2D PIC simulations of backward and forward stimulated Raman side-scattering in direct-drive ignition-scale plasmas

The experimental observation of stimulated Raman side-scattering (SRSS) in planar-target experiments at the National Ignition Facility (NIF) renews research and interest of SRSS. Side scatter requires scattered light propagating perpendicularly to the density gradient. Therefore, for oblique incidence, the propagation direction of side-scattered light is either backward or forward with respect to the incident beam. Here, the competition between backward SRSS and forward SRSS are investigated in direct-drive ignition-scale plasmas where laser intensity is ∼ 10¹⁴ W/cm²,  densities range from 0.15 n_c to 0.2 n_c, electron temperature is 4 keV, and the density scale-length is 800 μm. Linear analysis shows that both BSRSS and FSRSS are absolute (temporally growing) at higher densities (n_e>0.17n_c), and, at lower densities (n_e<0.15n_c) , BSRSS becomes convective with a large gain, but FSRSS is still absolute for conditions relevant to direct drive.  2D PIC simulations show that BSRSS and FSRSS coexist at both higher densities and lower densities, and backward side-scattered light is stronger than forward side-scattered light in the nonlinear stage. The effects of pump depletion on the growth rate of FSRSS and kinetic enhancement and saturation of BSRSS are presented.