Measurements of laser-imprint-induced shock velocity nonuniformities in plastic targets with the Nike KrF laser

We report results of direct-drive laser imprint experiments measuring velocity perturbation profiles of shock waves produced by the Nike krypton fluoride laser. A new high-resolution two-dimensional velocimeter system was successfully implemented on the Nike laser facility and used for sensitive optical measurements of the velocity perturbations. Planar polystyrene targets with and without a thin high-Z overcoat (400 A Au or 600 A Pd) were irradiated by four, eight, and sixteen Nike laser beams to examine laser imprint and its mitigation. The results from the uncoated targets showed that the shock velocity perturbations decreased with an increasing number of laser beams overlapped on target, precisely as anticipated by the beam averaging effect on laser imprint. In the experiment on the shocks driven in the high-Z coated targets, the shock velocity perturbations were further reduced by a factor of 2–6 compared to their counterparts in the uncoated experiment, with the amplitude of the velocity fluctuations measured as small as 20 m/s rms for shock velocities of 20 km/s. These experiments allowed more direct measurements of laser imprint effects without relying on the Rayleigh–Taylor hydrodynamic amplification, providing valuable quantitative data for calibrating radiation-hydrodynamic simulations of laser imprint.