Core performance and mix in direct-drive spherical implosions with high uniformity

The performance of gas-filled, plastic-shell implosions has significantly improved with advances in on-target uniformity on the 60-beam OMEGA laser system [T. R. Boehly, D. L. Brown, R. S. Craxton , Opt. Commun. 133, 495 (1997)]. Polarization smoothing (PS) with birefringent wedges and 1-THz-bandwidth smoothing by spectral dispersion (SSD) have been installed on OMEGA. The beam-to-beam power imbalance is less than or equal to5% rms. Implosions of 20-mum-thick CH shells (15 atm fill) using full beam smoothing (1-THz SSD and PS) have primary neutron yields and fuel areal densities that are similar to 70% larger than those driven with 0.35-THz SSD without PS. They also produce similar to 35% of the predicted one-dimensional neutron yield. The results described here suggest that individual-beam nonuniformity is no longer the primary cause of nonideal target performance. A highly constrained model of the core conditions and fuel-shell mix has been developed. It suggests that there is a clean fuel region, surrounded by a mixed region, that accounts for half of the fuel areal density. (C) 2001 American Institute of Physics.