Assessing explanations for unexpected fuel-ablator mixing measurements in HDC implosions at the NIF*

Inertial confinement fusion (ICF) implosions using high-density-carbon (HDC) ablators have achieved ignition and delivered record fusion yields. At the same time, there are signs that these implosions have significant mixing of the HDC ablator into the dense fuel. Such mixing can reduce compression and might also end up limiting the fuel burn-up fraction as designs push to higher performance. Mixing of fuel and ablator is a complex process that depends on the integrated impact of many factors, including the radiation drive history, ablator and fuel material properties, and instability seeds. Using simulations that vary these factors within expected uncertainties, here we show how in-flight mixing data from high-resolution radiography of HDC implosions at the National Ignition Facility (NIF) challenge current modeling of instability. While high-resolution mixing simulations reasonably match experiments for implosions with undoped ablators or ablators with a buried tungsten dopant layer, this is not the case for so-called W-inner capsules where the dopant extends all the way to the fuel-ablator interface. High dopant W-inner implosions are predicted by simulations to be much more stable than observations suggest, resulting in significant optical depth discrepancies between simulation and experiment. These results suggest more pernicious instability seeds than expected, or unexpected deviations in modeling inputs such as ablator opacity or drive history. We introduce a new planned series of in-flight radiography measurements, designed to help disambiguate potential mechanisms for these surprising results, in part by constraining in-flight shell opacities.