Analysis of 4-strap ICRF antenna performance in Alcator C-Mod

A 4-strap ICRF antenna was designed and fabricated for plasma heating and current drive in the Alcator C-Mod tokamak. Initial upgrades were carried out in 2000 and 2001, which eliminated surface arcing between the metallic protection tiles and reduced plasma-wall interactions at the antenna front surface. A boron nitride septum was added at the antenna midplane to intersect electric fields resulting from rf sheath rectification, which eliminated antenna corner heating at high power levels. The current feeds to the radiating straps were reoriented from an Eparallel toB to Eperpendicular toB geometry, avoiding the empirically observed similar to15 kV/cm field limit and raising antenna voltage holding capability. Further modifications were carried out in 2002 and 2003. These included changes to the antenna current strap, the boron nitride the mounting geometry, and shielding the BN-metal interface from the plasma. The antenna heating efficiency, power and voltage characteristics under these various configurations will be presented.