Behavior of RF sheath admittance across Ion Cyclotron Resonance analyzed with fluid theory and large-scale Particle-in-Cell simulations*

The next generation of tokamaks (SPARC, ARC, FNSF, DEMO) and several current tokamaks (WEST, etc.) operate with ICRH-only as auxiliary heating technique, because it transfers power directly to the ions, and it involves the cheapest Radio-Frequency (RF) actuators. However, RF sheaths are associated with increased levels of impurities and other drawbacks such as hot-spot formation. RF sheaths not only form on the ICRH antenna itself, but on far-field electrically connected surfaces. In this study, we highlight an interesting case where the cyclotron frequency of ions is on par with the frequency of an RF sheath in an oblique magnetic case. Due to the 1/R scaling of the magnetic field in a tokamak, such cases are possible in front of material surfaces placed in proximity of the radial location of the ICRH resonance. The problem has been analyzed using large-scale Particle-in-Cell simulations using the hPIC2 code and semi-analytical fluid models. We found that the combination of RF sheath rectification and cyclotron resonance generate trends of the RF sheath impedance analogous to the classical driven damped harmonic oscillator, where charge density damps ion admittance. We show that the resulting resonance can lead to increased ion flux at the wall and localized material sputtering.