Analytical and numerical modeling of RF sheath boundaries in ICRF heating experiments

Sheath potentials formed on the material surfaces of plasma facing components are dramatically enhanced by the application of radio-frequency (RF) power in the ion-cyclotron (IC) range of frequencies. The resulting rectified sheath potentials lead to several important effects such as the sputtering of impurities from the wall material and the formation of convective cells. An analytical model is presented to describe the self-consistent enhancement of the sheath potential together with the global electromagnetic field sourced by an ICRF antenna. Explicit formulas are obtained, yielding the sheath voltage in the antenna near field for varying input power. Nonlinear features associated with RF sheath-plasma interactions are elucidated and their importance for experimental regimes is assessed. DC effects are incorporated into the model and their importance in sheath regulation is highlighted. Utilizing the finite element code Petra-M, high-fidelity modeling of RF sheath rectification due to ICRF in the WEST tokamak is conducted, including realistic antenna geometry and plasma parameters.