Kα X-ray emission spectra from highly charged Fe ions in EBIT

Abstract

A detailed spectral model has been developed for the computer simulation of the 2p → 1s Kα X-ray emission from highly charged Fe ions in the electron beam ion trap (EBIT). The spectral features of interest occur in the range 1.84–1.94 Å. The fundamental radiative emission processes associated with radiationless electron capture or dielectronic recombination, inner-shell electron collisional excitation, and inner-shell-electron collisional ionization are taken in account. For comparison, spectral observations and simulations for high-temperature magnetic-fusion (tokamak) plasmas are reviewed. In these plasmas, small departures from steady-state corona-model charge-state distributions can occur because of ion transport processes, while the assumption of equilibrium (Maxwellian) electron energy distributions is expected to be valid. Our investigations for EBIT have been directed at the identification of spectral features that can serve as diagnostics of extreme nonequilibrium or transient ionization conditions, and allowance has been made for general (non-Maxwellian) electron energy distributions. 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 individual fine-structure components of the Kα radiative transitions in the ions from Fe XVIII to Fe XXV. At electron densities higher than the validity range of the corona-model approximation, collisionally induced transitions among low-lying excited states can play an important role. It is found that inner-shell-electron 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. PACS Nos.: 52.72.+v, 32.80.Dz, 32.70.Fw, 32.30.Rj