Relative Significance of the Stimulated Raman Scattering and Two-Plasmon--Decay Instabilities at Quarter-Critical Density

In direct-drive experiments on OMEGA, correlated signals of half-harmonic light and hot-electron production have usually been ascribed to two-plasmon decay (TPD). However, as scale lengths and temperatures increase, absolute stimulated Raman scattering (SRS) is expected to play a larger role in generating hot electrons and half-harmonic light. This may already be occurring in more-recent OMEGA experiments.\footnote{W. Seka \textit{et al}., Phys. Rev. Lett. \textbf{112}, 145001 (2014). } Both instabilities occur at quarter-critical density, and for obliquely incident light, they can merge into a ``hybrid'' instability with a threshold differing from SRS and TPD thresholds considered separately.\footnote{ B. B. Afeyan and E. A. Williams, Phys. Plasmas \textbf{4}, 3845 (1997).} This talk analyzes how the thresholds of the quarter-critical instabilities vary with the incidence angle and polarization of the incident light, as well as the plasma parameters, and the expected significance for direct-drive experiments. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.