Modeling stimulated Brillouin backscatter from the inner laser cones during indirect drive inertial confinement fusion experiments at the National Ignition Facility*

We report progress modeling stimulated Brillouin scatter (SBS) at the National Ignition Facility (NIF). For indirect-drive, ignition-relevant hohlraum experiments, backward SBS light is a longstanding concern due to its potential for damaging laser optics as well as affecting the symmetry of the x-ray field that drives capsule implosions. To model SBS, our current approach is to use plasma maps from hydrodynamics simulations of the hohlraum to run backscatter simulations of NIF quads in the paraxial approximation with the code pF3D [Berger et al, POP26 (2019)].  For the outer cone quads, our approach recovers the experimentally measured SBS reflectivity, as well as spectra, in the modern Hybrid-E hohlraum platform [Kemp et al POP 31(2024)]. However, for the inner cone quads of designs that utilize significant wavelength detuning (i.e., those that use cross-beam energy transfer to control implosion symmetry), this approach typically over-estimates both peak power and duration of SBS measured in experiments. In this work we discuss how modifications to our approach, including multi-species hydrodynamics simulations, can lead to better agreement with experiments through changes to the simulated plasma conditions and resulting SBS growth rates.