Experimental evidence for sheath effects at the ICRF antenna and ensuing changes in the plasma boundary during ICRF on TEXTOR

Measurements of a generated dc current between a Faraday-shielded ICRF antenna and the wall (liner) were performed on the ICRF-heated TEXTOR plasma. It is observed that the dc current, corresponding to electrons drawn by the antenna, increases by a factor of 8, reaching a value of 80 A for 200 kW ICRF power applied to the antenna. It is shown that this increased dc current is mainly due to the plasma sheath effect at the ICRF antenna. The resulting changes in the plasma edge are very similar to those observed during negative biasing of the ALT-I limiter on TEXTOR. The most important consequences of the sheath effects are the occurrence of a density bump in the scrape-off layer (SOL) due to particle transport changes, and the generation of waves at harmonics of the RF generator frequency. (orig.)