38 / 2020-02-12 00:08:29

Polarization-dependent absorption of laser radiation in an inhomogeneous underdense plasma

laser plasma interaction,Stimulated Raman Scattering,two-plasmon decay,shock ignition,kinetic simulation

全体主题 > 激光和粒子束聚变

Kinetic plasma simulations in a multi-dimensional geometry and with realistic parameters are essential for detailed understanding of the processes of laser absorption, hot electron generation and development of parametric instabilities. In the domain of parameters relevant for launching the strong shock wave in the shock ignition scheme, these processes play a very significant role. We study these processes in the domain of parameters corresponding to recent experiments at PALS laser facility with a relatively high laser irradiance in two- and three-dimensional geometry using particle-in-cell code EPOCH.
The excitation of plasma waves and electron acceleration are taking place in plasma density filaments which develop in the initial phase of laser-plasma interaction. Different parametric instabilities dominate the interaction depending on the direction of propagation of the daughter waves with respect to the laser field polarization (electric field vector). In the plane containing laser electric field the interaction is dominated by the two-plasmon decay instability and the beating of large amplitude electron plasma waves produces periodic ion density perturbations. In this case, stimulated Raman scattering is inefficient in the quarter critical density zone and it takes place in a less dense plasma only weakly contributing to laser energy absorption. In the plane perpendicular to laser polarization, two-plasmon decay is suppressed and the stimulated Raman scattering dominates. Strong Raman side-scattering is suppressed as its amplification length is limited by the size of the density filaments while the backscattering from the quarter critical density is found to be the dominant mechanism producing short intense bursts of backscattered light. These bursts eventually decay into density cavities trapping the backscattered light. Two-dimensional simulations demonstrate that a stronger absorption and heating of hot electrons takes place in the plane perpendicular to laser polarization leading to asymmetry in hot electron energy distribution. This assertion is confirmed in the dedicated three-dimensional simulations.