Measuring dark mix at stagnation of cryogenically layered capsule implosions at the NIF

Understanding the physics and quantifying the impact of ice-ablator mix has been a decadal effort in Inertial Confinement Fusion. Here we present a measurement of the 'dark' mix that resides in the cool and dense fuel layer surrounding the fusion hotspot at capsule stagnation. The mix mass that we infer to be located outside of an ignition seeding hotspot can reduce fuel compression at stagnation and adversely affect burn-propagation when it turns into hot mix as the burn-wave propagates through the initially cool, mix contaminated, ice layer. These experiments are enabled by combining separated nuclear reactants and x-ray emission measurements which probe the burn-weighted amount of mix mass present in the hotspot periphery and the hotspot itself. Systematically changing the ice layer thickness effectively probes how much ablator material mixes into different ice layer depths of an equivalent full layer thickness ignition capsule and demonstrates how increasing the ice layer thickness is a promising lever for cleaner, more efficient burn propagation.