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Title: Insight into bulk niobium superconducting RF cavities performances by tunneling spectroscopy

Abstract

Point contact tunneling (PCT) spectroscopy measurements are reported over wide areas of cm-sized cut outs from niobium superconducting RF cavities. A comparison is made between a high-quality, conventionally processed (CP) cavity with a high field Q drop for acceleration field E $>$ 20 MV/m and a nitrogen doped (N-doped) cavity that exhibits an increasing Q up to fields approaching 15 MV/m. The CP cavity displays hot spot regions at high RF fields where Q-drop occurs as well as unaffected regions (cold spots). PCT data on cold spots reveals a near ideal BCS density of states (DOS) with gap parameters, $$\Delta$$ as high as 1.62 meV, that are among the highest values ever reported for Nb. Hot spot regions exhibit a wide distribution of gap values down to $$\Delta \sim$$ 1.0 meV and DOS broadening characterized by a relatively large value of pair-breaking rate, $$\Gamma$$, indicating surface regions of significantly reduced superconductivity. In addition, hot spots commonly exhibit Kondo tunneling peaks indicative of surface magnetic moments attributed to a defective oxide. N-doped cavities reveal a more homoegeneous gap distribution centered at $$\Delta \sim$$ 1.5 meV and relatively small values of $$\Gamma/\Delta$$. The absence of regions of significantly reduced superconductivity indicates that the N interstitials are playing an important role in preventing the formation of hydride phases and other macroscopic defects which might otherwise severely affect the local, surface superconductivity that lead to hot spot formation. The N-doped cavities also display a significantly improved surface oxide, i.e., increased thickness and tunnel barrier height, compared to CP cavities. These results help explain the improved performance of N-doped cavities and give insights into the origin of the initial increasing Q with RF amplitude.

Authors:
 [1];  [2];  [3];  [4];  [4];  [2];  [5]
  1. Argonne
  2. IIT, Chicago
  3. Jefferson Lab
  4. Fermilab
  5. IRFU, Saclay
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States); 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)
OSTI Identifier:
1462216
Report Number(s):
arXiv:1805.06359; FERMILAB-PUB-18-237-TD
1673360
DOE Contract Number:  
AC02-07CH11359
Resource Type:
Journal Article
Journal Name:
TBD
Additional Journal Information:
Journal Name: TBD
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS

Citation Formats

Groll, N. R., Becker, C., Ciovati, G., Grassellino, A., Romanenko, A., Zasadzinski, J. F., and Proslier, T. Insight into bulk niobium superconducting RF cavities performances by tunneling spectroscopy. United States: N. p., 2018. Web.
Groll, N. R., Becker, C., Ciovati, G., Grassellino, A., Romanenko, A., Zasadzinski, J. F., & Proslier, T. Insight into bulk niobium superconducting RF cavities performances by tunneling spectroscopy. United States.
Groll, N. R., Becker, C., Ciovati, G., Grassellino, A., Romanenko, A., Zasadzinski, J. F., and Proslier, T. Wed . "Insight into bulk niobium superconducting RF cavities performances by tunneling spectroscopy". United States. https://www.osti.gov/servlets/purl/1462216.
@article{osti_1462216,
title = {Insight into bulk niobium superconducting RF cavities performances by tunneling spectroscopy},
author = {Groll, N. R. and Becker, C. and Ciovati, G. and Grassellino, A. and Romanenko, A. and Zasadzinski, J. F. and Proslier, T.},
abstractNote = {Point contact tunneling (PCT) spectroscopy measurements are reported over wide areas of cm-sized cut outs from niobium superconducting RF cavities. A comparison is made between a high-quality, conventionally processed (CP) cavity with a high field Q drop for acceleration field E $>$ 20 MV/m and a nitrogen doped (N-doped) cavity that exhibits an increasing Q up to fields approaching 15 MV/m. The CP cavity displays hot spot regions at high RF fields where Q-drop occurs as well as unaffected regions (cold spots). PCT data on cold spots reveals a near ideal BCS density of states (DOS) with gap parameters, $\Delta$ as high as 1.62 meV, that are among the highest values ever reported for Nb. Hot spot regions exhibit a wide distribution of gap values down to $\Delta \sim$ 1.0 meV and DOS broadening characterized by a relatively large value of pair-breaking rate, $\Gamma$, indicating surface regions of significantly reduced superconductivity. In addition, hot spots commonly exhibit Kondo tunneling peaks indicative of surface magnetic moments attributed to a defective oxide. N-doped cavities reveal a more homoegeneous gap distribution centered at $\Delta \sim$ 1.5 meV and relatively small values of $\Gamma/\Delta$. The absence of regions of significantly reduced superconductivity indicates that the N interstitials are playing an important role in preventing the formation of hydride phases and other macroscopic defects which might otherwise severely affect the local, surface superconductivity that lead to hot spot formation. The N-doped cavities also display a significantly improved surface oxide, i.e., increased thickness and tunnel barrier height, compared to CP cavities. These results help explain the improved performance of N-doped cavities and give insights into the origin of the initial increasing Q with RF amplitude.},
doi = {},
journal = {TBD},
number = ,
volume = ,
place = {United States},
year = {2018},
month = {5}
}