Mitigating LPI and gold bubble expansion using ultra low density foams

We consider designs for indirect drive that partially fill the hohlraum with an ultra-low density foam (1.0 mg/cc SiO2 aerogel). We compare the foam-filled designs with standard gas-filled hohlraums, paying particular attention to LPI and gold bubble expansion. We find that the best designs for mitigating both LPI and gold bubble expansion use a combination of high-Z foam and low-Z gas. The placement of the foam at the location where the outer beams strike the wall tamps the expansion of the gold bubble into the hohlraum. This widens the channel between the capsule and the wall, allowing propagation of the inners to the hohlraum waist for more time. The foam design has significantly lower inner beam SRS than the gas-only design, which is achieved via the presence of high-Z material in the foam. The high-Z atoms lead to a greater inverse-brem absorption rate, increasing the electron temperature and Landau damping in the region of high SRS gain. This results in reduced SRS gains and reflectivities, as simulated using FLIP and pF3D. SBS is kept in check via low-Z atoms in the foam, which facilitate efficient Landau damping of ion acoustic waves.