Investigation of Performance Limiting Phenomena in a Variable Phase ICRF Antenna in Alcator C-Mod

High power density, phased antenna operation can often be limited by antenna voltage handling and/or impurity and density production. Using a pair of two-strap antennas for comparison, the performance of a four-strap, fast wave antenna is assessed for a variety of configurations and antenna phase in Alcator C-Mod. To obtain robust voltage handling, the antenna was reconfigured to eliminate regions where the RF E-field is parallel to B or reduce the RF E-field to <1.0 MV/m. To limit impurity generation, BN tiles have replaced the original Mo tiles, a BN clad septum was inserted to limit field line connection length, and BN-metal interfaces are shielded from the plasma. With these modifications, the antenna heating efficiency and impurity generation are nearly identical to the 2-strap antennas and independent of antenna phase in L-mode discharges. This antenna has achieved 11 MW/m in both heating and current drive phase in both L-mode and H-mode discharges.