Development of Buffered
Electropolishing on Nb 

Surface condition of Nb Superconducting Radio Frequency (SRF) cavities used in particle accelerators is one of the crucial factors in determining their performance. Traditionally, Buffered Chemical Polishing (BCP) using an acid mixture of hydrofluoric, nitric, and phosphoric acids with ratio of either 1:1:1 or 1:1:2 is employed to chemically etch away about 100 m m of the damaged layer as well as any evaporated niobium scale deposited during welding on the surfaces of Nb SRF cavities as the final step of the surface Nb removal process. However, this traditional BCP treatment normally produces enhanced etching rate at grain boundaries and defect locations, leading to a relatively rougher Nb surface in comparison with that obtained by the conventional ElectroPolishing (EP) using an acid mixture of H 2SO 4 and HF by a factor of 2.5. Although it is still unclear at this moment how the performance of a Nb SRF cavity is affected by its inner surface roughness, Nb cavities obtained through EP have shown some advantages over those by BCP. Especially, an accelerating gradient of 40 MV/m can be routinely obtained on Nb cavities treated by EP. Furthermore, the enhanced etching rate at the grain boundaries can produce sharp steps on the order of 12 m m. These sharp surface features have been shown as the possible sources responsible for the degradation of the quality factor Q o of Nb SRF cavities.

As part of our R&D effort, scientists and engineers at JLab have initiated a study to explore a surface treatment technique named Buffered Electropolishing (BEP) that uses an acid mixture of lactic, sulphuric, hydrofluoric acids as electrolyte. It has been demonstrated that BEP can produce the smoothest surface finish on Nb. Besides, it has a polishing rate of 0.646 m m/min that is much higher than 0.381 m m/min by the EP process widely used in the SRF community. This is certainly beneficial since it will enable us to cut the surface processing time by half, which can result in a huge saving in the capital costs in Nb surface treatments. Furthermore, due to the partial replacement of sulphuric acid by lactic acid it is safer to handle the electrolyte.