Mind the gap: Small features can have large effects in HED experiments

The LANL Multi-Shock (MShock) campaign on the NIF explored the Richtmyer-Meshkov (RM) growth of initially solid, prescribed perturbation profiles after undergoing multiple strong shocks, including from the same side. We also developed a scaled experimental analog for the Omega-EP laser to test the growth of the direct-drive portion. We explored the growth of single- and multi-mode perturbations in a heavy-to-light geometry under the effects of small-scale roughness to test whether the growth cases predicted by ideal theory still hold.

Post-shot simulations explored the effect of additional machining features, specifically the inclusion of a gap between the tracer layer and the foam in shock tube to see how this affected evolution. We find that the addition of the gap significantly changes the evolution of the RM features seen in the x-ray radiographs from these experiments. Synthetic radiographs from simulations with the gap match the experimental data, suggesting that the gap is indeed important. Simulations with the gap show that the smaller-scale features initially in the troughs of the carrier mode perturbation are washed out during the first shock while features closer to the foam interface are retained, which complicates the inference of mode structure after the second shock. We will present these simulations results with comparison to the experiments and discuss relation to the ideal theory.