Experimental investigation of the effects of neon injection in TCV

Abstract

This contribution describes the results of impurity seeding experiments using neon gas injection into the private flux zone of an ohmically heated TCV single null lower, deuterium fuelled, open divertor configuration. In these first experiments, neon has been injected under feedback control on the intensity of the UV NeVIII line at 77 nm. Measurements of the dependence of edge temperature and density on n ̄ e at the outboard divertor target show the divertor in the absence of neon to be operating in the low recycling regime. Neon is observed to penetrate easily into the confined plasma at all densities, increasing core radiation and suppressing radiation in the divertor. This suppression is accompanied by reductions of power and particle flux to the divertor target and D α emission in the divertor volume. The most intense emission is observed in the X-point vicinity and stable discharges have been obtained at radiation fractions, P R /P Ω , up to 0.75. Values of 〈 Z eff〉=1.5–2 and 〈Δ Z eff〉=0.25–0.75 are observed at the highest densities for which the lowest core neon concentrations of ≈0.5–1.0% are obtained, with 〈 Z eff〉 appearing to scale with P R and and n ̄ e according to the empirical relationship derived by Matthews (G.F. Matthews et al., J. Nucl. Mater. 241–243 (1997) 450). There is evidence for an increase in particle confinement when neon is injected, with the effect being most marked at the highest densities when the radiated power fractions are highest. Neon injection is also found to be effective in suppressing ohmic H-modes.