SRF2017 - List of Authors (Koufalis, P.N.)

Paper	Title	Page
TUXBA01	Low Temperature Doping of Niobium Cavities: What is Really Going on?	353
	P.N. Koufalis, M. Liepe, J.T. Maniscalco Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Initial work, first at Fermilab and subsequently at Cornell, has shown that low temperature heat treatments (120 - 160 C) in a low pressure atmosphere can lead to a 'Q-rise' and high quality factors similar to that of cavities nitrogen-doped at high temperatures (~800 C). It was suggested that the low-temperature baking effect is a result of nitrogen doping or 'infusion'. We conducted a systematic study of this effect, using both RF measurements of cavities treated at different doping temperatures as well as detailed SIMS analysis of the surface layer. We match RF performance and extracted material parameters (especially electron mean free path) to the measured doping concentration profiles. We conclusively show that the low-temperature baking is drastically lowering the mean free path in the penetration layer, and that this is not the result of nitrogen doping or infusion. Instead, other interstitial impurities (specifically oxygen and carbon) are diffused into the surface in the low temperature heat treatment and are the source of lowering of the mean free path and, thus, of the observed Q-rise.
	Slides TUXBA01 [4.153 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUXBA01
Export •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUYAA01	The Importance of the Electron Mean Free Path for Superconducting RF Cavities	359
	J.T. Maniscalco, P.N. Koufalis, M. Liepe Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Theoretical results offer a potential explanation for the anti-Q-slope, the phenomenon of decreasing microwave surface resistance with increasing radiofrequency electromagnetic field strength. This effect has been observed in niobium doped with impurities, chiefly nitrogen, and has been put to use in the Linac Coherent Light Source II (LCLS-II) accelerator currently under construction. Our work, presented here, finds a strong link between the electron mean free path, the main measure of impurity doping, to the overheating of quasiparticles in the RF penetration layer. This is an important effect that adjusts the magnitude of the theoretical anti-Q-slope by providing a mechanism to counteract it and introduce a surface resistance that increases with field strength. We discuss our findings in a study of niobium cavities doped at high temperature (800-990 °C) as well as new analysis of low-temperature-doped cavities.
	Slides TUYAA01 [6.988 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUYAA01
Export •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

THPB004	Impact of the Duration of Low Temperature Doping on Superconducting Cavity Performance	750
	P.N. Koufalis, F. Furuta, J.J. Kaufman, M. Liepe Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Low temperature treatments of superconducting cavities in a low pressure ambient atmosphere have been shown to introduce a 'Q-rise' up to moderate surface fields and an overall increase in quality factor. However, the effect of varying the doping time at a fixed temperature on cavity performance has not been systematically examined. We present results of such an investigation for cavities prepared at 120 and 160 C in a continuously flowing low pressure atmosphere for various amounts of time. We show that the introduction of impurities to the RF penetration layer can improve cavity performance and investigate the relationship between electron mean free path and the temperature-dependent component of the surface resistance.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB004
Export •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

Paper

Title

Page

Low Temperature Doping of Niobium Cavities: What is Really Going on?

353

P.N. Koufalis, M. Liepe, J.T. Maniscalco
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Initial work, first at Fermilab and subsequently at Cornell, has shown that low temperature heat treatments (120 - 160 C) in a low pressure atmosphere can lead to a 'Q-rise' and high quality factors similar to that of cavities nitrogen-doped at high temperatures (~800 C). It was suggested that the low-temperature baking effect is a result of nitrogen doping or 'infusion'. We conducted a systematic study of this effect, using both RF measurements of cavities treated at different doping temperatures as well as detailed SIMS analysis of the surface layer. We match RF performance and extracted material parameters (especially electron mean free path) to the measured doping concentration profiles. We conclusively show that the low-temperature baking is drastically lowering the mean free path in the penetration layer, and that this is not the result of nitrogen doping or infusion. Instead, other interstitial impurities (specifically oxygen and carbon) are diffused into the surface in the low temperature heat treatment and are the source of lowering of the mean free path and, thus, of the observed Q-rise.

Slides TUXBA01 [4.153 MB]

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUXBA01

Export •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUYAA01

The Importance of the Electron Mean Free Path for Superconducting RF Cavities

359

J.T. Maniscalco, P.N. Koufalis, M. Liepe
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Theoretical results offer a potential explanation for the anti-Q-slope, the phenomenon of decreasing microwave surface resistance with increasing radiofrequency electromagnetic field strength. This effect has been observed in niobium doped with impurities, chiefly nitrogen, and has been put to use in the Linac Coherent Light Source II (LCLS-II) accelerator currently under construction. Our work, presented here, finds a strong link between the electron mean free path, the main measure of impurity doping, to the overheating of quasiparticles in the RF penetration layer. This is an important effect that adjusts the magnitude of the theoretical anti-Q-slope by providing a mechanism to counteract it and introduce a surface resistance that increases with field strength. We discuss our findings in a study of niobium cavities doped at high temperature (800-990 °C) as well as new analysis of low-temperature-doped cavities.

Slides TUYAA01 [6.988 MB]

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUYAA01

Export •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

THPB004

Impact of the Duration of Low Temperature Doping on Superconducting Cavity Performance

750

P.N. Koufalis, F. Furuta, J.J. Kaufman, M. Liepe
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Low temperature treatments of superconducting cavities in a low pressure ambient atmosphere have been shown to introduce a 'Q-rise' up to moderate surface fields and an overall increase in quality factor. However, the effect of varying the doping time at a fixed temperature on cavity performance has not been systematically examined. We present results of such an investigation for cavities prepared at 120 and 160 C in a continuously flowing low pressure atmosphere for various amounts of time. We show that the introduction of impurities to the RF penetration layer can improve cavity performance and investigate the relationship between electron mean free path and the temperature-dependent component of the surface resistance.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB004

Export •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)