Polar Direct Drive—Proof-of-Principle Experiments on OMEGA and Prospects for Ignition on the National Ignition Facility

Since the recent suggestion3 that the PDD option be reconsidered on account of the cost and complexity of rerouting half of the NIF beams, a number of two-dimensional (2-D) hydrodynamic PDD simulations have been reported. Simulations4,5 of the all-DT capsule design of Refs. 6 and 7 were carried out using the hydrodynamics code SAGE, which includes fully self-consistent 3-D ray tracing.8 These simulations used sets of optimized repointings of the four rings of NIF beams and elliptical far-field focal spots for some rings to increase the drive on the capsule equator. Skupsky et al.2 used the 2-D code DRACO9,10 to examine PDD designs for wetted-foam capsules,11 which are attractive because of increased laser absorption. They concluded that PDD enhances the capability of the NIF to explore ignition conditions and found that the primary cause of gain reduction was the time-dependent drive deficit on the equator due to target compression.12 The previous article (p. 61) describes simulations of a new “Saturn” target concept for PDD in which a low-Z ring is placed around the capsule in the equatorial plane. The plasma produced around the ring (by a combination of light refracted from the capsule and light directly intercepted by the ring) grows so that, at later times, laser rays that would otherwise miss the critical surface in the equatorial region of the capsule are now refracted by the ring plasma to provide stronger irradiation of this region. With appropriately chosen ring dimensions, the capsule can be driven with a uniformity (~1%) approaching that of a symmetrically driven capsule.