Experimental investigation of the effects of neon injection in TCV

This contribution describes the results of impurity seeding experiments usi ng neon gas injection into the private fl zone of an ohmically heated TCV s ingle null lower, deuterium fuelled, open divertor configuration. In these first experiments, neon has been injected under feedback control on the int ensity of the UV NeVIII line at 77 nm. Measurements of the dependence of ed ge temperature and density on ii, 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 dens ities, increasing core radiation and suppressing radiation in the divertor. This suppression is accompanied by reductions of power and particle Bur to the divertor target and D-alpha 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-Omega, up to 0.75. Values of (Z(eff))= 1.5-2 and (Delta Z(eff)) = 0.25-0.75 are observed at the highe st densities for which the lowest core neon concentrations of approximate t o 0.5-1.0% are obtained, with (Z(eff)) appearing to scale with P-R and and (n) over bar(e) according to the empirical relationship derived by Matthews (G.F. Matthews et al., J. Nucl. Mater. 241-243 (1997) 450). There is evide nce 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 supp ressing ohmic H-modes. (C) 1999 Elsevier Science B.V. All rights reserved.