First Results from Cryogenic Target Implosions on OMEGA

LLE Review, Volume 90 49 Introduction The base-line direct-drive ignition target design for the National Ignition Facility (NIF)1 is a thick cryogenic DT-ice layer enclosed in a thin CH shell.2,3 In direct-drive inertial confinement fusion (ICF)4 a spherical target is illuminated by a large number of laser beams to provide a spherically symmetric implosion. Target implosions with cryogenic DT fuel are planned using the 60-beam OMEGA laser system5 to validate the theoretically determined3 levels of laser and target uniformity required to achieve direct-drive ignition and gain on the NIF. The OMEGA cryogenic target designs are energy scaled from the NIF ignition designs.2,3 In particular, the OMEGA cryogenic targets, driven by an energy-scaled ignition pulse, are designed to be as “hydrodynamically equivalent” as possible to the ignition capsule designs. In this context, the constraints placed on the OMEGA cryogenic target designs include peak shell velocities, hot-spot convergence, in-flight aspect ratio, and stability properties similar to those of the NIF designs. To compare igniting and non-igniting target designs, we use the hot-spot convergence ratio, defined as the ratio of the radius containing 90% of the yield when propagating burn was deactivated compared to the initial ice–gas interface. In addition, the principle sources of nonuniformity on OMEGA, which lead to a degradation in target performance, are similar to the NIF. For direct-drive ICF these sources are single-beam nonuniformity (“laser imprint”), drive asymmetry, inner ice surface, and outer-surface roughness.