Observing the onset of pressure-driven K-shell delocalization*

We have developed an experimental platform for x-ray Thomson scattering (XRTS) at NIF to characterize plasma conditions in ICF indirectly-driven capsule implosions near stagnation [1,2]. This enabled us to investigate up to 30 times compressed ablator materials reaching pressures above 3 Gigabars, at conditions where the distance between the nuclei becomes comparable to the extent of the core shell bound states, which will eventually lead to their pressure ionization. In this talk we will present results from experiments with beryllium shells. We observe reduced elastic scattering for the most extreme conditions [2]. We interpret this reduction as the precursor of pressure ionization of the remaining K-shell electrons, that is, a strongly modified bound state. The beryllium charge state inferred from the data is considerable higher than standard models predict but agrees well with results from DFT simulations [2,3]. Accurate modelling of the K-shell occupation of light elements is imperative for creating predictive capabilities for ICF implosions. Our experiments yield valuable benchmarks for this process and demonstrating a complex pathway of pressure ionization.