Results and status of the Alcator C-Mod Tokamak

Recent Alcator C-Mod experimental campaigns have focused upon the study of the Advanced Tokamak regimes, which includes characterization of the RF heating, the formation and dynamics of internal barriers, H-mode edge pedestal, and divertor and scrape-off physics. The ICRF system has been recently upgraded with the improved performance of the 4-strap antenna. Total ICRF power in excess of 5 MW has been launched successfully into the plasma during this campaign. Due to the compact nature of C-Mod, the power feeds for the antenna are vacuum strip lines. Their orientation, to the tokamak B-field, is governed by maintaining E < 15 kV/cm in locations where the RF E-field is parallel to tokamak B-field. Other modifications included improved protection tile grounding and installation of protective shields for Faraday screen ceramic isolators. The antennas also make use of BN protection tiles to eliminate high Z impurities from the antennas. The present empirical power limit results from arcing in a region of the antenna strap where E ∼ 15 kV/cm and parallel to B and injections from the metallic fasteners used to attach the BN tiles to the antenna. In preparation for 2 MA plasma operation, the inner divertor is being modified to a more open design and the lower inner wall is being reinforced. For future 5-10 sec experiments, successful 3 seconds discharges were performed at the end of the last campaign. These experiments were designed to test the magnet power supplies, plasma and supply controls, divertor heat load handling, and density control. The experiments were successful in that the systems performed flawlessly and no uncontrolled density source was observed.