Parasitic Antenna Loading Measurements and Comparison between Shielded and Unshielded Antenna Excitation During Alfven-Wave Heating in Tca

The paper presents experimental measurements of the parasitic loading resulting from a radiofrequency (RF) Langmuir or sheath current which flows from an unshielded antenna to the scrape-off layer (SOL) during Alfven wave heating (AWH) in TCA. The measurements are of relevance to ion cyclotron resonance heating where RF currents drawn by either electrostatic screens or side limiters lead to parasitic loading and harmonic generation in the SOL. An experimental study of the plasma response to AWH after the installation of screens on the AWH antennas is also presented. The results demonstrate that the RF sheath current and some of the parasitic phenomena observed in the SOL during AWH with unshielded antennas are eliminated by screens. The plasma bulk response, however, is dominated by a large density rise which is unaffected by screens.