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

Abstract

Radio frequency (RF) sheaths occur under a wide variety of conditions when RF waves, material surfaces and plasma coexist. RF sheaths are of special importance in describing the interaction of ion cyclotron range of frequency (ICRF) waves with the boundary plasma in tokamaks, stellarators and other magnetic confinement devices. In this article the basic physics of RF sheaths is discussed in the context of magnetic fusion research. Techniques for modelling RF sheaths, their interaction with RF wave fields and the resulting consequences are highlighted. The article is intended as a guide for the early-career ICRF researcher, but it may equally well serve to provide an overview of basic RF sheath concepts and modelling directions for any interested fusion scientist.

The geometry of the ICRF-induced wave–SOL interaction. A multi-machine experimental review in view of the ITER operation

“…Most of the phenomenology described below is common with more familiar biasing experiments using electrodes, except that the DC bias is not controlled from outside but imposed by the RF waves. This 'selfbiasing' is described in textbooks for non-magnetized sheaths in RF discharges [7,8] but is more complicated in the presence of a static magnetic field tilted with respect to the wall [3]. In tokamaks, the sheath oscillations are hardly documented experimetally.…”

“…If the sheaths at the two extremities are subject to different levels of rectification, basic double Langmuir probe theory shows that all along the flux tube, V DC adapts to the electrode with larger |V RF | and I DC flows from the high-|V RF | boundary to the low-|V RF | one. Reference [3] calls this type of setup asymmetric RF sheath. If I DC flows only in the parallel direction, one obtains [4]…”

“…As dielectrics behave capacitively, this leads to V RF reduction at the sheath, provided that the sheath is capacitive too. This arises mainly at low density such that the ion plasma frequency is smaller than the RF wave frequency [3,7]. As the dielectric layer becomes thicker, all RF currents can be suppressed locally.…”

“…Reproducing the measured patterns also puts strong constraints on interpretative RF sheath models. Reference [3] proposes a recent tutorial on RF sheath modelling in magnetic fusion plasmas. References [4][5][6] reviewed some comparisons of experiments with modelling in realistic geometry.…”

Main Challenges of Heating Plasma with Waves at the Ion Cyclotron Resonance Frequency (ICRF)

“…This acceleration often boosts ions incident energies up to critical levels for exposed components (i.e., above sputtering yields threshold values). For deeper understanding of the topic, the reader is highly encouraged to refer to excellent tutorial [49] and reviews of key experimental and modelling [51] results.…”