Work performed under the auspices of the U.S. Recent efforts have focused on understanding the discrepancy between simulated and measured symmetry and on alternate designs for symmetry control through varying cone fraction, trade-offs between laser power and energy, and modifications to case-to- capsule ratio. Nominal radiation-hydrodynamics simulations have not yet proven predictive on symmetry of the final hotspot, and experiments show more prolate symmetry than preshot calculations. In simulations, laser propagation is degraded suddenly by hohlraum wall expansion interacting with ablated capsule material. Extending this platform to high-yield designs with high-density carbon capsules requires achieving adequate symmetry control throughout the pulse. Because this is a low laser-plasma interaction platform, implosion symmetry is controlled via pulse-shaping adjustments to laser power balance.

Recent experiments with near- vacuum hohlraums, which utilize a minimal but non-zero helium fill, have demonstrated performance improvements relative to conventional gas-filled (0.96 - 1.6 mg/cc helium) hohlraums: minimal backscatter, reduced capsule drive degradation, and minimal suprathermal electron generation. Ongoing work is focused on developing a physical model which captures this enhanced propagation and on utilizing the enhanced propagation to drive longer laser pulses than originally predicted in order to reach alpha-heating dominated neutron yields.Д®xploring symmetry in near- vacuum hohlraumsД«erzak Hopkins, L. Unexpectedly, experiments have demonstrated that the inner laser beam propagation is better than predicted by nominal simulations, and an enhanced beam propagation model is required to match measured hot spot symmetry. Two mitigation methodologies have been explored, dynamic beam phasing and increased case-to- capsule ratio (larger hohlraum size relative to capsule). Long pulse symmetry control, though, remains a challenge, as the hohlraum volume fills with material. Given these advantages, near- vacuum hohlraums are a promising choice for pursuing high neutron yield implosions. These hohlraums show improved efficiency relative to conventional gas-filled hohlraums in terms of minimal backscatter, minimal generation of suprathermal electrons, and increased hohlraum-capsule coupling. Recent experiments at the National Ignition Facility have explored driving high-density carbon ablators with near- vacuum hohlraums, which use a minimal amount of helium gas fill. Near- vacuum hohlraums for driving fusion implosions with high density carbon ablatorsa)Д«erzak Hopkins, L.

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