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.
Highlights
Advances in the processing of bulk superconducting radio frequency (SRF) niobium cavities in recent years via interior surface impurity diffusion of either titanium or nitrogen have resulted in significant improvements in their quality factor, Q0 [1,2,3,4]
This study indicates that the surface compositions are similar in coupon samples after N doping and subsequent buffer chemical polishing (BCP) or EP
EP produces much smoother surfaces than BCP, which could be due to removal rates in grain boundaries and grains with different orientations produced in BCP using the traditional recipes
Summary
Advances in the processing of bulk superconducting radio frequency (SRF) niobium cavities in recent years via interior surface impurity diffusion of either titanium or nitrogen have resulted in significant improvements in their quality factor, Q0 [1,2,3,4]. Normal conducting niobium nitride precipitates are produced on the surface and must be subsequently removed using standard chemical methods, such as electropolishing (EP) or buffer chemical polishing (BCP). An alternative “nitrogen infusion” technique uses a lower temperature heat treatment (120 °C–200 °C) for longer durations (∼48 hours) in the presence of nitrogen and this resulted in a higher quality factor without any degradation in accelerating gradient [5,6,7]. Treatment at a lower temperature over the high temperature nitrogen doping is the absence of postdoping electropolishing because no normal conducting precipitates are formed at the surface [7]. BCP has been used to treat cavity surfaces, there are no results reported on the use of BCP to remove the precipitate layers after the nitrogen doping process. BCP/EP’ed samples are compared to investigate the key differences in performance
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