Investigation of C v line ratio variations in a tokamak with an application to neutral hydrogen measurement

Significant differences in the ratio between the He-like carbon triplet lines 1s2s ${}_{}{}^3{}_{}{}^{}$${\mathit{S}}_{1\mathrm{-}}$1s2p ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_2$ (2271.59 Å) and 1s2${\mathit{p}}^3$${\mathit{S}}_{1\mathrm{-}}$1s2p ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_{0,1}$ (2278.41 Å) have been observed in line of sight measurements across the TJ-I tokamak. Such variations, which deviate from the 2 ${}_{}{}^3{}_{}{}^{}$P level statistical ratios, have not been reported for this line ratio in similar fusion plasmas. We show that such differences can be explained by charge exchange, involving collisions between H-like carbon ions and neutral hydrogen atoms, while ruling out excitation and deexcitation by electron collisions and line blending effects. Finally we discuss similar transitions in other low Z ions and propose monitoring these variations as a spectroscopic tool for estimating relative neutral concentrations in plasmas. (c) 1995 The American Physical Society