The effect of charge exchange with neutrals on the saturation of the spectral line intensities of multicharged ions in plasma

The influence of charge-exchange processes on the spectral line intensities of impurity ions in the edge and core plasmas of fusion devices is considered. It is found that, at a sufficiently high density of neutrals, the rate at which the atomic states are populated through charge exchange becomes independent of the neutral density, which results in the saturation of the spectral line intensities. This effect can substantially limit the efficiency of impurity-ion spectroscopy. Conditions under which the saturation effect manifests itself are examined for both the edge and core plasma regions in the presence of fast neutral beams. The results of calculations for the edge plasma are used to interpret the experimental data from the TORE SUPRA tokamak. It is shown that, in the central plasma region, the intensities of the visible spectral lines associated with the charge exchange of impurity ions in the course of neutral beam injection decrease (rather than increase, as was expected earlier) with increasing ion charge.