A novel dielectric barrier discharge (DBD) plasma-actuator module with an exposed electrode and two covered electrodes was developed to enhance electrohydrodynamic force generation based on the concept that it separates the ionization and acceleration processes. The conventional three-electrode configuration of the DBD plasma actuator suffers from unexpected spark discharge between the exposed electrodes, thereby failing to strengthen the electric field intensity for accelerating charged particles or generating a stable ionic wind. In this study, a third electrode was embedded in the dielectric layer to prevent spark discharge. Furthermore, an alternating current (AC) waveform was employed as the bias voltage, which was applied to the third electrode, instead of the direct current (DC) voltage used in a conventional DBD plasma actuator. Induced flow visualization using particle image velocimetry technique revealed that the DC bias voltage forms a weak ionic wind in the proposed DBD plasma actuator owing to the electric field screening effect, and the ionic wind periodically appears when the polarity of the voltage is reversed by applying an AC-bias voltage. The velocity of the ionic wind increases with increasing frequency and the AC bias voltage amplitude. Also, decreasing the distance between the second and third electrodes results in ionic wind enhancement. The results obtained in this study provide insights into the drastic improvement in the performance of DBD plasma actuators with the enhancement of the electric field intensity for charged particle acceleration.

Abstract An improved model was proposed to reduce a computational cost for subcritical millimeter-wave discharge. The proposed model was able to reproduce the plasma-front propagation via radiation transport as similar to the conventional model, and the plasma-front propagation speed was in agreement with the previous simulation. An electron transport effect by neutral fluid advection, which has been introduced into the conventional model, does not affect the propagation speed. By using the presented model, a computational time was reduced by 35%, which was suitable for a multi-dimensional simulation in the future.

A double layer (DL) is created in a plasma when the plasma is perturbed in the presence of a temperature anisotropy. We derive a new simple theory for the existence of an unstable, non-oscillatory electrostatic DL-like structure in the presence of a magnetic field gradient in a collisionless plasma with a temperature anisotropy in the direction perpendicular to the magnetic field. The DL is treated as a wave perturbation in the plasma using kinetic theory with a gyro-kinetic approximation to obtain a dispersion relation. The presence of an electron temperature anisotropy is the necessary condition to obtain an exponentially growing instability, and the corresponding growth rate is found to be the ratio of the electron kinetic energy and the electric field energy across the DL region. The theoretical predictions are then validated against a one-dimensional electrostatic particle simulation carried out in a traveling magnetic field thruster environment. An anisotropy ratio parameter was introduced, and the theoretical growth rates were found to be in good agreement with the simulation for different anisotropy ratios. An ion beam, associated with the DL dynamics, is observed within the simulation domain. A parametric study revealed that the DL-like structure loses its ambipolar shape for temperature ratios less than 10. It has been found that a stronger anisotropy is required to obtain the DL-like structure.

A novel method for fast simulation of discharge, which uses a plasma fluid model, is proposed with a focus on the small space-charge variation in the calculation of electric field. A simple scalar equation is employed in the proposed method to calculate the electric field variation instead of solving Poisson's equation at every time step when the dielectric relaxation time is relatively large compared to the time step interval. We perform numerical simulations of surface dielectric-barrier-discharge at atmospheric pressure, which is utilized in developing an active flow control device referred to as a plasma actuator, and demonstrate significant reduction of solve count of the Poisson's equation throughout the simulation. The simulation result indicates that the CPU time required for the electric-field calculation becomes one order of magnitude smaller than that of the conventional approach within an error margin of one percent when several-10 kHz sinusoidal voltage is applied for the discharge. Moreover, a cost reduction of almost two orders of magnitude is achieved in the lower-frequency case with the same order of the error in the several-10 kHz case. Our approach enables large-scale discharge simulation and simulation of multi-physics phenomenon including the discharge physics with a practical computational cost.

© 2018 IEEE. The propagation velocity of developing breakdown plasma head generated on the straight beam was measured for 203 GHz / 170 GHz / 137 GHz using a multi-frequency gyrotron. As a result, the propagation velocity has small dependence on frequency. The numerical calculation was conducted for similar condition and the gas heating by plasma was found dominant for growth process of plasma development for this power density range and numerical results presented similar propagation velocity dependence.

Radiation hydrodynamics simulations based on a single fluid two-temperature model may violate the law of energy conservation, because the governing equations are expressed in a nonconservative formulation. In this study, we maintain the important physical requirements by employing a strategy based on the key concept that mathematical structures associated with conservative and nonconservative equations are preserved, even at the discrete level. To this end, we discretize the conservation laws and transform them using exact algebraic operations. The proposed scheme maintains global conservation errors within the round-off level. In addition, a numerical experiment concerning the shock tube problem suggests that the proposed scheme agrees well with the jump conditions at the discontinuities regulated by the Rankine–Hugoniot relationship. The generalized derivation allows us to employ arbitrary central difference, artificial dissipation, and Runge–Kutta methods.

Two-dimensional Navier-Stokes equation was numerically solved to reproduce dynamics of a flow separation under supersonic flow with Mach number of 1.7. The repetitive pulses were irradiated to the under surface of the diamond wing to control the flow separation induced on the upper surface. A strong blast wave was generated from a focal point of the repetitive pulses because of a rapid gas heating. The expansion wave was induced from the trailing edge when the blast wave with the supersonic speed propagated from the under to upper surfaces. The separation region on the upper surface became smaller by irradiating the repetitive pulses on the under surface because the expansion wave induced from the trailing edge interacted with a recompres-sion wave and relaxed an inverse pressure gradient on the upper surface. A lift-to-drag ratio of the wing was improved by utilizing an energy deposition of the repetitive pulses because of a decrease in the pressure on the upper surface and an increase in the pressure on the under surface. An effectiveness of a “contactless” flow control device using the repetitive pulses was revealed by our numerical simulation.

The flow field induced by a dielectric-barrier-discharge (DBD) plasma actuator is complicated because the electrohydrodynamic (EHD) force depends on the applied voltage waveform and the electrode configuration. In this study, fluid-discharge coupling simulation was performed to reproduce the induced flow field driven by the DBD plasma actuator in the atmospheric pressure. The induced flow structure was reproduced with the detailed discharge simulation when a DC-voltage combined with repetitive nanosecond pulses was applied. The DC voltage generates a large EHD force at the beginning however, the EHD force decreases with time because the electric field is screened due to the surface charge accumulation. The negative pulse voltage ignites a pulsed discharge and neutralizes the dielectric surface, which is positively charged during the DC phase, forming a two-stroke charge cycle. This operation method repetitively generates a large EHD force. A wall jet parallel to the dielectric surface is induced with the two-stroke charge cycle operation. The peak velocity is approximately 4 m/s when the 8-kV DC voltage combined with the -8 kV nanosecond pulses is applied. Shock waves are also repetitively generated due to the fast gas heating at the exposed electrode tip during the pulse superposition phase.

Shock-wave propagation around a microwave-rocket nozzle was numerically reproduced using a computational fluid dynamics code when an electron cyclotron resonance condition was satisfied by irradiating 25 GHz microwaves onto the rocket nozzle under an external magnetic field. The pulse-duration and nozzle-shape dependence for the thrust performance of the microwave rocket was evaluated by changing the normalized heating length and the normalized nozzle radius. The open-front approach usinga butterfly valve was newly proposed to improve the thrust performance by eliminating the negative thrust and removing the stagnation region during the gas-refilling process. The thruster only received the positive pressure of the shock wave by eliminating the contribution of the negative pressure using the open-front approach, which improved the thrust performance. In addition to the thrust improvement, the gas-refilling performance was also improved because the ambient gas flowed from the open front to the nozzle inside without forming the stagnation region. The improvement of the thrust performance and the gas-refilling performance can improve the payload transportation efficiency and further reduce the cost of launch using beamed-energy propulsion.

We developed a three-dimensional radiative transfer code for an ultra-relativistic background flow-field by using the Monte Carlo (MC) method in the context of gamma-ray burst (GRB) emission. For obtaining reliable simulation results in the coupled computation of MC radiation transport with relativistic hydrodynamics which can reproduce GRB emission, we validated radiative transfer computation in the ultra-relativistic regime and assessed the appropriate simulation conditions. The radiative transfer code was validated through two test calculations: (1) computing in different inertial frames and (2) computing in flow-fields with discontinuous and smeared shock fronts. The simulation results of the angular distribution and spectrum were compared among three different inertial frames and in good agreement with each other. If the time duration for updating the flow-field was sufficiently small to resolve a mean free path of a photon into ten steps, the results were thoroughly converged. The spectrum computed in the flow-field with a discontinuous shock front obeyed a power-law in frequency whose index was positive in the range from 1 to 10MeV. The number of photons in the high-energy side decreased with the smeared shock front because the photons were less scattered immediately behind the shock wave due to the small electron number density. The large optical depth near the shock front was needed for obtaining high-energy photons through bulk Compton scattering. Even one-dimensional structure of the shock wave could affect the results of radiation transport computation. Although we examined the effect of the shock structure on the emitted spectrum with a large number of cells, it is hard to employ so many computational cells per dimension in multi-dimensional simulations. Therefore, a further investigation with a smaller number of cells is required for obtaining realistic high-energy photons with multidimensional computations. (C) 2017 Elsevier Inc. All rights reserved.

The generation mechanism of counter-streaming plasmas, by irradiating an inner-surface of the 1st-plane of double-plane target, is investigated experimentally using optical imaging and collective Thomson scattering (CTS) ion-term measurement. The CTS measurement reveals that counter-streaming plasmas exist at the same time at the same position, which is the evidence for the collisionless interaction between the counter streaming plasmas. The generation of the 2nd-plane plasma is in two steps: First, the 2nd-plane is ablated nearly at the laser timing by radiation from the 1st-plane plasma. Then similar or equal to 5-7 ns later, when the plasma from the 1st-plane reaches the 2ns-plane, it induces higher-density plasma generation. (C) 2017 Elsevier B.V. All rights reserved.

An experimental demonstration was carried out in a ballistic range at high Mach numbers with the low specific heat ratio gas hydrofluorocarbon HFC-134a to observe the unstable bow-shock wave generated in front of supersonic blunt objects. The shadowgraph images obtained from the experiments showed instability characteristics, in which the disturbances grow and flow downstream and the wake flow appears wavy because of the shock oscillation. Moreover, the influence of the body shape and specific heat ratio on the instability was investigated for various experimental conditions. Furthermore, the observed features, such as wave structure and disturbance amplitude, were captured by numerical simulations, and it was demonstrated that computational fluid dynamics could effectively simulate the physical instability. In addition, it was deduced that the shock instability is induced by sound emissions from the edge of the object. This inference supports the dependence of the instability on the specific heat ratio and Mach number because the shock stand-off distance is affected by these parameters and limits the sound wave propagation.

An approach for electrohydrodynamic (EHD) force production is proposed with a focus on a charge cycle on a dielectric surface. The cycle, consisting of positive-charging and neutralizing strokes, is completely different from the conventional methodology, which involves a negative-charging stroke, in that the dielectric surface charge is constantly positive. The two-stroke charge cycle is realized by applying a DC voltage combined with repetitive pulses. Simulation results indicate that the negative pulse eliminates the surface charge accumulated during constant voltage phase, resulting in repetitive EHD force generation. The time-averaged EHD force increases almost linearly with increasing repetitive pulse frequency and becomes one order of magnitude larger than that driven by the sinusoidal voltage, which has the same peak-to-peak voltage. Published by AIP Publishing.

Breakdown physics induced by a subcritical microwave was numerically reproduced by using a two-dimensional effective diffusion model for plasma transport and combining it with Maxwell's equations and a neutral gas dynamics equation. A discrete plasma structure was obtained when E-0,E- rms / E-c >= 0.69, where E-0,E- rms is the root-meansquare of the incident electric field and Ec is the breakdown threshold, because an overcritical field that exceeded the breakdown threshold was formed in a region away from the bulk plasma by the wave reflection when the plasma reflectivity was increased by joule heating. However, a continuous plasma structure with a branching pattern was formed when E-0,E- rms = E-c < 0.69 because the enhanced electric-field region away from the bulk plasma never exceeded the breakdown threshold even when the plasma reflectivity increased. The propagation speed of the plasma front drastically decreased when E-0,E- rms = E-c < 0.69 because the plasma propagation was sustained by strong gas expansion, which required more time than wave-reflection and ionization processes. (C) 2017 Author(s).

Numerical analysis was conducted for a hypersonic flow around a 5-degree half-angle sharp cone in order to examine wave propagation in the hypersonic boundary layer by computational fluid dynamics (CFD) and global stability analysis based on the computed flow field. Periodic signals were clearly found in the pressure fluctuations on the wall and moved downstream. The power spectrum of the wall pressure suggested that characteristic frequency appeared in the region predicted by linear stability theory for the second-mode waves, while the lower mode was also enhanced in the downstream side. Dynamic mode decomposition (DMD) as a global stability analysis was applied to the CFD data for finding the characteristic mode of which the frequency may be identified in the wall pressure spectrum since the DMD can extract the predominant modes with their growth rates and frequencies. The predominant dynamic modes obtained from the pressure field and those from the velocity field were different because the former depends on acoustic waves but the latter indicates entropy waves. The correlation of these modes may suggest the mode transfer in the hypersonic boundary layer.

Two-dimensional numerical simulations of the discharge process in a dielectric-barrier-discharge (DBD) plasma actuator were carried out toward the detailed understanding of the electrohydrodynamic (EHD) force characteristics and proposal of the suitable voltage input in view of the EHD force production. In this study, the inuence of the voltage amplitude on the discharge regime and EHD force is investigated when a triangle voltage waveform is applied. The streamer discharges are obtained with high amplitude in the positive-going phase in a voltage cycle, whereas no streamer propagation is observed with low amplitude. In the negative-going phase, the frequency of the repetitive current pulses decreases with decreasing the amplitude because the voltage slope decreases with the same voltage frequency. The charge on the dielectric surface has a quite important role in the discharge inception voltage the discharge is ignited in the negative-going phase even though the applied voltage is positive. The time-averaged EHD force increases with increasing the voltage amplitude and can be divided into two-power law regions. The increment rate of the EHD force changes sharply when the discharge regime transits to the streamer discharge. A wall jet induced by the DBD plasma actuator in quiescent air is reproduced by using the EHD force distribution obtained from the discharge simulation, which shows qualitatively agreement with experimental result.

This paper explores a suitable position for a dust-capturing device on the surface of an aeroflyby hypersonic vehicle traveling in the Martian atmosphere at an altitude of 36km. The feasibility of the Mars aeroflyby sample collection as a mission for capturing Martian dust particles has been investigated by the Japan Aerospace Exploration Agency. In our previous work, trajectories and thermal analyses for spherical dust particles reaching the capturing device were computed. However, Martian dust particles are not all spherical but have several shape variations. In this study, the trajectories and the temperature elevations of disk-shaped particles were numerically investigated. The trajectories were found to deviate and the temperatures were elevated whether the particles faced the disk or the lateral side to the relative flow velocity. For the disk-shaped and spherical particles, the tendencies of the obtained density maps were similar. In contrast, the obtained temperature maps indicated an area divided into high- and low-temperature regions based on differences in the projection area or the stable flight attitude of the disk-shaped particles that cannot be seen in the spherical particles. This study concludes that the leeward and the side surfaces of the aeroflyby vehicle are suitable positions for the dust-capturing device.

Although it has been demonstrated that high-Z doped plastic can suppress the Rayleigh-Taylor instability, its usability in direct-drive implosion design on mega-joule class reactors is still controversial. In this study, the radiation hydrodynamics code was validated by a planar target experiment of a brominated plastic target, since the result including high-Z strongly depends on the opacity model. Opacity for bromine ion based on the detailed configuration accounting model has better agreement with the experimental results compared to that of the average-ion model. Two-dimensional implosion simulations assuming a mega-joule driver were also conducted to estimate whether a brominated plastic ablator can suppress the hydrodynamic instability. It was revealed that a brominated plastic, which has an appropriate fraction of doping, can assist the generation of a high-density core by suppression of the hydrodynamic instability. A high-Z doped target can suppress the Rayleigh-Taylor instability at the foot-drive phase when the laser intensity is relatively low. Published by AIP Publishing.

A filamentary plasma is reproduced based on a fully kinetic model of electron and ion transports coupled with electromagnetic wave propagation. The discharge plasma transits from discrete to diffusive patterns at a 110-GHz breakdown, with decrease in the ambient pressure, because of the rapid electron diffusion that occurs during an increase in the propagation speed of the ionization front. A discrete plasma is obtained at low pressures when a low-frequency microwave is irradiated because the ionization process becomes more dominant than the electron diffusion, when the electrons are effectively heated by the low-frequency microwave. The propagation speed of the plasma increases with decrease in the incident microwave frequency because of the higher ionization frequency and faster plasma diffusion resulting from the increase in the energy-absorption rate. An external magnetic field is applied to the breakdown volume, which induces plasma filamentation at lower pressures because the electron diffusion is suppressed by the magnetic field. The thrust performance of a microwave rocket is improved by the magnetic fields corresponding to the electron cyclotron resonance (ECR) and its higher-harmonic heating, because slower propagation of the ionization front and larger energy-absorption rates are obtained at lower pressures. It would be advantageous if the fundamental mode of ECR heating is coupled with a lower frequency microwave instead of combining the higher-harmonic ECR heating with the higher frequency microwave. This can improve the thrust performance with smaller magnetic fields even if the propagation speed increases because of the decrease in the incident microwave frequency. Published by AIP Publishing.

Simple models based on two-dimensional simulations are proposed to estimate intervals of periodically observed current pulses with a positive-going voltage in a dielectric barrier discharge plasma actuator. There are two distinct peaks in one streamer discharge; one is related to the formation of an ion cloud and the other is related to a filamentary discharge that is identified as a streamer. Simulation results show that the intervals of the current pulses depend on the slope of the applied voltage. For the ion-cloud formation phase, we model the time evolution of electron number density at the exposed electrode with ionization frequency. For the ion-cloud expansion phase, a positive ion cylinder model is proposed to estimate the electric field generated by surface charge on the dielectric. These models well reproduce the discharge intervals obtained in the numerical simulations.

Filamentary plasma induced by microwave beam irradiation was reproduced in nitrogen and argon by combining fluid or particle plasma models with electromagnetic wave propagation. Transport coefficients used in the fluid model are estimated from particle simulation to maintain consistency of the breakdown structure between the fluid and particle models. A discrete structure was obtained using the one-dimensional (1D) fluid model, because a standing wave is generated in front of the plasma when the incident microwave beam is reflected by the overcritical plasma, which agrees with the breakdown structure obtained using the 1D particle model. A 2D plasma filament was also reproduced using the fluid model in nitrogen and argon. Reflection of the incident microwave in argon becomes stronger than that in nitrogen because of the denser argon plasma. Change in filament shape is induced in argon because the electric field is deformed at the plasma tip owing to stronger wave reflection from the neighboring filament. The propagation speed of the plasma front becomes larger in argon breakdown because of the larger ionization frequency and the larger diffusion coefficient.

Time-resolved compression of a laser-driven solid deuterated plastic sphere with a cone was measured with flash K alpha x-ray radiography. A spherically converging shockwave launched by nanosecond GEKKO XII beams was used for compression while a flash of 4.51 keV Ti K alpha x-ray backlighter was produced by a high-intensity, picosecond laser LFEX (Laser for Fast ignition EXperiment) near peak compression for radiography. Areal densities of the compressed core were inferred from two-dimensional backlit x-ray images recorded with a narrow-band spherical crystal imager. The maximum areal density in the experiment was estimated to be 87 +/- 26 mg/cm(2). The temporal evolution of the experimental and simulated areal densities with a 2-D radiation-hydrodynamics code is in good agreement. Published by AIP Publishing.

A petawatt laser for fast ignition experiments (LFEX) laser system [N. Miyanaga et al., J. Phys. IV France 133, 81 (2006)], which is currently capable of delivering 2 kJ in a 1.5 ps pulse using 4 laser beams, has been constructed beside the GEKKO-XII laser facility for demonstrating efficient fast heating of a dense plasma up to the ignition temperature under the auspices of the Fast Ignition Realization EXperiment (FIREX) project [H. Azechi et al., Nucl. Fusion 49, 104024 (2009)]. In the FIREX experiment, a cone is attached to a spherical target containing a fuel to prevent a corona plasma from entering the path of the intense heating LFEX laser beams. The LFEX laser beams are focused at the tip of the cone to generate a relativistic electron beam (REB), which heats a dense fuel core generated by compression of a spherical deuterized plastic target induced by the GEKKO-XII laser beams. Recent studies indicate that the current heating efficiency is only 0.4%, and three requirements to achieve higher efficiency of the fast ignition (FI) scheme with the current GEKKO and LFEX systems have been identified: (i) reduction of the high energy tail of the REB; (ii) formation of a fuel core with high areal density using a limited number (twelve) of GEKKOXII laser beams as well as a limited energy (4 kJ of 0.53-mu m light in a 1.3 ns pulse); (iii) guiding and focusing of the REB to the fuel core. Laser-plasma interactions in a long-scale plasma generate electrons that are too energetic to efficiently heat the fuel core. Three actions were taken to meet the first requirement. First, the intensity contrast of the foot pulses to the main pulses of the LFEX was improved to > 10(9). Second, a 5.5-mm-long cone was introduced to reduce pre-heating of the inner cone wall caused by illumination of the unconverted 1.053-mu m light of implosion beam (GEKKO-XII). Third, the outside of the cone wall was coated with a 40-mu m plastic layer to protect it from the pressure caused by imploding plasma. Following the above improvements, conversion of 13% of the LFEX laser energy to a low energy portion of the REB, whose slope temperature is 0.7 MeV, which is close to the ponderomotive scaling value, was achieved. To meet the second requirement, the compression of a solid spherical ball with a diameter of 200-mu m to form a dense core with an areal density of similar to 0.07 g/cm(2) was induced by a laser-driven spherically converging shock wave. Converging shock compression is more hydrodynamically stable compared to shell implosion, while a hot spot cannot be generated with a solid ball target. Solid ball compression is preferable also for compressing an external magnetic field to collimate the REB to the fuel core, due to the relatively small magnetic Reynolds number of the shock compressed region. To meet the third requirement, we have generated a strong kilo-tesla magnetic field using a laser-driven capacitor-coil target. The strength and time history of the magnetic field were characterized with proton deflectometry and a B-dot probe. Guidance of the REB using a 0.6-kT field in a planar geometry has been demonstrated at the LULI 2000 laser facility. In a realistic FI scenario, a magnetic mirror is formed between the REB generation point and the fuel core. The effects of the strong magnetic field on not only REB transport but also plasma compression were studied using numerical simulations. According to the transport calculations, the heating efficiency can be improved from 0.4% to 4% by the GEKKO and LFEX laser system by meeting the three requirements described above. This efficiency is scalable to 10% of the heating efficiency by increasing the areal density of the fuel core. Published by AIP Publishing.

A cause of an overestimation of the computed surface pressure on a blunted cone in high-temperature hypersonic flow is explored. The overestimation was observed in a free-piston shock tunnel at the stagnation of H-0 = 15.6 and 10.1 MJ/kg. The sensitivity analysis reveals that a reduction of the upstream translational temperature in the range of 100 to 300 K substantially improves the agreement of the surface pressure with the measured data. As the cause of the lower upstream translational temperature, radiative cooling effect is included in the thermochemical nonequilibrium calculation in the nozzle, the translational temperature at the nozzle exit is reduced to about 250 K. Using the obtained flow variables as the upstream boundary condition, the computed pressure agrees quite well with the experimental data. In order to clarify whether other variables such as translational-vibrational relaxation time, chemical reaction rates, and upstream chemical composition could be the cause of the discrepancy, uncertainty quantification is employed. It is shown that these parameters of the thermochemical model and upstream chemical composition have minor effect on the agreement of surface pressure. It is concluded that the observed discrepancy in the surface pressure is due to radiative cooling effect of high temperature gas in the nozzle region. (C) 2015 Elsevier Masson SAS. All rights reserved.

A model experiment of magnetic field amplification (MFA) via the Richtmyer-Meshkov instability (RMI) in supernova remnants (SNRs) was performed using a high-power laser. In order to account for very-fast acceleration of cosmic rays observed in SNRs, it is considered that the magnetic field has to be amplified by orders of magnitude from its background level. A possible mechanism for the MFA in SNRs is stretching and mixing of the magnetic field via the RMI when shock waves pass through dense molecular clouds in interstellar media. In order to model the astrophysical phenomenon in laboratories, there are three necessary factors for the RMI to be operative: a shock wave, an external magnetic field, and density inhomogeneity. By irradiating a double-foil target with several laser beams with focal spot displacement under influence of an external magnetic field, shock waves were excited and passed through the density inhomogeneity. Radiative hydrodynamic simulations show that the RMI evolves as the density inhomogeneity is shocked, resulting in higher MFA. (C) 2016 AIP Publishing LLC.

<p>A particle model of plasma was coupled with electromagnetic wave propagation to reproduce a filamentary structure and diffusive pattern in a microwave breakdown. The thrust performance of a microwave rocket was estimated by combining discharge calculation and computational fluid dynamics (CFD). An external magnetic field is applied to the discharge volume to improve thrust performance for low ambient pressure by combination of magnetic confinement and resonance heating technique. The magnetic field suppresses the propagation speed of the ionization front because the electron cannot cross the magnetic field at lower pressure, whereas propagation speed increases with decreased ambient pressure when no magnetic field is applied. The energy absorption rate increases when electron cyclotron resonance (ECR) occurs with the corresponding magnetic field, which improves the thrust performance of the microwave rocket.</p>

<p>レーザープラズマの1流体2温度モデルに代表される非保存型流体方程式は，離散化時に保存則が破れるという問題を有する．本研究では保存型方程式を離散化レベルで展開し，非保存型方程式がエネルギー保存を保証するのに必要な差分オペレータや誤差項を導出した．衝撃波管問題による数値実験においても丸め誤差程度で保存性を確認した．今後は本手法を1流体2温度モデルに展開し，物理的に無矛盾な人工粘性の開発を行う．</p>

A reversely-induced azimuthal current has been found in two-dimensional particle simulations with moderately screened rotating electric field (REF) though an ideally penetrating REF drives a "positive" azimuthal current following rotating E x B drifts. This brings us an alternative acceleration concept, called a negative-moving response (NMR) acceleration, of the helicon plasma under practical conditions using a converging magnetic field because the internal electric potential, formed by the plasma response against the external field, drives the "negative" azimuthal current. Under realistic experimental conditions, e.g., a magnetic field of 0.2 T, AC frequency of &lt;100 MHz, and AC voltage of &lt;1000V, the resultant thrust can be estimated at an observable level of &gt;0.1mN with the NMR acceleration. Moreover, the reverse REF is more favorable to the NMR acceleration than the conventional forward one because the reverse field produces a Lissajous acceleration in the converging magnetic field. (C) 2016 The Japan Society of Applied Physics

A one-dimensional compressible fluid calculation was coupled with a finite difference time-domain code and a particle-in-cell code with collision to reproduce propagation of electromagnetic wave, ionization process of plasma, and shock wave formation in atmospheric microwave discharge. Plasma filaments are driven toward the microwave source at 1 atm, and the distance between each filament is one-fifth of the wavelength of the incident microwave. The strong shock wave is generated due to the high plasma density at the atmospheric pressure. A simple analysis of the microwave propagation into the plasma shows that cut-off density of the microwave becomes smaller with the pressure decrease in a collisional plasma. At the lower pressure, the smaller density plasma is obtained with a diffusive pattern because of the smaller cut-off density and the larger diffusion effect. In contrast with the 1-atm case, the weak shock wave is generated at a rarefied condition, which lowers performance of microwave thruster.

We conducted two-dimensional simulations of inertial confinement fusion targets to evaluate effects of high-Z doping on implosion hydrodynamics. It was found that an ablation structure drastically changes with concentration of dopant material. We also confirmed that even a lightly-doped target can suppress Rayleigh-Taylor instability on short wavelength, while a long-wavelength perturbation is difficult to be suppressed with any dopant concentration. The high-Z doping is thus only effective for a spherical implosion with high-mode perturbations.

We investigate spherical collisionless shocks in the presence of an external magnetic field. Spherical collisionless shocks are common resultant of interactions between a expanding plasma and a surrounding plasma, such as the solar wind, stellar winds, and supernova remnants. Anisotropies often observed in shock propagations and their emissions, and it is widely believed a magnetic field plays a major role. Since the local observations of magnetic fields in astrophysical plasmas are not accessible, laboratory experiments provide unique capability to investigate such phenomena. We model the spherical shocks in the universe by irradiating a solid spherical target surrounded by a plasma in the presence of a magnetic field. We present preliminary results obtained by shadowgraphy.

One of the important and interesting problems in astrophysics and plasma physics is collimation of plasma jets. The collimation mechanism, which causes a plasma flow to propagate a long distance, has not been understood in detail. We have been investigating a model experiment to simulate astrophysical plasma jets with an external magnetic field [Nishio et al., EPJ. Web of Conferences 59, 15005 (2013)]. The experiment was performed by using Gekko XII HIPER laser system at Institute of Laser Engineering, Osaka University. We shot CH plane targets (3 mm x 3 mm x 10 mu m) and observed rear-side plasma flows. A collimated plasma flow or plasma jet was generated by separating focal spots of laser beams. In this report, we measured plasma jet structure without an external magnetic field with shadowgraphy, and simultaneously measured the local parameters of the plasma jet, i.e., electron density, electron and ion temperatures, charge state, and drift velocity, with collective Thomson scattering.

We have developed a two-dimensional magneto-radiation hydrodynamics code with an induction equation solver to analyze laboratory-astrophysics experiments relevant to a supernova remnant (SNR) environment. The computed magnetic field was amplified by a order of magnitude from the background level as expected for accelerating fields of cosmic rays in the SNR context. A part of the amplification of the magnetic field is caused by Richtmyer-Meshkov instability (RMI) that stretches a contact discontinuity. However, the maximum magnetic field is smaller than the theoretically predicted limit since the non-linear RMI cannot sufficiently grow and the field is amplified by the compression effect rather than the stretching in the present experimental condition.

This paper discusses active laser control using a genetic algorithm and a mirror-actuating system to achieve kilometer-order in-air flight using a laser propulsion vehicle while riding a beam. A 10 kg vehicle reaction driven by a strong shock wave is examined using our flight simulator to analyze interaction with unsteady blast wave propagation based on coupling calculation between hydrodynamic simulation of the shock wave propagation and orbital simulation of the vehicle flight motion, and the generated impulses are characterized by the spherically symmetric Sedov solution. Beam-riding flight with an initial y offset of 5 mm is successfully sustained by controlling angular offset, while vehicle acceleration is kept low for safe launch to the target altitude. A system delay of laser control is introduced into the flight simulator, and beam-riding flight is maintained for a 20 ms system delay using delay correction following prediction by six-degree-of-freedom equations of motion. This study also examines robustness of the flying technique for wind perturbation, and an active control scheme that can ensure stable flight with a wind of up to 40 m/s. The stability of flight control is assessed when there is a positioning error of laser incident light, and flight is stably sustained by keeping the angular offset small for the positioning error with a standard deviation 1 cm. This paper examines the lower limit of repetition frequency of multiple pulses for stable flight, and found that a repetition frequency exceeding 15 Hz should be selected in order to maintain posture stability. Stable flight is not maintained for the combination of larger system delay and positioning error because translational velocity and angular offset simultaneously increase. However, the vehicle can fly to kilometer-order altitude while maintaining posture stability if the translational and angular impulses are adjusted. Flight performance can be improved by adjusting impulse direction and size using our flight simulator as an impulse design tool.

We have conducted radiation magneto-hydrodynamics (RMHD) simulations of Richtmyer-Meshkov instability (RMI) in a magnetized counter flow produced by intense lasers. A jet-like plasma from a planar plastic target is formed and maintained in several-tens of nanoseconds by expanding plasma from rear side of two separated laser spots, and parallelly located another target is ablated by the radiation from the plasma, reproducing past experimental works. A planar shock driven by the radiation interacts with the jet as a nonuniform density structure, resulting in the RMI. The magnetic field is amplified up to similar to 40 times greater than the background value at the interface at which the instability occurs. However, a certain extent of the amplification results from the compression effect induced by the counter flow, and the obtained amplification level is difficult to be measured in the experiments. An experiment for observing a clear amplification must be designed through the RMHD simulations so that the RMI takes place in the low-density area between two targets. (C) 2014 Elsevier B.V. All rights reserved.

Some high-energy photons are thought to be produced by the inverse Compton scattering process in ultrarelativistic flows, and the high-energy component of spectra in gamma-ray bursts can be interpreted by the process. To examine numerically the trajectory of photons traveling in relativistic jets in detail, a coupled computation method of radiative transport with relativistic hydrodynamics is required. We have developed a three-dimensional code of radiative transport on a background with a relativistic flow using Monte Carlo method. Radiative transfer simulations have been implemented in different inertial frames which are described as a shock rest frame or shock moving frames, and obtained results are compared in the shock rest frame to identify a consistent transformation among different frames. Optical depth tau for every directions agrees among each frame if a time duration of the computation is small enough to resolve photon path close to a shock front with almost the speed of light. Although the obtained results of the direction distribution and the spectrum of the escaped photons from the computational domain in each frame show discrepancies due to different flow velocities, they are identical after Lorentz transforming to the shock rest frame. We found the second peak of energy in the high-energy side of the spectra if the simulation condition is determined to allow the scattering process in the upstream side of the shock, and this peak is formed by the inverse Compton scattering process. (C) 2014 Elsevier B.V. All rights reserved.

Bow-shock instability has been experimentally observed in a low-gamma flow. To clarify its mechanism, a parametric study was conducted with three-dimensional numerical simulations for specific heat ratio gamma and Mach number M. A critical boundary of the instability was found in the gamma-M parametric space. The bow shock tends to be unstable with low gamma and high M, and the experimental demonstration was designed based on this result. The experiments were conducted with the ballistic range of the single-stage powder gun mode using HFC-134a of gamma = 1.12 at Mach 9.6. Because the deformation of the shock front was observed in a shadowgraph image, the numerical prediction was validated to some extent. The theoretical estimation of vortex formation in a curved shock wave indicates that the generated vorticity is proportional to the density ratio across the shock front and that the critical density ratio can be predicted as similar to 10. A strong slipstream from the surface edge generates noticeable acoustic waves because it can be deviated by the upstream flow. The acoustic waves emitted by synchronizing the vortex formation can propagate upstream and may trigger bow-shock instability. This effect should be emphasized in terms of unstable shock formation around an edged flat body. (C) 2015 AIP Publishing LLC.

This paper explores an appropriate position for the dust-capturing device on the surface of an aeroflyby capsule traveling at a velocity of 4.4 km/s in the Martian atmosphere at an altitude of 36 km. The equation of motion and the heat-transfer equation for dust particles are solved for particle sizes of 0.5 and 0.1 mu m. A thermochemical nonequilibrium flowfield over the vehicle is computed using a prismatic unstructured mesh method. Analysis indicates that placing a dust-capturing device on the leeward frustum edge results in less aerodynamic drag and lower surface heat flux than placing the same device on the windward frustum edge. The lower heat flux exerted on the surface of the dust-capturing device is preferable because the aerogel on the surface of the device is less damaged. The temperature of dust particles of 0.5 mu m diameter is elevated to almost the phase-change temperature when the dust-capturing device is on the leeward frustum edge, due to longer flight time in the high-temperature shock layer. The temperature of dust particles reaching the device on the windward frustum edge is well below the phase-change temperature. However, this study could not find any position to capture dust particles of 0.1 mu m diameter before reaching the phase-change temperature, regardless of the position of the dust-capturing device.

A high order CFD code was developed for LES computation of turbulent heat flux in hypersonic flow. Aeroheating measurement tests with forced transition on an Apollo capsule model and an HTV-R which was a manned space capsule under development by JAXA were performed by the free-piston shock tunnel HIEST in JAXA Kakuda Space Center. Measured data set indicate that heat flux on the forebody of the Apollo capsule model was 1.5-2 times larger than one in laminar flow. On the other hand, the heat flux on the afterbody of HTV-R became significantly larger. Furthermore, in the separation region which exists afterbody of capsules, the Baldwin-Lomax model which can reproduce turbulent heat flux in the attached flow, tends to underestimate. In order to reproduce the turbulent heat flux on the after body by numerical simulation, high order CFD code towards LES in hypersonic flow is needed. Since the robustness near the shock and high resolution in the boundary layer is needed for LES in hypersonic flow, improved WENO method is employed. We calculated the hypersonic flow (M∞ = 17) around cylinder and investigated the robustness for strong shock by the method. Smooth pressure distribution was obtained agree well with the calculated heat flux by NASA LAURA code. Our developed code applied to the hypersonic flowfield around HTV-R with a trip and the heat flux was examined.

To estimate the flight reactions of a full-scale vehicle from reduced-scale tests, we constructed a scaling theory for the vehicle size, input energy, moment of inertia, and pulse frequency needed to maintain dynamic equivalence between a laboratory-scale and full-scale launch of a laser propulsion vehicle. The dynamic scaling law for a single pulse was constructed using translational and angular equations of motion. The analytical scaling was confirmed for a single-pulse incident using a fluid-orbit coupling simulator for the interaction between the blast wave and the vehicle. Motion equivalence was maintained for multiple pulses by adjusting the repetition frequency of the pulse incident to correct for the effect of aerodynamic drag during the free flight of the pulse-to-pulse interval. The flight of a full-scale vehicle can be estimated for single-and multiple-pulse operations from the flight data for a small-scale vehicle using the proposed scaling theory, which provides correlations between the characteristics of small-scale and large-scale flight systems. Small-scale tests were shown to be useful in estimating the flight of a full-scale vehicle using dynamic scaling theory.

Unexpected heat flux augmentation in a free-piston high-enthalpy shock tunnel (HIEST) was numerically analyzed. Since a previous experimental study implied that the radiation heating from the shock layer caused the augmentation, a three-dimensional thermochemical non-equilibrium CFD code including radiation transport calculation in the shock layer was developed. This calculation was conducted under the following models: 1) Radiation heating from the air species in the shock layer was calculated by a solving radiative transport equation using tangent slab approximation; and 2) Radiation heating from impurities such as carbon soot and metal particulates, which could be included in the upstream test gas, was calculated by assuming the shock layer as a grey body with averaged shock layer temperature for a trial calculation. The calculations were performed at the stagnation enthalpy and stagnation pressure from 7 to 21MJ/kg and 31 to 55 MPa, respectively. For air species radiation, radiative heat flux was too small to contribute heat flux augmentation. On the other hand, for grey body assumption, we could find that abnormal heat flux augmentation could be expected by epsilon sigma T-ave(4) for an engineering technique, where epsilon denotes the emissivity epsilon = 0.132 and T-ave(4) was the average shock layer temperature.

Plasma filamentation is induced by an external magnetic field in an atmospheric discharge using intense microwaves. A discrete structure is obtained at low ambient pressure if a strong magnetic field of more than 1 T is applied, due to the suppression of electron diffusion, whereas a diffusive pattern is generated with no external field. Applying a magnetic field can slow the discharge front propagation due to magnetic confinement of the electron transport. If the resonance conditions are satisfied for electron cyclotron resonance and its higher harmonics, the propagation speed increases because the heated electrons easily ionize neutral particles. The streamer velocity and the pattern of the microwave plasma are positively controlled by adjusting two parameters-the electron diffusion coefficient and the ionization frequency-through the resonance process and magnetic confinement, and hot, dense filamentary plasma can be concentrated in a compact volume to reduce energy loss in a plasma device like a microwave rocket. (C) 2014 AIP Publishing LLC.

Two-dimensional simulations of Lissajous acceleration were conducted by a code based on Particle-In-Cell (PIC) method and Monte Carlo Collision (MCC) method for understanding plasma motion inaccelerationarea of an electrodeless plasma thruster and for finding the optimal condition. Obtained results showthat azimuthal current depends on a ratio of electron drift radius to plasma region length, and peak value is in proportion to AC frequency. To eliminate a disturbance on rotating mode due to electron cyclotron motion, the cyclotron frequency should be much higher than the AC frequency. When the high density plasma reduces electric field penetration, the azimuthal current is suppressed in a low level. In order to improve the electric field penetration, we should apply the high magnetic field and the high AC frequency. When a ratio of the collision frequency between electron and neutral argon to the cyclotron frequency is lower than 0.01, collisional loss of the azimuthal current is little or nothing.

To present a feasibility of high-altitude flight of a laser-propelled vehicle at supersonic speed, we have developed a flight simulator which has fluid-orbit coupling calculation module to reproduce impulsive flight reaction driven by blast waves. By high-power energy transmission through arrayed lasers together with the genetic algorithm (GA) controlled sub-laser, the supersonic flight is successfully achieved in the simulation for 32.5-g vehicle, while the angular offset should be suppressed as small as possible. Rather than translational position, controlling angular offsets by the GA operation is especially important to attain the km-order flight on the premise of the active control. Additionally, the vehicle weight, the vehicle size, and the input energy are scaled up to assess the stable flight of 10-kg vehicle. The active control technique has enough possibility to launch kg-order vehicle at supersonic regime with the optimized beaming strategy.

We present numerical attempts of radiative transfer in a relativistic scattering flow that can produce gamma rays using a three-dimensional Monte Carlo code. We prepared an initial background flowfield obtained from hydrodynamical simulation of a relativistic jet in which Thomson scattering dominates compared to absorption, and solved the radiative transfer equation for the background evolved by a simple expansion model. Since a large number of sample particles is required for an accurate computation, we have parallelized the Monte Carlo code in order to obtain solutions in a practical computational time even for a long-term simulation coupled with a time-dependent flowfield. Using this code, higher parallel efficiency is achieved with larger number of particles. The obtained light curve from the simple model shows a signal of the transition from the opaque post-shock flow to the transparent regime as the flow expands, and the high-energy photons are generated by not only the Doppler boosting but also the inverse Compton scattering. (c) 2013 Elsevier B.V. All rights reserved.

A fitting formula for radiative cooling with collisional-radiative population for air plasma flowfield has been developed. Population number densities are calculated from rate equations in order to evaluate the effects of nonequilibrium atomic and molecular processes. Many elementary processes are integrated to be applied to optically-thin plasmas in the number density range of 10(12)/cm(3) &lt;= N &lt;= 10(19)/cm(3) and the temperature range of 300 K &lt;= T &lt;= 40,000 K. Our results of the total radiative emissivity calculated from the collisional-radiative population are fitted in terms of temperature and total number density. To validate the analytic fitting formula, numerical simulation of a laser-induced blast wave propagation with the nonequilibrium radiative cooling is conducted and successfully reproduces the shock and plasma wave front time history observed by experiments. In addition, from the comparison between numerical simulations with the radiation cooling effect based on the fitting formula and those with a gray gas radiation model that assumes local thermodynamic equilibrium, we find that the displacement of the plasma front is slightly different due to the deviation of population probabilities. By using the fitting formula, we can easily and more accurately evaluate the radiative cooling effect without solving detailed collisional-radiative rate equations.

We conducted a comparative study among the Bi-CG family with incomplete LU factorization employed for solving diffusion-type radiative transfer equation in radiation hydrodynamics (RHD) simulations for laser-produced plasmas. The result suggests that Bi-CGSTAB is the most suitable method thanks to its high convergence stability and low computational cost. RHD simulations were performed for Rayleigh-Taylor instability experiments of an inertial confinement fusion target. We found that higher harmonics of static pressure perturbation become important at an intermediate wavelength.

Using a fluid-orbit coupling simulator, we numerically solve the three-dimensional Navier-Stokes equations with exchanging information of six-degree-of-freedom reactions for predicting impulsive flight motions of a laser propulsion vehicle driven by blast waves. By feedback of angular and translational velocities into the flowfield, pressure and viscous drags induced by the unsteady vehicle motion are introduced to provide precise motion analysis. In the impulsive-motion estimation of the laser-boosted vehicle, restoring forces and moments are underestimated if the vehicle motion effect is modeled using aerodynamic coefficients of steady flow. Also, a simple model using impulse data examined by experiments for predicting the impulsive motion is compared with our coupling approach which can reproduce instantaneous acceleration resulting from the interaction between the vehicle and the blast wave. Velocity overshoot is generated by evaluating sharp thrust through the coupling calculation, and the flight height becomes 6% larger than conventional prediction using the impulse data.

We have proposed a method for reproducing an accurate solution of low-density ablation plasma by properly treating anisotropic radiation. Monte-Carlo method is employed for estimating Eddington tensor with limited number of photon samples in each fluid time step. Radiation field from ablation plasma is significantly affected by the anisotropic Eddington tensor. Electron temperature around the ablation surface changes with the radiation field and is responsible for the observed emission. An accurate prediction of the light emission from the laser ablation plasma requires a careful estimation of the anisotropic radiation field.

Process of counterstreaming plasma generation for laser irradiation of the innner-surface of the first plane of a double-plane target is investigated. The image taken by streaked self-emission optical pyrometer and radiation hydrodynamic simulation show the plasma from the second plane is ablated by radiation almost at the laser timing. After similar to 5 ns, increase in brightness and the generation of a plasma on the second plane are observed. According to the contemporary measurement of streaked interferometry, this is caused by the ablation of the second plane by the first plane plasma.

We have developed a two-dimensional PIC-MCC code which can describe interactions between input microwave and charged particles and collisions among the gas particles including neutrals in order to examine the mechanism of formation of a filamentary- structure observed in a microwave-beaming thruster. Assuming that an initial discharge spot is created by an air-breakdown, an incident microwave is reflected and scattered at the spot, and a strong electric field is formed ahead of it. The region of the strong field is discretisely located at a quarter of microwave wavelength from the original spot as predicted by a simple model, and a newly discharged spot is created through electron impact ionization originating in background electrons in the strong field region.

The primary hydrodynamic flow feature of early explosion phases of a core-collapse supernova is a spherical shock. This shock is born deep in the central regions of the collapsing stellar core, stalls shortly afterward, and in case of a successful explosion is revived and becomes the supernova shock. The revival process involves a standing accretion shock instability, SASI. This shock instability is considered the key processes aiding the core-collapse supernova (ccSN) explosion. The aim of our study is to identify feasible conditions and parameters for an experimental system that is able to capture the essential characteristics of SASI. We use semi-analytic methods and high-resolution hydrodynamic simulations in multidimensions to investigate a possible experimental design on the National Ignition Facility. The experimental configuration involves a steady, spherical shock. We explore a viable region of parameters and obtain limits on the shocked flow geometry. We study the stability properties of the shock and its post-shock region. We compare properties of the experimental design and the ccSN environment. The obtained model experimental flow field closely resembles converging nozzle flow. The post-shock region, in contrast to the supernova setting, is found to be stably stratified and stable against to perturbations upstream of the shock. We conclude that it is not possible to capture the characteristics of the ccSN SASI for the converging shocked flow configuration considered here. However, such configuration offers a very stable setting for precision studies of dense, high-temperature plasmas requiring finely-controlled conditions and long lifetimes. (C) 2012 Elsevier B. V. All rights reserved.

A radiation hydrodynamics code has been developed for more accurate prediction of laser-produced low-density ablation plasmas with appropriately describing anisotropic radiation field by using Monte-Carlo technique for variable Eddington tensor with reasonable computational loads. The Eddington tensor estimated by thousand of sample particles per single fluid step can reproduce the anisotropic radiation field in the low-density region and will not result in large computational consumption. Prediction of the emitted light from ablation plasma can be corrected by the proposed method. Ablation structure sustained by a compact radiation source, which is sometimes found in experiments of collisionless shock relevant to laboratory astrophysics, may also be changed by anisotropic transfer computation in optically thin region. (C) 2012 Elsevier B. V. All rights reserved.

We report our recent efforts on the experimental investigations related to the origins of cosmic rays. The origins of cosmic rays are long standing open issues in astrophysics. The galactic and extragalactic cosmic rays are considered to be accelerated in non-relativistic and relativistic collisionless shocks in the universe, respectively. However, the acceleration and transport processes of the cosmic rays are not well understood, and how the collisionless shocks are created is still under investigation. Recent high-power and high-intensity laser technologies allow us to simulate astrophysical phenomena in laboratories. We present our experimental results of collisionless shock formations in laser-produced plasmas.

We have developed a computational code based on the axisymmetric Navier-Stokes equations with thermochemical kinetics for assessing wave drag reduction and other effects in pulse-energy deposition ahead of a bow shock by means of full simulations from generation of a laser-induced blast wave to interaction with the bow shock. Thermochemical nonequilibrium computations can reproduce the process of blast wave formation with laser ray tracing, and the computed low-density core inside the blast wave has a teardrop-like shape, depending on the laser input condition. The flowfield interacting with a bow shock formed in Mach 5 flow was computed. The result suggests that the shape of the low-density core affects the resultant wave drag, and parameters of an incident laser beam should be taken into account in exploring the optimal condition of the proposed wave-drag scheme.

To improve the in-air flight performance of a laser propulsion vehicle, it is helpful to develop a computational model that can reproduce full dynamics, including impulsive vehicle motion driven by interaction with a blast wave. The authors have developed a three-dimensional hydrodynamics code coupled with six-degree-of-freedom equations of motion of a laser propulsion vehicle for analyzing flight dynamics through numerically simulating flowfield interacting with unsteady motion of the vehicle. Using ray tracing, asymmetrically deposited energy corresponding to laser offsets was initially added to the flowfield around the vehicle to estimate beam riding mechanics against lateral or angular offset, and a combination of them for a single pulse. The centering performance of the "lightcraft" is excellent for lateral and combined offsets, but the recentering force becomes weak with a large angular offset. If the vehicle inclines toward the axis of the incident laser beam, an angular restoring moment is generated to reduce the angular offset. Also, flight dynamics simulations were performed for multiple pulses by integrating aerodynamic responses based on single-pulse results, and spiral-flight motion and deviation from the laser path were successfully reproduced. When the laser axis is rotated following the gyro motion of the vehicle, the angular restoring moment is maintained for a longer time, and the vehicle is able to fly to a higher altitude.

We report the experimental results of a turbulent electric field driven by Kelvin-Helmholtz instability associated with laser produced collisionless shock waves. By irradiating an aluminum double plane target with a high-power laser, counterstreaming plasma flows are generated. As the consequence of the two plasma interactions, two shock waves and the contact surface are excited. The shock electric field and transverse modulation of the contact surface are observed by proton radiography. Performing hydrodynamic simulations, we reproduce the time evolutions of the reverse shocks and the transverse modulation driven by Kelvin-Helmholtz instability.

We investigate a proto-neutron star kick velocity estimated from kinetic momentum of a flow around the proto-neutron star after the standing accretion shock instability grows. In this study, ten different types of random perturbations are imposed on the initial flow for each neutrino luminosity. We found that the kick velocities of proto-neutron star are widely distributed from 40 km s(-1) to 180 km s(-1) when the shock wave reaches 2000 km away from the center of the star. The average value of kick velocity is 115 km s(-1), whose value is smaller than the observational ones. The kick velocities do not depend on the neutrino luminosity.

For longer lifetime of electric propulsion system, an electrodeless plasma thruster with rotating electric field have been proposed utilizing a helicon plasma source. The rotating electric field may produce so-called Lissajous acceleration of helicon plasma in the presence of diverging magnetic field through a complicated mechanism originating from many parameters. Two-dimensional simulations of the Lissajous acceleration were conducted by a code based on Particle-In-Cell (PIC) method and Monte Carlo Collision (MCC) method for understanding plasma motion in acceleration area and for finding the optimal condition. Obtained results show that azimuthal current depends on ratio of electron drift radius to plasma region length, AC frequency, and axial magnetic field. When ratio of cyclotron frequency to the AC frequency is higher than unity, reduction of the azimuthal current by collision effect is little or nothing.

We developed a particle code based on the PIC-MCC method and a plasma fluid code solving drift-diffusion equations. Fluid simulation produces a streamer-type discharge for a positively biased input applied to the exposed electrode against to the buried electrode as obtained in particle simulation. However, we found differences in micro-discharge formation around the exposed electrode between the results of the two codes, suggesting that a hybrid approach based on particle and fluid models may be needed for a long-term precise simulation. Our hybrid code and perspective models are presented for reliably predicting the entire dynamics from micro-discharge to flow generation in a DBD plasma actuator.

We investigate explosive nucleosynthesis in a non-rotating 15 M <SUB>sun</SUB> star with solar metallicity that explodes by a neutrino-heating supernova (SN) mechanism aided by both standing accretion shock instability (SASI) and convection. To trigger explosions in our two-dimensional hydrodynamic simulations, we approximate the neutrino transport with a simple light-bulb scheme and systematically change the neutrino fluxes emitted from the protoneutron star. By a post-processing calculation, we evaluate abundances and masses of the SN ejecta for nuclei with a mass number &lt;=70, employing a large nuclear reaction network. Aspherical abundance distributions, which are observed in nearby core-collapse SN remnants, are obtained for the non-rotating spherically symmetric progenitor, due to the growth of a low-mode SASI. The abundance pattern of the SN ejecta is similar to that of the solar system for models whose masses range between (0.4-0.5) M <SUB>sun</SUB> of the ejecta from the inner region (&lt;=10, 000 km) of the precollapse core. For the models, the explosion energies and the <SUP>56</SUP>Ni masses are ~= 10<SUP>51</SUP>erg and (0.05-0.06) M <SUB>sun</SUB>, respectively; their estimated baryonic masses of the neutron star are comparable to the ones observed in neutron-star binaries. These findings may have little uncertainty because most of the ejecta is composed of matter that is heated via the shock wave and has relatively definite abundances. The abundance ratios for Ne, Mg, Si, and Fe observed in the Cygnus loop are reproduced well with the SN ejecta from an inner region of the 15 M <SUB>sun</SUB> progenitor....

By performing three-dimensional (3D) simulations that demonstrate the neutrino-driven core-collapse supernovae aided by the standing accretion shock instability (SASI), we study how the spiral modes of the SASI can impact the properties of the gravitational-wave (GW) emission. To see the effects of rotation in the nonlinear postbounce phase, we give a uniform rotation on the flow advecting from the outer boundary of the iron core, the specific angular momentum of which is assumed to agree with recent stellar evolution models. We compute fifteen 3D models in which the initial angular momentum and the input neutrino luminosities from the protoneutron star are changed in a systematic manner. By performing a ray-tracing analysis, we accurately estimate the GW amplitudes generated by anisotropic neutrino emission. Our results show that the gravitational waveforms from neutrinos in models that include rotation exhibit a common feature; otherwise, they vary much more stochastically in the absence of rotation. The breaking of the stochasticity stems from the excess of the neutrino emission parallel to the spin axis. This is because the compression of matter is more enhanced in the vicinity of the equatorial plane due to the growth of the spiral SASI modes, leading to the formation of the spiral flows circulating around the spin axis with higher temperatures. We point out that recently proposed future space interferometers like Fabry-Perot-type DECIGO would permit the detection of these signals for a Galactic supernova.

A time-dependent collisional-radiative model for air plasma has been developed to study the effects of non-equilibrium atomic and molecular processes on population densities in a weakly ionized high enthalpy flow. This model consists of 15 species: e(-), N, N+, N2+, O, O+, O2+, O-, N-2, N-2(+), NO, NO+, O-2, O-2(+), and O-2(-) with their major electronic excited states. Many elementary processes are considered in the number density range of 10(12)/cm(3) &lt;= N &lt;= 10(19)/cm(3) and the temperature range of 300 K &lt;= T &lt;= 40,000 K. We then compare our results with an existing collisional-radiative code to validate our model. Additionally, the unsteady nature of pulsively heated air plasma is investigated. When the ionization relaxation time is of the same order as the time scale of a heating pulse, the effects of unsteady ionization are important for estimating air plasma states. From parametric computations, we determine the appropriate conditions for the collisional-radiative steady state, local thermodynamic equilibrium, and corona equilibrium models in that density and temperature range.

Collisionless shocks in counter-streaming plasmas, created by the high-power laser system Gekko XII HIPER, are investigated. The shock structure and density are measured by optical diagnostics such as shadowgraphy, interferometry, and streaked interferometry. The plasma density and temperature are estimated from self-emission measurements with visible light by streaked optical pyrometer and gated optical imager. Brightness temperatures are calculated considering the efficiency of the detectors, and electron temperatures are estimated.

We have developed a three-dimensional hydrodynamics code coupling equation of motion of a rigid body for analyzing posture stability of laser propulsion vehicle through numerical simulations of flowfield interacting with unsteady motion of the vehicle. Asymmetric energy distribution is initially added around the focal spot (ring) in order to examine posture stability against an asymmetric blast wave resulting from a laser offset for a lightcraft-type vehicle. The vehicle moves to cancel out the offset from initial offset. However, the Euler angle grows and never returns to zero in a time scale of laser pulse. Also, we found that the vehicle moves to cancel tipping angle when the laser is irradiated to the vehicle with initial tipping angle over the wide angle range, through the vehicle cannot get sufficient restoring force in particular angle, and the tipping angle does not decrease from the initial value for that case.

Collisionless shock wave generation in counter-streaming plasmas for several target materials (Al, C, Cu, and Pb) is investigated using a high-power laser system. Counter-streaming plasmas are produced by irradiating an inner surface of a double-plane target. For Al, C, and Pb, a shock wave is observed in self-emission measurements similar to the previous experiment using a CH target [Y. Kuramitsu et al., J. Phys.: Conf. Series. 24, 042008 (2010)], and the width of the transition region is much shorter than the ion-ion collision mean-free-paths. The mean-free-paths tend to be longer for heavier materials, because the ionization degrees Z are not so different among these materials. © 2011 The Japan Society of Plasma Science and Nuclear Fusion Research.

An efficient implicit procedure for the Discontinuous Galerkin (DG) method is developed utilizing a pointwise relaxation algorithm. In the pointwise relaxation, those contributions from the degrees of freedom in own computational cell are accounted for in the implicit matrix inversion. The resulting scheme is shown to be stable with very large CFL numbers for both the Euler and the Navier-Stokes equations for typical test problems. In order to achieve a faster convergence, efforts are also made to reduce computing time of the present method by utilizing a p-multigrid scheme and also by solving a simplified matrix instead of a fully loaded dense matrix in the implicit matrix inversion. A superior performance of the present implicit DG method on the parallel computer using up to 128 PEs is shown in terms of readily achievable scalability and high parallel efficiency. The RANS simulation of turbulent flowfield over AGARD-B model is carried out to show the convergence property and numerical stability of the present implicit DG method for engineering applications.

The propagation of a laser-driven heat-wave into a Ti-doped aerogel target was investigated. The temporal evolution of the electron temperature was derived by means of Ti K-shell X-ray spectroscopy, and compared with two-dimensional radiation hydrodynamic simulations. Reasonable agreement was obtained in the early stage of the heat-wave propagation. In the later phase, laser absorption, the propagation of the heat-wave, and hydrodynamic motion interact in a complex manner, and the plasma is mostly re-heated by collision and stagnation at the target central axis. (C) 2009 Elsevier B.V. All rights reserved.

We present a computational method for solving time-dependent radiation moment equations with an Eddington tensor which represents an anisotropic field of radiation. For estimating the Eddington tensor, we propose a conical-ray method covering the whole solid angle with which the steady-state radiative transfer equation is solved. Computed radiation flux shows an excellent agreement with an analytical solution using the proposed conical-ray method even if a light source is located at a distant place from the computed point.

We propose a ray-tracing method to estimate gravitational waves (GWs) generated by anisotropic neutrino emission in supernova cores. To calculate the gravitational waveforms, we derive analytic formulae in a useful form, which are applicable also for three-dimensional computations. Pushed by evidence of slow rotation prior to core-collapse, we focus on asphericities in neutrino emission and matter motions outside the protoneutron star. Based on the two-dimensional models, which mimic standing accretion shock instability (SASI)-aided neutrino heating explosions, we compute the neutrino anisotropies via the ray-tracing method in a post-processing manner and calculate the resulting waveforms. For simplicity, neutrino absorption and emission by free nucleons, dominant processes outside the protoneutron stars, are only taken into account, while the neutrino scattering and the velocity-dependent terms in the transport equations are neglected. With these computations, it is found that the waveforms exhibit more variety in contrast to the ones previously estimated by the ray-by-ray analysis. In addition to a positively growing feature, which was predicted to determine the total wave amplitudes predominantly, the waveforms are shown to exhibit large negative growth for some epochs during the growth of SASI. These features are found to stem from the excess of neutrino emission in lateral directions, which can be precisely captured by the ray-tracing calculation. Reflecting the nature of SASI which grows chaotically with time, there is little systematic dependence of the input neutrino luminosities on the maximum wave amplitudes. Due to the negative contributions and the neutrino absorptions appropriately taken into account by the ray-tracing method, the wave amplitudes become more than one order of magnitude smaller than the previous estimation, thus making their detections very hard for a Galactic source. On the other hand, it is pointed out that the GW spectrum from matter motions have its peak near similar to 100 Hz, reflecting the SASI-induced matter overturns of O(10) ms. Such a feature could be characteristic for the SASI-induced supernova explosions. The proposed ray-tracing method will be useful for the GW prediction in the first generation of three-dimensional core-collapse supernova simulations that do not solve the angle-dependent neutrino transport equations as part of the numerical evolution.

The study of standing accretion shock instability (SASI) in core-collapse supernova cores has been done with three-dimensional (3D) computer simulations. Rotations with various perturbations were introduced from outer boundary of an initial steady accreting flow. We found that one or two armed spiral accreting flow onto the proto-neutron star (PNS) is formed inside the shock wave depending on perturbations. The linear growth of spiral modes are clearly diagnosed by the mode analysis of the shock surface, and the lower m modes grow quickly in the linear regime.

We present the results of numerical analysis of wave drag reduction by a single-pulse energy deposition in a supersonic flow field around a sphere. The wave drag for the sphere was reduced as a result of the interaction between a low-density core following the blast wave produced by the energy deposition and the bow shock developed in front of the sphere. We investigated the drag reduction mechanism in terms of the unsteady flow field induced by the interaction. The effects of deposited energy and deposition location on energy reduction were examined by parametric study. From the obtained results, we refined the parameters, utilizing the baroclinic source term that produced vorticity in the vortex equation when the gradients of density and pressure were not parallel. The baroclinic vortex driven by Richtmyer-Meshkov-like instability was strong enough to contribute to the temporary low-entropy shock formation that caused low wave drag for the supersonic object. We determined that the reduced energy had a linear dependence on the radius of the low-density core formed in the blast wave and was proportional to the square of the freestream Mach number. Such dependencies could be predicted with the assumption that the energy was consumed by the baroclinic vortex generation and advected downward without thermalization in an inviscid shock layer.

For the aim of computing compressible turbulent flowfield involving shock waves, an implicit large eddy simulation (LES) code has been developed based on the idea of monotonically integrated LES. We employ the weighted compact nonlinear scheme (WCNS) not only for capturing possible shock waves but also for attaining highly accurate resolution required for implicit LES. In order to show that WCNS is a proper choice for implicit LES, a two-dimensional homogeneous turbulence is first obtained by solving the Navier-Stokes equations for incompressible flow. We compare the inertial range in the computed energy spectrum with that obtained by the direct numerical simulation (DNS) and also those given by the different LES approaches. We then obtain the same homogeneous turbulence by solving the equations for compressible flow. It is shown that the present implicit LES can reproduce the inertial range in the energy spectrum given by DNS fairly well. A truncation of energy spectrum occurs naturally at high wavenumber limit indicating that dissipative effect is included properly in the present approach. A linear stability analysis for WCNS indicates that the third order interpolation determined in the upwind stencil introduces a large amount of numerical viscosity to stabilize the scheme, but the same interpolation makes the scheme weakly unstable for waves satisfying k Delta x approximate to 1. This weak instability results in a slight increase in the energy spectrum at high wavenumber limit. In the computed result of homogeneous turbulence, a fair correlation is shown to exist between the locations where the magnitude of del x omega becomes large and where the weighted combination of the third order interpolations in WCNS deviates from the optimum ratio to increase the amount of numerical viscosity. Therefore, the numerical viscosity involved in WCNS becomes large only at the locations where the subgrid-scale viscosity can arise in ordinary LES. This suggests the reason why the present implicit LES code using WCNS can resolve turbulent flowfield reasonably well.

We study the properties of gravitational waves (GWs) based on three-dimensional (3D) simulations, which demonstrate neutrino-driven explosions aided by standing accretion shock instability (SASI). Pushed by evidence supporting slow rotation prior to core collapse, we focus on the asphericities in neutrino emissions and matter motions outside the protoneutron star. By performing a ray-tracing calculation in 3D, we estimate accurately the gravitational waveforms from anisotropic neutrino emissions. In contrast to the previous work assuming axisymmetry, we find that the gravitational waveforms vary much more stochastically because the explosion anisotropies depend sensitively on the growth of SASI which develops chaotically in all directions. Our results show that the GW spectrum has its peak near similar to 100 Hz, reflecting SASI-induced matter overturns of similar to O(10) ms. We point out that the detection of such signals, possibly visible to the LIGO-class detectors for a Galactic supernova, could be an important probe into the long-veiled explosion mechanism.

We have studied nonaxisymmetric standing accretion shock instabilities, or SASI, using three-dimensional (3D) hydrodynamical simulations. This is an extension of our previous study of axisymmetric SASI. We have prepared a spherically symmetric and steady accretion flow through a standing shock wave onto a proto-neutron star, taking into account a realistic equation of state and neutrino heating and cooling. This unperturbed model is meant to represent approximately the typical postbounce phase of core-collapse supernovae. We then added a small perturbation (similar to 1%) to the radial velocity and computed the ensuing evolutions. Both axisymmetric and nonaxisymmetric perturbations have been imposed. We have applied mode analysis to the nonspherical deformation of the shock surface, using spherical harmonics. We have found that (1) the growth rates of SASI are degenerate with respect to the azimuthal index m of the spherical harmonics Y-l(m), just as expected for a spherically symmetric background; (2) nonlinear mode couplings produce only m = 0 modes for axisymmetric perturbations, whereas m not equal 0 modes are also generated in the nonaxisymmetric cases, according to the selection rule for quadratic couplings; (3) the nonlinear saturation level of each mode is lower in general for 3D than for 2D, because a larger number of modes contribute to turbulence in 3D; (4) low-l modes are dominant in the nonlinear phase; (5) equipartition is nearly established among different m modes in the nonlinear phase; (6) spectra with respect to l obey power laws with a slope slightly steeper for 3D; and (7) although these features are common to the models with and without a shock revival at the end of the simulation, the dominance of low-l modes is more remarkable in the models with a shock revival.

A trajectory-based analysis for obtaining aerodynamic heating environment for the Huygens probe is carried out using the thermochemical nonequilibrium computational fluid dynamics code. Radiative heat transfer is accounted for in computational fluid dynamics calculations where a modern multiband radiation model is employed. In this study, we first compare the radiative heat flux obtained by the ray-tracing approach in three-dimensional space with that given by the tangent-slab approximation, to determine how the difference in obtaining radiative heat flux can alter the overall aerodynamic heating environment. We then explore the radiative cooling effect on surface heat flux through radiation coupled computational fluid dynamics calculations. It is shown that the radiative heat flux value at the stagnation point obtained by the ray-tracing approach becomes about 17-19% smaller than that given by the tangent-slab approximation due to body curvature at all the chosen trajectory points. Furthermore, it is also shown that the radiative cooling effect can reduce the surface radiative heat flux by almost the same amount at the stagnation point when computational fluid dynamics calculation coupled with radiation is conducted. It is therefore confirmed that, even for the relatively lower radiative heating rate such as for the Huygens entry flight, we need to employ the ray-tracing approach instead of the tangent-slab approximation, and also need to account for radiative cooling effect through radiation coupled computational fluid dynamics calculation, to evaluate the surface heating condition accurately.

We have performed axisymmetric simulations in order to investigate the thrust generation resulting from the interference between the projectile and the blast wave produced by a pulsed laser. The results obtained by our numerical code well agree for the pressure history and the momentum coupling coefficient with the experimental data. In such analysis, it is found that the approximate impulse estimated only by the pressure history at the projectile base is difficult to predict the actual one. Since the shock wave rapidly attenuates in low fill pressure, and the interaction with the projectile almost finishes in the shroud, a high momentum coupling coefficient can be achieved unlike the case of high fill pressure in which the projectile experiences the subsequent negative thrust.

Standing accretion shock instability (SASI) is one of the candidates to solve the mystery of why we cannot reproduce the explosion with the present core-collapse supernova models. We have studied this phenomenon with including neutrino heating and realistic EOS and found that SASI may enhance neutrino heating. Although g-mode of proto-neutron star may enhance the SASI growth, the simulations just including the pressure perturbation as a mimic of g-mode induced sound wave reveal no significant effect on the shock dynamics. Moreover, we discuss the required conditions toward the possible laboratory experiment of SASI.

We studied the standing accretion shock instability (SASI) for a core-collapse supernova explosion. SASI induces a nonspherically symmetric motion of a standing spherical shock wave. In order to investigate the growth of SASI, we solved the three-dimensional compressible Euler equations using ZEUS-MP/2 code based on the finite-difference method with a staggered mesh of spherical polar geometry. Although the von Neumann and Richtmyer artificial viscosity is used in ZEUS-MP/2 code to capture shock waves, we propose utilizing a tensor artificial viscosity in order to overcome the numerical instability regarded as the carbuncle phenomenon. This numerical instability emerges around the grid polar axis and precludes mode analysis of SASI.

We have carried out atomistic simulations of grain-grain collisions for spherical grains of 1.4 and 4 nm radii, with relative velocities of 3.6-6.1 km/s and a number of impact parameters. Since the initial grains are crystallites without any pre-existing defects, grain shattering due to nucleation of cracks was not observed in our simulations. We find grain fusion in some events, but generally melting occurs, leading to nucleation, growth and linkage of voids in the melt, which then leads to production of small clusters. The size distribution does not obey a simple power law and can be considered as having four different regimes, where each regime can be fitted as a power law.

We present the results of the numerical simulation of laser-induced blast wave coupled with rate equations to clarify the unsteady property of ionization processes during pulse heating. From comparison with quasi-steady computations, the plasma region expands more widely, which is sustained by the inverse-bremsstrahlung since an ionization equilibrium does not establish at the front of the plasma region. The delayed relaxation leads to the rapid expansion of the driving plasma and enhances the energy conversion efficiency from a pulse heating laser to the blast wave.

We have observed supersonic heat wave propagation in a low-density aerogel target (rho similar to 3.2 mg/cc) irradiated at the intensity of 4 x 1014 W/cm(2). The heat wave propagation was measured with a time-resolved x-ray imaging diagnostics, and the results were compared with simulations made with the two-dimensional radiation-hydrodynamic code, RAICHO. Propagation velocity of the ionization front very slightly decreased as the wave propagates into the target. The reason of decrease is due to increase of laser absorption region as the front propagates and interplay of hydrodynamic motion and reflection of laser propagation in the target. These features are well reproduced with the simulation.

We present the results of numerical experiments, in which we have investigated the influence of the inelastic neutrino-helium interactions on the standing accretion shock instability supposed to occur in the postbounce supernova core. The axisymmetric hydrodynamical simulations of accretion flows through the standing accretion shock wave onto the proto-neutron star show that the interactions are relatively minor and the linear growth of the shock instability is hardly affected. The extra heating given by the inelastic reactions becomes important for the shock revival after the instability enters the nonlinear regime, but only when the neutrino luminosity is very close to the critical value at which the shock would be revived without the interactions. We have also studied the dependence of the results on the initial amplitudes of perturbation and the temperatures of mu and tau neutrinos.

The acoustic-revival mechanism in core-collapse supernovae, proposed recently by the Arizona group led by A. Burrows, is an interesting new scenario. With an aim to understanding the elementary processes involved in this mechanism, we have calculated the eigenfrequencies and eigenfunctions for the g-mode oscillations of a nonrotating proto-neutron star. The possible excitation of these modes by the standing accretion shock instability, or SASI, is discussed. We have formulated the forced oscillations of g-modes due to external pressure perturbations exerted on the proto-neutron star's surface. The driving pressure fluctuations have been adopted from our previous computations of axisymmetric SASI in the nonlinear regime. We pay particular attention to low-l modes, since these are the modes that are dominant in SASI and what Burrows et al. claim to have played an important role in their acoustic-revival scenario. Here l is the index of the spherical harmonic functions, Y-l(m). Although the frequency spectrum of nonlinear SASI is broadened substantially by nonlinear couplings, the typical frequency is still much smaller than those of the g-modes, leading to a severe impedance mismatch. As a result, the excitations of various g-modes are rather inefficient, and the energy of the saturated g-modes is similar to 10(50) ergs or less, with the g(2) mode being the largest in our model. Here the g(2) mode has two radial nodes and is confined to the interior of the convective region. The energy transfer rate from the g-modes to outgoing sound waves is estimated from the growth of the g-modes to be similar to 10(51) ergs s(-1) in the models studied in this paper.

Two-dimensional (2D) radiation hydrodynamics simulations have been performed to investigate the generation and the refraction influence in the plasma medium of a fully coherent x-ray laser at 13.9 nm by the double-target configuration. The local energy deposition of the main laser pulse generates a blast wave near the critical density surface and the density dip structure is gradually formed behind the blast wave. The size of the density dip structure is about 10 mu m after 50 ps of the main pulse. The three-dimension (3D) ray-trace calculation using the result of the 2D simulation shows the x-rays pass through the density dip with less refraction. The size and the position of the density dip area are similar to the light source of the fully coherent x-ray laser.

We present the results of numerical experiments in which we study how asphericities induced by the growth of the standing accretion shock instability ( SASI) produce gravitational waveforms in the postbounce phase of core-collapse supernovae. To obtain the neutrino-driven explosions, we parameterize the neutrino fluxes emitted from the central proto-neutron star and approximate the neutrino transfer by a light-bulb scheme. We find that the waveforms due to anisotropic neutrino emissions show a monotonic increase with time, whose amplitudes are up to 2 orders of magnitude larger than those from convective matter motions outside proto-neutron stars. We point out that the amplitudes begin to become larger when the growth of the SASI enters the nonlinear phase, in which the deformation of the shocks and the neutrino anisotropy become large. From the spectrum analysis of the waveforms, we find that the amplitudes from the neutrinos are dominant over those from the matter motions at frequencies below similar to 100 Hz, which should be within the detection limits of next-generation detectors such as LCGT and the advanced LIGO for a supernova at 10 kpc. As a contribution to the gravitational wave background, we show that the amplitudes from this source could be larger at frequencies above similar to 1 Hz than the primordial gravitational wave backgrounds but, unfortunately, invisible to the proposed space-based detectors.

Two-dimensional (2D) radiation hydrodynamics simulations have been performed to investigate the refraction influence and the gain property in the plasma medium of the x-ray laser. The local energy deposition of the main pumping pulse generates a blast wave near the critical density surface, and the density dip structure is gradually formed behind the blast wave. The three-dimensional (3D) ray-trace calculation using the result of the 2D simulation shows the x-rays pass through the density dip with less refraction.

We have numerically studied the instability of the spherically symmetric standing accretion shock wave against nonspherical perturbations. We have in mind the application to collapse-driven supernovae in the postbounce phase, where the prompt shock wave generated by core bounce is commonly stalled. We take an experimental standpoint in this paper. Using spherically symmetric, completely steady, shocked accretion flows as unperturbed states, we have clearly observed both the linear growth and the subsequent nonlinear saturation of the instability. In so doing, we have employed a realistic equation of state, together with heating and cooling via neutrino reactions with nucleons. We have performed a mode analysis based on the spherical harmonics decomposition and found that the modes with l 1; 2 are dominant not only in the linear regime but also after nonlinear couplings generate various modes and saturation occurs. By varying the neutrino luminosity, we have constructed unperturbed states both with and without a negative entropy gradient. We have found that in both cases the growth of the instability is similar, suggesting that convection does not play a dominant role, which also appears to be supported by the recent linear analysis of the convection in accretion flows by Foglizzo et al. The oscillation period of the unstable l 1 mode is found to fit better with the advection time rather than with the sound crossing time. Whatever the cause may be, the instability favors a shock revival.

To achieve a higher thrust performance in the laser-driven in-tube accelerator operation, numerical analysises have been carried out. The computational code covers from the generation of the blast wave to its interactions with the projectile and the acceleration wall. The thrust history and the momentum coupling coefficient evaluated from the numerical simulation depend on the fill pressure and the projectile shape. The confinement effect can be clearly found using the projectile attached with a shroud.

Flow visualizations of the interaction between a laser-pulse-generated plasma and a shock wave driven by it have been experimentally conducted. The configuration of the experimental set-up corresponds to the laser-driven, in-tube accelerator. Primary-mode deformation of the plasma is governed by Richtrnyer-Meshkov instability which is produced by the vector product between the pressure and density gradients, which in turn correspond to a reflected shock wave and to the plasma, respectively. Higher-mode contact surface deformations are supposedly originated in Rayleigh-Taylor instability in the shrinkage phase of the plasma, and is enhanced due to the passage of the reflected shock wave.

We report a current status of our radiation-magnetohydrodynamic code for the study of core-collapse supernovae. In this contribution, we discuss the accuracy of our newly developed numerical code by presenting the test problem in a static background model. We also present the application to the spherically symmetric core-collapse simulations. Since close comparison with the previously published models is made, we are now applying it for the study of magnetorotational core-collapse supemovae.

We report a current status of our radiation-magnetohydrodynamic code for the study of core-collapse supernovae. In this contribution, we discuss the accuracy of our newly developed numerical code by presenting the test problem in a static background model. We also present the application to the spherically symmetric core-collapse simulations. Since close comparison with the previously published models is made, we are now applying it for the study of magnetorotational core-collapse supernovae.

近年, DengらによってWeighted Compact Nonlinear Scheme (WCNS) と呼ばれる高次精度スキームが開発された.このWCNSは, 数値振動を生じることなく衝撃波を単調に捉えることができるため, 陰的LESに適用できると考えられる.本研究では, WCNSを用いた陰的LESによりBurgers乱流の数値計算を行い, エネルギースペクトルを求めた.得られたエネルギースペクトルはDNSの結果と良い一致を示したが, 従来型のLESでは, 高波数域に非物理的な振る舞いが現れた.

Two schemes that are designed to capture contact surface sharply are examined in this study. The first one is an overset mesh method in which a submesh system moves on the background Cartesian mesh system and follows the contact surface. The other scheme is a fully conservative Eulerian scheme that introduces two inert gases to identify the coexisting region where two gases coexist in a computational cell. The numerical flux function at the boundary of coexisting region is then modified bill extrapolation. The computed results for the Richtmyer-Meshkov instability problem indicate that these two schemes can capture contact surface quite sharply and accurately.

A trajectory-based heating analysis of the Galileo probe entry flowfield is attempted to reproduce the heat-shield recession data obtained during the entry flight. In the calculation, the mass conservation equations for the freestream gas (hydrogen-helium gas mixture) and the ablation product gas are solved with an assumption of thermochemical equilibrium. The ablation process is assumed to be quasi steady and is coupled with the flowfield calculation. The radiative energy transfer calculation is tightly coupled with the flowfield calculation, where the absorption coefficients of the gas mixture are given by the multiband radiation model having 4181 wavelength points for wavelength range from 750 to 15,000 Angstrom. The injection-induced turbulence model proposed by Park is employed to account for the enhanced turbulence effect due to the ablation product gas. It is shown that the final recession profile of the flight data at the frustum region can be closely reproduced if the injection-induced turbulence model is employed, although that at the stagnation region is overestimated. The cause of the enhanced radiative heating that occurs at the frustum region is given in connection with the enhanced turbulence effect in the shock layer.

A scheme to suppress the ablative Rayleigh-Taylor (RT) instability using high-Z doped plastic target (brominated polystyrene;CHBr) has been proposed for a directly laser-driven IFE target. When an intense laser irradiates directly onto a high-Z doped target, radiation emitted from a corona plasma propagates and deposits locally its energy inside the target. The enhanced radiation forms the double-ablation structure, which consists of primaryelectron conduction ablation front and secondary radiative ablation front. The radiative ablation in the double-ablation structure has many advantages to suppress the growth of the RT instability in analogy of the indirect-drive approach, i.e. large mass ablation rate, long density scale length and low peak density. Two-dimensional (2D) hydrodynamic simulation code shows strong suppression of the RT instability in a brominated plastic (CHBr) target compared with that in an undoped polystyrene (CH) target. RT growth rates evaluated theoretically using the Betti-Goncharov procedure with one-dimensional(1D) radiation-hydrodynamic simulation are in good agreement with 2D simulation results. Several experiments were performed at the GEKKO XII- HIPER (High Intensity Plasma Experimental Research) laser facility. A trajectory of a laser-driven CHBr target observed in experiment was reproduced fairly well by 1D simulation code. The double-ablation structure formed inside a directly laser-driven CHBr target was clearly observed in experiments for the first time The strong suppression of the RT instability in the CHBr target was confirmed in experiments with face-on and side-on x-ray backlighting technique.

A scheme to suppress the Rayleigh-Taylor instability has been investigated for a direct-drive inertial fusion target. In a high-Z doped-plastic target, two ablation surfaces are formed separately-one driven by thermal radiation and the other driven by electron conduction. The growth of the Rayleigh-Taylor instability is significantly suppressed on the radiation-driven ablation surface inside the target due to the large ablation velocity and long density scale length. A significant reduction of the growth rate was observed in simulations and experiments using a brominated plastic target. A new direct-drive pellet was designed using this scheme.

Suppression of hydrodynamic instabilities is very crucial for the ultimate goal of inertial fusion energy (IFE). A high-Z doped plastic of CHBr (brominated polystyrene) ablator is a very promising candidate to suppress the ablative Rayleigh-Taylor (RT) instability in a directly laser-driven IFE target. When a CHBr target is irradiated by intense laser beams, bromine atoms in the corona plasma emit strong radiation. The strong radiation drives the radiative ablation front inside the CHBr targets. This radiative ablation in the high-Z doped plastic target has many advantages for the suppression of the growth of the RT instability in analogy to the indirect-drive approach, i.e., large mass ablation rate, long density scale length and low peak density. Two-dimensional (2D) hydrodynamic simulation shows significant suppression of the RT instability in a CHBr target compared to an undoped polystyrene (CH) target. RT growth rate, calculated theoretically using the Betti-Goncharov procedure with a one-dimensional radiation-hydrodynamic simulation code, is in good agreement with the 2D calculations. Experiments were performed at the GEKKO XII- [C. Yamanaka , IEEE J. Quantum Electron. QE-17, 1639 (1981)] HIPER (High Intensity Plasma Experimental Research) laser facility. The trajectory of a laser-driven CHBr target observed in the experiment was reproduced fairly well by the simulation. The radiative ablation front formed inside a directly laser-driven CHBr target was clearly observed for the first time. The strong suppression of the RT instability in the CHBr target was confirmed using the face-on and side-on x-ray backlighting technique. The high-Z doped ablator can be applied to high density cryogenic deuterium-deuterium and deuterium-tritium compression, because the hydrogen-isotopes are nearly transparent to x rays, which are transmitted through the ablator from the laser-irradiation side. (C) 2004 American Institute of Physics.

Two-dimensional numerical simulations of an accretion flow in a close binary system are performed by solving the Euler equations with radiative transfer. In the present study, the specific heat ratio is assumed to be constant while the radiative cooling effect is included as a non-adiabatic process. The cooling effect of the disc is considered by discharging energy in the vertical directions from the top and bottom surfaces of the disc. We use the flux-limited diffusion approximation to calculate the radiative heat flux values. Our calculations show that a disc structure appears and spiral shocks are formed on the disc. These features are similar to those observed in the case of an adiabatic gas with a lower specific heat ratio, gamma= 1.01. It is found that, when the radiative cooling effect is accounted for, the mass of the disc becomes larger than that assuming gamma= 5/3, and smaller than that assuming gamma= 1.01. We conclude that employing an adiabatic gas with a lower specific heat ratio is almost a valid assumption for simulating an accretion disc with the radiative cooling effect.

Physics of the inertial fusion is based on a variety of elements such as compressible hydrodynamics, radiation transport, non-ideal equation of state, non-LTE atomic process, and laser plasma interaction. In addition, implosion process is not in stationary state and fluid dynamics, energy transport and instabilities should be solved simultaneously. In order to study such complex physics, an integrated implosion code including all physics important in the implosion process should be developed. Before starting this work, an integrated code based on Hirt's ALE method had been developed. But it needed sophisticated rezoning/remapping algorithm and less dissipative ALE method in hardly distorted mesh. In this work, we have developed 2-D integrated implosion code based on CIP method which was described in ALE formation. In the IFE research, the fast ignition scheme is one of the epoch making new scheme. In the scheme, the formation of the high density core plasma is one of the problem to be solved. In this paper non-spherical implosion for fast ignition is solved using the integrated code.

This paper presents a new parallel strategy for radiative transfer calculation coupled with hypersonic flow computations. The cell-interface division parallel strategy is developed based on the distributed-memory parallelization, and applied to the calculation of three-dimensional radiative transfer in axisymetric hypersonic flowfield around the forebody of Fire II vehicle. The developed-parallel code realizes-fair scalability up to 128 processors. The computational speed achieved by the present parallel strategy using 128 processors of SGI ORIGIN 2000 is approximately 20 Gflops that is 115 times faster than that of a single processor. It is shown that cell-interface division strategy is suitable for large-scale radiation coupling calculations.

Indirect-direct-hybrid irradiation scheme has been proposed for suppressing the initial imprint of the laser irradiation nonuniformities. The target is irradiated by a low intensity x-ray radiation prior to the direct-drive laser pulse. The x-ray irradiation generates a plasma expansion layer on the target surface. The thermal smoothing effect is expected to take place in the preformed plasma when the direct-drive laser pulse comes onto the target, and then, the initial imprint can be significantly reduced. Planar target experiments on the indirect-direct-hybrid irradiation scheme were performed. The preformed plasma profile was measured by using x-ray side-on backlighting method. The reduction of the initial imprint was demonstrated by the indirect-direct-hybrid irradiation scheme on planar target experiments. The imprint is suppressed by a factor of 1.5-7 depending on x-ray preirradiation conditions. Results are in good agreement with the cloudy-day model with parameters derived from one-dimensional simulation. (C) 2002 American Institute of Physics.

The initial imprint caused by the spatial non-uniformity of laser intensity which generates a mass perturbation is one of the critical issues for directly driven laser fusion research. To mitigate such laser imprint a foam-hybrid target, which has low-density foam layer and is coated with high-Z material, was proposed. In such targets, the X-ray radiation plays a significant role in the formation of the initial imprint. For short wavelength perturbations, a clear suppression of the perturbation growth is observed in the foam-hybrid target. The perturbation growth is reduced by radiation preheating since the ablation surface is smoothed. Moreover, the emitted X-rays from the coated high-Z material suppresses the hydrodynamic instability at the interface of plastic and foam. However, the ablation structure is sensitive to the opacity model. Thus, it is important to analyze the role different radiation models play in the hydrodynamics. In this work we will compare both CRE and LTE models. (C) 2001 Elsevier Science Ltd. All rights reserved.

The initial imprint of mass perturbation on a target due to the spatial nonuniformity of laser intensity is a critical issue for laser fusion research. A foam-hybrid target with a low-density foam layer, and that was coated with high-Z material, was proposed to mitigate the laser imprint. In such a target, x-ray radiation played significant roles in the formation of the initial imprint. The perturbation in the absorbed laser energy is diffused by the radiation transport. For short wavelengths, a clear suppression of the perturbation growth was observed in the foam-hybrid target, although the growth rate of Rayleigh-Taylor instability was almost the same as that observed in conventional plastic targets. Hydrodynamic instability at the interface of the material was also suppressed with high-Z coating. Good agreement between the numerical results and the experimental findings suggests the validity of radiation hydrodynamic modeling.

The initial imprint of mass perturbation due to spatial nonuniformity of laser intensity is one of the most important issues in laser fusion research. Several imprint mitigation schemes by the use of soft X-ray radiation have been proposed to enhance the thermal smoothing effect within the conduction region. One of the schemes uses external X-ray irradiation prior to laser incidence to produce preformed plasma. Another has a low-density foam layer and high-Z material to heat the foam radiatively and make it uniform in density.

A new fusion capsule drive scheme was investigated. The capsule is illuminated by a low intensity thermal X ray pulse prior to the main drive pulse. This leads to a noticeable suppression of initial imprinting by the drive beam because of thermal smoothing in the preformed plasma. Of the several types of indirect-direct hybrid target, the authors investigated the hybrid effect for two types. One is a foam hybrid, in which the fuel capsule has a low density foam layer attached directly on its surface and where pulsed radiation generated from a thin high Z layer on the foam propagates through the foam, creating a preformed plasma. The other is an external hybrid, in which the capsule is illuminated by external X ray radiation generated using different beams from the capsule drive beams. The hybrid effect was demonstrated for both types by imposing an initial imprint on a planar target with an intensity modulated beam, and subsequent non-uniformity growth due to Rayleigh-Taylor instability was observed by means of face-on backlighting. The observed suppression due to the presence of the preformed plasma is interpreted by a cloudy day model for both hybrids. Capsule implosion experiments have also been started. The overall implosion dynamics observed is replicated by 1-D hydrocode simulations. Preliminary results from the implosion experiments are presented.

The initial-imprint of density perturbation due to spatial nonuniformity of laser intensity is one of the most important issues in laser fusion research. Several imprint mitigation scheme by means of soft x-ray radiation have been proposed to reduce the induced perturbation through the thermal conduction region. One of the schemes uses an external x-ray source prior to laser incidence to produce preformed plasma. Another has a low-density foam layer and high-Z material to heat the foam radiatively and make it uniform. We present the dynamics of these schemes and the perturbation growth with nonuniform laser from the results of two-dimensional simulation using our integrated code.

Recent work on laser produced plasmas is presented focusing on the theoretical studies of four topics which have been carried out at the Institute of Laser Engineering (ILE), Osaka University, after a brief explanation of the six physics issues to be studied for plasma physics related to the laser fusion. The importance of integrated code development is emphasized and it is shown that the growth of the Rayleigh-Taylor instability at the ablation front is reduced partly due to non-local electron transport. Non-uniformity of implosion dynamics is studied by comparing a two-dimensional simulation with a spectroscopic post-processing package. It is concluded that all the available experimental data are consistently explained. The fast-ignition with ultra-high power laser is studied. The ignition criteria are clarified with a two-dimensional burning wave simulation. The possibility to study astrophysics by designing model experiments with intense lasers is introduced. The computational design of models of the ejecta-ring collision of SN1987A is shown by comparing with the event in the space.

Present status of ignition and high-gain target design is presented. A simple scalings of target design by one-dimensional hydrodynamic code is shown in the range from 1 kJ to 10 MJ of driver energy. The sensitivity of the pellet gain is studied with a two-dimensional code and the Rayleigh-Taylor growth factor is evaluated. It is concluded that the mixing may play an important role even in the present high-gain target design. As a way to ignite a mixed, compressed core externally, we have studied an ignition condition for the fast ignition scheme with two dimensional hydrodynamic code. Requirement for heating the spark is discussed to find the energy and duration of heating pulse. (C) 1999 Elsevier Science S.A. All rights reserved.

In Inertial Confinement Fusion (ICF) studies, the development of a multi-dimensional hydrodynamic computational code with the sophisticated physics of high temperatures and densities is essential to promote the research. A two-dimensional integrated ICF simulation code, ILESTA-2D, which includes physical models of thermal relaxation, multi-group radiation transport, realistic equation of state, laser absorption, etc. has been developed by one of the authors. Recently, some modifications of the code have been carried out to improve the reliability of numerical simulations and increase the robustness against numerical algorithms. In this work, two parts of the numerical methods were mainly improved. One is a rezoning/remapping algorithm, in which the Arbitrary Lagrangian Eulerian method (ALE) was applied to keep accuracy in space and time without a tangled computational mesh. Also, the use of numerical algorithms in solving diffusion-type equations was improved with a nine-point approximation algorithm. These numerical algorithms and the computational results of Rayleigh-Taylor instability solved by the new ILESTA-2D are presented in this paper. (C) 1999 Elsevier Science S.A. All rights reserved.

When an intense laser light is irradiated on a solid target, high temperature (a few keV) and high density (g/cm(3)-mg/cm(3)) plasmas are produced. The physics of the plasmas has been studied intensively relating to the laser fusion research. The physics is the same as that of plasmas inside and near the stellar objects. We briefly describe the physics of laser plasmas and the possibility of studying astrophysical plasmas with the laser plasmas.