Quantifying the effectiveness of different laser beam smoothing techniques on mitigating inflationary Stimulated Raman scattering*

Kinetic inflation exacerbates the threat of the stimulated Raman scattering instability (SRS) to inertial confinement fusion. Continued growth of the instability requires phase matching between the incident light wave and the decay products, a scattered light wave and an electron plasma wave (EPW). In principle, a density inhomogeneity can disrupt the phase matching by changing the frequency of the EPW along the gradient. In reality, electron trapping in the EPW produces a frequency shift that can compensate this change. This autoresonance, or kinetic inflation, can substantially enhance the SRS reflectivity. Here we demonstrate that large laser bandwidth and small laser speckles can mitigate inflationary SRS and limit the reflectivity to non-inflationary levels by restricting the growth of SRS EPW. While the instantaneous reflectivity depends on the local chirp and local intensity of the incident light, the inflationary SRS threshold depends on the beam smoothing techniques, for example, smoothing by spectral dispersion and induced spatial incoherence. This leads to distinct scaling laws which can help guide SRS mitigation efforts.