Modeling stimulated Brillouin scattering across multiple NIF hohlraum designs

Stimulated Brillouin backscatter (SBS) is a potential risk for laser damage in experiments at the National Ignition Facility and, by altering the energy deposition pattern in hohlraums, it affects the symmetry of indirect-drive ICF implosions. We have surveyed SBS backscatter across NIF integrated hohlraums of various platforms numerically, using three-dimensional simulations with the backscatter code pF3D [R. L. Berger, C. H. Still, E. A. Williams, and A. B. Langdon, Phys. Plasmas 5, 4337 (1998)] and ray-based gain calculations with FLIP [M. Belyaev, priv.comm.]. Measured reflected powers and energies, drawn from two separate diagnostics, as well as the spectrum of reflected light all compare favorably with pF3D simulations. Ray-based calculations of exponential SBS amplification ("gain"), which assume a strongly damped plasma wave and steady-state response, are performed using a novel method that includes the 3D speckled field of the laser that drives SBS. This approach is useful for understanding qualitative differences between hohlraum designs and identifying regions susceptible to SBS within hohlraums. However, gains are in general not found to correlate with SBS reflectivities in 3D, necessitating time-dependent calculations using pF3D.