Experimental investigation of SiO₂ foam-filled hohlraums for inertial fusion

The performance of SiO₂ foam-filled hohlraums is experimentally assessed as a candidate for ignition-relevant hohlraums. Such targets could potentially overcome symmetry challenges at low case-to-capsule ratio (CCR) by tamping the hohlraum wall motion, and help mitigate Laser Plasma Interactions (LPI). Building upon a promising experimental campaign at the Omega laser facility [1], a scaled-up experiment was conducted at the Laser Megajoule (LMJ) facility [2], comparing a novel foam-fill design target [3,4] with a gas-filled one. Time resolved x-ray images show successful reduction in the expansion of the hohlraum wall in the foam-filled target, while measurements from Full-Aperture Backscatter Stations (FABS) indicate backscatter levels from Stimulated Brillouin Scattering (SBS) comparable to the gas-filled target. However, Near Backscatter Imager (NBI) plates [5] show a 20 fold reduction in Stimulated Raman Scattering (SRS) in the outer cone beams for the foam target, despite both targets having the same electron number density. Such reduction could be due to the enhanced density fluctuations caused by the foam structure when ionized, which introduces an effective collision term. This effective collision term then enhances the damping rate, increases laser absorption and reduces SBS by destroying the phase matching.