RF current distribution and topology of RF sheath potentials in front of ICRF antennae

Abstract

The 2D (radial/poloidal) spatial topology of RF-induced convective cells developing radially in front of ion cyclotron range of frequency (ICRF) antennae is investigated, in relation to the spatial distribution of RF currents over the metallic structure of the antenna. This is done via a Green's function, determined from the ICRF wave coupling equations, and well-suited to open field lines extending toroidally far away on both sides of the antenna. Using such formalism, combined with a full-wave calculation using the 3D antenna code ICANT (Pécoul S. et al 2000 Comput. Phys. Commun. 146 166-87), two classes of convective cells are analysed. The first one appears in front of phased arrays of straps, and depending on the strap phasing, its topology is interpreted using the poloidal profiles of either the RF current or the RF voltage of the strip line theory. The other class of convective cells is specific to antenna box corners and is evidenced for the first time. Based on such analysis, general design rules are worked out in order to reduce the RF-sheath potentials, which generalize those proposed in the earlier literature, and concrete antenna design options are tested numerically. The merits of aligning all strap centres on the same (tilted) flux tube, and of reducing the antenna box toroidal conductivity in its lower and upper parts, are discussed.

ICRF specific plasma wall interactions in JET with the ITER-like wall

“…This implies a modification of density in front of the antennas by ICRF power [5] which could be attributed to convective cells [19,20]. The spectroscopic observations of the increased Be sputtering at the limiters are consistent with the spatial sputtering patterns moving in accordance with magnetic field connections to active ICRF antennas.…”

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.…”

Development of steady-state operation using ion cyclotron heating in the Large Helical Device

“…6 In previous works to decrease impurity generation during ICH, a low-Z coating on plasma-facing components, 7 reducing the sheath potential by the antenna phase, 8 decreasing the inductive imaginary current on the Faraday screen (FS) caused by misalignment 9 and rotating the center conductor 10 to a magnetic field line (MFL) have been attempted, and a favorable ICH with low impurity generation and relatively high heating efficiency was achieved with a phasing dipole. [11][12][13][14] It has been reported that field alignment for the ICRF antenna should theoretically decrease the average RF sheath potential along magnetic field lines in the Alcator C-Mod, 15 and that low-impurity production is achieved by dipole phasing using a field-aligned ICRF antenna. 16 In steady-state operation, dipole phasing may, indeed, be one of the candidate methods for decreasing impurity generation during ICH, and a toroidal phasing experiment was planned to execute a stable steady-state plasma using a field-aligned ICRF antenna in the LHD.…”