Development of Impedance Sheath Boundary Condition in Stix Finite Element RF Code

TL;DR

This paper develops and implements a finite impedance sheath boundary condition in the Stix finite element RF code to better model RF sheath effects at material boundaries in fusion devices.

Contribution

It introduces a non-linear RF sheath boundary condition based on a model by Myra and D'Ippolito into the Stix finite element solver, enabling more accurate boundary modeling.

Findings

Successful implementation of the sheath boundary condition in Stix.

Verification against existing results shows improved boundary modeling.

Parallelized solver handles pseudo-1D and pseudo-2D geometries.

Abstract

Ion cyclotron radio frequency range (ICRF) power plays an important role in heating and current drive in fusion devices. However, experiments show that in the ICRF regime there is a formation of a radio frequency (RF) sheath at the material and antenna boundaries that influences sputtering and power dissipation. Given the size of the sheath relative to the scale of the device, it can be approximated as a boundary condition (BC). Electromagnetic field solvers in the ICRF regime typically treat material boundaries as perfectly conducting, thus ignoring the effect of the RF sheath. Here we describe progress on implementing a model for the RF sheath based on a finite impedance sheath BC formulated by J. Myra and D. A. D'Ippolito, Physics of Plasmas 22 (2015) which provides a representation of the RF rectified sheath including capacitive and resistive effects. This research will discuss the results from the development of a parallelized cold-plasma wave equation solver Stix that implements this non-linear sheath impedance BC through the method of finite elements in pseudo-1D and pseudo-2D using the MFEM library. The verification and comparison of the sheath BC from Stix with results from H. Kohno and J. Myra, Computer Physics Communications 220 129-142 (2017) will also be discussed.