The Effectiveness of Different Laser Smoothing Techniques for Mitigating Inflationary Stimulated Raman Scattering

Kinetic inflation exacerbates the threat of the stimulated Raman scattering (SRS) instability to inertial confinement fusion (ICF). 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 present a semi-analytic theory and supporting particle-in-cell simulations that describe how laser bandwidth can inhibit inflationary SRS by rapidly moving the location of exact phase matching and reducing the interaction time. The theory predicts the instantaneous SRS reflectivity and the extent to which different laser bandwidth formats, such as random or smoothing by spectral dispersion, can mitigate inflationary SRS. These predictions can inform designs of next-generation, broadband ICF drivers. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.