SRF2017 - List of Authors (Hall, D.L.)

Paper	Title	Page
TUPB009	High-frequency SRF Cavities	400
	T.E. Oseroff, D.L. Hall, M. Liepe, J.T. Maniscalco Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Historically, the frequency of SRF cavities has been limited by cryogenic power dissipation increasing rapidly with frequency, due to the BCS surface resistance having a quadratic dependence on frequency. Now, new SRF surfaces using doped niobium and compound superconductors like Nb3Sn can drastically reduce the BCS part of the surface resistance. The temperature independent part of the surface resistance (residual resistance) can therefore become dominant, and has its own, different frequency dependence. We have developed a model to analyze cryogenic cooling power requirements for SRF cavities as function of operating frequency, temperature, and trapped flux to evaluate the impact of the novel low-loss SRF surfaces on the questions of optimal operating frequency and frequency limit. We show that high-frequency SRF cavities now become a realistic option for future SRF driven accelerators. As the transverse cavity size decreases inversely with respect to its resonant frequency, such high-frequency SRF cavities could greatly reduce cost.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB009
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WEXA01	High Performance Nb3Sn Cavities	667
	D.L. Hall, M. Liepe, R.D. Porter Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA T. Arias, P. Cueva, D. Liarte, D.A. Muller, J.P. Sethna, N. Sitaraman Cornell University, Ithaca, New York, USA
	In recent years, 1.3 GHz single-cell cavities coated with Nb3Sn at Cornell University have repeatedly demonstrated quality factors of >10¹⁰ at 4.2 K and >15 MV/m. Ongoing research is currently focussed on the impact of intrinsic and extrinsic factors that limit the quality factor and quench field in these cavities. New single-cell cavities have been commissioned to enable further exploration of the coating parameter space. Experimental studies on both cavities and sample coupons have been supplemented by theoretical work done on layer growth, trapped vortex motion and flux entry. In this paper, we provide a comprehensive overview of the latest developments on Nb3Sn cavities, including work conducted in collaboration with the new NSF Centre for Bright Beams, with a brief summary on work being done in the field at large.
	Slides WEXA01 [10.681 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-WEXA01
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WEXA03	High-performance Thin-film Niobium Produced via Chemical Vapor Deposition (CVD)	674
	R.D. Porter, D.L. Hall, M. Liepe, J.T. Maniscalco Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA V.M. Arrieta, S.R. McNeal, W.E. Williams Ultramet, Pacoima, California, USA
	Bulk niobium cavities have been the standard for superconducting particle accelerators for many years. However, the cost of high RRR niobium start materials makes them expensive. The use of Chemical Vapor Deposition (CVD) processing technologies to produce thin Nb films on low-cost substrates (e.g. copper) offers a method to significantly reduce the cost of accelerator cavity fabrication while increasing cavity performance capabilities. Recent optimization of CVD niobium processes for high RRR Nb films has led to RF performance approaching that of bulk Nb. In collaboration with Ultramet, Cornell continues to explore the potential of CVD techniques. This paper presents results from a detailed study of CVD thin film Nb materials produced by Ultramet on 5-inch diameter copper and molybdenum substrates, including RF performance results with T-mapping and detailed surface analysis of performance limiting regions. Our work shows that CVD-based cavity fabrication methods are a promising alternative to sheet-formed bulk cavities, and to other thin Nb film techniques, warranting further development. Additional results from the field will be discussed.
	Slides WEXA03 [1.503 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-WEXA03
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WEXA05	Dirty Layers, Bi-layers and Multi-layers: Insights from Muon Spin Rotation Experiments
	T. Junginger, R.E. Laxdal, D.W. Storey, E. Thoeng TRIUMF, Vancouver, Canada D.L. Hall, M. Liepe Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA T. Junginger HZB, Berlin, Germany S. Posen Fermilab, Batavia, Illinois, USA T. Prokscha, Z. Salman, A. Suter PSI, Villigen PSI, Switzerland D.W. Storey Victoria University, Victoria, B.C., Canada T. Tan, W.K. Withanage, M.A. Wolak, X. Xi Temple University, Philadelphia, USA E. Thoeng UBC & TRIUMF, Vancouver, British Columbia, Canada A-M. Valente-Feliciano JLab, Newport News, Virginia, USA W.W. Wasserman UBC, Vancouver, B.C., Canada
	Funding: This research was supported by a Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007-2013). The multilayer approach is being investigated for SRF applications since 2006 "". More recently the option of using a bilayer system of two superconductors has been considered as an alternative approach to reach accelerating gradients beyond bulk niobium or to explain the gradient enhancement from a 120°C bake by introduction of a 'dirty layer ""'. In this talk results are presented from two muon spin rotation experiments at TRIUMF and PSI. The former measures the field of first entry Hentry. It will be shown that MgB2 and Nb3Sn on top of Nb both push Hentry above Hc1 to a value consistent with Hsh, independent of the layer thickness. 120°C baking increases Hentry slightly but significantly above Hc1. Using the low energy muon beam at PSI we show that there is a long range proximity effect in a bilayer system of NbTiN on Nb. This effect yields a stronger decay of the RF field with depth as expected for pure NbTiN, opposite to what has been predicted for a bi-layer system due to counter current flow at the superconductor-superconductor interface "*". An insulating layer suppresses this proximity effect. Gurevich, A. APL 88.1 (2006) Checchin, M. Diss. Illinois Institute of Technology, 2016. Kubo, T. Superconductor Science and Technology 30.2 (2016) * Kubo, T et al. APL 104.3 (2014)
	Slides WEXA05 [3.716 MB]
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WEXA07	Theoretical Estimates of Maximum Fields in Superconducting Resonant Radio Frequency Cavities: Stability Theory, Disorder, and Laminates
	D. Liarte, M. Liepe, J.P. Sethna Cornell University, Ithaca, New York, USA G. Catelani Forschungszentrum Jülich, Peter Gruenberg Institut, Jülich, Germany D.L. Hall Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA S. Posen Fermilab, Batavia, Illinois, USA M.K. Transtrum Brigham Young University, Provo, USA
	Funding: This work was supported by the US National Science Foundation under Award OIA-1549132, the Center for Bright Beams. Theoretical limits to the performance of superconductors in high magnetic fields parallel to their surfaces are of key relevance to current and future accelerating cavities. We present intuitive arguments and simple estimates for Hsh, and combine them with rigorous calculations. We explore the effects of materials anisotropy and the danger of disorder in nucleating vortex entry. Will we need to control surface orientation in the layered compound MgB2? Can we estimate theoretically whether dirt and defects make these new materials fundamentally more challenging to optimize than niobium? We discuss and analyze recent proposals to use thin superconducting layers or laminates to enhance the performance of superconducting cavities. Flux entering a laminate can lead to so-called pancake vortices; we consider the physics of the dislocation motion and potential re-annihilation or stabilization of these vortices after their entry.
	Slides WEXA07 [2.975 MB]
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THPB040	SRF Theory Developments from the Center for Bright Beams	835
	D. Liarte, T. Arias, M. Liepe, J.P. Sethna, N. Sitamaran Cornell University, Ithaca, New York, USA D.L. Hall Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA A.R. Pack, M.K. Transtrum Brigham Young University, Provo, USA
	Funding: This work was supported by the US National Science Foundation under Award OIA-1549132, the Center for Bright Beams. We present theoretical studies of SRF materials from the Center for Bright Beams. First, we discuss the effects of disorder, inhomogeneities, and materials anisotropy on the maximum parallel surface field that a superconductor can sustain in an SRF cavity, using linear stability in conjunction with Ginzburg-Landau and Eilenberger theory. We connect our disorder mediated vortex nucleation model to current experimental developments of Nb3Sn and other cavity materials. Second, we use time-dependent Ginzburg-Landau simulations to explore the role of inhomogeneities in nucleating vortices, and discuss the effects of trapped magnetic flux on the residual resistance of weakly-pinned Nb3Sn cavities. Third, we present first-principles density-functional theory (DFT) calculations to uncover and characterize the key fundamental materials processes underlying the growth of Nb3Sn. Our calculations indicate that the observed tin-depleted regions may be the direct result of an exothermic reaction between Nb3Sn and Nb at the growing Nb/Nb3Sn interface. We suggest new growth protocols to mitigate the formation of tin depleted regions.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB040
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THPB041	Cavity Quench Studies in Nb3Sn Using Temperature Mapping and Surface Analysis of Cavity Cut-outs	840
	D.L. Hall, M. Liepe, R.D. Porter Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA P. Cueva, D. Liarte, D.A. Muller, J.P. Sethna Cornell University, Ithaca, New York, USA
	Previous experimental studies on single-cell Nb3Sn cavities have shown that the cause of quench is isolated to a localised defect on the cavity surface. Here, cavity temperature mapping has been used to investigate cavity quench behaviour in an Nb3Sn cavity by measuring the temperature at the quench location as the RF field approaches the quench field. The heating profile observed at the quench location prior to quench appears to suggest quantised vortex entry at a defect. To investigate further, the quench region has been removed from the cavity and analysed using SEM methods. These results are compared to theoretical models describing two vortex entry defect candidates: regions of thin-layer tin-depleted Nb3Sn on the cavity surface that lower the flux entry field, and grain boundaries acting as Josephson junctions with a lower critical current than the surrounding material. A theoretical model of layer growth developed using density functional theory is used to discuss alterations to the coating process that could mitigate the formation of such defects.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB041
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THPB042	Field-dependence of the Sensitivity to Trapped Flux in Nb3Sn	844
	D.L. Hall, M. Liepe, R.D. Porter Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA D. Liarte, J.P. Sethna Cornell University, Ithaca, New York, USA
	The amount of residual resistance gained per unit of trapped flux ' referred to as the trapped flux sensitivity ' in Nb3Sn cavities has been found to be a function of the amplitude of the RF field. This behaviour is consistent with a scenario in which the trapped vortex dynamics are described by collective weak pinning. A model has been developed to describe this, and results in the observed linear dependence of trapped flux sensitivity with RF field. The model is used to discuss cavity preparation methods that might suppress this dependence, which would reduce the trapped flux requirements necessary to operate an Nb3Sn cavity at simultaneous high quality factors and accelerating gradients.
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THPB043	Effects of Chemical Treatments on the Surface Roughess and Surface Magnetic Field Ehancement of Niobium-3 Tin Films for Superconducting Radio-Frequency Cavities	848
	R.D. Porter, F. Furuta, D.L. Hall, M. Liepe, J.T. Maniscalco Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Current niobium-3 tin (Nb3Sn) films produced via vapor diffusion have rougher surfaces than typical electropolished niobium surfaces causing significantly enhancement of the surface magnetic fields. Reducing surface roughness of Nb3Sn surfaces may be necessary to achieve higher gradient accelerator cavities with high Q. Previous work at Cornell has shown the impact of several chemical treatments on the surface roughness of Nb3Sn films; however, it had not been evaluated how the changes in surface roughness impact the surface magnetic field enhancement. In this paper we present simulations of the surface field enhancement of oxipolished Nb3Sn, which was shown to be effective at reducing the surface roughness of Nb3Sn. The surface magnetic field enhancement data is compared to those of unetched Nb3Sn to find that the surface magnetic field enhancement (and surface roughness) has been roughly halved.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB043
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THPB053	Surface Resistance Characterization of Nb3Sn Using the HZB Quadrupole Resonator	863
	S. Keckert, J. Knobloch, O. Kugeler HZB, Berlin, Germany D.L. Hall, M. Liepe Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Funding: This work is part of EuCARD-2, partly funded by the European Commission, GA 312453. Nb3Sn is a very promising candidate material for future SRF cavities. With a critical temperature more than twice as the one of bulk niobium, higher operational temperatures with still lower surface resistance are theoretically possible. A sample prepared by Cornell University was characterized towards its SRF properties using the HZB Quadrupole Resonator. In comparison to a coated cavity this device enables SRF measurements at an extended parameter space (frequency, temperature and RF field) and easy access to physical quantities such as critical field and penetration depth. In this contribution we present surface resistance and RF critical field measurements.
	Poster THPB053 [2.725 MB]
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THPB079	Simulations of RF Field-induced Thermal Feedback in Niobium and Nb3Sn Cavities	920
	J. Ding, D.L. Hall Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA M. Liepe Cornell University, Ithaca, New York, USA
	Thermal feedback is a known limitation for SRF cavities made of low-purity niobium, as the increased losses at higher temperature described by BCS theory create a feedback mechanism that can eventually result in a runaway effect and associated cavity quench. In a similar manner, niobium cavities coated with Nb3Sn may also be subject to increased losses from thermal feedback, as Nb3Sn is possessed of a much lower thermal conductivity than niobium, although this effect will be mitigated by the thin film nature of the coating. In order to better understand the degree to which thermal feedback plays a role in the performance of Nb3Sn cavities, it is necessary to understand how the various components of the problem play a role in the outcome. In this paper, we present the first results from simulations performed at Cornell University that model RF induced thermal feedback in both conventional niobium cavities and niobium cavities coated with a thin film of Nb3Sn. The impacts of layer thickness, niobium substrate thermal conductivity, and trapped flux on the performance of the cavity are discussed.
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Paper

Title

Page

High-frequency SRF Cavities

400

T.E. Oseroff, D.L. Hall, M. Liepe, J.T. Maniscalco
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Historically, the frequency of SRF cavities has been limited by cryogenic power dissipation increasing rapidly with frequency, due to the BCS surface resistance having a quadratic dependence on frequency. Now, new SRF surfaces using doped niobium and compound superconductors like Nb3Sn can drastically reduce the BCS part of the surface resistance. The temperature independent part of the surface resistance (residual resistance) can therefore become dominant, and has its own, different frequency dependence. We have developed a model to analyze cryogenic cooling power requirements for SRF cavities as function of operating frequency, temperature, and trapped flux to evaluate the impact of the novel low-loss SRF surfaces on the questions of optimal operating frequency and frequency limit. We show that high-frequency SRF cavities now become a realistic option for future SRF driven accelerators. As the transverse cavity size decreases inversely with respect to its resonant frequency, such high-frequency SRF cavities could greatly reduce cost.

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB009

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WEXA01

High Performance Nb3Sn Cavities

667

D.L. Hall, M. Liepe, R.D. Porter
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
T. Arias, P. Cueva, D. Liarte, D.A. Muller, J.P. Sethna, N. Sitaraman
Cornell University, Ithaca, New York, USA

In recent years, 1.3 GHz single-cell cavities coated with Nb3Sn at Cornell University have repeatedly demonstrated quality factors of >10¹⁰ at 4.2 K and >15 MV/m. Ongoing research is currently focussed on the impact of intrinsic and extrinsic factors that limit the quality factor and quench field in these cavities. New single-cell cavities have been commissioned to enable further exploration of the coating parameter space. Experimental studies on both cavities and sample coupons have been supplemented by theoretical work done on layer growth, trapped vortex motion and flux entry. In this paper, we provide a comprehensive overview of the latest developments on Nb3Sn cavities, including work conducted in collaboration with the new NSF Centre for Bright Beams, with a brief summary on work being done in the field at large.

Slides WEXA01 [10.681 MB]

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WEXA03

High-performance Thin-film Niobium Produced via Chemical Vapor Deposition (CVD)

674

R.D. Porter, D.L. Hall, M. Liepe, J.T. Maniscalco
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
V.M. Arrieta, S.R. McNeal, W.E. Williams
Ultramet, Pacoima, California, USA

Bulk niobium cavities have been the standard for superconducting particle accelerators for many years. However, the cost of high RRR niobium start materials makes them expensive. The use of Chemical Vapor Deposition (CVD) processing technologies to produce thin Nb films on low-cost substrates (e.g. copper) offers a method to significantly reduce the cost of accelerator cavity fabrication while increasing cavity performance capabilities. Recent optimization of CVD niobium processes for high RRR Nb films has led to RF performance approaching that of bulk Nb. In collaboration with Ultramet, Cornell continues to explore the potential of CVD techniques. This paper presents results from a detailed study of CVD thin film Nb materials produced by Ultramet on 5-inch diameter copper and molybdenum substrates, including RF performance results with T-mapping and detailed surface analysis of performance limiting regions. Our work shows that CVD-based cavity fabrication methods are a promising alternative to sheet-formed bulk cavities, and to other thin Nb film techniques, warranting further development. Additional results from the field will be discussed.

Slides WEXA03 [1.503 MB]

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WEXA05

Dirty Layers, Bi-layers and Multi-layers: Insights from Muon Spin Rotation Experiments

T. Junginger, R.E. Laxdal, D.W. Storey, E. Thoeng
TRIUMF, Vancouver, Canada
D.L. Hall, M. Liepe
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
T. Junginger
HZB, Berlin, Germany
S. Posen
Fermilab, Batavia, Illinois, USA
T. Prokscha, Z. Salman, A. Suter
PSI, Villigen PSI, Switzerland
D.W. Storey
Victoria University, Victoria, B.C., Canada
T. Tan, W.K. Withanage, M.A. Wolak, X. Xi
Temple University, Philadelphia, USA
E. Thoeng
UBC & TRIUMF, Vancouver, British Columbia, Canada
A-M. Valente-Feliciano
JLab, Newport News, Virginia, USA
W.W. Wasserman
UBC, Vancouver, B.C., Canada

Funding: This research was supported by a Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007-2013).
The multilayer approach is being investigated for SRF applications since 2006 "*". More recently the option of using a bilayer system of two superconductors has been considered as an alternative approach to reach accelerating gradients beyond bulk niobium or to explain the gradient enhancement from a 120°C bake by introduction of a 'dirty layer "**"'. In this talk results are presented from two muon spin rotation experiments at TRIUMF and PSI. The former measures the field of first entry Hentry. It will be shown that MgB2 and Nb3Sn on top of Nb both push Hentry above Hc1 to a value consistent with Hsh, independent of the layer thickness. 120°C baking increases Hentry slightly but significantly above Hc1. Using the low energy muon beam at PSI we show that there is a long range proximity effect in a bilayer system of NbTiN on Nb. This effect yields a stronger decay of the RF field with depth as expected for pure NbTiN, opposite to what has been predicted for a bi-layer system due to counter current flow at the superconductor-superconductor interface "***". An insulating layer suppresses this proximity effect.
* Gurevich, A. APL 88.1 (2006)
** Checchin, M. Diss. Illinois Institute of Technology, 2016.
Kubo, T. Superconductor Science and Technology 30.2 (2016)
*** Kubo, T et al. APL 104.3 (2014)

Slides WEXA05 [3.716 MB]

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WEXA07

Theoretical Estimates of Maximum Fields in Superconducting Resonant Radio Frequency Cavities: Stability Theory, Disorder, and Laminates

D. Liarte, M. Liepe, J.P. Sethna
Cornell University, Ithaca, New York, USA
G. Catelani
Forschungszentrum Jülich, Peter Gruenberg Institut, Jülich, Germany
D.L. Hall
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
S. Posen
Fermilab, Batavia, Illinois, USA
M.K. Transtrum
Brigham Young University, Provo, USA

Funding: This work was supported by the US National Science Foundation under Award OIA-1549132, the Center for Bright Beams.
Theoretical limits to the performance of superconductors in high magnetic fields parallel to their surfaces are of key relevance to current and future accelerating cavities. We present intuitive arguments and simple estimates for Hsh, and combine them with rigorous calculations. We explore the effects of materials anisotropy and the danger of disorder in nucleating vortex entry. Will we need to control surface orientation in the layered compound MgB2? Can we estimate theoretically whether dirt and defects make these new materials fundamentally more challenging to optimize than niobium? We discuss and analyze recent proposals to use thin superconducting layers or laminates to enhance the performance of superconducting cavities. Flux entering a laminate can lead to so-called pancake vortices; we consider the physics of the dislocation motion and potential re-annihilation or stabilization of these vortices after their entry.

Slides WEXA07 [2.975 MB]

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THPB040

SRF Theory Developments from the Center for Bright Beams

835

D. Liarte, T. Arias, M. Liepe, J.P. Sethna, N. Sitamaran
Cornell University, Ithaca, New York, USA
D.L. Hall
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
A.R. Pack, M.K. Transtrum
Brigham Young University, Provo, USA

Funding: This work was supported by the US National Science Foundation under Award OIA-1549132, the Center for Bright Beams.
We present theoretical studies of SRF materials from the Center for Bright Beams. First, we discuss the effects of disorder, inhomogeneities, and materials anisotropy on the maximum parallel surface field that a superconductor can sustain in an SRF cavity, using linear stability in conjunction with Ginzburg-Landau and Eilenberger theory. We connect our disorder mediated vortex nucleation model to current experimental developments of Nb3Sn and other cavity materials. Second, we use time-dependent Ginzburg-Landau simulations to explore the role of inhomogeneities in nucleating vortices, and discuss the effects of trapped magnetic flux on the residual resistance of weakly-pinned Nb3Sn cavities. Third, we present first-principles density-functional theory (DFT) calculations to uncover and characterize the key fundamental materials processes underlying the growth of Nb3Sn. Our calculations indicate that the observed tin-depleted regions may be the direct result of an exothermic reaction between Nb3Sn and Nb at the growing Nb/Nb3Sn interface. We suggest new growth protocols to mitigate the formation of tin depleted regions.

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB040

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THPB041

Cavity Quench Studies in Nb3Sn Using Temperature Mapping and Surface Analysis of Cavity Cut-outs

840

D.L. Hall, M. Liepe, R.D. Porter
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
P. Cueva, D. Liarte, D.A. Muller, J.P. Sethna
Cornell University, Ithaca, New York, USA

Previous experimental studies on single-cell Nb3Sn cavities have shown that the cause of quench is isolated to a localised defect on the cavity surface. Here, cavity temperature mapping has been used to investigate cavity quench behaviour in an Nb3Sn cavity by measuring the temperature at the quench location as the RF field approaches the quench field. The heating profile observed at the quench location prior to quench appears to suggest quantised vortex entry at a defect. To investigate further, the quench region has been removed from the cavity and analysed using SEM methods. These results are compared to theoretical models describing two vortex entry defect candidates: regions of thin-layer tin-depleted Nb3Sn on the cavity surface that lower the flux entry field, and grain boundaries acting as Josephson junctions with a lower critical current than the surrounding material. A theoretical model of layer growth developed using density functional theory is used to discuss alterations to the coating process that could mitigate the formation of such defects.

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB041

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THPB042

Field-dependence of the Sensitivity to Trapped Flux in Nb3Sn

844

D.L. Hall, M. Liepe, R.D. Porter
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
D. Liarte, J.P. Sethna
Cornell University, Ithaca, New York, USA

The amount of residual resistance gained per unit of trapped flux ' referred to as the trapped flux sensitivity ' in Nb3Sn cavities has been found to be a function of the amplitude of the RF field. This behaviour is consistent with a scenario in which the trapped vortex dynamics are described by collective weak pinning. A model has been developed to describe this, and results in the observed linear dependence of trapped flux sensitivity with RF field. The model is used to discuss cavity preparation methods that might suppress this dependence, which would reduce the trapped flux requirements necessary to operate an Nb3Sn cavity at simultaneous high quality factors and accelerating gradients.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB042

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THPB043

Effects of Chemical Treatments on the Surface Roughess and Surface Magnetic Field Ehancement of Niobium-3 Tin Films for Superconducting Radio-Frequency Cavities

848

R.D. Porter, F. Furuta, D.L. Hall, M. Liepe, J.T. Maniscalco
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Current niobium-3 tin (Nb3Sn) films produced via vapor diffusion have rougher surfaces than typical electropolished niobium surfaces causing significantly enhancement of the surface magnetic fields. Reducing surface roughness of Nb3Sn surfaces may be necessary to achieve higher gradient accelerator cavities with high Q. Previous work at Cornell has shown the impact of several chemical treatments on the surface roughness of Nb3Sn films; however, it had not been evaluated how the changes in surface roughness impact the surface magnetic field enhancement. In this paper we present simulations of the surface field enhancement of oxipolished Nb3Sn, which was shown to be effective at reducing the surface roughness of Nb3Sn. The surface magnetic field enhancement data is compared to those of unetched Nb3Sn to find that the surface magnetic field enhancement (and surface roughness) has been roughly halved.

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB043

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THPB053

Surface Resistance Characterization of Nb3Sn Using the HZB Quadrupole Resonator

863

S. Keckert, J. Knobloch, O. Kugeler
HZB, Berlin, Germany
D.L. Hall, M. Liepe
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Funding: This work is part of EuCARD-2, partly funded by the European Commission, GA 312453.
Nb3Sn is a very promising candidate material for future SRF cavities. With a critical temperature more than twice as the one of bulk niobium, higher operational temperatures with still lower surface resistance are theoretically possible. A sample prepared by Cornell University was characterized towards its SRF properties using the HZB Quadrupole Resonator. In comparison to a coated cavity this device enables SRF measurements at an extended parameter space (frequency, temperature and RF field) and easy access to physical quantities such as critical field and penetration depth. In this contribution we present surface resistance and RF critical field measurements.

Poster THPB053 [2.725 MB]

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB053

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THPB079

Simulations of RF Field-induced Thermal Feedback in Niobium and Nb3Sn Cavities

920

J. Ding, D.L. Hall
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
M. Liepe
Cornell University, Ithaca, New York, USA

Thermal feedback is a known limitation for SRF cavities made of low-purity niobium, as the increased losses at higher temperature described by BCS theory create a feedback mechanism that can eventually result in a runaway effect and associated cavity quench. In a similar manner, niobium cavities coated with Nb3Sn may also be subject to increased losses from thermal feedback, as Nb3Sn is possessed of a much lower thermal conductivity than niobium, although this effect will be mitigated by the thin film nature of the coating. In order to better understand the degree to which thermal feedback plays a role in the performance of Nb3Sn cavities, it is necessary to understand how the various components of the problem play a role in the outcome. In this paper, we present the first results from simulations performed at Cornell University that model RF induced thermal feedback in both conventional niobium cavities and niobium cavities coated with a thin film of Nb3Sn. The impacts of layer thickness, niobium substrate thermal conductivity, and trapped flux on the performance of the cavity are discussed.

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB079

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