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.
The rate at which atoms and ions within a plasma are further ionized by collisions with the free electrons is a fundamental parameter that dictates the dynamics of plasma systems at intermediate and high densities. While collision rates are well known experimentally in a few dilute systems, similar measurements for nonideal plasmas at densities approaching or exceeding those of solids remain elusive. Here we describe a spectroscopic method to study collision rates in solid-density aluminium plasmas created and diagnosed using the Linac Coherent light Source free-electron X-ray laser, tuned to specific interaction pathways around the absorption edges of ionic charge states. We estimate the rate of collisional ionization in solid-density aluminium plasmas at temperatures B30 eV to be several times higher than that predicted by standard semiempirical models.
The possibility of creating hot-dense plasma samples by isochoric heating of solid targets with high-intensity femtosecond X-ray lasers has opened up new opportunities in the experimental study of such systems. A study of the X-ray spectra emitted from solid density plasmas has provided significant insight into the X-ray absorption mechanisms, subsequent target heating, and the conditions of temperature, electron density, and ionization stages produced (Vinko et al. Nature 482, 5962). Furthermore, detailed analysis of the spectra has provided new information on the degree of ionization potential depression in these strongly coupled plasmas (Ciricosta et al. PRL 109, 065002). Excellent agreement between experimental and simulated spectra has been obtained, but a full outline of the procedure by which this has been achieved has yet to be documented. We present here the details and approximations concerning the modelling of the experiment described in the above referenced work. We show that it is crucial to take into account the spatial and temporal gradients in simulating the overall emission spectra, and discuss how aspects of the model used affect the interpretation of the data in terms of charge-resolved measurements of the ionization potential depression.
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