On Stimulated Scattering of Laser Light in Inertial Fusion Energy Targets

Abstract

Propagation of a laser light through regions of an underdense plasma is an active research topic in laser fusion. In particular, a large effort has been invested in studies of stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS), which can reflect laser energy and produce energetic particles to preheat a fusion energy target. Experiments, theory, and simulations agree on a complex interplay between various laser-plasma instabilities. By particle-in-cell simulations of an underdense electron plasma, apart from the standard SRS, a strong backscattering was found near the electron plasma frequency at densities beyond the quarter critical. This novel instability, recognized in recent experiments as stimulated laser scattering on a trapped electron-acoustic mode (SEAS), is absent from a classical theory of laser-parametric instabilities. A parametric excitation of SEAS instability is explained by a three-wave resonant decay of the incident laser light into a standing backscattered wave and a slow trapped electron-acoustic wave ([omega] < [omega]_p). Large SEAS pulsations, eventually suppressed by relativistic heating of electrons, are observed in these simulations. This phenomenon seems relevant to future hohlraum target and fast ignition experiments.