[en] A thrust power of a gas-driven laser-propulsion system is obtained through interaction with a propellant gas heated by a laser energy. Therefore, understanding the nonequilibrium nature of laser-produced plasma is essential for increasing available thrust force and for improving energy conversion efficiency from a laser to a propellant gas. In this work, a time-dependent collisional-radiative model for air plasma has been developed to study the effects of nonequilibrium atomic and molecular processes on population densities for an air-driven type laser propulsion. Many elementary processes are considered in the number density range of 10¹²/cm³≤N≤10¹⁹/cm³ and the temperature range of 300 K≤T≤40,000 K. We then compute the unsteady nature of pulsively heated air plasma. When the ionization relaxation time is 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.

[en] 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

[en] The initial imprint of mass perturbation to a target due to spatial nonuniformity of laser intensity is one of critical issues for laser fusion targets. The foam-hybrid target, which has low-density foam layer and is coated high-Z material, has been proposed to mitigate the laser imprint. In such a target, the x-ray radiation plays significant roles on the formation of the initial imprint. One of the radiation effects is to preheat the foam layer. The wave-front perturbation of the rippled shock attenuates faster than the case without preheating. The ablation structure is also affected by radiation transport. This leads to the suppression of the perturbation grows on the ablation front. (author)

[en] 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. (paper)

[en] Fundamentals of radiation hydrodynamics simulation applied so far to astrophysical plasmas and laser-produced plasmas and its set of equations to be solved are reviewed. Typical methods for the radiation transfer calculation; their merits and demerits are discussed. At present, the simulation based on the radiation moment equations is most popular. Various numerical methods such as the flux-limited diffusion method, P₁ method, P_1/3 method, the maximum entropy method, Levermore-Pomraning method and M₁ method are introduced. They depend on different evaluations of the Eddington factor. (author)

[en] 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. (author)

[en] 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

[en] Present status and plan for developing an integrated implosion code are briefly explained by focusing on motivation, numerical scheme and issues to be developed more. Highly nonlinear stage of Rayleigh-Taylor instability of ablation front by laser irradiation has been simulated so as to be compared with model experiments. Improvement in transport and rezoning/remapping algorithms in ILESTA code is described. (author)

[en] The initial imprint of density perturbation due to spatial non-uniformity of laser intensity is one of the most important issues in laser fusion research. The imprint mitigation scheme by use of soft x-ray radiation has been proposed to reduce the laser non-uniformity through the thermal conduction region. We present the results of two-dimensional simulation concerning this issue using our integrated code. (author)

[en] 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. (author)