Measurements of Continuum Lowering in Strongly Coupled Plasmas of Elements and Compounds

We have used the X-ray pulse of the Linac Coherent Light Source to perform a charge-resolved measurement of continuum lowering in solid-density plasmas, at temperatures up to 200 eV, from Mg, Al, Si, alumina, silica and mica. The comparison between Al or Si and their respective compounds shows that the K-edges for the same element in different plasma environments is unaffected by the considerable density differences, contrary to the predictions of any analytical continuum lowering model. Conversely, the K-edges for all of the materials can be approximated surprisingly well by pure atomic-physics calculations, consistent with recent DFT predictions [1]. The results provide strong evidence that the ion-sphere models, used to describe continuum lowering as well as to calculate the equation of state of materials, may need to be revisited for strongly coupled systems. \\[4pt] [1] S.M. Vinko {\it et al.}, Nat. Comm. {\bf 5}, 3533 (2015).