Effect of self-bias on cylindrical capacitive discharge for processing of inner walls of tubular structures—Case of SRF cavities

Cylindrical capacitive discharge is a convenient medium for generating reactive ions to process inner walls superconductive radio-frequency (SRF) cavities. These cavities, used in particle accelerators, presents a three-dimensional structure made of bulk Niobium, with axial cylindrical symmetry. Manufactured cavity walls are covered with Niobium oxides and scattered particulates, which must be removed for desired SRF performance. Cylindrical capacitive discharge in a mixture of Ar and Cl2 is a sole and natural non-wet acid choice to purify the inner surfaces of SRF cavities by reactive ion etching. Coaxial cylindrical discharge is generated between a powered inner electrode and the grounded outer electrode, which is the cavity wall to be etched. Plasma sheath voltages were tailored to process the outer wall by providing an additional dc current to the inner electrode with the help of an external compensating dc power supply and corrugated design of the inner electrode. The dc bias potential difference is established between two electrodes to make the set-up favorable for SRF wall processing. To establish guidelines for reversing the asymmetry and establishing the optimal sheath voltage at the cavity wall, the dc self-bias potential and dc current dependence on process parameters, such as gas pressure, rf power and chlorine content in the Ar/Cl2 gas mixture was measured. The process is potentially applicable to all concave metallic surfaces.