The deduction of low-Z ion temperature and densities in the JET tokamak using charge exchange recombination spectroscopy

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Abstract
A charge exchange recombination spectroscopy (CXRS) diagnostic has been established on JET to study fully stripped low-Z species. Ion temperature in the plasma centre is measured from visible lines of helium, carbon and oxygen excited by charge exchange with heating neutral beam particles. Coincident cold components produced at the plasma edge are apparent on helium and carbon spectra and most spectra are subject to accidental blending from other species' edge plasma emission. The charge exchange feature can be isolated from the various composite lines and all three impurities agree on the same temperature within experimental error. Observed column emissivities are converted into absolute impurity densities using a neutral beam attenuation code and charge exchange effective rate coefficients. The models take detailed account of cascading and the influence of the plasma environment in causing l-mixing, and allow the n-dependence of the rate coefficients to be addressed experimentally. Some illustrative results are shown for helium (helium discharge or minority r.f. heating), carbon and oxygen concentrations monitored during characteristic operating regimes.