MEASUREMENTS OF LASER-IMPRINTED PERTURBATIONS AND RAYLEIGH-TAYLOR GROWTH WITH THE NIKE KRF LASER

Nike is a 56 beam Krypton Fluoride (KrF) laser system using Induced Sp atial Incoherence (ISI) beam smoothing with a measured focal nonunifor mity [Delta I/I] of 1% rms in a single beam [S. Obenschain et al., Phy s. Plasmas 3, 1996 (2098)]. When 37 of these beams are overlapped on t he target, we estimate that the beam nonuniformity is reduced by root 37, to (Delta I/I)congruent to 0.15% (excluding short-wavelength beam- to-beam interference). The extraordinary uniformity of the laser drive , along with a newly developed x-ray framing diagnostic, has provided a unique facility for the accurate measurements of Raylejgh-Taylor amp lified laser-imprinted mass perturbations under conditions relevant to direct-drive laser fusion. Data from targets with smooth surfaces as well as those with impressed sine wave perturbations agree with our tw o-dimensional (2-D) radiation hydrodynamics code that includes the tim e-dependent ISI beam modulations. A 2-D simulation of a target with a 100 Angstrom rms randomly rough surface finish driven by a completely uniform beam gives final perturbation amplitudes similar to the experi mental data for the smoothest laser profile. These results are promisi ng for direct-drive laser fusion.