Measurement of rf voltages on the plasma‐touching surfaces of ICRF antennas

Measurements of the rf voltages on Faraday shields and protection bumpers have been made for several loop antennas, including the mock‐up antenna and A1 for JET, the original antenna for Tore Supra, the present ASDEX‐U antenna, and the folded waveguide. The loop antennas show voltages that scale to ≊12 kV for a maximum input voltage of 30 kV with 0/0 phasing. The voltages are dramatically reduced for 0/π phasing. These voltages are significant in that they can substantially increase the rf sheath potential beyond the levels associated with the simple electromagnetic field linkage from the current straps that results in plasma heating. In this paper, we investigate and measure the source of these voltages, their scaling with antenna impedance, and the differences between the loop arrays.