Laser imprint measurement and mitigation experiments

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 EP and NIF. The high-Z coating imprint mitigation pioneered on Nike [Obenschain et al. PoP 9, 2234 (2002)] has now been demonstrated on EP through an x-ray prepulse to pre-expand the coating. Laser prepulse, while successfully used with ISI smoothing on Nike, appears to seed imprint when using an unsmoothed beam on EP, implying that smoothing on the prepulse will be necessary when moving to implosions on OMEGA. Experiments on Nike will diagnose expansion of metal coating under intensities of prepulse that can be generated with SSD on OMEGA. Experiments on the NIF aim to measure laser imprint in an ignition-scale, low f/# multibeam direct dive to benchmark the simulations and assess the need for imprint mitigation there. Non-uniformity amplified by RM/RT instability is measured using radiography of cone-in-shells. Pre-imposed modulations benchmark amplification of the laser-seeded perturbations. Results indicate significantly higher non-uniformity for ignition-relevant low-adiabat pulses, as expected. The latest measurements and possible mitigation strategies will be discussed.