Electropolishing of Nb for Superconducting Radio-Frequency (SRF) Cavities

Niobium superconducting radio-frequency (SRF) cavities are widely used in modern particle accelerators, because of higher operating efficiency than normal-conducting cavities made of copper. Since the superconducting RF current flows in a surface layer about 40 nm deep, a set of preparation techniques have been developed over the years to assure good superconducting properties of the surface layer.  Electropolishing (EP) is the technique of choice being developed for high–field SRF cavities, which result in a typical surface roughness of about 0.1 μm.  Currently, the 1:10 volume ratio HF (49%) and H₂SO₄ (96%) electrolyte was widely used for high field SRF cavity production. Electrochemical impedance spectroscopy (EIS) and related techniques point to the electropolishing mechanism of Nb in a sulfuric and hydrofluoric acid electrolyte of controlled by a compact surface salt film under F- diffusion-limited mass transport control. In addition, data indicates that above 25^oC electrode kinetics become competitive with the mass transfer current limitation and increase dramatically with temperature. These findings are guiding the optimization of EP process parameters for achieving controlled, reproducible and uniform nano-smoothes surface finishing for SRF cavities.  The problem with using HF is that it is dangerous both to people and to the environment. Alternative approaches recently were developed, such pulse reverse EP which catholic pulse is used to remove the oxide layer rather than HF, and results are promising.  Also ionic solution such as acid-free vitamin B₄ electrolyte has been introduced and has shown promising results. Those results will be briefly introduced here.