Calculation of RF sheath properties from surface wave-fields: a post-processing method

Abstract

In ion cyclotron range of frequency (ICRF) experiments in fusion research devices, radio frequency (RF) sheaths form where plasma, strong RF wave fields and material surfaces coexist. These RF sheaths affect plasma material interactions such as sputtering and localized power deposition, as well as the global RF wave fields themselves. RF sheaths may be modeled by employing a sheath boundary condition (BC) in place of the more customary conducting wall BC; however, there are still many legacy ICRF computer codes that do not implement the sheath BC. In this paper we present a method for post-processing results obtained with the conducting wall BC. The post-processing method produces results that are equivalent to those that would have been obtained with the RF sheath BC, under certain assumptions. The post-processing method is also useful for verification of sheath BC implementations and as a guide to interpretation and understanding of the role of RF sheaths and their interactions with the waves that drive them.

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

“…Although implementation of the sheath BC in full wave RF codes is required for accurate modelling, it is possible under some limitations to gain information about RF sheaths from codes which implement the more conventional perfectly conducting wall (CW) BC, E t = 0 using a sheath post-processing method (Myra & Kohno 2019a). The basic idea relies on the assumption that, observing from the surface, the incoming waves are not affected by whether the BC is the CW BC or the sheath BC.…”

“…Fixed-point iteration has been implemented in several codes including a COMSOL workflow (Beers et al 2021), codes under development in the US RF SciDAC project (Bonoli et al 2020) and SSWICH in the EU (Tierens et al 2019). It has been shown to converge rapidly (fewer than a dozen iterations) in one-, two- (Myra & Kohno 2019a) and three-dimensional (Beers et al 2021) cases where the boundary shapes were relatively simple and parameters where not close to multiple root conditions ( § 6.2). Convergence properties for a realistic three-dimensional antenna tokamak geometry remain to be investigated.…”

Recent progress in modeling ICRF-edge plasma interactions with application to ASDEX Upgrade

“…The recent development of the SSWICH code allows it to handle contributions both from the slow wave (SW) and the fast wave (FW) [23]. More recently, the study of RF sheath theory and models have been significantly developed by parameterizing the RF sheath impedance, the sheath-rectified DC potential and net DC current flow through the sheath [24][25][26] and using oblique magnetic field lines at the sheath entrance [27]. By incorporating this sheath impedance model with the 2D selfconsistent finite element code rfSOL [28,29], the interaction between waves in plasmas and sheaths was studied [29].…”