The Impact of Target-Layer Imperfections on Performance Sensitivity to Laser Imprint for Direct-Drive Inertial Confinement Fusion (DDICF) Implosions on OMEGA.*

Recent experimental results showed that the highest performing class of DDICF implosions on OMEGA at adiabat ~4.5 are not dominated by degradation from laser imprint¹, but are significant at adiabat~3.5 .  This was shown by measuring changes in neutron yield and neutron-averaged areal density (ρR) to varying levels of laser spot smoothing by spectral dispersion (SSD) and observing performance saturation for sufficiently high SSD bandwidth. We report on a 2-D simulation study with DRACO² seeking to reproduce these trends by introducing target imperfections via a surface-roughness spectrum, in addition to laser imprint.  While previous DRACO simulation results with laser imprinting alone did not show performance saturation at the SSD levels observed, a mechanism to reproduce it could be done through an additional, high-mode perturbation source that is independent of SSD. The perturbation added is characterized by <1 um amplitude, high-mode imperfections (l=60 to 120 spectrum suggested by 2D VISAR measurements³) at the fuel-ablator interface, in addition to a NIF standard surface roughness spectrum. The results from this study are valuable for determining what perturbation seeds need to be included in simulations in order to create scientifically credible performance extrapolations for future target designs.