Kα emission in equilibrium and non-equilibrium plasmas

Abstract

A detailed spectral model has been under development for the computer simulation of 2 p → ls Kα x-ray emissions from highly-charged Fe ions in plasmas. Particular interest has been directed at the Ka emissions produced by the ions from Fe XVIII to Fe XXIV, which occur in the spectral range from 1.84 to 1.94 Å. Account has been taken of the fundamental radiative emission processes associated with radiationless electron capture or dielectronic recombination, inner-shell-electron collisional excitation, and inner-shell-electron collisional ionization. In high-temperature plasmas, small or moderate departures from steady-state corona-model charge-state distributions may occur due to the effects of ion transport processes. However, the assumption of equilibrium (Maxwellian) electron energy distributions is usually accepted to be valid. Particular emphasis has now been directed at the identification of spectral features that can serve as diagnostics of extreme non-equilibrium or transient-ionization conditions, which can occur in both low-density and high-density plasmas as well as in electron-ion beam interactions. In the development of the general theoretical description of the emission spectra, it has been necessary to allow for an arbitrary (non-Maxwellian) electron energy distribution. In order to provide a microscopic investigation of the fundamental Ka line-formation processes that can play a dominant role under extreme non-equilibrium conditions, the observed x-ray emission spectra from Fe ions in the Electron Beam Ion Trap (EBIT) facility at the Lawrence Livermore National Laboratory have been simulated. For the precise interpretation of the high-resolution x-ray observations, which may involve the analysis of blended spectral features composed of many lines, it has been necessary to take into account the multitude of fine-structure components of the Ka radiative transitions in the ions from Fe XVIII to Fe XXIV. A complex situation can be encountered for densities that are higher than the validity range of the simple corona-model approximation. With increasing density, collisionally-induced transitions among the low-lying fine-structure states can play an important role. We have developed a hierarchy of simplified statistical-population models for the distribution of the initial ions among the different low-lying fine-structure states. The inadequacies of this simple approximation can be fundamentally remedied only by the application of a detailed (and possibly time-dependent) collisional-radiative-model description of the fine-structure excitation, de-excitation, and ionization processes. It has been found that inner-shell-electron collisional excitation and ionization processes involving the complex intermediate ions from Fe XVIII to Fe XXI produce spectral features, in the wavelength range from 1.89 to 1.94 Å, which are particularly sensitive to density variations and transient-ionization conditions. For a precise analysis of the x-ray observations produced by anisotropic (directed) electron collisional excitations, it will be necessary in a future extension of this investigation to take into account the angular distribution and polarization of the emitted radiation.