Chemical Polishing of Niobium for Superconducting Radio Frequency Cavity

Abstract:

The performance of niobium superconducting radiofrequency (SRF) accelerator cavities strongly depends on its interior surface state. This paper describes the experimental characteristics of chemical polishing technology for niobium metal, namely polishing rate, surface topography, chemical composition and grain orientation, which have been conducted to optimize process parameters. The mixed solution of hydrofluoric acid, Nitric acid and prthoposphoric acid, 1: 1: 2 in parts by volume is proposed as appropriate polishing reagents with rate equation h = 29.359 × t _0.9247. Polishing for 6 ~ 15 min can remove 154 ~ 360 μm of material using this solution, corresponding roughness Ra and Nb2O5 layer thickness of polished surface is less than 0.65 μm and 10 nm respectively. Meanwhile, we discussed the polishing mechanism using electron back scatter diffration (EBSD) and laser scanning confocal microscope (LSCM), found that the outmost surface of over-polishing sample consists of many fine high-index planes which intersects each other to form sharp egdes in grains or at boundaries.

Key words:SRF cavity; chemical polishing; surface state