Large enhancement in high-energy photoionization of Fe XVII and missing continuum plasma opacity

TL;DR

This paper reveals significant high-energy photoabsorption in Fe XVII due to core photo-excitations, substantially increasing plasma opacity models and aligning them more closely with experimental measurements.

Contribution

It introduces extensive R-Matrix calculations for Fe XVII, uncovering previously neglected photo-excitation effects that greatly enhance opacity estimates.

Findings

Up to orders of magnitude increase in photoionization cross sections.

Identification of new core photo-excitation resonances.

35% increase in Rosseland Mean Opacity.

Abstract

Aimed at solving the outstanding problem of solar opacity, and radiation transport plasma models in general, we report substantial photoabsorption in the high-energy regime due to atomic core photo-excitations not heretofore considered. In extensive R-Matrix calculations of unprecedented complexity for an important iron ion Fe xvii (Fe$^{16+}$), with a wave function expansion of 99 Fe xviii (Fe$^{17+}$) LS core states from $n \leq 4$ complexes (equivalent to 218 fine structure levels), we find: i) up to orders of magnitude enhancement in background photoionization cross sections, in addition to strongly peaked photo-excitation-of-core resonances not considered in current opacity models, and ii) demonstrate convergence with respect to successive core excitations. The resulting increase in the monochromatic continuum, and 35% in the Rosseland Mean Opacity, are compared with the "higher-than-predicted" iron opacity measured at the Sandia Z-pinch fusion device at solar interior conditions.