Measuring Isotropic Fuel-Ablator Mix at Stagnation of Cryogenically Layered Capsule Implosions at the NIF

Fuel-ablator mix in ICF implosions is a major degradation mechanism of nuclear fusion performance. At the same time, it is one of the most challenging mechanisms to measure at capsule stagnation. While significant progress has been made to quantify localized mix, previous studies of isotropic fuel-ablator mix have been limited to the early implosion phase. Here we report on the first measurements carried out at the NIF, directly probing isotropic fuel-ablator mix at capsule stagnation between a CH shell with a roughened inner surface and a cryogenic fuel layer. We use the separated reactants technique which employs an HT fuel layer of varying thickness and an inner layer of deuterated plastic (CD) such that the DT neutron signal at stagnation represents a quantitative measure of mixing between shell and fuel. We observe a significant increase in isotropically mixed ablator material when reducing the ice layer thickness from 29 to 17 micrometer because of increased propagation of fuel-ablator instabilities through the thinner ice layer. These measurements improve our understanding of fuel-ablator mix at stagnation by deriving an experimental scaling with ice layer thickness and benchmarking various mix models in-line with state-of-the art radiation hydrodynamics simulations.