Advanced Hohlraum Designs on the NIF for High Coupling Efficiency

The coupling of 3ω laser light to the capsule in hohlraum designs for the NIF is typically <10%, resulting in an absorbed energy E_cap of 200 kJ or less in cylindrical hohlraums driven at 2 MJ. Ignition thresholds or performance margins scale linearly with E_cap, so that a significant improvement in these metrics may require considering non-standard hohlraum shapes, e.g., rugbys [1] or a “frustraum”. The frustraum is formed by joining a pair of frusta (or truncated right-circular cones) above the capsule. The low surface area of the frustraum allows for a tradeoff in increased volume above the capsule to facilitate inner beam propagation and accommodate a far larger capsule than the nominal ~1 mm radius size. Integrated hohlraum simulations suggest E_cap~ 500 kJ at only 1.8 MJ laser energy and 460 TW peak power for HDC capsules of radius 1.5 mm may be achievable under conditions of adequate drive symmetry and low backscatter risk. The greatest modeling uncertainty in these candidate hohlraum geometries is the relative balance of specular and diffuse reflection of the outer cones from the oblique hohlraum wall and its high impact on drive symmetry. [1] P. Amendt et al., PoP 21, 112703 (2014)