[en] A recent paper reported an enhancement of the boron K-shell photon ionization cross-section per electron at high photon energies along the principal Hugoniot. Here, it is shown that the enhancement is due to known properties of isolated ions rather than subtle plasma effects. In addition, approximations and corrections to the photon absorption cross-section are examined.

[en] Spectral line Stark broadening calculations in the “standard theory” typically retain only the long-ranged dipole term in the interaction between the perturbing plasma and emitting or absorbing atom. Thus, penetrating collisions as well as higher multipoles are neglected. The full Coulomb interaction is applied to hydrogen lines using a quantum mechanical treatment. Furthermore, it is found that the line widths from the quantum mechanical approach moderately disagree with earlier semi-classical treatment of penetrating collisions.

[en] It was recently shown that a quantum kinetic theory approach to Stark broadening of spectral lines yields corrections to the standard impact theory that may resolve discrepancies between theoretical and experimental widths of isolated lines. Furthermore, the kinetic theory method predicts, contrary to the standard impact theory, different emission and absorption line widths for plasmas in non-thermal equilibrium. This difference presents new opportunities for experimentally resolving the disagreements in electron-impact line broadening. Quantitative predictions suggest that such experiments are feasible

[en] A recent study of Fe xvii R-matrix calculations aimed at resolving outstanding opacity problems claimed that substantial photon absorption from atomic core ionization processes was not previously considered. It is shown, however, that major opacity models already include cross-sections that are equivalent to the enhancements reported by the R-matrix method. Furthermore, the R-matrix calculations neglected important cross-sections that help to explain why the resultant opacity is lower than other models in the spectral range measured in transmission experiments relevant to the solar interior.

[en] For many decades optical interferometers have been used to measure the electron density of plasmas. During the last ten years X-ray lasers in the wavelength range 14-47 nm have enabled researchers to use interferometers to probe even higher density plasmas. The data analysis assumes that the index of refraction is due only to the free electrons, which makes the index of refraction less than one and the electron density proportional to the number of fringe shifts. Recent experiments in Al plasmas observed plasmas with an index of refraction greater than one and brought into question the validity of the usual formula for calculating the index of refraction. Recent calculations showed how the anomalous dispersion from the bound electrons can dominate the index of refraction in many types of plasma and make the index greater than one or enhance the index such that one would greatly overestimate the electron density of the plasma using interferometers. In this work we calculate the index of refraction of C, Al, Ti, and Pd plasmas for photon energies from 0 to 100 eV (12.4 nm) using a new average-atom code. The results show large variations from the free electron approximation under many different plasma conditions. We validate the average-atom code against the more detailed OPAL code for carbon and aluminum plasmas. During the next decade X-ray free electron lasers and other sources will be available to probe a wider variety of plasmas at higher densities and shorter wavelengths so understanding the index of refraction in plasmas will be even more essential