High-voltage pulse electropolishing of niobium and 3.9 GHz SRF cavity in ionic liquid electrolytes without hydrofluoric acid

Superconducting radio frequency (SRF) cavities' inner surfaces must be buffer chemical polished (BCP) or electropolished (EP) to remove the damaged layer. The better smoothness of the SRF cavity's inner surface can improve its performance. This paper is the first to use ionic liquids and high-voltage pulse electropolishing (hp-EP) for niobium and 3.9 GHz cavity, breaking through the limitations of traditional BCP and EP which must use HF and other concentrated acids, and significantly improving polishing efficiency. It systematically studies the main parameters of hp-EP, such as voltage, duty cycle, and ammonium fluoride concentration, on the effects of current density, polishing rate, and surface roughness. By summarizing hp-EP rules, it achieves controllable electropolishing of niobium. Surface analysis of Nb samples after hp-EP shows a reduction in surface roughness, resulting in a uniform, mirror-like niobium surface. Niobium samples achieve a rapid electropolishing rate under the combined action of electrochemical mechanisms and mechanical mechanisms. To observe the surface state of the 3.9 cavity after hp-EP, the tube and the cell part are treated separately. The electric field of the cell part is simulated by COMSOL Multiphysics, and different cathode electrodes were designed. The results demonstrate that the cathode electrode shape significantly affects the hp-EP effect. Finally, the inner surface of the tube and cell is polished to achieve a bright surface. This work provides a new perspective in the SRF field.