Pushered Single Shell (PSS) implosions for mix and radiation trapping studies using high-Z layers on National Ignition Facility*

Hydrodynamic instabilities and mix are of major interest in the field of High Energy Density physics. This talk reviews the first Pushered Single Shell (PSS) experiments on the National Ignition Facility to measure high-mode instabilities and mixing in the deceleration phase of indirectly-driven spherical implosions with gas-filled plastic shells. In PSS, high-Z Ge dopant been added at the inner surface of the shell to increase core radiation trapping and influence ablator-gas mix near peak compression of implosions. While the radiation trapping can lower the threshold core temperature for gas ignition, the high-Z mix can also cool the core through radiation losses. PSS addresses mix at ignition relevant core temperatures and hottest gas-ablator mix region, unveiling a new regime in the diffusive and turbulent mix balance, for the first time. The effect of partially ionized dopant on the relative importance of the diffusion vs hydrodynamic turbulence was also studied. Implosion performance and mix were assessed in plastic shells filled with hydrogen-tritium gas, with and without deuterated and Ge-doped inner layers by means of nuclear, monochromatic x-ray imaging and spectroscopy diagnostics. Neutron yield and ion temperature of the DT fusion reactions give a measure of shell-gas mix, while yield of the TT fusion reactions assess the implosion performance. In addition, Ge K-shell absorption and emission spectra further constrain radiation trapping and mix. Experimental results and comparisons with simulations, including mix models, will be presented. Validated models are used in future graded Be/Cr shell PSS designs with enhanced radiation trapping and with additional low-Z anti-mix layers to control core cooling.