Three-dimensional simulations of the angular and polarization dependence of stimulated Brillouin backscattering from NIF hohlraums

The National Ignition Facility (NIF) groups its 192 beams in 48 quads, 2/3 of which are 'outer' beams and 1/3 'inner' beams. Half of the outer quads are focused at the laser entrance hole (LEH) at an mean angle of $44^{\circ} $ and the other half at $50^{\circ}$ with respect to the hohlraum axis. The majority of the stimulated Brillouin scatter (SBS) is reflected into the $50^{\circ} $ quads, and most of that into the $52^{\circ}$ beams. That observation we reproduce with our simulations that use the wave propagation code, pF3D.[\textit{Berger et al., Phys.Plasmas {\bf5}, 4337 (1998)}] Simulations considered a number of different pulse shapes, wall materials, capsule materials, and initial fill gas density with the plasma properties taken from 2D cylindrically-symmetric, radiation-hydrodynamic simulations of the hohlraum, capsule included. The simulations predict that different hohlraum designs have different fractions, between 20\% and 50\%, of the total SBS reflected into the backscattered light collection optics (the so-called FABS). The amount of light backscattered outside of FABS is not currently measured but is assumed to be 70\% of the light backscattered. That assumption is a reasonable but not accurate estimate.