K-ALPHA EMISSION IN EQUILIBRIUM AND NONEQUILIBRIUM PLASMAS

A detailed spectral model has been under development for the computer simulation of 2p --> 1s K alpha x-ray emissions from highly-charged Fe ions in plasmas. Particular interest has been directed at the K alpha emissions produced by the ions from Fe XVIII to Fe XXIV, which occur in the spectral range from 1.84 to 1.94 Angstrom. Account has been tak en of the fundamental radiative emission processes associated with rad iationless 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 d ue to the effects of ion transport processes. However, the assumption of equilibrium (Maxwellian) electron energy distributions is usually a ccepted to be valid. Particular emphasis has now been directed at the identification of spectral features that can serve as diagnostics of e xtreme non-equilibrium or transient-ionization conditions, which can o ccur in both low-density and high-density plasmas as well as in electr on-ion beam interactions. In the development of the general theoretica l description of the emission spectra, it has been necessary to allow for an arbitrary (non-Maxwellian) electron energy distribution. In ord er to provide a microscopic investigation of the fundamental K alpha l ine-formation processes that can play a dominant role under extreme no n-equilibrium conditions, the observed x-ray emission spectra from Fe ions in the Electron Beam Ion Trap (EBIT) facility at the Lawrence Liv ermore National Laboratory have been simulated. For the precise interp retation of the high-resolution x-ray observations, which may involve the analysis of blended spectral features composed of many lines, it h as been necessary to take into account the multitude of fine-structure components of the K alpha radiative transitions in the ions from Fe X VIII to Fe XXIV. A complex situation can be encountered for densities that are higher than the validity range of the simple corona-model app roximation. 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 mo dels for the distribution of the initial ions among the different low- lying fine-structure states. The inadequacies of this simple approxima tion can be fundamentally remedied only by tie application of a detail ed (and possibly time-dependent) collisional-radiative-model descripti on of the fine-structure excitation, de-excitation, and ionization pro cesses. It has been found that inner-shell-electron collisional excita tion and ionization processes involving the complex intermediate ions from Fe XVIII to Fe XXI produce spectral features, in the wavelength r ange from 1.89 to 1.94 Angstrom, which are particularly sensitive to d ensity variations and transient-ionization conditions. For a precise a nalysis of the x-ray observations produced by anisotropic (directed) e lectron collisional excitations, it will be necessary in a future exte nsion of this investigation to take into account the angular distribut ion and polarization of the emitted radiation. Published by Elsevier S cience Ltd.