Automated Time-Step Control of Radiation-Hydrodynamic Imprint Simulations of Inertial Confinement Fusion Implosions

Inertial confinement fusion (ICF) facilities like the Omega Laser System directly drive an implosion target with overlapping beams with a controlled speckled envelope. The ICF target outer shell ablates from direct laser illumination, and any aggregate nonuniformity imprints in the shell. As ablation accelerates the shell, hydrodynamic instability growth of nonuniformity occurs in the converging shell, which impedes fusion reactions and can even disintegrate the shell. Speckle smoothing is paramount in direct-drive ICF experiments, and options include time-based and instantaneous smoothing. Time-based smoothing effectively averages the highly modulated speckle by dynamically altering the spatial distribution with coherence times shorter than the hydrodynamic response time, ~100ps. Radiation-hydrodynamic simulations of this process are time-consuming due to the required spatial and temporal resolution of speckle motion. However, the exploding plasma creates a heat conduction zone that separates the ablation surface from the laser deposition region. The conduction zone thickness limits the imprint efficacy of short wavelengths, which impose the strictest time-step constraints. As the ablation zone expands to kd ~ 1, these modes cease imprinting. By monitoring the growth of the conduction zone, this information relaxes the imprinting time-step constraint during a simulation. This poster explores this process and demonstrates the speed increase afforded to rad-hydro simulations.