The relationship between gas fill density and hohlraum drive performance at the National Ignition Facility

Indirect drive inertial confinement fusion experiments were conducted at the National Ignition Facility to investigate the performance of the hohlraum drive as a function of hohlraum gas fill density by imploding high-density-carbon capsules using a 2-shock laser pulse. Measurements characterized the backscatter behavior, the production of hot electrons, the motion and brightness of the laser spots on the hohlraum wall, and the efficiency of the hohlraum x-ray drive as a function of gas fill density ρ_gf between 0.03 mg/cc ("near vacuum") and 1.6 mg/cc. For hohlraums with ρ_gf up to 0.85 mg/cc, very little stimulated Raman backscatter (SRS) was observed. For higher ρ_gf, significant SRS was produced and was observed to occur during the rise to peak laser power and throughout the main pulse. The efficiency with which laser energy absorbed by the hohlraum is converted into drive energy was measured to be the same for ρ_gf ≥ 0.6 mg/cc once the laser reached peak power. However, for the near vacuum case, the absorbed energy was converted to drive energy more efficiently throughout the pulse and maintained an efficiency ∼10% higher than the gas filled hohlraums throughout the main pulse.