Plasma-material interactions in TFTR

Abstract

This paper presents a summary of plasma-material interactions which influence the operation of TFTR with high current (less than or equal to 2 MA), ohmically heated and high power (approx. 10 MW), neutral-beam-heated plasmas. The conditioning procedures which are applied routinely to the first-wall hardware are reviewed. Fueling characteristics during gas, pellet, and neutral beam fueling are described. Recycling coefficients near unity are observed for most gas-fueled discharges. Gas-fueled discharges after helium discharge conditioning of the toroidal bumper limiter and discharges fueled by neutral beams and pellets show R < 1. In the vicinity of the gas-fueled density limit (at n/sub e/ = 5 to 6 x 10/sup +19/ m/sup -3/) values of less than or equal to 1.5. Increases in Z/eff of less than or equal to 1 have been observed with neutral beam heating of 10 MW. The primary low-Z impurity is carbon with concentrations decreasing from approx.10% to <1% with increasing n/sub e/. Oxygen densities tend to increase with n/sub e/, and at the ohmic plasma density limit oxygen and carbon concentrations are comparable. Chromium getter experiments and He/sup + +//D/sup +/ plasma comparisons indicate that the limiter is the primary source of carbon and that the vessel wall is a significant source of the oxygen impurity. Metallic impurities, consisting of the vacuum vessel metals (Ni, Fe, Cr), have significant (approx. 10/sup -4/ n/sub e/) concentrations only at low plasma densities (n/sub e/ < 10/sup +19/ m/sup -3/). The primary source of metallic impurities is most likely ion sputtering from metals deposited on the carbon limiter surface.