ANALYSIS OF RF SHEATH INTERACTIONS IN TFTR

New theoretical and experimental tools are applied to the analysis of ICRF antenna-edge plasma interactions in the TFTR tokamak. A new numer ical method for computing the three dimensional (3-D) of sheath voltag e distribution is used, and the quantitative predictions of rf sheath theory are compared with measurements of the edge density profile obta ined by microwave reflectometry and with titanium impurity concentrati on data. It is shown that the local density depletion at the antenna i s consistent with density pump-out by strong E x B convection into the Faraday screen (FS). Modelling of the FS impurity influx shows that t he calculated titanium impurity concentration based on this direct inf lux agrees with the measured concentration for ii 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 sh ots, a fraction of the metal impurities migrates around the machine an d is deposited on the limiters, providing a secondary source of titani um. The data show that the central titanium concentration is strongly dependent on antenna phasing. Possible explanations for this phasing d ependence are discussed.