Hollow-Shell Implosion Studies on the 60-Beam, UV OMEGA Laser System

82 LLE Review, Volume 78 Direct-drive inertial confinement laser fusion is accomplished by uniformly illuminating spherical fuel-bearing targets with high-power laser beams, ablatively driving implosions that result in large increases in density and temperature. Current large laser systems such as the University of Rochester’s OMEGA laser, which is capable of both directand indirectdrive implosion experiments,1,2 and the Lawrence Livermore National Laboratory’s Nova laser,3,4 which is designed primarily for indirect-drive implosions, are smaller in size and total output energy than what is believed necessary to obtain ignition and gain. Attaining conditions for ignition to occur (densities of ~200 g/cm3 and temperatures of ~3 to 4 keV) awaits the completion of the National Ignition Facility5 (NIF) and other megajoule-class drivers currently being planned. In addition to the high temperatures and densities, ignition requires fuel areal densities (density–radius product) 0.3 g/cm2 to stop the 3.5-MeV alpha particles in order to obtain thermonuclear burn propagation.6,7 To reach these conditions in direct-drive implosions requires controlling the growth of the Rayleigh–Taylor (RT) instability, which is seeded by nonuniformities in the laser illumination. The RT instability can lead to shell breakup and mixing of shell material into the gas-fill or central voided region in the case of evacuated targets. We are currently studying the attainment of nearignition-scale areal densities on OMEGA and the effects of beam smoothing and pulse shaping thereon, by using surrogate cryogenic targets where the shell acts as the fuel layer. These will be followed by actual cryogenic (DD or DT) targets, when the cryogenic target–handling facility is completed.