SYMMETRY EXPERIMENTS IN GAS-FILLED HOHLRAUMS AT NOVA

Unerstanding drive symmetry in gas-filled hohlraums is currently of in terest because the baseline design of the indirect drive ignition targ et for the planned National Ignition Facility uses a gas-filled hohlra um. This paper reports on the results of a series of experiments perfo rmed at the Nova laser [C. Bibeau et al. Appl. Opt. 31, 5799 (1992)] f acility at Lawrence Livermore National Laboratory with the goal of und erstanding time-dependent drive symmetry in gas filled hohlraums. Time -dependent symmetry data from capsule implosions and reemission target s in gas-filled hohlraums are discussed. Results of symmetry measureme nts using thin wall gas filled hohlraums are also discussed. The resul ts show that the gas is effective in impeding the motion of the wall b lowoff material, and that the resulting implosion performance of the c apsule is not significantly degraded from vacuum results. The implosio n symmetry in gas differs from vacuum results with similar laser point ing indicating a shift in beam position on the hohlraum wall and hotte r drive at the capsule's poles than at the equator. A theory has been proposed to explain the observed shift as a plasma physics effect: bea m steering due to filamentation and transverse plasma flows. (C) 1996 American Institute of Physics.