Buffered electrochemical polishing of niobium

Abstract

The standard preparation of superconducting radio-frequency (SRF) cavities made of pure niobium include the removal of a “damaged” surface layer, by buffered chemical polishing (BCP) or electropolishing (EP), after the cavities are formed. The performance of the cavities is characterized by a sharp degradation of the quality factor at high surface magnetic field, a phenomenon referred to as “Q-drop”. In some cases, the Q-drop can be significantly reduced by a low-temperature (~120 °C) “in situ” baking of the cavity. As part of the effort to understand this phenomenon, the effect of introducing a polarization potential during BCP, creating a process which is between the standard BCP and EP, was investigated. The focus of this contribution is on the characterization of this novel electrochemical process by measuring polarization curves, etching rates, surface finish, and electrochemical impedance. In particular, it is shown that the anodic potential of Nb during BCP has a plateau region in the polarization curve and the impedance diagrams on the plateau can be described with a “surface charge” model found in the literature. By applying an anodic potential to Nb, a lower etching rate and better the surface finish than by standard BCP process have been obtained.