Numerical Investigation of Two Plasmon Decay Instability with Zakharov Model

Jiayu Zhang, Ph.D. Defense

Hopeman 224

Two plasmon decay instability (TPD) is a three wave parametric decay instability where a light wave decays into two electron plasma waves (plasmons). It is important in direct-drive inertial confinement fusion (ICF) because large amplitude plasma waves are able to generate hot electrons. These electrons might penetrate and preheat the ignition target while success of the scheme requires that the fuel is sufficiently cold. We investigated the TPD instability in 3D geometry driven by multiple laser beams for the first time with a new developed code called ZAK3D. ZAK3D is a fluid code solving driven and extended Zakharov equations. The stability properties and nonlinear evolution of TPD have been examined for various laser beam configurations that are relevant to ICF experiments. We find that absolute threshold of the TPD instability is determined by the total laser intensity of multiple beams if they could drive a common Langmuir wave in small wave number region.

Half harmonic emission is one of the few important experimental diagnostics of the TPD instability. Historically, analysis of half harmonic emission has proven to be very difficult because of its complicated generation mechanisms. By extending ZAK3D, we developed a self-consistent model, which can simultaneously calculate parametric instabilities and the generation of half harmonic light wave via linear mode conversion, nonlinear mode conversion, Thomson down scattering and stimulated Raman scattering. In the regime of low laser intensities (below TPD absolute threshold), we observed a two-peaked feature which is consistent with some features of OMEGA experiments, while in the high laser intensity regime (above TPD absolute threshold), half harmonic emission spectrum becomes broader. These results have been compared with experiments.