SRF2017 - Table of Session: TUXBA (Fundamental-Nb I)

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

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TUXBA02

Flux Expulsion Studies on Niobium

S. Posen
Fermilab, Batavia, Illinois, USA

Increased requirements for high Q0 in SRF linac applications have made it crucial to avoid Q0 degradation resulting from trapped flux. In this contribution, we show results from a new systematic study of cavities at Fermilab that show a strong correlation between the temperature gradient required to expel flux during cooldown and the material history, including 1) niobium vendor processing and 2) heat treatment after purchase from the vendor. Early results from these studies indicated that it would be important to evaluate the niobium purchased for production of high Q0 cavities for the LCLS-II project. We present measurements of both expulsion and Q0 on single-cell and bare 9-cell cavities made from this material, which led to the decision to increase the heat treatment temperature in production. We present data showing the significant improvement in flux expulsion behavior of production cavities after this modification was introduced. In addition, we present new understanding of the physics of flux expulsion, and we discuss the ramifications for procurement of niobium for future applications requiring high Q0.

Slides TUXBA02 [3.340 MB]

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TUXBA03

N-H Interaction Study of Nitrogen Doping-treatment on Nb Samples

Z.Q. Yang, X.Y. Lu, W.W. Tan, D.Y. Yang, Y. Yang, J. Zhao
PKU, Beijing, People's Republic of China

Fundamental understanding of the nitrogen doping mechanism has been carried out extensively, but yet remains unclear. We studied the nitrogen doping effect through a series of experiments on niobium samples at both room and low temperature circumstances to seek the physical explanation based on the proposed nitrogen-hydrogen interaction. The time of flight secondary ion mass spectrometry (TOF-SIMS) revealed that the diffused nitrogen plays a key role in the nitrogen doping effect. X-ray photoelectron spectroscopy (XPS) measurements showed the chemical distribution of it. The effects of different concentrations of diffused nitrogen on the samples' magnetic property were studied by the magnetic property measurement system (MPMS). Direct observation of the surface topography of both doped and undoped samples by using scanning electron microscope (SEM) at liquid nitrogen temperature has proved that the diffused nitrogen can prevent or retard the hydrides formation to varying degrees with different amounts of material removal. A possible mechanism of nitrogen-doping effect is proposed based on our study.

Slides TUXBA03 [3.235 MB]

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TUXBA04

A Unified Theory of Surface Resistance and the Residual Resistance of SRF Cavities at Low Temperatures

T. Kubo
KEK, Ibaraki, Japan
A.V. Gurevich
ODU, Norfolk, Virginia, USA

The Mattis-Bardeen (MB) theory has been widely used for the calculation of the surface resistance of superconductors at weak RF fields. It is well known that the observed surface resistance decreases exponentially as the temperature decreases but tends to a constant residual resistance which is not described by the MB theory. Using the quasiclassical formalism of the microscopic BCS theory, we have developed a unified theory of the surface resistance which, in addition to the MB contribution, includes the residual resistance. Based on this theory, possible materials treatments to reduce the surface resistance are discussed.

Slides TUXBA04 [2.207 MB]

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TUXBA05

Hydrogen Distribution and Hydride Precipitation in SRF Nb Revealed by Metallographic Techniques

S. Balachandran, P.J. Lee
NHMFL, Tallahassee, Florida, USA
S. Chetri, D.C. Larbalestier
ASC, Tallahassee, Florida, USA
P. Dhakal
JLab, Newport News, Virginia, USA

The current issues that are of importance for SRF Nb cavities include: flux trapping depending on temperature gradient at superconducting transition, variation in flux trapping depending on precursor Nb material, and quality or process control of SRF Nb sheets that would lead to consistent performance. Our work relates to exploring microstructure correlations relevant to the above topics using coupon SRF Nb samples. Of particular importance are: a) hydride precipitation which could occur during cavity cooling below cryogenic temperatures, b) variation in surface superconducting properties and N doping, c) extent of surface damage in as received Nb sheets, and d) distribution of dislocation and substructures which lead to lattice strains in the material. The main results of our studies suggest the role of GB's in hydride formation, direct evidence of N doping preventing hydrides, and occurrence of strained lattice features depending on the surface treatment of SRF Nb. We also look at how this type of study can supplement and strengthen the effort to include quantitative microstructure based features in our understanding, and modelling of SRF superconductivity in Nb.

Slides TUXBA05 [12.981 MB]

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TUXBA06

Analysis of Flux Pinning Variability with Nb Stock Material

A.D. Palczewski
JLab, Newport News, Virginia, USA

Within Nb material batches which otherwise conform to specification, a distinct variability of susceptibility to pinning of residual magnetic field and thus enhanced RF losses and lower Q has been observed. Systematic investigation of the various lots of material procured for use in the fabrication of LCLS-II cavities is underway to distinguish the critical contributing material characteristic. The aim is clarification of a revision to the material specification in order to reliably obtain lower residual resistance in operational SRF cavities for any application.

Slides TUXBA06 [1.530 MB]

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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)

TUXBA02	Flux Expulsion Studies on Niobium
	S. Posen Fermilab, Batavia, Illinois, USA
	Increased requirements for high Q0 in SRF linac applications have made it crucial to avoid Q0 degradation resulting from trapped flux. In this contribution, we show results from a new systematic study of cavities at Fermilab that show a strong correlation between the temperature gradient required to expel flux during cooldown and the material history, including 1) niobium vendor processing and 2) heat treatment after purchase from the vendor. Early results from these studies indicated that it would be important to evaluate the niobium purchased for production of high Q0 cavities for the LCLS-II project. We present measurements of both expulsion and Q0 on single-cell and bare 9-cell cavities made from this material, which led to the decision to increase the heat treatment temperature in production. We present data showing the significant improvement in flux expulsion behavior of production cavities after this modification was introduced. In addition, we present new understanding of the physics of flux expulsion, and we discuss the ramifications for procurement of niobium for future applications requiring high Q0.
	Slides TUXBA02 [3.340 MB]
Export •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUXBA03	N-H Interaction Study of Nitrogen Doping-treatment on Nb Samples
	Z.Q. Yang, X.Y. Lu, W.W. Tan, D.Y. Yang, Y. Yang, J. Zhao PKU, Beijing, People's Republic of China
	Fundamental understanding of the nitrogen doping mechanism has been carried out extensively, but yet remains unclear. We studied the nitrogen doping effect through a series of experiments on niobium samples at both room and low temperature circumstances to seek the physical explanation based on the proposed nitrogen-hydrogen interaction. The time of flight secondary ion mass spectrometry (TOF-SIMS) revealed that the diffused nitrogen plays a key role in the nitrogen doping effect. X-ray photoelectron spectroscopy (XPS) measurements showed the chemical distribution of it. The effects of different concentrations of diffused nitrogen on the samples' magnetic property were studied by the magnetic property measurement system (MPMS). Direct observation of the surface topography of both doped and undoped samples by using scanning electron microscope (SEM) at liquid nitrogen temperature has proved that the diffused nitrogen can prevent or retard the hydrides formation to varying degrees with different amounts of material removal. A possible mechanism of nitrogen-doping effect is proposed based on our study.
	Slides TUXBA03 [3.235 MB]
Export •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUXBA04	A Unified Theory of Surface Resistance and the Residual Resistance of SRF Cavities at Low Temperatures
	T. Kubo KEK, Ibaraki, Japan A.V. Gurevich ODU, Norfolk, Virginia, USA
	The Mattis-Bardeen (MB) theory has been widely used for the calculation of the surface resistance of superconductors at weak RF fields. It is well known that the observed surface resistance decreases exponentially as the temperature decreases but tends to a constant residual resistance which is not described by the MB theory. Using the quasiclassical formalism of the microscopic BCS theory, we have developed a unified theory of the surface resistance which, in addition to the MB contribution, includes the residual resistance. Based on this theory, possible materials treatments to reduce the surface resistance are discussed.
	Slides TUXBA04 [2.207 MB]
Export •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUXBA05	Hydrogen Distribution and Hydride Precipitation in SRF Nb Revealed by Metallographic Techniques
	S. Balachandran, P.J. Lee NHMFL, Tallahassee, Florida, USA S. Chetri, D.C. Larbalestier ASC, Tallahassee, Florida, USA P. Dhakal JLab, Newport News, Virginia, USA
	The current issues that are of importance for SRF Nb cavities include: flux trapping depending on temperature gradient at superconducting transition, variation in flux trapping depending on precursor Nb material, and quality or process control of SRF Nb sheets that would lead to consistent performance. Our work relates to exploring microstructure correlations relevant to the above topics using coupon SRF Nb samples. Of particular importance are: a) hydride precipitation which could occur during cavity cooling below cryogenic temperatures, b) variation in surface superconducting properties and N doping, c) extent of surface damage in as received Nb sheets, and d) distribution of dislocation and substructures which lead to lattice strains in the material. The main results of our studies suggest the role of GB's in hydride formation, direct evidence of N doping preventing hydrides, and occurrence of strained lattice features depending on the surface treatment of SRF Nb. We also look at how this type of study can supplement and strengthen the effort to include quantitative microstructure based features in our understanding, and modelling of SRF superconductivity in Nb.
	Slides TUXBA05 [12.981 MB]
Export •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUXBA06	Analysis of Flux Pinning Variability with Nb Stock Material
	A.D. Palczewski JLab, Newport News, Virginia, USA
	Within Nb material batches which otherwise conform to specification, a distinct variability of susceptibility to pinning of residual magnetic field and thus enhanced RF losses and lower Q has been observed. Systematic investigation of the various lots of material procured for use in the fabrication of LCLS-II cavities is underway to distinguish the critical contributing material characteristic. The aim is clarification of a revision to the material specification in order to reliably obtain lower residual resistance in operational SRF cavities for any application.
	Slides TUXBA06 [1.530 MB]
Export •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)