Abstract
Raman microscopy/spectroscopy measurements are presented on high purity niobium (Nb) samples, including pieces from hot spot regions of a tested superconducting rf cavity that exhibit a high density of etch pits. Measured spectra are compared with density functional theory calculations of Raman-active, vibrational modes of possible surface Nb-O and Nb-H complexes. The Raman spectra inside particularly rough pits in all Nb samples show clear differences from surrounding areas, exhibiting enhanced intensity and sharp peaks. While some of the sharp peaks are consistent with calculated NbH and ${\mathrm{NbH}}_{2}$ modes, there is better overall agreement with C-H modes in chain-type hydrocarbons. Other spectra reveal two broader peaks attributed to amorphous carbon. Niobium foils annealed to $>2000\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ in high vacuum develop identical Raman peaks when subjected to cold working. Regions with enhanced C and O have also been found by SEM/EDX spectroscopy in the hot spot samples and cold-worked foils, corroborating the Raman results. Such regions with high concentrations of impurities are expected to suppress the local superconductivity and this may explain the correlation between hot spots in superconducting rf (SRF) cavities and the observation of a high density of surface pits. The origin of localized high carbon and hydrocarbon regions is unclear at present but it is suggested that particular processing steps in SRF cavity fabrication may be responsible.
Highlights
Superconducting radio frequency (SRF) cavities, having Q values orders of magnitude higher than normal Cu, are an enabling device for a host of linear particle accelerator applications [1]
The Raman spectra observed in particular types of macroscopic pit defects that appear very rough under an optical microscope showed reproducible features of enhanced intensity and sharp peaks, in stark contrast with the featureless spectra of surrounding regions
The close agreement of some peaks with calculated NbH and NbH2 modes indicates that these compounds cannot be ruled out, the overall Raman spectra, especially the high frequency modes near 2900 cmÀ1, more closely resemble that of CH2 chain compounds such as stearic acid
Summary
Superconducting radio frequency (SRF) cavities, having Q values orders of magnitude higher than normal Cu, are an enabling device for a host of linear particle accelerator applications [1]. One concern is that such spectroscopic tools cannot measure hydrogen, a gas which diffuses in Nb and is a well-known contaminant potentially affecting the quality factor Q of the cavity [5] These techniques measure an average composition over a relatively large electron probing depth, >100 nm, and would be insensitive to subtle changes in surface oxide composition occurring over $5 nm. There is the possibility that stable hydrocarbons are already present in the Nb as a consequence of processing such as deep drawing These results provide a reasonable explanation of the correlation of hot spots in SRF cavities with the observation of a high density of etch pits
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callMore From: Physical Review Special Topics - Accelerators and Beams
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
