Polar direct drive: Proof-of-principle experiments on OMEGA and prospects for ignition on the National Ignition Facility

Polar direct drive (PDD) [S. Skupsky et al., Phys. Plasmas 11 2763 (2004)] shows promise for achieving direct-drive ignition while the National Ignition Facility (NIF) [E. M. Campbell and W. J. Hogan, Plasma Phys. Control. Fusion 41 B39 (1999)] is initially configured for indirect drive. Experiments have been carried out using 40 repointed beams of the 60-beam OMEGA laser system [T. R. Boehly et al., Rev. Sci. Instrum. 66 508 (1995)] to approximate the NIF PDD configuration. Backlit x-ray framing-camera images of D2-filled spherical CH capsules show a characteristic nonuniformity pattern that is in close agreement with predictions. Saturn targets (similar capsules surrounded by a plastic ring) increase the drive on the equator, suggesting that highly symmetric PDD implosions may be possible with appropriate tuning. Two-dimensional (2D) simulations reproduced the approximately threefold reduction in yield found for the non-Saturn PDD capsules. Preliminary simulations for a NIF Saturn design predict a high gain close to the 1D prediction. These results increase the prospects of obtaining direct-drive ignition with the initial NIF configuration.