Coupled modes analysis of SRS backscattering, with Langmuir decay and possible cascadings

Abstract

Recent experiments aimed at understanding stimulated Raman scattering (SRS) in ICF laser-plasma interactions, suggest that SRS is coupled to the Langmuir decay interaction (LDI). The effects of LDI on the saturation of the SRS backscattering have been investigated, considering typical parameters from recent experiments. Detailed simulations with the coupled mode equations in a finite length plasma, with real wave envelopes and no wave dephasing, are explored here for the first time. A detailed description and analysis of such simulations is provided. The excitation of LDI is found to reduce the SRS reflectivity; the reduction is appreciable in the weak EPW damping limit. The reflectivity is also observed to increase with the damping of ion acoustic waves, the length of the plasma, the intensity of the laser, and the initial amplitude of the noise fluctuations. Possible cascadings of LDI have also been investigated. While the cascading of LDI is found to increase the SRS backscattering, the cascading of SRS is found to reduce it. Considering only the coupling to LDI, our model fails to quantitatively predict the experimental SRS backscattering; however, the calculated backscattering is found to vary in a manner similar to the experimental observations, and our simulations explain interesting physics in the ICF laser-plasma interactions.