Nonlinear development of absolute Raman backscatter in ignition-scale direct-drive coronal conditions

Abstract

Motivated by a resurgence of interest in stimulated Raman scattering (SRS) in direct-drive inertial confinement fusion, we use Vlasov-Maxwell simulations to investigate the nonlinear saturation and associated hot electron production for absolute SRS backscatter near the quarter-critical density. The simulated plasma conditions are relevant to recent planar target experiments on the National Ignition Facility [Rosenberg et al., Phys. Rev. Lett. 120, 055001 (2018)]. It is discovered that wave-wave interactions dominate over kinetic effects in the nonlinear development of instability with clear signatures of the Langmuir decay instability (LDI) cascade and modulational instability of the primary electron plasma wave. Spectral broadening of the scattered light is shown to occur by two mechanisms: via a long wavelength modulation of the electron density - caused by the ponderomotive action of the near-turning point Airy-like structure of the SRS light - and by seeding of SRS by the LDI cascade. Hot electrons are produced at a level that is ≲1% of the incident laser energy by acceleration in the turbulent spectrum associated with the LDI cascade. Despite the high electron plasma temperature, collisions are shown to be important and have the effect of moderating the LDI cascade. The effect of collisions is shown to have a more dramatic effect on hot electron production than on the SRS reflectivity.