Mix at the interface – Diffusion-dominated mixing phenomena probed by high-resolution separated reactant experiments

Mix is one of the most challenging problems in inertial confinement fusion (ICF). High-Z shell material mixed into the fuel degrades inertial fusion implosions by decreasing compressibility, increasing radiative and conductive losses and reducing confinement time. Because the processes that drive mix span from the scale of the full system down to Kolmogorov scale, it is extremely expensive to do complete simulations and instead simplified models must be used. To benchmark models, highly specific experimental data can be used to constrain and understand which mix mechanisms are important.

By placing a thin (150 nm) deuterated plastic layer in the shell of OMEGA capsules and measuring the deuterium-tritium (DT) yield as a mix metric, one obtains a high level of mix sensitivity through the fusion yield and high initial spatial specificity. Over the past five years, 65 implosions studying the spatial source of shell mix within the 1^st micron of shell-fuel interface on moderate convergence implosions has revealed a complex mix landscape unseen with historically thick separated reactant campaigns. The higher resolution data shows that plasma diffusion on the interface, driven through gradients of temperature and density - a kinetic process - is an important effect in even moderate convergence implosions. Furthermore, the close interface is sensitive across the transition away from diffusion to traditional hydrodynamically driven instabilities, as well as the mix effects from capsule mounting stalks, placement offsets, capsule fabrication and laser delivery variations. Comparisons of the dataset with Fokker-Planck, Zimmerman–Paquette–Kagan–Zhdanov (ZPKZ) and bubble-drag mix models demonstrates the applicability and trade-offs of such models. The relatively large and specific dataset offers novel insights into mix mechanisms for ICF capsules.