Laser imprint suppression using high-{Z} layers at high foot intensities

Laser imprint experiments are carried out on the Nike KrF laser with induced spatial incoherence (ISI) smoothing. Most of the imprint occurs during the initial low-intensity (``foot'') part of the pulse, which is necessary to compress the target to achieve high gain. It has been found previously that a thin high-Z overcoat on the laser side of the target can be effective in suppressing imprint [S. P. Obenschain et al. Phys. Plasmas 9, 2234 (2002)]. The present experiments are designed to extend this method to higher foot intensities $(\sim10^{13} W/cm^2)$, approaching those of the current high gain pellet designs. Measurements of Raleigh-Taylor (RT) amplified areal mass non-uniformity are made by face-on x-ray radiography using Bragg reflection from a curved crystal coupled to an x-ray streak camera. X-ray flux from the high-Z layer is monitored using absolutely calibrated time-resolved x-ray spectrometers. Simultaneous side-on radiography using a curved crystal allows target trajectory measurement for comparison with simulations. The effect of the high-Z layers of varying thicknesses on ISI imprint as well as re-imposed ripple growth will be presented for two different materials (Au and Pd). This work is supported by US DOE/NNSA.