Direct-drive laser imprint experiment measuring shock velocity perturbations at Nike$^*$

Using a high resolution 2D VISAR$^{a,b}$, we have performed a laser imprint experiment measuring velocity modulations in shock wave produced by the Nike laser. The 2D VISAR takes snapshot images of the velocity field across the shock front that is imprinted and then decouples from the ablation surface before the target gets accelerated. Hence the VISAR experiment measures imprint effects without relying on Rayleigh-Taylor amplification during the acceleration providing a complementary means to study laser imprint. In this campaign, the measurements were made on shocks driven in planar CH targets by various numbers of overlapped Nike beams to deliver a broad range of illumination uniformities. Each Nike beam utilizes 1 THz bandwidth ISI beam smoothing to achieve a more uniform illumination than any other ICF laser. We also made measurements on shocks driven in high-Z coated CH targets to explore the high-Z imprint-reduction strategy pioneered in radiography experiments on Nike. The current experiment has observed the multibeam irradiation effect on imprinting when varying the number of overlapping beams and confirmed less velocity perturbations in the shock when using a high-Z overcoat. $^a$ Celliers et al, Rev Sci Instr 81, 035101 (2010). $^b$ Oh et al., APS DPP, GP11.119 (2018).