Reduction of RF sheaths potentials by compensation or suppression of parallel RF currents on ICRF antennae

Abstract

Radio Frequency (RF) sheaths are suspected to limit the performance of present‐day Ion Cyclotron Range of Frequencies (ICRF) antennae over long pulses and should be minimized in future Fusion devices. Within the simplest models, RF sheath effects are quantified by the integral VRF = ∫E//⋅dl where the parallel RF field E// is linked with the slow wave. On “long open field lines” with large toroidal extension on both sides of the antenna it was shown that VRF is excited by parallel RF currents j// flowing on the antenna structure. We thus propose two ways to reduce |VRF| by acting on j// on the antenna front face. The first method, more adapted for protruding antennae, consists in avoiding the j// circulation on the antenna structure, by slotting the antenna frame on its horizontal edges and by cutting partially the Faraday screen rods. The second method, well suited for recessed antennae, consists in compensating j// of opposite signs along long flux tubes, with parallelepiped antennae aligned with tilted flux tubes. The different concepts are assessed numerically on a 2‐strap Tore Supra antenna phased [0, π] using near RF fields from the antenna code TOPICA. Simulations stress the need to suppress all current paths for j// to reduce substantially |VRF| over the whole antenna height.