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 long-standing 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, we use maps of underdense plasma conditions, i.e., temperatures, densities, materials, charge states, and flows, from hydrodynamics simulations of the hohlraum to run backscatter simulations with the code pF3D [Berger et al., Phys. Plasmas 26, 012709 (2019)]; the latter calculates propagation of incident- and backscattered light of a NIF quad in the paraxial approximation. For the inner cone quads, in designs that utilize significant wavelength detuning, i.e., those that use cross-beam energy transfer to control implosion symmetry, this approach typically has overestimated both the peak power and the duration of SBS measured in experiments. In this work, we discuss how multi-species hydrodynamics simulations can lead to better agreement with experiments through changes to the simulated plasma conditions and the resulting SBS growth rates. Specifically, we discuss SBS reflectivity in the 23° and 30° inner cone quads, compare simulated spectra to FABS measured ones, and discuss how the time history of the backscattered light is related to the absorption/refraction of the incoming light off density features in the hohlraum plasma.