K. Mima

R. Horiuchi

S. Miyamoto

T. Sato

National Institute for Fusion Science (NIFS-Repository)

§20. Simulation on Laser-Plasma Interaction and Fluid Dynamics in Inertial Confinement Fusion Mirna, K., Miyamoto, S. (Inst. of Laser Eng., Osaka Univ.) Horiuchi, R. and Sato, T. When the laser photon pressure exceeds the plasma pressure, the ultra intense laser pulse accelerates ions by the photon pressure [1]. To investigate the ion acceleration mechanism, we have done the 1-D PIC simulation which includes three species of particle (electron, deuteron and carbon). Figure 1 shows simulation results of carbon phase space plot for the intensity IL = 6.3 X 1019W I cm2 . In the figure, the laser is injected from left side, so that the plasma is pushed to right and compressed. From the carbon phase plot, it is clearly seen that a shock wave is formed and ions are reflected at the shock front (waxlc = 30.5). The reflection of ion is due to the jump of potential at the shock front. The ions in the upstream of the shock mainly flows into the downstream. But, owing to the thermal energy spread, a part of ion can't pass through the potential barrire so that it is reflected. (c) 20 IQ-3r--:----:-----.,---..,.....----, 15 1 (.) 101 s -o.." 5.0 1 ro x/c 0 Fig. 1. Simulation result for shock acceleration at IL = 6.3 X 1019 Wicm2 , W 0 i = 258. Figure 2 shows the results for IL = 3.97 X 1021 WI cm2 . The ion phase plot is completery different from that in Fig. 1. In this case, 270 the ion is reflected at the laser-plasma interface (waxlc = 34). (c) 0.1 0.0 0.0 (.) s 0.0 -0: 0.0 -0.02 30 ro x/c 0 Fig. 2. Simulation result for direct acceleration at IL = 3.97 X 1021 Wicm2 , W 0 t = 184. The transition of ion acceleration mechanism from the shock acceleration to the direct accel-eration is explained by the excess of the shock Mach Rumber over the critical Mach number [2]. The mach number M must satisfy the condition 1 < M < 1.6 (1) As increased the laser intensity, plasma pres-sure increases in proportion to the electron tem-prature. But the laser radiation pressure in-creases more rapidly than the plasma pressure, the Mach number increases. Then the Mach number is out of the range of eq. 1 and the accel-eration mechanism changes from the shock ac-celeration to the direct acceleration. In conclusion, we found that the ion accelera-tion mechanism changes from the shock acceler-ation to the direct acceleration. As increases the laser intensity, the ion acceleration mechanism changes at the critical laser intensity a2 ~ 100. In the case of the shock acceleration either ion acceleration energy or conversion efficiency is higher than those in the case of the direct ac-celeration. Refarences 1) J. Denavit, Phys. Rev. Lett. 69, 3052 (1992). 2) F. F. Chen, Introduction to Plasma Physics, (Plenum Press, 1974). 

Simulation on Laser-Plasma Interaction and Fluid Dynamics in Inertial Confinement Fusion

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