Numerical Analysis of Expanding Nonequilibrium Plasma in a Gas-Driven Laser Propulsion

Abstract

Understanding the dynamics of laser-produced plasma is essentially important for in- creasing available thrust force in a gas-driven laser propulsion system such as laser-driven in-tube accelerator. A computer code is developed to explore the formation of expand- ing nonequilibrium plasma produced by laser irradiation. Various properties of the blast wave driven by the nonequilibrium plasma are examined. It is found that the blast wave propagation is substantially affected by radiative cooling effect for lower density case. Recently, laser propulsion systems that acquire thrust force by laser en- ergy are extensively examined by using various facilities. A typical example of such gas-driven laser propulsion system is the Laser-driven In-Tube Accelerator: LITA, 1 which is developed at the Institute of Fluid Science, Tohoku University. In this propulsion system, relatively high thrust can be obtained by placing a projectile in an acceleration tube which is filled by inert propellant gas. 2,3 A schematic illustration of LITA operation is shown in Fig. 1. The projectile placed in the acceleration tube consists of a centerbody, a shroud and struts. Since the base of the centerbody is shaped as parabola, the incident pulse laser beam from the rear side of the projectile is reflected at the base and focused behind the centerbody. Then a breakdown occurs at the focal point and free electrons appear. These free electrons absorb the laser beam energy by inverse bremsstrahlung. As a result, high temperature and high pressure plasma core region is formed, which then induces a blast wave. This blast wave impinges on the projectile base and a thrust force is given to the projectile. Therefore the performance of the gas-driven laser propulsion system highly depends on the blast wave dynamics as well as on the plasma state that sustains the blast wave. In our previous work, a two-dimensional computational method was devel- oped to clarify the physical phenomena in the LITA acceleration tube, where a blast wave was sustained by the laser-produced nonequilibrium plasma. 4 Al- though various features of the blast wave were successfully reproduced, the computed pressure value at the acceleration tube wall was found to be sub- stantially higher than that measured in the experiment. A possible cause of this discrepancy is suggested in Fig. 2, where the emission spectra from the focal point obtained in the experiment are compared with that