RF Plasma Edge Interactions and Their Impact on ICRF Antenna Performance in Alcator C-Mod

In Alcator C-Mod, we have investigated the compatibility of high power ion cyclotron range of frequencies antenna with high performance plasmas and all high-Z plasma facing components, to provide operational information for future devices such as ITER. Boronization appears to be critical achieving low radiated power plasmas and is eroded rapidly such that the best performance is lost when the integrated injected RF energy reaches ~50 MJ. Since Ohmic H-modes with similar integrated input energy show a significantly slower degradation, an RF erosion mechanism is playing a significant role. RF-enhanced sheaths on flux tubes connected from the antennas to the top of the outer divertor are the most likely erosion mechanism. Antenna operation without a Faraday screen was found to degrade antenna performance through increased impurity sources. ______________________________________________ JNM keywords: Impurities, Plasma-materials interaction PSI-17 keywords: Alcator C-Mod, ICRF, RF, Sheaths PACS: 52.40.Fd, 52.40.Hf, 52.40.Kh, 52.40.Qt *Corresponding author address: MIT Plasma Science and Fusion Center, NW21-103, 190 Albany St. Cambridge, MA 02139 *Corresponding author e-mail: [email protected] Presenting author: Stephen J Wukitch