Laser imprint measurement and mitigation experiments in spherical geometry on OMEGA and NIF

NRL in collaboration with LLE is conducting a broad effort in laser imprint measurement and mitigation, with experiments on the Nike KrF laser and Nd:glass lasers Omega and NIF. An experimental platform is being developed on OMEGA to test imprint mitigation in spherical geometry using a high-Z coating technique pioneered in planar experiments on Nike [Obenschain et al. PoP 9, 2234 (2002)]. As demonstrated in our Omega EP experiments [M. Karasik, et al., to be published], a smooth prepulse is required to pre-expand the coating for order-of-magnitude imprint reduction. A low intensity, ISI-smoothed prepulse was used in the original Nike experiments, while an x-ray prepulse was used in the EP experiments. The OMEGA experiments aim to test both pre-expansion methods - an SSD-smoothed, few-ns duration laser prepulse as well as an x-ray prepulse, in spherical geometry. The NIF experiments intend to measure and mitigate laser imprint for direct drive at ignition scale. Areal mass non-uniformity amplified by RM/RT instability is measured using radiography of spherical shells.