Initial Operation of the Alcator C-Mod ICRF Antennas with High-Z Metal Antenna Guards

The Alcator C-Mod ICRF antennas have been operated with BN antenna guards since 2000. This modification had followed the observation that metallic impurities in the plasma increased with increasing ICRF power. Systematic improvements to the antenna structure have allowed the launched power to be raised to the 6 MW level, with good heating efficiency and few deleterious effects on the plasma. BN is inherently fragile, and disruption mechanical shocks have resulted in fracturing of the antenna tiles, exposing the supporting metal structure. Since the antennas are now electrically in reasonable shape, and one of C-Mod's goals is to study the behavior of plasmas with all-metal plasma facing components, it was decided to replace the BN tiles with molybdenum. High power operation was resumed in March, 2005, and the behavior of the antennas during the present run campaign will be reported