Signatures of Laser Imprint in OMEGA Cryogenic Implosions

The growth of single-beam nonuniformity or laser imprint can compromise target performance in direct-drive implosions. With increasing in-flight aspect ratio (IFAR is the ratio of the shell radius to shell thickness), the imprint front can penetrate through the shell, reducing the in-flight shell density. This results in a less-efficient piston for assembling the hot spot. The role of laser imprint on target performance is discussed in this presentation. Simulations of OMEGA cryogenic implosions with varying IFAR are presented. These DRACO simulations include a 3-D ray trace with the effect of cross-beam energy transfer, nonlocal heat conduction, and calculated first‑principles equations of state and opacities of the ablator and ice. Observables including ablation-surface trajectories, neutron yields, areal densities, neutron rates, ion temperatures, and hot-spot images are compared to experiment. Specific signatures including broadened neutron rates, reduced areal densities, and thicker shells relative to spherically symmetric simulations are presented.