Recombination mechanisms in He-like argon spectra measured in low-density plasmas

The formation mechanisms of K a emission from hot low-density plasmas are analysed on the basis of accurate measurements of the He-like tokamak spectra carried out over a wide range of plasma conditions. The analysis is supported by the improved calculations of the relevant atomic data. The main focus was made on a study of recombination processes responsible for ionization balance and atomic levels population. In distinction to astrophysical plasma, the simulation of fusion spectra in the coronal approximation showed conflicting results. The derived density of Li-like ions is highly overestimated, while the density of H-like ions appears to be close to the corresponding coronal distribution. The complex influence of charge-exchange recombination (CXR) on the intensity of He-like lines is shown to be a main reason for such confusing interpretation of the measurements. A simple analytical consideration is proposed to explain the behaviour of H-like ions in ohmically heated plasmas. The results of the systematic study of the ionization balance for such plasmas based on the spectra measured at the tokamak TEXTOR have been used to provide a quantitative description of the influence of the background of neutral particles and anomalous transport on the intensity of the spectral lines. The possibility of separating the influence of CXR and impurity transport from the analysis of the spectra is also shown.