Applying the plasma physical sputtering process to SRF cavity treatment: Simulation and Experiment Study

Abstract

Plasma processing is a fast-emerging surface treatment technology for superconducting radio frequency (SRF) cavities: plasma interacts with the impurity contamination and tiny burrs, resulting in an etched clean-and-smooth RF surface. Among all the variations of plasma processing method, the physical sputtering process might be applied to SRF cavities but has no clear etching profile (including distribution uniformity, ion flux, energy, and etching rate). To investigate that, this paper utilizes a two-dimensional self-consistent model combining the fluid equations and the Monte Carlo collision method. With the model, we introduced the sputtering-yield probability distribution to evaluate the physical sputtering process quantitatively; optimized the plasma distribution characteristics with three electrode structures to attain uniformly distributed plasma; built the correlation between ion flux and ion-energy distribution function (IEDF) under varied power and frequency; and studied the physical sputtering process at different positions. In the end, we performed the plasma physical sputtering experiment on small niobium samples, seeing an etching and smoothing effect. This research successfully evaluated the physical sputtering process for SRF cavities, and indicated the feasibility of pure physical sputtering etching.