Comparison between average-atom and detailed-configuration-type calculations of the photoionization cross section of hot and dense aluminum plasmas.

Abstract

Photoionization cross sections in hot and dense aluminum plasmas have been obtained within the framework of a detailed-configuration (DC) model and two versions of the average-atom (AA) model: In the first the same non--local thermodynamic equilibrium (non-LTE) population distribution was used as that for the DC one (non-LTE-AA), whereas in the second an atom with LTE occupation numbers was used [Thomas-Fermi type (TF)-AA]. Even using the same non-LTE occupation numbers, significant differences were found in the photoionization cross sections as calculated by means of the non-LTE-AA and the DC models particularly near the edge regions. These differences include mainly the relative shift of the edges and their splitting which leads to a larger tip-to-dip ratio near the edge in the AA than that in the DC model, as well as a structured edge region in the DC model relative to the simple sharp edge in the AA model. Far from the edges in the high-photon-energy region, there are only minor differences in the predictions of the two models. The comparison between the non-LTE-AA and the TF-AA models reveals huge quantitative differences in low-density plasmas where a coronal equilibrium prevails; these originate from the different occupation numbers used in the two models. Such differences become smaller in a high-density plasma where the non-LTE-AA model has, practically, an LTE population similar to that of the TF-AA model.