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Title: Detection of surface carbon and hydrocarbons in hot spot regions of niobium superconducting rf cavities by Raman spectroscopy

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 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°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. Finally, the origin of localized high carbon and hydrocarbon regionsmore » is unclear at present but it is suggested that particular processing steps in SRF cavity fabrication may be responsible.« less
Authors:
 [1] ;  [2] ;  [3] ;  [4] ;  [1] ;  [5] ;  [1] ;  [1] ;  [1] ;  [3] ;  [6] ;  [1] ;  [2]
  1. Illinois Inst. of Technology, Chicago, IL (United States). Dept. of Physics
  2. (ANL), Argonne, IL (United States). Materials Science Division
  3. Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States). Superconducting Materials Dept., Technical Division
  4. (United States). Dept. of Chemical and Biological Engineering
  5. Argonne National Lab. (ANL), Argonne, IL (United States). Materials Science Division
  6. Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States)
Publication Date:
Report Number(s):
FERMILAB-PUB-13-596-TD; ANL-MSD-JA-76935
Journal ID: ISSN 1098-4402; PRABFM; 1281330
Grant/Contract Number:
AC02-07CH11359; AC02-06CH11357
Type:
Published Article
Journal Name:
Physical Review Special Topics. Accelerators and Beams
Additional Journal Information:
Journal Volume: 16; Journal Issue: 6; Journal ID: ISSN 1098-4402
Publisher:
American Physical Society (APS)
Research Org:
Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States); Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States)
Sponsoring Org:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25); USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
Language:
English
Subject:
43 PARTICLE ACCELERATORS
OSTI Identifier:
1104393
Alternate Identifier(s):
OSTI ID: 1431009

Cao, C., Argonne National Lab., Ford, D., Northwestern Univ., Evanston, IL, Bishnoi, S., Proslier, T., Albee, B., Hommerding, E., Korczakowski, A., Cooley, L., Ciovati, G., Zasadzinski, J. F., and Argonne National Lab.. Detection of surface carbon and hydrocarbons in hot spot regions of niobium superconducting rf cavities by Raman spectroscopy. United States: N. p., Web. doi:10.1103/PhysRevSTAB.16.064701.
Cao, C., Argonne National Lab., Ford, D., Northwestern Univ., Evanston, IL, Bishnoi, S., Proslier, T., Albee, B., Hommerding, E., Korczakowski, A., Cooley, L., Ciovati, G., Zasadzinski, J. F., & Argonne National Lab.. Detection of surface carbon and hydrocarbons in hot spot regions of niobium superconducting rf cavities by Raman spectroscopy. United States. doi:10.1103/PhysRevSTAB.16.064701.
Cao, C., Argonne National Lab., Ford, D., Northwestern Univ., Evanston, IL, Bishnoi, S., Proslier, T., Albee, B., Hommerding, E., Korczakowski, A., Cooley, L., Ciovati, G., Zasadzinski, J. F., and Argonne National Lab.. 2013. "Detection of surface carbon and hydrocarbons in hot spot regions of niobium superconducting rf cavities by Raman spectroscopy". United States. doi:10.1103/PhysRevSTAB.16.064701.
@article{osti_1104393,
title = {Detection of surface carbon and hydrocarbons in hot spot regions of niobium superconducting rf cavities by Raman spectroscopy},
author = {Cao, C. and Argonne National Lab. and Ford, D. and Northwestern Univ., Evanston, IL and Bishnoi, S. and Proslier, T. and Albee, B. and Hommerding, E. and Korczakowski, A. and Cooley, L. and Ciovati, G. and Zasadzinski, J. F. and Argonne National Lab.},
abstractNote = {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 NbH2 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°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. Finally, 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.},
doi = {10.1103/PhysRevSTAB.16.064701},
journal = {Physical Review Special Topics. Accelerators and Beams},
number = 6,
volume = 16,
place = {United States},
year = {2013},
month = {6}
}