Tuesday, November 16, 2021
As ion cyclotron radio frequency range (ICRF) heating becomes increasing used in fusion devices, the urgency of predicting and mitigating impurity generation that arises from it becomes increasingly important. In the ICRF regime, rectified RF sheaths are known to form at antenna and material edges that influence negative effects like sputtering and a decrease in heating efficiency. 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. A novel model for the RF sheath based on the a finite impedance sheath BC formulated by J. Myra et al. 2015 now provides a more comprehensive representation of the RF rectified sheath including capacitive and resistive effects in numerical RF simulations. This research will discuss 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 aimed at investigating the fundamental behavior of sheaths. With the RF sheath BC implemented in Stix and verified against literature, Stix is currently being adapted to investigate the RF sheath potential behavior seen on Alcator C-Mod when the antenna’s ratio of center strap to outer strap power was varied. The ultimate aim of this study will be to couple Stix’s RF sheath potentials to an impurity generation code, RustBCA, as an integrated model to qualitatively reproduce the trend in the impurity flux measured in this C-Mod experiment.