1ANALYSIS OF RF SHEATH INTERACTIONS IN TFTR

New theoretical and experimental tools are applied to the analysis of ICRF antenna-edge plasma interactions in the Tokamak Fusion Test Reactor (TFTR) tokamak. A new numerical method for computing the three-dimensional rf sheath voltage distribution is used, and the quantitative predictions of rf sheath theory are compared with measurements of the edge density profile obtained by microwave reflectometry and with titanium impurity concentration data. It is shown that the local density depletion at the antenna is consistent with density pump-out by strong E · B convection into the Faraday screen. Modeling of the Faraday screen impurity influx shows that the calculated Ti impurity concentration based on this direct influx agrees with the measured concentration for p phasing. It is also shown that screening of impurity neutrals by ionization in the SOL is a large effect and increases with rf power. At high power over many shots, a fraction of the metal impurities migrates around the machine and is deposited on the limiters, providing a secondary source of titanium. The data shows that the central Ti concentration is strongly dependent on antenna phasing. Possible explanations for this phasing dependence are discussed