Symmetry tuning and high energy coupling for an Al capsule in a Au rugby hohlraum on NIF

Abstract

Experiments on imploding an Al capsule in a Au rugby hohlraum with up to 1.5 MJ laser drive were performed on the National Ignition Facility (NIF). The capsule diameter was 3.0 mm with ∼ 1 MJ drive and 3.4 mm with ∼ 1.5 MJ drive. Effective symmetry tuning by modifying the rugby hohlraum shape was demonstrated, and good shell symmetry was achieved for 3.4 mm capsules at a convergence of ∼10. The nuclear bang time and the shell velocity from simulations agree with experimental data, indicating ∼500 kJ coupling with 1.5 MJ drive, or ∼30% efficiency. The peak velocity reached above 300 km/s for a 120 µm-thick Al capsule. The laser backscatter inside the low-gas-fill rugby hohlraum was very low (<4%) at both scales. The high energy coupling allows implosion designs with increased adiabat which in turn increases the tolerance to detrimental effects of instabilities and asymmetries. These encouraging experimental results open new opportunities for both the mainline single-shell scheme and the double-shell design toward ignition.