MEASUREMENTS OF ICRF LOADING ON DIII-D WITH AND WITHOUT A FARADAY SHIELD

Measurements of radiofrequency (RF) loading on the Doublet III D shape (DIII-D) tokamak using a fast wave ion cyclotron antenna with and wit hout a Faraday shield present indicate that the loading resistance see n by the antenna is significantly higher at low values of RF power (P less than or equal to 20 kW with no Faraday shield) than is predicted by standard fast wave antenna coupling theory. Measurements of the den sity profile in the plasma edge region allow a direct comparison of ca lculated and measured loading. At high power, the calculated fast wave loading agrees with the measurements. The high loading observed at lo w power can be explained by direct losses in an RF enhanced sheath. Th e density value at the current strap of; approximate to 4 x 10(10) cm( -3) needed to give the observed loading behaviour is in agreement with the measured density values in the plasma scrape-off region, extrapol ated to the current strap location using a simple Bohm diffusion model .