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 LT 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.