Density and RF-drive scalings of an idealized one-dimensional RF sheath model and implications for impurity sputtering

Abstract Radio frequency (RF) sheath formation at the plasma edge is a critical factor for impurity generation during ion cyclotron range of frequency (ICRF) heating. Motivated by EAST observations that correlate core impurity contamination with edge plasma density and ICRF power, this paper presents a parametric study of RF sheath characteristics using an idealized one-dimensional model. The model couples a hydrodynamic sheath description with an equivalent circuit framework, with the RF driving current prescribed through an RF-drive parameter P_RF via a fixed empirical mapping and without antenna-loading feedback. Within the scanned parameter space (edge plasma density n_0=0.5×10^18 to 1.0×10^18 m^(-3)and P_RF=1 to 5 MW), spanning the evanescent regime relevant to typical ICRF conditions, the nonlinear balance between displacement and conduction currents yields well-defined scaling laws. The results indicate that, within the scanned density interval, the time-averaged sheath potential scales inversely with the square of the plasma density (n_0^(-2)). With respect to P_RF, the sheath potential exhibits a stronger-than-classical scaling (P_RF^(b_fit )) where the exponent b_fit decreases monotonically from approximately 1.10 at low densities to 0.76 at high densities, approaching the classical P_RF^(1/2) limit. Total Harmonic Distortion (THD) analysis provides a physical interpretation of these scaling behaviors in terms of the density-dependent suppression and P_RF-driven enhancement of the displacement current. The implications of these scalings for impurity sputtering are assessed by estimating the physical sputtering yields of metallic impurities (Fe, Cu) under the computed sheath potentials. The physical sputtering yields are shown to be highly sensitive to both parameters. Such results establish the intrinsic parametric dependence of sheath formation within the idealized model, providing theoretical insights into the nonlinear RF sheath effects that can help interpret impurity behaviors in high-power ICRF-heated plasmas.