Modelling of radio frequency sheath and fast wave coupling on the realistic ion cyclotron resonant antenna surroundings and the outer wall

Abstract

In order to model the sheath rectification in a realistic geometry over the size of Ion Cyclotron Resonant Heating (ICRH) antennas, the Self-consistent Sheaths and Waves for ICH (SSWICH) code couples self-consistently the RF wave propagation and the DC SOL biasing via non-linear RF and DC sheath boundary conditions (SBCs) applied at plasma/wall interfaces. A first version of SSWICH had 2D (toroidal and radial) geometry, rectangular walls either normal or parallel to the confinement magnetic field B0 and only included the evanescent slow wave (SW) excited parasitically by the ICRH antenna. The main wave for plasma heating, the fast wave (FW) plays no role on the sheath excitation in this version. A new version of the code, 2D SSWICH-Full Wave, was developed based on the COMSOL software, to accommodate full RF field polarization and shaped walls tilted with respect to B0. SSWICH-Full Wave simulations have shown the mode conversion of FW into SW occurring at the sharp corners where the boundary shape varies rapidly. It has also evidenced "far-field" sheath oscillations appearing at the shaped walls with a relatively long magnetic connection length to the antenna, that are only accessible to the propagating FW. Joint simulation, conducted by SSWICH-Full Wave within a multi-2D approach excited using the 3D wave coupling code (RAPLICASOL), has recovered the double-hump poloidal structure measured in the experimental temperature and potential maps when only the SW is modelled. The FW contribution on the potential poloidal structure seems to be affected by the 3D effects, which was ignored in the current stage. Finally, SSWICH-Full Wave simulation revealed the left-right asymmetry that has been observed extensively in the unbalanced strap feeding experiment s , suggesting that the spatial proximity effects in RF sheath excitation, studied for SW only previously, is st ill important in the vicinity of the wave launcher under full wave polarizations.

Filament-assisted mode conversion in magnetized plasmas

“…It also naturally explains the observed origin of the parallel RF power flow from the SOL in front of the antenna rather than from the antenna surface itself 26 . It does not contradict any of the existing proposed explanations: Wavefilament bound states could act in synergy with the "opening" of the boundary plasma to FW propagation 27 , and with sheath rectification and the subsequent parallel spread of DC bias via DC current transport [28][29][30] . In future work, we will use Finite Element calculations to ascertain to what extent the claims in this paper depend on the unrealistic assumptions made in the Mie solution, such as non-circular filament cross sections, steep but continuous density gradients at the filament interface, and nonconstant background density.…”

Optimization of discharges with ion cyclotron range of frequencies using local gas injection in EAST

“…Note we assumed ideal dipole phasing which is far from guaranteed considering absence of real-time control, so that the real situation might even be worse. Further details would lead too far beyond the scope of the present study and will be the object of another paper devoted to characterization of the EAST ICRF antenna near-field patterns both through experiments and modelling with other devoted codes like SSWICH [36].…”

A tutorial on radio frequency sheath physics for magnetically confined fusion devices

“…But there are other factors, which influence the DC sheath potential. In particular, if there is good DC electrical conductivity between the antenna sheath contact point and the magnetically connected far-field sheath, the antenna sheath which is generally stronger may dominate the DC sheath potential at the remote location (Lu et al 2018;Myra et al 2020). Also, to the extent permitted by finite DC resistivity, a DC potential difference may be established between the antenna sheath and the far-field sheath governed in part by the DC plasma current which will flow between these two surfaces.…”