Design of folded waveguide antenna for Alcator C-Mod

The Oak Ridge National Laboratory (ORNL) ion cyclotron range of frequencies (ICRF) antenna for Alcator C-Mod is a folded waveguide (FWG) antenna designed to determine whether the FWG can serve as a high power density, ceramic-free antenna for both present heating and fast wave current drive (FWCD) applications and for future tokamaks such as the International Thermonuclear Experimental Reactor (ITER) and the Tokamak Physics Experiment (TPX). The FWG is particularly attractive because it has a low internal electric field per unit power coupled to the plasma. This results in more power capability and has been demonstrated by 1-MW (unloaded) tests on the Radio Frequency Test Facility (RFTF). The experiment will characterize the impact of an FWG on impurity control in the presence of high power density and on central heating. The antenna is designed to withstand the tokamak environment, including high heat fluxes, high-temperature bakeout, and major disruptions, without vacuum leaks. The front face is curved to fit the plasma outline. Two front plates are fabricated for the antenna, one with full-width slits at every other location between vanes and the other with alternating right and left half-width slits at every location between vanes for pi-phasing.