MEASUREMENTS OF LASER-PLASMA INSTABILITY RELEVANT TO IGNITION HOHLRAUMS

The potential for laser-plasma instability is a serious concern for in direct-drive inertial confinement fusion (ICF), where laser beams illu minate the interior of a cavity (called a hohlraum) to produce x-rays for imploding a fusion capsule symmetrically. The speckled nature of l aser beams used in ICF is an important factor in laser-plasma instabil ity processes. For example, models which calculate the spatial growth of convective instability by properly accounting for the laser speckle s successfully predict the observed onsets of backscattering due to st imulated Brillouin and Raman scattering instabilities (SBS and SRS). A ssuming pump depletion as the only saturation mechanism in these model s results in very large predicted levels of SBS and SRS backscattering from the long-scale plasmas expected in ignition hohlraums. However, in the long-scale plasmas studied in the Nova and Trident lasers [E. M . Campbell, Rev. Sci. Instrum. 57, 2101 (1986) and N. K. Moncur et al. , Appl. Opt. 34, 4274 (1995)], SRS and SBS are observed to saturate mu ch below the levels expected from pump depletion. While the mechanism of SBS saturation is not understood at present, the observations of SR S saturation are qualitatively understood. (C) 1997 American Institute of Physics.