Abstract
The effect of a dense plasma environment on the energy levels of an embedded ion is usually described in terms of the lowering of its continuum level. For strongly coupled plasmas, the phenomenon is intimately related to the equation of state; hence, an accurate treatment is crucial for most astrophysical and inertial-fusion applications, where the case of plasma mixtures is of particular interest. Here we present an experiment showing that the standard density-dependent analytical models are inadequate to describe solid-density plasmas at the temperatures studied, where the reduction of the binding energies for a given species is unaffected by the different plasma environment (ion density) in either the element or compounds of that species, and can be accurately estimated by calculations only involving the energy levels of an isolated neutral atom. The results have implications for the standard approaches to the equation of state calculations.
Highlights
The effect of a dense plasma environment on the energy levels of an embedded ion is usually described in terms of the lowering of its continuum level
Discussion atomic-like features in the electron density are predicted by both IPD10,27,29 and equation of state (EOS) numerical models[30,31], the physical picture just described is incompatible with models ignoring the overlap of atomic orbitals of neighbouring ions
This band-like behaviour poses relevant problems for the widespread EOS/ionization potential depression (IPD) models based on a single ion confined in a neutral sphere of radius rWS7–10
Summary
The effect of a dense plasma environment on the energy levels of an embedded ion is usually described in terms of the lowering of its continuum level. A correct description of how atoms and ions interact with each other in a dense system is of fundamental importance across a wide range of disciplines: in the case of a plasma, the effect of the charged environment on a single ion is usually represented in terms of a lowering of its continuum level[1,2] This energy shift results in a modification of the ionization balance in the plasma, directly related to important properties of the system such as its opacity and equation of state (EOS)[3], which play a pivotal role in our understanding of solar[4], planetary[5] and inertial-fusion[6] systems. Our results imply that a correct interpretation of continuum-lowering effects cannot be based only on the average plasma density (defined as the average ionization times the ion density) and suggest that orbital overlap effects between neighbouring ions need to be included in the numerical models
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