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

Abstract

Radio frequency (RF) sheaths are suspected of limiting the performance of present-day ion cyclotron range of frequencies (ICRFs) antennas over long pulses and should be minimized in future fusion devices. Within the simplest models, RF-sheath effects are quantified by the integral V RF = ∫ 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 V RF is excited by parallel RF currents j ∥ flowing on the antenna structure. In this paper, the validity of this simple sheath theory is tested experimentally on the Tore Supra (TS) ITER-like antenna prototype (ILP), together with antenna simulation and post-processing codes developed to compute V RF. The predicted poloidal localization of high-|V RF| zones is confronted to that inferred from experimental data analysis. Surface temperature distribution on ILP front face, as well as ILP-induced modifications of RF coupling and hot spots on a magnetically connected lower hybrid current drive antenna, indicates local maxima of dc plasma potential in both the upper and lower parts of the ILP. This result, qualitatively conforming to V RF simulations, is interpreted in terms of j ∥ flowing on ILP frame. Once the validation is done, such reliable theoretical models and numerical codes are then employed to provide predictive results. Indeed, we propose two ways to reduce |V RF| by acting on j ∥ on the antenna front face. The first method, more adapted for protruding antennas, consists of avoiding the j ∥ circulation on the antenna structure, by slotting the antenna frame on its horizontal edges and by partially cutting the Faraday screen rods. The second method, well suited for recessed antennas, consists of compensating j ∥ of opposite signs along long flux tubes, with parallelepiped antennas aligned with (tilted) flux tubes. The different concepts are assessed numerically on a two-strap TS 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 substantially reduce |V RF| over the whole antenna height.

Radio-frequency sheaths physics: Experimental characterization on Tore Supra and related self-consistent modeling

“…The new electrical design proposed to mitigate RF sheaths by minimizing the integrated parallel RF electric on the "long field lines" in the SOL. 7 Multiple diagnostics on TS were available: infrared (IR) thermography, 8 two-dimensional mappings by reciprocating Langmuir probes (RCPs), 5,9 Vacuum Utraviolet (VUV) spectroscopy, 10 calorimetry of actively cooled plasma facing components. 11 Complementary to the experiments on Tore Supra, a better theoretical understanding of the first principles mechanisms driving RF sheaths is needed.…”

ICRF operation with improved antennas in ASDEX Upgrade with W wall

“…In Tore Supra, modifications of the Faraday screen have been made [7] in order to affect the pattern of the parallel RF currents on the antenna frame and this way minimize the integrated E || on the long field lines. However, experimental data [8] suggests that the minimization of the integrated E || on the long field lines is not enough to reduce the wave-plasma interactions on the antenna itself.…”

Self consistent radio-frequency wave propagation and peripheral direct current plasma biasing: Simplified three dimensional non-linear treatment in the “wide sheath” asymptotic regime

“…This procedure, although clearly not self-consistent, was widely implemented as the only tool able to model realistic wave-launching structures. [5][6][7][8][9][10][11][12][13][14][15] It was shown to reproduce qualitatively some RF-induced SOL modifications observed around powered antennae. 6,8,10,11,16 Yet its quantitative validity is questionable.…”

“…[5][6][7][8][9][10][11][12][13][14][15] It was shown to reproduce qualitatively some RF-induced SOL modifications observed around powered antennae. 6,8,10,11,16 Yet its quantitative validity is questionable. 17 Moreover, contradictions appeared with recent measurements, e.g., the scaling of rectified DC plasma potential with RF power, 18 the radial penetration of the plasma bias, 19 or the non-linear generation of edge DC currents by RF waves.…”