Exploring New Methods For Increasing Coupling Efficiency in Indirect Drive Ignition Targets for the National Ignition Facility (NIF)

Coupling efficiency, the ratio of capsule absorbed energy to driver energy, is a key parameter in ignition targets. The hohlraums originally proposed for the NIF coupled $\sim $11{\%} of the absorbed laser energy to the capsule as x-rays. We describe here a second generation of hohlraum targets which has higher coupling efficiency, $\sim $17.5{\%}. Because an ignition capsule's ability to withstand 3D effects increases so rapidly with absorbed energy, the additional coupling can significantly increase the likelihood of ignition. The new targets include laser-entrance-hole (LEH) shields as a principal method for increasing coupling efficiency while controlling symmetry in indirect-drive ICF targets. The LEH shields are high Z disks placed inside the hohlraum to block the capsule's view of the cold LEH's. 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 hole, 2) because the capsule poles do not have a direct view of the cold LEH's, good symmetry requires a shorter hohlraum with less wall area. Current integrated simulations of this class of target couple $\sim $140 kJ of x-rays to a target from $\sim $800 kJ of absorbed laser light and produce over 10 MJ of thermonuclear yield. In addition to shields, this design utilizes a low density (1mg/cc) aerogel foam to provide symmetry control. This replaces the He-H fill gas in previous targets and provides significant operational advantages. We describe the new targets, provide a quantitative design analysis of the radiation hydrodynamics properties as well as estimates of the laser-plasma interaction environment and properties. We also describe the diagnostic challenges presented by these targets and how we propose to meet those challenges on NIF.