RF-Sheath Assessment of ICRF Antenna Geometry for Long Pulses

Monitoring powered Ion Cyclotron Resonance Frequency (ICRF) antennas in magnetic fusion devices has revealed localized modifications of the plasma edge in the antenna shadow, most of them probably related to an enhanced polarization of the Scrape‐Off Layer (SOL) through Radio‐Frequency (RF) sheath rectification. Although tolerable on present short RF pulses, sheaths should be minimized, as they may hinder proper operation of steady‐state antennas and other subsystems connected magnetically to them, such as lower hybrid grills. As a first step towards mitigating RF sheaths in the design of future antennas, the present paper analyses the spatial structure of sheath potential maps in their vicinity, in relation with the 3D topology of RF near fields and the geometry of antenna front faces. Various combinations of poloidal radiating straps are first considered, and results are confronted to those inferred from transmission line theory. The dependence of sheath potentials on RF voltages or RF currents is studied. The role of RF near‐field symmetries along tilted field lines is stressed to interpret such effects as that of strap phasing. A generalization of the “dipole effect” is proposed. With similar arguments, the behavior of Faraday screen corners, where hot spots concentrate on Tore Supra (TS), is then studied. The merits of aligning the antenna structure with the tilted magnetic field are thus discussed. The effect of switching from TS (high RF voltage near corners) to ITER‐like electrical configurations of the straps (high voltage near equatorial plane) is also analyzed.