tc superconductors during overloaded AC states are theoretically investigated to understand the basic physical mechanisms, which are characteristic for the stable formation of the operating modes when the peak current exceeds the critical current of a superconductor during AC modes. It is shown that there exist characteristic time windows defining the existence of stable overloaded AC states. They identify the stability boundary of the overloaded AC states. Therefore, there is the maximum allowable value of a peak current of stable overloaded AC regimes at the given charging rate, cooling conditions and properties of a superconductor and a matrix. The results obtained prove that the limiting peak current is higher than the corresponding quench current defining the stability margin of DC states. It monotonically increases with the charging rate. Besides, in the stable overloaded AC states, the peak values of the electric field and temperature may be also noticeably higher than the corresponding quench values. They depend on the peak current and charging rate at the given cooling conditions. As a result, high-Tc superconducting tapes can stably work under intensive AC modes without instability onset when the peak of applied currents may significantly exceed not only the critical current but also the corresponding values of DC-quench currents. © 2013 Elsevier B.V. All rights reserved.

The strain and the critical current of the GdBCO coated conductor were investigated in order to understand the mechanism of the strain effect for a coated conductor. The internal strain behaviors of the longitudinal, width, and thickness directions were measured by the X-rays of the synchrotron radiation. From these results, the residual strains of a, b, and c-axes for the GdBCO crystal were estimated. In addition, the strain dependence of the critical current and the n-value at 4.2 and 77 K were measured under the tensile strain. The relationship between superconducting properties and the triaxial strain in the GdBCO crystal of the coated conductors is discussed. © 2012 IEEE.

Nb3Al superconducting wires produced by rapid heating and quenching (RHQ) have been developed for high-field accelerator magnets. It is known that critical currents of A15 superconductors (e.g., Nb3Al and Nb3Sn) have a dependence on stress/strain. It is thus important to clarify the stress/strain behavior of the RHQ-Nb3Al wires for the development of high-field accelerator magnets. We recently started experimentally investigating the strain dependence of the critical current of the RHQ-Nb3Al wires with a Ta interfilament by measuring their critical currents under longitudinal tensile strains. To evaluate the effect of residual strain in Nb3Al filaments induced by thermal contraction of the materials in the wire, neutron diffraction measurements were performed at room temperature. (C) 2012 Elsevier Ltd. All rights reserved.

Effects of Zr addition and magnetic field process on the superconducting property of HoBa2Cu3O7-x films fabricated by a chemical vapor deposition technique in magnetic fields have been investigated. It is found that Zr additions lead to improvement of the critical current density J(c) in-field, while a further J(c) enhancement along the c-axis of the HoBa2Cu3O7-x films is obtained in magnetic field processed samples. The nano-particles and nano-rods induced by the Zr addition are thought to account for the in-field J(c) improvement. In addition, it seems that there are more columnar defects in the samples deposited under magnetic field than the normal samples, which in turn enhance the pinning ability along the c-axis. (C) 2012 Elsevier B. V. All rights reserved.

The strain dependence of I-c and I-c for the GdBCO coated conductor was measured at 77 and 4.2 K in magnetic fields. T-c decreased with increasing applied tensile strain. For the I-c measurements, the magnetic field was applied in the direction parallel to the c-axis (perpendicular to the tape surface). I-c in fields was degraded with increasing strain, and the strain dependence of I-c increased with increasing magnetic field. The ratio of the I-c degradation at 4.2 K was smaller than that at 77 K. These results mean that the T-c degradation is one of the reasons for I-c degradation at 77 K at least. Further, in order to understand the strain effect at 4.2 K, the internal strain of the GdBCO coated conductor under tensile strain was measured using synchrotron radiation. It was found that the strain behavior for the a-axis is different from that for the b-axis. This result suggests that the strain dependence of I-c for GdBCO coated conductors under external tensile strain is related with the I-c degradation behavior of the crystal.

We measured the time evolution of voltage and temperature of a CVD-Y123 coated conductor monolayer coil with Cu stabilizer under the cryocooled condition. The thermal runaway occurred locally at operate currents of 1.05-1.3 I-c at 38 K and 1-10 T. In this case, the hot spot originated from the inhomogeneity of I-c was observed. The thermal runaway generated from different parts of the tape at high and low fields at 38 K, respectively. This is due to the different heating behavior related to the I-c distribution. The critical heating power of the thermal runaway obtained in this study were about 30 mW at 38 K and 10 T. It weakly depends on magnetic filed but is independent to the temperature. The analysis suggests that the cooling condition for the transient phenomena such as the thermal runaway is poor.

Internally reinforced strands Nb3Sn with CuNb-re-inforcing stabilizer (CuNb/Nb3Sn) have been developed for realizing a compact high-field superconducting magnet. The Rutherford flat cables composed of sixteen strands were made with a 0.8 mm diameter CuNb/Nb3Sn strand. The designed critical current of the CuNb/Nb3Sn Rutherford flat cable is I-c = 1890 A at 13 T and 4.2 K in case of an ordinary heat-treatment at 670 degrees C for 96 h, which is two times higher than an operation current desired for safety margin. In addition, mechanical characteristics for CVD-processed YBa2Cu3O7 (YBCO) coated-conductor tapes were measured at 4.2 K in fields up to 18 T. It was found that YBCO tape exhibits the irreversible strain 0.5% and the irreversible stress 1140 MPa for the critical current at 4.2 K and 18 T for B / / c. The cabling-and-react+prebending-processed CuNb/Nb3Sn Rutherford flat cables and CVD-processed YBCO tapes will be used as a react-and-wind method for the development of a 20 T superconducting magnet with a wide room temperature bore. We design a 20 T-440 mm warm-bore superconducting magnet consisting of YBCO inner coils, middle coils, Nb3Sn middle coils, and NbTi outer coils, which has the coil parameters of inner diameter 480 mm, outer diameter 1283 mm, and coil height 1302 mm. Since the coil inductance is estimated to be 232 H, the magnetic stored energy is 94 MJ at an operation current 900 A. An energy-saving 47 T hybrid magnet can be constructed in combination with a wide-bore 20 T superconducting outsert and a 15 MW-27 T water-cooled resistive insert.

We have studied for upgrading an 18 T super-conducting magnet conductively cooled by a GM/JT cryocooler at Tohoku University in Japan to a 23 T superconducting magnet. In order to upgrade the cryogen-free high field superconducting magnet up to 23 T, an YBa2Cu3O7(Y123) coated conductor insert is intended for replacing the insert using Bi2Sr2Ca2Cu3O10(Bi2223) tape with stainless steel reinforcement tape. We have produced double pancake coils with almost the same size as a double pancake coil for the Bi2223 insert, and have investigated the performance of the coils. And the design of the double pancake coil was reviewed from the result of the R&D coils.

Internal strains under a tensile load for CuNb/Nb3Sn wires with and without the prebending treatment were measured directly by neutron diffraction at room temperature and 7 K. The axial residual strains changed by 0.30% at room temperature and by 0.38% at 7 K to the tensile side due to the prebending treatment. The effective Poisson's ratio evaluated from the relationship between axial and lateral strains under an axial stress was about 0.36 at 7K with or without the prebending treatment. The applied tensile strain dependences of the deviatoric and hydrostatic strains were estimated, and the tensile strain dependence of the superconducting properties were discussed on the basis of the three-dimensional strain model. A control of the residual strains such as the prebending treatment plays an important role in the superconducting properties of Nb3Sn wires under axial stress/strain.

SmBCO and YBCO coated conductors (CC) showed different critical current, I-c degradation behavior with strain showing different I-c peak position under self field. Strain effect on I-c under magnetic field in REBCO CC tapes was investigated in order to measure and to describe its tolerance and degradation behavior. Katagiri-type tensile testing apparatus was adopted in which magnetic field was applied parallel to the c-axis of coating film using 10 T superconducting magnet at 77 K. Both strain gauge and Nyilas double extensometers were used to measure the strain. In all the REBCO CC tapes investigated, I-c(epsilon, B) strongly depends on magnetic field and their degradation behavior depends on the kind of HTS film and fabrication technique adopted. Differently with YBCO CC tapes, SmBCO CC tapes showed increasing I-c strain sensitivity with increasing magnetic field at 77 K and the irreversible strain limit was magnetic field independent over the entire field range studied.

The effect of Zr and Sn addition on the critical current density (J(c)) and the microstructure of filamentary SmBa2Cu3Oy (Sm123) has been investigated. Samples with additions fabricated by a solution spinning method were partially melted under an atmosphere gas containing 0.1-3.0% O-2. After heat-treatment, homogeneous dispersion of fine Sm2BaCuO5 (Sm211) particles was observed in the Zr-doped sample. The 0.2 at% Zr-doped Sm123 showed the depressed J(c) value of 1.8 x 10(4) A/cm(2) at 77 K and self-field compared with the undoped sample. From the J(c) measurement in magnetic fields, J(c) value higher than 10(3) A/cm(2) at 77 K was maintained up to 8 T. On the other hand, 0.2 at% Sn-added sample showed high J(c) value of 5.0 x 10(4) A/cm(2) at 77 K and self-field, and the J(c) value higher than 10(3) A/cm(2) is maintained up to 17 T. A small amount of Sn doping was found to be effective in increasing the J(c) under high magnetic field and the irreversibility field of Sm123 filaments.

The internal strain of the practical Nb3Sn wires under tensile strain were measured by synchrotron radiation. The x-ray energy of 72 keV was selected to obtain the diffraction from the Nb3Sn in the composite wire. The axial and lateral strains of the as-reacted and prebent wire were investigated. The Nb3Sn 321 and 320 diffraction peaks were measured to obtain accurate strain data because of their high intensity. It was found that the relationship between internal and external strain under the tensile strain is slightly different. The strain of the Nb3Sn 321 plane reveals the similar total strain applied to the Nb3Sn wire. The ratio of lateral/axial strain for the as-reacted and prebent wires is 0.34 and 0.30, respectively.

Internal lattice strains under a tensile load for a seven-strand cable consisting of three pre-bent CuNb/Nb3Sn and four stainless steel strands were measured by neutron diffraction at room temperature in order to investigate the improvement of mechanical properties of Nb3Sn strands. Stress-lattice strain curves of Nb3Sn and stainless steel strands were obtained, and the effective stress for a Nb3Sn strand was estimated by considering the load sharing of a stainless strand. It was found that the reinforcement with stainless steel strands is an effective way to reduce an applied electromagnetic tensile load for Nb3Sn strands for superconducting magnet application.

SmFeAsO1-xFx tapes were prepared using three kinds of starting materials. This showed that the starting materials have an obvious effect on the impurity phases in the final superconducting tapes. Compared with the other samples, the samples fabricated with SmAs, FeO, Fe2As, and SmF3 have the smallest arsenide impurity phases and voids. As a result, these samples possess much denser structures and better grain connectivities. Moreover, among the three kinds of sample fabricated in this work, this kind of sample has the highest zero resistivity temperature, similar to 40 K, and the largest critical current density, similar to 4600 A cm(-2), in self-field at 4.2 K. This is the highest J(c) value reported so far for SmFeAsO1-xFx wires and tapes.

Internal lattice strains under a tensile load for CuNb/Nb3Sn wires with and without prebending treatment were measured directly by neutron diffraction at room temperature. In the axial direction of the wire, the residual lattice strain was changed by 0.29% to the tensile side, while, in the lateral direction, the change was 0.03% to the compressive side due to the prebending treatment. The relationships between the axial and lateral lattice strains under axial stress are almost linear with a slope of about 0.3 for both wires; however, the absolute value of the lateral lattice strain differed at the same axial lattice strain. Tensile load dependences of the axial and lateral lattice strains of Cu and Nb in the wires were also obtained. The prebending treatment was found to affect the elastic-plastic deformation behavior of Cu and to change the internal lattice strains of Nb3Sn. (C) 2012 American Institute of Physics. [doi:10.1063/1.3686692]

High-resolution electron backscatter diffraction (EBSD) technique and DC magnetization were used to characterize a Cu-Nb reinforced bronze route Nb3Sn superconducting multifilamentary wire. The results of DC-magnetization show an extended regime of magnetic reversibility in the operational magnetic field-temperature phase diagram. This observation is discussed in terms of microstructure characteristics of the A15 phase such as grain size, grain boundary misorientation angle distribution, tin gradient across the filaments and residual strain, in connection to the literature. (C) 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors.

The transport properties of a 5-mm-wide practical PLD-GdBa(2)Cu(3)Oy (GdBCO) coated conductor, which has a 100 mu m thickness copper stabilizer, was evaluated. Critical currents were measured up to 600 A in magnetic fields (parallel and perpendicular to the conductor surface) up to 12 T and in a temperature range of 4.2 to 85 K. The temperature dependence of n-value and the power law decay of J(c) in low field were evaluated. I-c anisotropy and pinning properties were also discussed. The results suggest that random pinning dominates the pinning mechanism of the practical GdBCO coated conductor.

The effect of c-axis correlated pinning on the critical current density at low temperatures under high magnetic fields was investigated on the basis of the transport critical current density and irreversibility field properties of an ErBa2Cu3Oy film with inclined columnar defects. We found that the contribution of correlated pinning on J(c) decreased drastically with decreasing temperature under high magnetic fields above the matching field, but still retained more than a 60% enhancement at 13K. These effects of correlated pinning can be described using a cooperative model based on a combination of correlated and random pinning. At low temperatures, the presence of a matching field limited the flux pinning force for correlated pinning in comparison with that of random pinning, while the ratio of the maximum pinning force of the correlated pinning to the random one was proportional to the inverse of the irreversibility field. This suggests that the low efficiency of correlated pinning at low temperatures under a strong magnetic field is a result of a high irreversibility field and a low matching field. In order to improve the critical current properties at low temperatures under high magnetic fields, an increase in the matching field and/or the introduction of strong random pinning are effective. (C) 2012 American Institute of Physics. [doi:10.1063/1.3675181]

The authors have explored mechanical and transport characteristics of coated conductors by hoop stress tests at 4.2 K, 11 T. Two monolayer coils, which were YBa(2)Cu(3)O(7) (YBCO) and GdBa(2)Cu(3)O(7) (GdBCO) coils, and a double-stacked single-pancake coil were tested. The mechanical and electrical behavior of monolayer coils were simple, thus they were well explained by simple forms. The stress and strain were easily evaluated by using BJR relation and strain gauge measurements. On the other hand, the behavior of the pancake coil was not simple. The analytically evaluated azimuthal strain was qualitatively consistent with measured strain. However, the measured radial strain was not explained by analytically because of non-monolithic deformation of the coil. The transport characteristic was influenced by the non-monolithic deformation. (C) 2011 Elsevier B.V. All rights reserved.

To investigate the influence of the crossing angles of columnar defects (CDs) on the in-field properties of the critical current density J(c) and the scaling parameter m derived from the isothermal current-voltage characteristics near the glass-liquid transition at B parallel to c-axis, YBCO thin films were irradiated using the 200 MeV Xe ions at two angles +/-theta(i) relative to the c-axis. For the thin films, the obvious effect of the crossing angle of CDs occurred on the vortex glass-liquid transition more than the J(c) properties. On the glass-liquid transition line, two inflection points induced by the c-axis correlated pinning were confirmed even for the samples of theta(i) = +/- 45 degrees. In the magnetic field dependence of m, the peak or kink appeared near B/B(phi) = 1/3 for the smaller crossing angles, whereas that was slightly visible for the samples of theta(i) = +/- 45 degrees. In addition, the values of m for the small crossing angle were larger than those for the parallel CD configuration, while those for the larger crossing angle became smaller. These results suggested that the morphologies of correlated pinning centers strongly affect the dynamics of flux lines even in the disordered system such as thin films. (C) 2011 Elsevier B.V. All rights reserved.

Carbon/silica composite films were prepared from colloidal silica aqueous solution in which vapour grown carbon nano-fibers were dispersed. The orientation of nano-fibers was attempted by dip-coating substrates under high magnetic field up to 10 T. The fibers started to align along the direction of magnetic field below 1 T. The degree of orientation was saturated at about 6 T. The anisotropic susceptibility, Delta chi = chi(parallel to) - chi(perpendicular to), was estimated to be (3.05 +/- 1.10) x 10(-7) cm(3)/mol from the fitting of numerical simulation. Optical anisotropy, the ratio between transmittances of light polarized perpendicular to and parallel to the direction of fiber orientation, increased immediately in the films prepared under applied fields below 1 T and almost saturated above 1 T. The optical anisotropy was in good agreement with the behaviour of magnetic orientation of fibers. (C)2011 The Ceramic Society of Japan. All rights reserved.

When designing accelerator magnets using high superconductor tapes, their current transport characteristics with anisotropic magnetic field dependence as well as their temperature dependence must be considered. The current transport characteristics of commercially-available GdBa(2)Cu(3)O(7) coated conductor made by IBAD and PLD process were measured in various magnetic fields and temperatures, and they were formulated with the percolation depinning model. Using the formulated current transport characteristics, dipole magnets wound with coated conductors were designed to show the feasibility of accelerator magnets operated at a high temperature, where cryo-coolers can be applied. First, a two-dimensional magnet cross-section was designed, and, then, a three-dimensional magnet design was made, based on the constant perimeter restriction and considering the bending and torsion tolerances of coated conductors. Their enthalpy margins, temperature margins, and current margins were estimated and they were compared with that of a NbTi magnet.

The flux pinning behaviors for the heavy ion irradiated Er123 films are investigated based on the detailed J(c)(B,T,theta) properties. The double peaks of F-p curves for B/ /c appear in case of the matching field of 0.3 T along c-axis and 1.7 T tilted from c-axis. In addition, the peaks on J(c)(theta) at theta = 0 degrees (B/ /c), which originate from the columnar defects, decrease with increasing magnetic field but increase again near the irreversibility field. We found that the cooperation model of the c-axis correlated and random pinning centers can describe the observed double peak behavior of F-p(B) and the angular dependence of J(c) related to the correlated pinning.

We have investigated critical current I-c properties of 600 A/cm-class GdBa2Cu3O7-delta (GdBCO) coated conductor (CC) deposited on IBAD template. 2.5 mu m thick GdBCO layer has been obtained by pulse laser deposition without degradation of critical current density by an improved heating condition in reel-to-reel deposition process. This enables stable production of long length CC with I-c (77 K, self-field) of 600 A/cm-w. In-filed I-c has also shown high value e.g., I-c@77K, 3 T = 33 A/cm-w, I-c@20 K, 27 T = 107 A/cm-w. These experimental results obtained in a wide range of bias conditions of temperature T, magnetic field beta and field angle theta are useful as a fundamental data for design of superconducting devices such as coil winding and magnet. Furthermore, we have also shown that our proposed analytical expression of electric field vs. current density (E-J) characteristics agrees well with the experimental results. This analytical expression allows us to predict I-c-B-Tproperty as well as E-J characteristics at arbitrary conditions of and

Mechanical property of GdBa2Cu3Oy (GdBCO) coated conductor was investigated at 4.2 K in a magnetic field by two ways, which are the tensile test and the hoop stress test. The tensile stress/strain dependence of critical current (I-c) of a 2 mm width conductor was explored at 4.2 K, 18 T. The result provided that I-c-reversible strain limit existed in between 0.43% and 0.46%, corresponding to 907 MPa and 960 MPa in stress, and the elastic constant was 203 GPa. The hoop stress test has been performed at 4.2 K, 11 T. A test coil was fabricated by winding a 5 mm width conductor on a 270 mm diameter GFRP bobbin by 1.5 turns. The maximum value of applied hoop stress, which was deduced from a product of magnetic field, current density and coil radius, was 1322 MPa. Five strain gauges glued on the conductor surface showed almost the same values, which were in a range of 0.64% to 0.67%, indicating the uniform longitudinal deformation. Furthermore, the hoop stress-strain characteristics were linear, suggesting an elastic deformation. The deduced elastic constants were in a range of 196-204 GPa. It was confirmed that the GdBCO coated conductor performance was deteriorated irreversibly by 1322 MPa hoop stress, whereas not by 1302 MPa hoop stress.

The three-dimensional strain dependence of the superconducting property for an internal-tin Nb3Sn wire was investigated, because three-dimensional strains are necessary to understand the strain effect of superconducting properties for Nb3Sn wires. The three-dimensional strain dependence of B-c2 and residual strains were measured by the strain gauge method and neutron diffraction, experimentally. As a result, the three-dimensional strain model based on the experimental results for bronze route Nb3Sn wires explained the strain dependence of superconducting properties for an internal-tin Nb3Sn wire. We found that the strain effect of B-c2 for bronze route and internal-tin Nb3Sn wires are expressed by three-dimensional strain model.

A superconducting magnetic energy storage system (SMES) for electric power system control has been developed using Yttrium (Y)-based coated conductor of high performance in I(c) and mechanical properties, in order to fulfill the requirements for large capacity and cost reduction of the SMES. The target stored energy of the coil required for the SMES system of 100 MVA output power is 2 GJ class. The conceptual designed coil of toroid type consists of one hundred eighty unit coils of 2.8 m outer diameter and each unit coil is connected to each converter of a multi-cell type. Due to this design concept, the main unit coil specifications of 2 kA current, 2 kV voltage, 600 MPa hoop stress tolerance and 3 W/m(2) heat flux around 20 K can realize the SMES system. Coiling technologies have been developing using Y-based wire for SMES. A small multi-layer coil was manufactured and its electromagnetic force characteristics were verified by 600 MPa class hoop stress tests. A bundled-conductor coil of 650 mm class diameter was also manufactured and its current characteristics were verified by 2 kA class large current tests.

From the viewpoint of a cryogen-free high-field superconducting magnet, the thermal stability properties of YBa2Cu3O7 (Y123) coated conductor tapes before thermal runaway are examined under the conduction-cooling condition by a GM-cryocooler. Under such cryocooling condition, Ag-sheathed Bi2Sr2CaCu2O8 (Bi2212) wires with J(c) similar to 10(4) A/cm(2) reveal stable behaviors of 3 times larger current properties before thermal runaway than critical currents, although conventional Nb3Sn wires with J(c) similar to 10(4) A/cm(2) quench almost at their critical currents under the cryocooling condition. In order to determine the maximum current-carrying capacity of the Y123 tape in high fields, we use a zero-dimensional heat balance model. On the basis of the measured basic properties for the Y123 tape, the calculations were done at B = 20 T, bath temperature T-0 = 4.2 K, J(c) similar to 10(7) A/cm(2), and cryocooling heat transfer coefficient h = 10(-3) W/cm(2)K. We found that the Y123 tape with J(c) similar to 10(7) A/cm(2) does not have the over-critical current but rather the sub-critical current before thermal runaway.

A new method for measuring the temperature dependence of the diamagnetic susceptibility is described. It is based on the Faraday method and employs a magnetic levitation technique. The susceptibility of a magnetically levitating diamagnetic sample is determined from the product of the magnetic flux density and the field gradient at the levitating position observed using a micro CCD camera. The susceptibility of a sample during containerless melting and solidification can be measured to a precision of better than +/-0.05%. The temperature dependence of the susceptibility of paraffin wax was measured by the magnetic levitation technique with an accuracy of +/-0.25%. This method enables sensitive and contactless measurements of the diamagnetic susceptibility across the melting point with in situ observations.

Carbon/silica composite films were prepared from colloidal silica aqueous solution in which vapour grown carbon nano-fibers dispersed. The orientation of nano-fibers was attempted by dip-coating substrates under high magnetic field up to 10 T. The fibers started to align along the direction of magnetic field below 1 T. The degree of orientation was saturated at about 6 T. The anisotropic susceptibility, Δχχ//-χ⊥, was estimated to be (3.051.10)×10-7 cm3/mol from the fitting of numerical simulation. It was smaller than that reported in previous studies. It was deduced that perturbation of fiber orientation in the sol during drawing and drying, and the existence of coupled fibers in the starting materials suppressed the orientation of fibers. © Published under licence by IOP Publishing Ltd.

The intrinsic pinning mechanism of YBa2Cu3Oy coated conductors was investigated in terms of the anisotropy of J(c). The dissipation in the intrinsic pinning is mainly due to the double-kink excitation of the pinned vortices in the intermediate temperature region between 70 and 40 K. In this temperature region, the n-value is as small as about 8-10. At low temperature below 20 K, however, n-values become large and J(c) is independent of the magnetic field because of the reduction of the double-kink excitation of the vortices. (C) 2011 The Japan Society of Applied Physics

We have successfully constructed an 18.1 T superconducting magnet conductively cooled by a GM/JT cryocooler. The double-pancake insert using Ag-sheathed Bi2Sr2Ca2Cu3O10 (Bi2223) tape with stainless steel reinforcement tape generated 2.5 T in a 15.6 T background magnet. In order to develop a cryogen-free high-field superconducting magnet producing over 20 T, an YBa2Cu3O7 (Y123) coated conductor insert is intended for upgrading the cryogen-free 18 T superconducting magnet. The Bi2223 insert, whose size is 176 mm outer diameter, 90 mm inner diameter, and 252 mm coil height, is now excited at 162 A operation current, and will be replaced by a new Y123 insert. We have already confirmed excellent mechanical properties of 1000 MPa hoop stress tolerance for Y123 coated conductor tape with Hastelloy substrate. This means that we no longer need stainless steel reinforcement for the insert. As a result, an Y123 insert with almost the same size as the Bi2223 insert is designed to generate 7.5 T at 187 A, because the number of turns can be improved extremely. A cryogen-free 23 T superconducting magnet can sufficiently be developed for a long-term experiment at a constant high magnetic field.

This study investigates superconducting and mechanical properties of monolayer GdBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>y</SUB>-coated conductor coils under large electromagnetic stress. We found that reversible superconducting properties can be achieved in the large hoop stress/strain states up to 1288 MPa/0.63% for the Hastelloy-outside-winding coil (coil A). This limit is larger than the results for the tensile test of the short sample. However, damage occurred near the current terminal at 640 MPa for the Hastelloy-inside-winding coil (coil B). It is considered that the degradation was due to the stress concentration at the edge of the current terminal. In the case of the Hasetelloy-inside-winding coil with a lap joint (coil C), the coil performance is limited by the shear stress of the lap joint.

The liquid helium-free hybrid magnet, which consists of an outer large bore cryocooled superconducting magnet and an inner water-cooled resistive magnet, was developed for magneto-science in high fields. The characteristic features of the cryogen-free outsert superconducting magnet are described in detail in this paper. The superconducting magnet cooled by Gifford-McMahon cryocoolers, which has a 360 mm room temperature bore in diameter, was designed to generate high magnetic fields up to 10 T. The hybrid magnet has generated the magnetic field of 27.5 T by combining 8.5 T generation of the cryogen-free superconducting magnet with 19 T generation of the water-cooled resistive magnet. The superconducting magnet was composed of inner Nb(3)Sn coils and outer NbTi coils. In particular, inner Nb(3)Sn coils were wound using high-strength CuNi-NbTi/Nb(3)Sn wires in consideration of large hoop stress. Although the cryocooled outsert superconducting magnet achieved 9.5 T. we found that the outsert magnet has a thermal problem to generate the designed maximum field of 10 Tin the hybrid magnet operation. This problem is associated with unexpected AC losses in Nb(3)Sn wires. (C) 2009 Elsevier B.V. All rights reserved.

A cryocooled 101 superconducting magnet with a 360 mm room temperature bore has been developed for a hybrid magnet. The superconducting magnet cooled by four Gifford-McMahon cryocoolers has been designed to generate a magnetic field of 10 T. Since superconducting wires composed of coils were subjected to large hoop stress over 150 MPa and Nb(3)Sn superconducting wires particularly showed a low mechanical strength due to those brittle property, Nb(3)Sn wires strengthened by NbTi-filaments were developed for the cryocooled superconducting magnet. We have already reported that the hybrid magnet could generate the resultant magnetic field of 27.5 T by adding 8.5 T from the superconducting magnet and 19 T from a water-cooled Bitter resistive magnet, after the water-cooled resistive magnet was inserted into the 360 mm room temperature bore of the cryocooled superconducting magnet. When the hybrid magnet generated the field of 27.5 T, it achieved the high magnetic-force field (B x partial derivative Bz/partial derivative z) of 4500 T(2)/m, which was useful for magneto-science in high fields such as materials levitation research. In this paper, we particularly focus on the cause that the cryocooled superconducting magnet was limited to generate the designed magnetic field of 101 in the hybrid magnet operation. As a result, it was found that there existed mainly two causes as the limitation of the magnetic field generation. One was a decrease of thermal conductive passes due to exfoliation from the coil bobbin of the cooling flange. The other was large AC loss due to both a thick Nb(3)Sn layer and its large diameter formed on Nb-barrier component in Nb(3)Sn wires. (c) 2010 Elsevier B.V. All rights reserved.

MgB2/Fe tapes were prepared by in situ powder-in-tube process with acetone doping. The magnetic field was applied parallel and perpendicular to the tape surface and the effects of acetone doping on the J(c) anisotropy were studied. It is found that the acetone doping not only improved Jc. in high fields, but also decreased the anisotropy ratios Gamma = J(c)(parallel to)/J(c)(perpendicular to). In comparison to pure samples, the anisotropy ratios Gamma for acetone-doped samples was decreased to 1.5 from 3.9 at 4.2 K and 10 T. The decreases of anisotropy ratios Gamma for acetone-doped tapes may be attributed to the small MgB2 grain sizes and the disorder induced by C substitution of B site. (c) 2010 Elsevier B.V. All rights reserved.

We found that YBCO films deposited by the controlled process showed improved superconducting properties in magnetic fields, even without any artificial pinning centers such as BaZrO3 and BaSnO3. The YBCO films with a thickness of 450 nm were deposited on hastelloy tapes with a textured MgO buffer deposited by IBAD (Ion Beam Assist Deposition) method (IBAD-MgO) by PLD method. In the YBCO films on the IBAD-MgO (IBAD-YBCO), the critical current density of 2.9 MA/cm(2) at 77 K in self-field and the ratio of J(c)(B//c)/J(c)(B//a, b) at 1 T was almost unity. Furthermore, we found that the J(c) anisotropy changed with surface morphology on underlying buffer layer and YBCO thickness. This means that some c-axis correlated pinning centers are included in the films. From the cross-sectional TEM images of the IBAD-YBCO, there were not so many defects such as dislocations, Y2O3 inclusions and so on. Thus it is considered that some defects such as oxygen heterogeneity, displacement of a local composition and so on are pinning centers, although such defects can not be detected by a present analytical technique. (c) 2010 Published by Elsevier B.V.

Tin-compounds were doped into YBa2Cu3O7-delta (YBCO) films as pinning centers to enhance J(c) in magnetic fields. YBCO films were grown by a metal organic deposition process using tri-fluoroacetates starting solutions. Tin-acetylacetonate salts were then dissolved into the starting solution as pinning centers. J(c) of the YBCO films with tin-acetylacetonates were enhanced in all magnetic field angles. Transmission electron microscopy (TEM) and TEM-EDS (Energy Dispersive X-ray Spectroscopy) observations revealed the existence of tin-compound particles with the size of about 30 nm in the YBCO film. These nano particles were distributed randomly in the film and were considered to act as 3-dimentional pinning centers. (C) 2010 Elsevier B.V. All rights reserved.

We investigated the dependences of the critical current density J(c) on the magnetic field angle 0 in YBa(2)Cu(3)O(7-delta) thin films with the crossed configurations of the columnar defects (CDs). To install the crossed CDs, the films were irradiated using the high energetic Xe ions at two angles relative to the c-axis. The additional peak around the c-axis appears in the J(c)(0) for all irradiated films. In lower magnetic fields, the height of the J(c)(0) peak caused by the crossed CDs with the crossing angles 0(i) = +/- 10 degrees was higher than that for the parallel CDs. It is considered that the correlation of the flux pinning by the crossed CDs along the c-axis occurs even in the case of 0(i) = +/- 25 degrees, which was also suggested by the kink behaviors of the scaling parameters of the current-voltage characteristics near 1/3 of the matching field. In higher magnetic fields, on the other hand, the height and width of the J(c)(0) peak for the crossed CD configurations rapidly reduce with increasing the magnetic field compared to the parallel ones. In the crossed CD configurations, the dispersion in the direction of CDs would prevent the correlation of flux pinning along the c-axis in high magnetic fields, which occurs in the parallel CD configurations due to the collective pinning of flux lines including the interstitial flux lines between the directly pinned flux lines by CDs. (C) 2010 Elsevier B.V. All rights reserved.

We have investigated current transport property in Gd(1)Ba(2)Cu(3)O(7-delta) coated conductor with artificial pinning centers in a wide range of temperature, magnetic field, B up to 27 T, and field angle. Due to the additional c-axis correlated pins, critical current density, J(c) in B//c was enhanced and the improvement was observed in wide range of B. On the other hand, around B perpendicular to c below 65 K. the angular dependence of n-value showed a valley-like behavior, although the J(c) was increasing. In addition, the temperature dependence of the pinning force density defined as J(c) x B was not scaled on an expected master curve. These results indicate the pinning in B perpendicular to c is governed by different mechanism below 65 K and high magnetic field. (C) 2010 Elsevier B.V. All rights reserved.

Transport properties of the MgB2 thin films on Si, MgO and ZnO substrates with Ti buffer layer prepared by molecular beam epitaxy were investigated to clarify effects of the substrates and the Ti buffer layer on flux pinning. The critical current density J(c) of each sample shows different dependence on magnetic fields parallel to c-axis. However, the scaling analysis of the macroscopic pinning force for all the measured samples implies that the grain boundaries work as the dominant pinning centers for B//c. The pinning parameter for MgB2/Ti/Si estimated from the electric field E vs. the current density./ characteristics shows the highest value among all the measured samples. This result is attributed to the high density of grain boundaries caused by the effect of both the Ti buffer and Si substrate in the growth process. Therefore, the selection of substrates and buffer layer strongly affects the flux pining properties of MgB2 thin films and plays an important role in the determination of performance for superconducting devices and wires. (C) 2010 Elsevier B.V. All rights reserved.

We evaluated three-dimensional residual strains for practical Nb3Sn wires with different architectures and residual strains at low temperature by neutron diffraction, and the upper critical fields under the tensile strains were discussed on the basis of the three-dimensional strain model. We found that the angular dependence of the residual strain can be described well by the two representative strains, i.e. the axial and lateral strains. The differences in the three-dimensional residual strains, which depend on the wire architecture and mechanical treatment history, play an important role in the superconducting properties of composite Nb3Sn wires under axial stress/strain.

In order to investigate quantitatively the effect of melt convection in an electromagnetically levitated molten droplet on the thermal conductivity of liquid silicon measured by the electromagnetic levitation (EML) technique superimposed with a static magnetic field, the numerical simulations for melt convection in the droplet and additionally, for the measurement of thermal conductivity were carried out. In addition, the thermal conductivity of molten silicon was measured by the EML technique, and then compared with those obtained numerically. In the numerical simulations of melt convection, the buoyancy force, thermocapillary force due to the temperature dependence of the surface tension on the melt surface, and electromagnetic force in the droplet were considered as the driving forces of convection. As a result, the numerical simulations could sufficiently explain the measurement of thermal conductivity by the EML technique under a static magnetic field. Also, it was suggested that a magnetic field of more than 4. T should be applied to measure the real thermal conductivity of molten silicon by the EML technique. © 2010.

An optimum design of a water-cooled Bitter magnet was investigated for containerless melting experiments under a magnetic levitation condition. In order to minimize the vertical movement of a levitating sample due to the magnetic susceptibility change during a heating process, the gradient of B(partial derivative B/partial derivative z) along the center axis of a magnet must be large around the levitating position. Concerning the modification of an existing double Bitter magnet with 52 mm warm bore at the HFLSM, lager gradient of B(partial derivative B/partial derivative z) is obtained by shortening the inner coil and making the outer coil longer.

We evaluated critical current density and mechanical properties of the Y123 coated conductor (CC) tapes on buffered Hastelloy substrates prepared by the chemical vapor deposition method in high magnetic fields and low temperatures. The J(c) values of the tape are about 1.1 MA/cm(2) at 77.3 K, 0 T and 1.8 MA/cm(2) at 4.2 K and 17 T for B//c. In addition, the hoop stress test of the single layer coil shows that the stress limit of the CVD-Y123 CC tapes on Hastelloy is over 1 GPa. On the basis of those experimental data, we designed the innermost high temperature superconducting insert coil for upgrading of the 18 T cryogen-free superconducting magnet (18 T-CSM) under the condition of the stress limit of 600 MPa. The coil, which consists of the 22 double pancake coils can generate 9.4 T with the operation current of 295 A in the backup field of 15.6 T. In this case, the central field of the 18 T-CSM can be improved up to 25 T from 18.1 T.

It is very important for the application of high field magnets to understand the strain effect of the superconducting properties for Nb3Sn wires. We found that the axial strain dependence of the superconducting properties for various kinds of the bronze route Nb3Sn wires can be described clearly, in the case that the deviatoric and hydrostatic strains are determined experimentally as the three-dimensional strain. In addition, we constructed a modified three-dimensional strain model of B-c2 for bronze route Nb3Sn wires based on the deviatoric and invariant strain models. This modified model can fit the axial strain dependence of B-c2 for various kinds of bronze route Nb3Sn wires by only a set of the parameters. It is suggested that three parameters such as axial residual strain and lateral one, and the ratio of the axial and the lateral strain determine various superconducting properties for Nb3Sn wires under the axial tensile strain.

Concerning the strain effect of superconducting properties for Nb3Sn wires, it is necessary to investigate a three-dimensional strain state that Nb3Sn superconductors truly experience in the composite wire. However, it is very difficult for Nb3Sn wires to obtain the three-dimensional residual strain components experimentally. We adopted the strain gauge that is directly glued onto the 1 mm outer diameter Nb3Sn wire, in order to quantitatively measure the three-dimensional distortions. To evaluate axial and lateral distortions of the wire, strain gauges were set in both axial and lateral directions. This measurement system can obtain the distortion detail in fields up to 27 T at temperatures ranging from 4.2 to 20 K. We measured the upper critical field B-c2 in a three-dimensional strain state for Nb3Sn wires. It was found that the wire architecture changes each residual strain in axial and lateral directions of the wire. Moreover, the B-c2 strain sensitivity that is related to the B-c2 variation is also affected by its architecture. We found that the axial tensile strain variation 0.3% roughly corresponded to the lateral compressive strain variation 0.1% for Nb3Sn wires. This means that the ratio of the lateral strain and the axial one for Nb3Sn wires is 0.3. The ratio of both residual strains in the axial and lateral directions is very important to examine the strain effect of Nb3Sn wires in detail.

In order to realize a hybrid magnet with no need of liquid helium for operation, we have designed a 28 T cryogen-free hybrid magnet with a 32 mm room temperature experimental bore, and actually tested to generate the peak field of 27.5 T. An easy-to-operate cryogen-free hybrid magnet is now operating for basic research in high fields up to 27 T at low temperatures down to 40 mK, using a dilution refrigerator. In addition, we are intending to develop a 100 mm wide bore water-cooled resistive insert magnet combined with a cryogen-free superconducting outsert magnet for X-ray diffraction measurements in steady magnetic fields up to 20 T. It found that YBa(2)Cu(3)O(7) (Y123) coated conductor tape with Hastelloy substrate has the excellent mechanical property of 1000 MPa hoop stress tolerance. We are carrying out the research and development of a 30 T all superconducting magnet immersed in liquid helium and a 23 T cryogen-free superconducting magnet, employing Y123 tape.

A relationship between microstructures of YBa2Cu3Oy (Y123) films with two-dimensional artificial pinning centers (2D APCs) and their critical current density (J(c)), especially under matching fields (B-phi), is discussed. Using a pulsed laser deposition (PLD) method, c-axis in-plane aligned a-axis-oriented Y123/Pr123 multilayer films were grown on substrates. The Pr123 interlayer distance was changed by controlling a target shape and its rotation speed in the PLD process. The multilayer structure was observed in cross-sectional transmission electron microscopy images. Critical temperatures of the films decrease as the Y123 layer becomes thinner. The magnetic field (B) dependence of J(c) showed that the interlayers act as 2D APCs in multilayer films. The matching effects were observed in two ways: J(c)(B parallel to c)/J(c)(B parallel to a)-B curves and irreversibility lines.

MgB2 tapes with organic rare-earth salt additions were prepared by the in situ powder-in-tube processing method. Compared to the undoped tapes, the transport J(c) are greatly improved by organic rare-earth salt doping. In addition, magnetization measurements showed an enhancement of flux pinning. Organic rare-earth salt doping not only enhances the upper critical field by increasing electron scattering, but also introduces precipitates and structural defects to improve the flux pinning. The results suggest that organic rare-earth salt has to be considered as a promising dopant for enhancing the current capacity of MgB2 conductors.

Residual strains for practical Nb3Sn superconducting wires were measured directly using neutron diffraction. Seven wires, stacked with epoxy resin, were measured under a tensile load at room temperature. As a result, the three-dimensional strain was obtained up to 0.5% axial tensile strain. We found that the neutron diffraction result is consistent with the strain measured using strain gauges and extensometers at the same time, although the strains increased due to the long measurement time. The ratio between the axial and lateral strains is about 0.33, although the Cu-Sn, Cu and CuNb were plastic in this region. The ratio is an important value for evaluating three-dimensional strain effects on the superconducting properties of Nb3Sn wires.

Thermal conductivity of liquid silicon is necessary for numerical process modeling. It is also of scientific interest. However, measuring thermal conductivity is a difficult task because of convections in the liquid and contamination from contact materials. To overcome these experimental difficulties, we have developed noncontact modulated laser calorimetry in a dc magnetic field to measure heat capacity, thermal conductivity and emissivity of high-temperature liquid metals. In this study, through improvement in temperature measurements, we considerably reduced the experimental uncertainty of measurements. Furthermore, the thermal conductivity and heat capacity of supercooled liquid silicon were measured. Thermal conductivity of liquid silicon agrees with the values calculated assuming the Wiedemann-Franz law near the melting point. This result suggests that free electrons play a dominant role in the thermal transport process in liquid silicon.

A significant improvement in J(c) at both 4.2 and 20 K has been achieved for MgB2 tapes using C9H11NO as a dopant. It is found that C9H11NO doping will cause a small depression in T-c while the J(c) and H-c2 values are strongly enhanced. The highest J(c) was obtained for a C9H11NO doped sample sintered at 800 degrees C, namely 3.7 x 10(4) A cm(-2), at 4.2 K, 10 T. This is the highest J(c) value obtained for a MgB2 tape using a carbohydrate as a doping material. Moreover, at 20 K, 5 T, a transport J(c) value as high as 1.9 x 10(4) A cm(-2) is observed, suggesting that MgB2 holds promise for practical applications. Besides the improvement of grain linkage, the clearly enhanced H-c2 is thought to be a major reason for the prominent J(c) performance, in a field, for C9H11NO doped MgB2 samples.

A high critical current density J(c) with a nearly isotropic angular dependence at 77.3 K was recently obtained by the introduction of nanoparticles to (Y, RE) Ba2Cu3Oy (RE123, RE = Sm, Gd)-coated conductors prepared by trifluoroacetate metal-organic deposition (TFA-MOD). The flux pinning properties were evaluated on the basis of detailed measurements of the upper critical field, the irreversibility field and the critical current density of the TFA-MOD RE123-coated conductor. We found that strong random pinning by the BaZrO3 nanoparticles and weak c-axis correlated pinning by twin boundaries functioned cooperatively in nanoparticles introduced to (Y, Gd)123-coated conductors prepared by TFA-MOD. The coexistence of two different pinning centers played an important role in the nearly isotropic angular dependence of Jc.

High-resolution electron backscatter diffraction was used to characterize a Nb3Sn-based superconducting multifilamentary wire produced by the bronze method. The niobium core has a clear &lt;1 1 0&gt; fiber texture in the wire axis direction. The Nb3Sn crystals also show a slight preference for the &lt;1 1 0&gt; direction parallel to the wire axis. The misorientation angle distribution of the Nb3Sn crystals are similar to the Mackenzie distribution for a randomly oriented set of crystals with cubic crystal symmetry. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A high critical current density J(c) with a nearly isotropic angular dependence at 77.3 K was recently obtained by the introduction of nanoparticles to (Y, RE) Ba2Cu3Oy (RE123, RE = Sm, Gd)-coated conductors prepared by trifluoroacetate metal-organic deposition (TFA-MOD). The flux pinning properties were evaluated on the basis of detailed measurements of the upper critical field, the irreversibility field and the critical current density of the TFA-MOD RE123-coated conductor. We found that strong random pinning by the BaZrO3 nanoparticles and weak c-axis correlated pinning by twin boundaries functioned cooperatively in nanoparticles introduced to (Y, Gd)123-coated conductors prepared by TFA-MOD. The coexistence of two different pinning centers played an important role in the nearly isotropic angular dependence of Jc.

We report the achievement of transport critical currents in Sr0.6K0.4Fe2As2 wires and tapes with a T-c = 34 K. The wires and tapes were fabricated through an in situ powder-in-tube process. Silver was used as a chemical addition as well as a sheath material. All the wire and tape samples have shown the ability to transport superconducting current. Critical current density J(c) was enhanced upon silver addition, and at 4.2 K, a largest J(c) of similar to 1200 A/cm(2) (I-c = 9 A) was achieved for 20% silver added tapes, which is the highest in iron-based wires and tapes so far. The J(c) is almost field independent between I T and 10 T, exhibiting a strong vortex pinning. Such a high transport critical current density is attributed to the weak reaction between the silver sheath and the superconducting core, as well as an improved connectivity between grains. We also identify a weak-link behavior from the apparent drop of J(c) at low fields and a hysteretic phenomenon. Finally, we found that compared to Fe, Ta and Nb tubes, Ag was the best sheath material for the fabrication of high-performance 122 type pnictide wires and tapes. (C) 2009 Elsevier B.V. All rights reserved.

The effects of columnar defects on the critical current density (<i>J</i>_c) and electric <i>(E)</i> <i>vs</i>. current density <i>(J)</i> characteristics were investigated for a YBa₂Cu₃O_<i>y</i> thin film prepared using a pulsed-laser deposition method. The YBa₂Cu₃O_<i>y</i> thin film was irradiated with 200 MeV Au ions from a direction &theta;=8.8&deg;off the <i>c</i>-axis. Two peaks in the angular dependence of <i>J</i>_c were observed at the angles of &theta;=8.8&deg;and &theta;=90&deg;. These angles correspond to the directions of the columnar defect and intrinsic pinning, respectively. The <i>E-J</i> characteristics at various magnetic field angles can be described by the percolation transition model. The value of pinning parameter m increases at the angles of &theta;=8.8&deg;and &theta;=90&deg;. This result indicates that the efficiency of flux pinning is enhanced because fluxoids are effectively pinned by the columnar defects and intrinsic pinning or stacking faults in these directions.

In order to develop a cryogen-free high-field superconducting magnet generating over 20 T, a YBa2Cu3O7 (Y123)-coated conductor insert coil is used to enhance a cryogen-free 18 T superconducting magnet. The Y123 tape does not require stainless steel reinforcement. The Y123 insert coil, which is almost the same size as the current Bi2Sr2Ca2Cu3O10 (Bi2223) insert coil, is designed to generate 7.5 T at 187 A. It is able to achieve this since without stainless steel reinforcement the number of turns can be greatly increased. A cryogen-free superconducting magnet can achieve a static magnetic field of 23 T, which is higher than that generated by practical superconducting magnets using liquid helium. (C) 2009 The Japan Society of Applied Physics

In this study, c-axis-correlated pinning centers, which are known to increase the critical current density (J(c)) for B parallel to c-axis, were produced by heavy-ion-irradiation in REBa2Cu3Oy (RE: rare earth element, RE123) coated conductors, and the subsequent changes in J(c) were measured. A peak in the angular dependence of J(c) induced by the defects was observed for applied magnetic fields of up to 13 T for Er123 films with matching fields of B-phi = 0.1 and 1 T. We found that the columnar defects led to improvements in J(c) even under applied magnetic fields more than 130 times higher than B-phi. (C) 2009 The Japan Society of Applied Physics

Flux pinning properties on the ErBa2Ca3O7 films with the nanoscaled and columnar-shaped BaMO3 (BMO, M=Zr,Sn) precipitates, i.e., nanorods, were investigated systematically based on the microstructure, critical current density J(c) and irreversibility field B-i properties in high magnetic fields. We found that the in-field J(c) and B-i increase monotonically with increasing the nanorod density until 3.5 wt % BZO and 6 wt % BSO additions, respectively, although the critical temperature T-c is reduced. According to the flux pinning analysis assuming of the linear summation, the BSO doped samples have the higher performance of the flux pinning than the BZO doped samples quantitatively even after the reduction in the T-c and size of the nanorods. It is considered that those differences originate from the effective pinning length of the nanorods and/or the condensation energy due to the difference of the carrier density. (C) 2009 American Institute of Physics. [doi:10.1063/1.3257260]

We report the effect of acetone addition on in situ powder-in-tube processed MgB2/Fe tapes by using acetone as liquid additive. The amount of acetone was varied from 0 to 15 wt.%. We found that a significant J (c)-B enhancement was easily achieved in high field by acetone doping. At 4.2 K, the transport J (c) for the best acetone added tapes (5 wt.%) reached up to 2.4x10(4) A/cm(2) at 10 T. With increasing acetone concentration, the a-axis lattice parameters, B (c2) and B (irr), were not much changed; however, the insulating MgO impurities and pores gradually increased, thereby resulting in the obvious decrease of J (c) values for 10 and 15 wt.% acetone-added samples, especially within low field region.

In order to study the degradation of critical temperature T-c for a coated conductor using nanorods, the relationship between Tc and nanorod shape was investigated by transmission electron microscopy for ErBa2Cu3Oy (Er123) films with BaZrO3 (BZO) and BaSnO3 (BSO) nanorods. The dependence of Tc on the BZO or BSO content was different, however, the nanorods interface density dependence of Tc was the same for both. Thus, the interface density of the nanorods controls Tc of the nanorod-added films. The different degradation in Tc between the BZO and BSO nanorod-added films originated in differences in the nanorod diameter and density. (C) 2009 The Japan Society of Applied Physics

We have been developing high-strength Nb(3)Sn strand cables to construct a high-field superconducting outsert for a 45 T hybrid magnet with a 25 T water-cooled resistive magnet. Evidence for cold work strengthening of repeated bending treatment (prebending effect) on Nb(3)Sn strands internally reinforced with CuNb stabilizer, which exhibits significant enhancement of the critical current density, has been found in the superconducting magnet fabrication process using a react-and-wind method. The strand cables were designed by controlling the constituent number of CuNb/Nb(3)Sn strands with the prebending effect and stainless steel strands, which are expected to have a stress limit 580 MPa at 0.4% strain. In order to design a compact superconducting outsert, high-strength strand cables are adopted in a magnetic field region below 14 T to maintain relatively large engineering current densities (J(e)). In a higher-field region above 14 T, YBa(2)Cu(3)O(7) (Y123) coated conductors are employed for an insert coil. Using combination of Y123, Nb(3)Sn and NbTi superconductors, a 20 T superconducting outsert with a room temperature bore of 400 mm consisting of three layers made of Y123, two layers of CuNb/Nb(3)Sn and two layers of NbTi was designed. The coil parameters are 440 mm inner diameter, 1080 mm outer diameter and 1138 mm coil height. A very compact 20 T superconducting outsert with a stored magnetic energy 72 MJ at an operation current 903 A can be developed for a 45 T hybrid magnet.

A program to develop a 30 T superconducting magnet based on novel concepts is now in progress at the High Field Laboratory for Superconducting Materials (HFLSM)at Tohoku University and the Tsukuba Magnet Laboratory (TML) at the National Institute for Materials Science. A 30 T superconducting magnet comprising a high-temperature superconducting (HTS) insert and a low-temperature superconducting (LTS) outsert was conceptually designed. For the high-field HTS insert, a YBCO coated conductor tape was adopted because of its high critical current density in high fields and its high mechanical strength. A relatively high tolerance limit of hoop stress in the insert coil can be assumed in the coil design according to its mechanical properties. The critical current density of the YBCO tape was analytically predicted as a function of temperature and magnetic field. To withstand a large electromagnetic force, the LTS outsert was composed of CuNb/Nb(3)Sn and NbTi coils. The CuNb/Nb(3)Sn coil was designed using high-strength cable consisting of internally reinforced Nb(3)Sn strands with a CuNb reinforcing stabilizer subjected to repeated bending treatment. The results of this design study show the potential for a compact high-field magnet employing an insert coil formed of YBCO coated conductor.

The irreversibility temperatures were measured for a-axis oriented and c-axis in-plane aligned Y123/Pr123 multilayer films in various magnetic fields. Pr123 layers act as two-dimensional (planer) c-axis correlated artificial pinning centers (APCs) in the multilayer films. In the irreversibility field -temperature (B(irr) - T) curves, the crossover was observed near matching field B(phi) for the multilayer films, when the magnetic fields were applied parallel to the - axis of the film (B//c). It is suggested the vortex state in Y123 films with two-dimensional APCs is Bose glass below the crossover field. The B(irr) - T curves under B//a shifted toward lower temperature and magnetic field regions.

A 7-strand Nb3Sn superconducting cable, which consists of three pre-bent CuNb/Nb3Sn strands, three stainless-steel (SS) strands, and a center SS strand, was fabricated. 2.5 turns of the cable was wound on a 262-mm diameter GFRP bobbin A triplet consisting of three pre-bent CuNb/Nb3Sn strands was also tested for comparison. The reinforcement effect of stainless-steel strands in a cable was investigated to develop a high-strength large-current superconducting cable. The hoop stress test result indicated that the stainless-steel reinforced CuNb/Nb3Sn cable showed better performance than the CuNb/Nb3Sn triplet under the hoop stress, because the stainless-steel strands reduced the hoop stress on Nb3Sn strands. Furthermore, the solder impregnation of the cable, which made the seven strands a monolithic conductor, improved its performance.

In order to understand the vortex pinning properties by the BaZrO3 (BZO) and BaSnO3 (BSO) nano-rods in Er123 films, the microstructure and the angular dependence of J(c) and resistivity were measured in detail for the BZO and the BSO added Er123 films with the similar matching field prepared by pulsed laser deposition. We found that the peak at B//c in the angular dependence of J(c) for the Er123 film with BSO nano-rods was higher in a wide magnetic field region up to the irreversible field than that for the Er123 film with BZO nano-rods, although the minimum values of the angular dependence of J(c) are similar each other. Hence, the BSO nano-rods work more effectively as the c-axis-correlated pinning center than the BZO nano-rods because of the different elementary pinning energy of nano-rods.

MgB2 was thought as a promising superconductor used at temperatures around 20 K for cryogen-free magnet. Nanostructure materials were often selected as MgB2 additives due to their high chemical reactivity. In this paper, the mixture of nano-C and nano-SiC was doped to MgB2 bulks and tapes. The co-doping effect of C and SiC on the phase formation, microstructure, and critical current density of MgB2 bulks and tapes were systematically investigated. The mechanisms for the superconducting properties improvement in co-doped MgB2 superconductor was analyzed based on the characterization and measuring results.

We have characterized nonlinear current transport properties in a coated conductor as a function of temperature, magnetic field vector and mechanical strain, and then have developed a thermally-electromagnetically-structurally coupled analysis code for a high-field magnet coil. The distributions of heat generation and electromagnetic force in the coil are computed by electromagnetic analysis. Then, the temperature distribution and the strain distribution are correspondingly calculated by thermal analysis and by structural analysis. Furthermore, both of them are fed back to the electromagnetic analysis. These analyses are based on finite element method, and are repeated until the convergence. By taking a design example of a 40 T class magnet coil using a GdBCO coated conductor, we have discussed the necessity of the consideration of thermally-structurally influenced transport properties in the coil for the coil design.

In order to study the mechanism of the small n-value for Bi2212, E - J characteristics were measured carefully for various kinds of Bi2212 tapes and round wires in high magnetic fields. Some of wires were heat-treated in a magnetic field of 5 T. We found that the n-values of all the samples increase by the in-field heat-treatment at 5 T. In addition, we analyzed the E - J properties by the combination of the percolation model based on the local J(c) distribution and the two directional J(c) distribution model. The improvement of the n-value by the in-field heat treatment can be understood by the change of the aspect-ratio of the grains. Therefore, the aspect-ratio of the grains plays an important role in the current transport influencing not only J(c) but also n-value.

Phthalocyanine-doped MgB2 tapes were prepared by the in situ powder-in-tube method. The relationships between the critical current properties, crystallinity and microstructure were studied as a function of the phthalocyanine doping level. It is found that both H-irr and H-c2 were improved when MgB2 samples were doped with phthalocyanine, which are mainly attributed to the effective carbon substitution and enhanced flux pinning strength caused by very fine grain sizes. Furthermore, compared to pure samples, the MgO content remained almost unchanged in all doped tapes, which is very beneficial to having better grain connectivity in MgB2. Significantly improved J(c) was obtained in the phthalocyanine-doped MgB2 tapes, especially under high magnetic fields.

A formation mechanism of a silicon hollow sphere via electromagnetic levitation method under high static magnetic field in hydrogen atmosphere is discussed in this paper. Since the convection in the levitated silicon melt which is caused by the electromagnetic field is restrained by a high static magnetic field, it is possible to examine a solidification of silicon from an equilibrium silicon melt with hydrogen except an effect of convection. The solidified silicon sphere has an extrusion which is formed during the solidification due to reduced stress in the melt. When a nucleation of pore occurs before the formation of the extrusion, a silicon hollow sphere is formed. Since the chance to form the nucleation of pore increases with increasing hydrogen concentration in the melt, a silicon hollow sphere is formed when the solidification is performed in higher hydrogen partial pressure. In the present case, a silicon hollow sphere is formed by the solidification in 50% H(2)-50%Ar atmosphere. However, non-porous silicon spheres are formed by the solidification in 25%H(2)-75% At or 100%Ar atmosphere. (C) 2008 Elsevier B.V. All rights reserved.

The effects of static magnetic field and hydrogen gas atmosphere on the solidification of silicon with hydrogen atom were examined using electromagnetic levitation method in hydrogen atmosphere. Since the convection in the levitated silicon melt which is caused by the electromagnetic field is restrained by a high static magnetic field, it is possible to examine a solidification of silicon from an equilibrium silicon melt with hydrogen without an inflluence of convection. The cooling rate of the melt increases with increasing hydrogen partial pressure. Since the undercooling of the melt does not change with the static magnetic field, the grain size on the surface does not change with the static magnetic field. However, the morphology of the grain surface drastically changed from flat and smooth to undulation. © 2009 IOP Publishing Ltd.