The Langmuir Decay Instability and Stimulated Raman Scattering in ICF-Relevant Plasmas

Kinetic simulations of stimulated Raman scattering (SRS) in ICF-relevant plasmas with long-scale-length density gradients have shown that SRS can grow strongly when the laser propagates above that density for which $k\lambda_D \approx 0.30$, where $k$ is the wavenumber of the daughter electron plasma wave and $\lambda_D$ is the electron Debye length. Simulations and experiments have shown that SRS saturation is dominated by kinetic effects for $k\lambda_D > 0.30$ and the Langmuir decay instability (LDI) for $k\lambda_D < 0.30$, but few kinetic simulations of SRS have explicitly explored the role of LDI in this regime or the transition in SRS behavior across this $k\lambda_D$ boundary. Here we present one- and two-dimensional PIC simulations of LDI in the midst of SRS dynamics for both single-laser-speckles as well as for lasers propagating up long-scale-length density gradients covering a range of $k\lambda_D$. We show the effect of LDI on SRS reflectivity and hot electron generation for variable $ZT_e/T_i$ and $k\lambda_D$, as well as the effect of LDI on the spatio-temporal behavior of electron plasma wave packets generated by SRS.