Surface characterization of nitrogen-doped high purity niobium coupons compared with superconducting rf cavity performance

Abstract

We report the results from the measurements of high purity Nb samples and superconducting radio-frequency (SRF) cavities doped with nitrogen and followed by either electropolishing (EP) or buffered chemical polishing (BCP), in order to understand the role of the postdoping treatment on the performance of SRF cavities. The samples characterization via scanning electron microscope, x-ray photoelectron spectroscopy and secondary ion mass spectroscopy showed topographical differences on the surface of the samples after EP versus BCP treatment, but similar surface composition. Radio-frequency measurements were done on single cell cavities made from fine-grain and large-grain Nb treated by nitrogen doping followed by BCP and showed that improved $Q_0$ in the medium field in both fine-grain and large-grain cavities is possible with BCP postprocessing. However, there are differences between performances of large-grain versus fine-grain cavities after BCP. A cavity made from large-grain Nb showed a larger increase in $Q_0$ and a lower quench field compared to cavities made from fine-grain Nb.