Studies of kinetic effects in shock-driven implosions using a comprehensive set of x-ray and nuclear diagnostics at OMEGA

In thin-glass, shock-driven implosions, a strong shock propagates through the plasma during the early phase of implosion.  This strong shock heats the low-density material causing the ions’ mean-free path to be larger than the hydrodynamic length scale. Previous studies demonstrated substantial yield degradation as the fill pressure was decreased. Radiation-hydrodynamic simulations using empirically-tuned ion kinetic models [M. J. Rosenberg et al. PRL 2014] and simulations solving the Vlasov-Fokker-Plank equations for the ions [O. Larroche et al. PoP 2016] reproduce the yield-degradation trend. Both simulation methods predict the dominant mechanism responsible for the yield degradation is kinetic ion-diffusion, which interchanges D³He gas with the glass shell. We report on a set of x-ray measurements from a series of implosions with varied D³He gas fill. The measurements show increased x-ray emission as the D³He gas-fill pressure decreases. In addition, x-ray images in multiple energy bands show a transition from a shell-like emission to a center-like emission structure as the gas-fill pressure decreases. Both observations support the prediction of a partial interchange between the glass and D³He due to the long mean-free paths. These x-ray measurements provide an additional constraint for the kinetic modeling of ICF implosions. Results are compared to iFP, xRage, and ARES simulation codes.