Using Laser Entrance Hole Shields to Increase Coupling Efficiency in Indirect Drive Ignition Targets for the National Ignition Facility (NIF) Using Laser Entrance Hole Shields to Increase Coupling Efficiency in Indirect Drive Ignition Targets for the Nati

Abstract: Coupling efficiency, the ratio of the capsule absorbed energy to the driver energy, is a key parameter in ignition targets. The hohlraum originally proposed for NIF coupled ~11% of the absorbed laser energy to the capsule as x-rays. We describe here a second generation of hohlraum target which has higher coupling efficiency, ~ 16%. Because the ignition capsule's ability to withstand 3D effects increases rapidly with absorbed energy, the additional energy can significantly increase the likelihood of ignition. The new target includes laser entrance hole (LEH) shields as a principal method for increasing coupling efficiency while controlling symmetry in indirect-drive ICF. The LEH shields are high Z disks placed inside the hohlraum to block the capsule's view of the cold LEHs. The LEH shields can reduce the amount of laser energy required to drive a target to a given temperature via two mechanisms: 1) keeping the temperature high near the capsule pole by putting a barrier between the capsule and the pole, 2) because the capsule pole does not have a view of the cold LEHs, good symmetry requires a shorter hohlraum with less wall area. Current integrated simulations of this class of target couple 140 kJ of x-rays to a capsule out of 865 kJ of absorbed laser energy and produce ~ 10 MJ of yield. In the current designs, which are not completely optimized, the addition of the LEH shields saves ~ 95 kJ of energy (about 10%) over hohlraums without LEH shields. Introduction