Plasma-material interactions in TFTR

This paper presents a summary of plasma-material interactions which influence the operation of TFTR with high current (≤ 2.2 MA) ohmically heated, and high-power (∼ 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_e = 5-6 × 10¹⁹ m^-3) values of Z_eff are ≦1.5. Increases in Z_eff of ≦1 have been observed with neutral beam heating of 10 MW. The primary low Z impurity is carbon with concentrations decreasing from ∼10% to <1% with increasing n_e. Oxygen densities tend to increase with n_e, and at the ohmic plasma density limit oxygen and carbon concentrations are comparable. Chromium getter experiments and He²⁺/D⁺ 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 (∼10^-4 n_e) concentrations only at low plasma densities (n_e <10¹⁹ m^-3). The primary source of metallic impurities is most likely ion sputtering from metals deposited on the carbon limiter surface.