SRF2019 - List of Authors (Porter, R.D.)

Paper	Title	Page
MOP010	Ab Initio Calculations on the Growth and Superconducting Properties of Nb₃Sn	39
SUSP023	use link to see paper's listing under its alternate paper code
	N. Sitaraman, T. Arias, P. Cueva, M.M. Kelley, D.A. Muller Cornell University, Ithaca, New York, USA J.M. Carlson, A.R. Pack, M.K. Transtrum Brigham Young University, Provo, USA M. Liepe, R.D. Porter, Z. Sun Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Funding: This research was funded by the Center for Bright Beams. In this work, we employ theoretical ab initio techniques to solve mysteries and gain new insights in Nb₃Sn SRF physics. We determine the temperature dependence of Nb₃Sn antisite defect formation energies, and discuss the implications of these results for defect segregation. We calculate the phonon spectral function for Nb₃Sn cells with different combinations of antisite defects and use these results to determine Tc as a function of stoichiometry. These results allow for the first-ever determination of Tc in the tin-rich regime, where experimental measurements are unavailable and which is critical to understanding the impact of tin-rich grain boundaries on superconducting cavity performance. Finally, we propose a theory for the growth mechanism of Nb₃Sn growth on a thick oxide, explaining the puzzling disappearing droplet behavior of Sn on Nb oxide and suggesting how in general an oxide layer reacts with Sn to produce a uniform Nb₃Sn layer.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP010
About •	paper received ※ 02 July 2019 paper accepted ※ 03 July 2019 issue date ※ 14 August 2019
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MOP011	High Frequency Nb₃Sn Cavities	44
SUSP020	use link to see paper's listing under its alternate paper code
	R.D. Porter, M. Liepe, J.T. Maniscalco Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Niobium-3 Tin (Nb₃Sn) is an alternative material to Nb for SRF cavities. This material is capable of higher temperature operation and has high theoretical maximum accelerating gradients. Cornell University is a leader in the development of this material for SRF applications, and current Nb₃Sn 1.3 GHz single cells produced at Cornell achieve quality factors above 10^zEhNZeHn at 4.2 K at medium fields, far above what can be reached with niobium. Most of the recent Nb₃Sn cavity development has been done at 1.3 GHz. In this paper, we present new results from Nb₃Sn cavities at 2.6 GHz and 3.9 GHz. We compare relative cavity performance and flux trapping sensitivities, and extract frequency dependencies. Results show that the frequency can be increased without degrading the performance of the cavities, opening the path towards a new generation of compact and efficient SRF cavities for a wide range of future applications.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP011
About •	paper received ※ 05 July 2019 paper accepted ※ 12 July 2019 issue date ※ 14 August 2019
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MOP013	Reducing Surface Roughness of Nb₃Sn Through Chemical Polishing Treatments	48
	H. Hu, M. Liepe, R.D. Porter Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Niobium-3 tin (Nb₃Sn) is a promising alternative material for SRF cavities, with theoretical limits for critical temperatures and superheating fields reaching twice that of conventional Nb cavities. However, currently achievable accelerating gradients in Nb₃Sn cavities are much lower than their theoretical limit. One limitation to the maximum accelerating gradient is surface magnetic field enhancement caused by the surface roughness of Nb₃Sn. However, there are currently no standard techniques used to reduce Nb₃Sn surface roughness. Since Nb₃Sn is only 2-3 microns thick, it is difficult to selectively polish Nb₃Sn without removing the entire layer. Here, we investigate reducing the surface roughness of Nb₃Sn through applying chemical polishing treatments, including modified versions of standard techniques such as Buffered Chemical Polishing (BCP) and Electropolishing (EP). Through data acquired from Atomic Force Microscope (AFM) scans, SEM scans, and SEM-EDS analysis, we show the effects of these chemical treatments in reducing surface roughness and consider the changes in the chemical composition of Nb₃Sn that may occur through the etching process. We find that BCP with a 1:1:8 solution is ineffective while EP halves the surface roughness of Nb₃Sn.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP013
About •	paper received ※ 01 July 2019 paper accepted ※ 04 July 2019 issue date ※ 14 August 2019
Export •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

MOP014	Electroplating of Sn Film on Nb Substrate for Generating Nb₃Sn Thin Films and Post Laser Annealing	51
SUSP036	use link to see paper's listing under its alternate paper code
	Z. Sun, M. Liepe, T.E. Oseroff, R.D. Porter Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA T. Arias, A.B. Connolly, J.M. Scholtz, N. Sitaraman, M.O. Thompson Cornell University, Ithaca, USA X. Deng University of Virginia, Charlottesville, Virginia, USA K.D. Dobson University of Delaware, Newark, Delaware, USA
	Controlling film quality of Nb₃Sn is critical to its SRF cavity performance. The state-of-the-art vapor diffusion approach for Nb₃Sn deposition observed surface roughness, thin grain regions, and misfit dislocations which negatively affect the RF performance. The Sn deficiency and non-uniformity at the nucleation stage of vapor deposition is believed to be the fundamental reason to cause these roughness and defects issues. Thus, we propose to pre-deposit a uniform Sn film on the Nb substrate, which is able to provide sufficient Sn source during the following heat treatment for Nb₃Sn nucleation and growth. Here, we demonstrated successful electrodeposition of a low-roughness, dendrite-free, excellent-adhesion Sn film on the Nb substrate. More importantly, we further achieved a uniform, low-roughness (Ra = 66 nm), pure-stoichiometric Nb₃Sn film through thermal treatment of this electroplated Sn film in the furnace. Additionally, we provide preliminary results of laser annealing as a post treatment for epitaxial grain growth and roughness reduction.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP014
About •	paper received ※ 22 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019
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TUFUB8	CVD Coated Copper Substrate SRF Cavity Research at Cornell University	381
	M. Ge, T. Gruber, J.J. Kaufman, M. Liepe, J.T. Maniscalco, T.E. Oseroff, R.D. Porter, Z. Sun Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA V.M. Arrieta, S.R. McNeal Ultramet, Pacoima, California, USA
	Chemical vapor deposition (CVD) is a promising alternative to conventional sputter techniques for coating copper substrate cavities with high-quality superconducting films. Through multiple SRF-related DOE SBIR projects, Ultramet has developed CVD processes and CVD reactor designs for SRF cavities, and Cornell University has conducted extensive RF testing of CVD coated surfaces. Here we report results from thin-film CVD Nb₃Sn coated copper test plates, and for thick-film CVD niobium on copper including full-scale single cell 1.3 GHz copper substrate cavities. Detailed optical inspection and surface characterization show high-quality and well-adhered coatings. No copper contamination is found. The Nb₃Sn coated plates have a uniform Nb₃Sn coating with a slightly low tin concentration (19 -22%), but a BCS resistance well in agreement with predictions. The CVD Nb coatings on copper plates demonstrate excellent adhesion characteristics and exceeded surface fields of 50 mT without showing signs of a strong Q-slope that is frequently observed in sputtered Nb cavities. Multiple single-cell 1.3 GHz copper cavities have been coated to date at Ultramet, and results from RF testing of these are presented and discussed.
	Slides TUFUB8 [12.488 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUFUB8
About •	paper received ※ 01 July 2019 paper accepted ※ 05 July 2019 issue date ※ 14 August 2019
Export •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

THFUA5	Field Limitation in Nb₃Sn Cavities
	R.D. Porter Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Niobium-3 Tin (Nb₃Sn) is the most promising alternative material to Nb for SRF cavities, allowing for twice the operating temperature and potentially twice the accelerating gradient compared to Nb. According to the superheating field, an elliptical Nb₃Sn cavity could reach 96 MV/m. However, current Cornell Nb₃Sn cavities quench between 14 and 18 MV/m in CW operation. Previous work has shown that cavity quench occurs at a thermal surface defect, but the details of the defect are not yet understood. Here we present further studies of the defect/quench mechanism conducted at Cornell and with collaborators. These studies suggest new quench mechanisms and rule out older hypotheses.
	Slides THFUA5 [12.734 MB]
Export •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

Paper

Title

Page

MOP010

Ab Initio Calculations on the Growth and Superconducting Properties of Nb₃Sn

39

SUSP023

use link to see paper's listing under its alternate paper code

N. Sitaraman, T. Arias, P. Cueva, M.M. Kelley, D.A. Muller
Cornell University, Ithaca, New York, USA
J.M. Carlson, A.R. Pack, M.K. Transtrum
Brigham Young University, Provo, USA
M. Liepe, R.D. Porter, Z. Sun
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Funding: This research was funded by the Center for Bright Beams.
In this work, we employ theoretical ab initio techniques to solve mysteries and gain new insights in Nb₃Sn SRF physics. We determine the temperature dependence of Nb₃Sn antisite defect formation energies, and discuss the implications of these results for defect segregation. We calculate the phonon spectral function for Nb₃Sn cells with different combinations of antisite defects and use these results to determine Tc as a function of stoichiometry. These results allow for the first-ever determination of Tc in the tin-rich regime, where experimental measurements are unavailable and which is critical to understanding the impact of tin-rich grain boundaries on superconducting cavity performance. Finally, we propose a theory for the growth mechanism of Nb₃Sn growth on a thick oxide, explaining the puzzling disappearing droplet behavior of Sn on Nb oxide and suggesting how in general an oxide layer reacts with Sn to produce a uniform Nb₃Sn layer.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP010

About •

paper received ※ 02 July 2019 paper accepted ※ 03 July 2019 issue date ※ 14 August 2019

Export •

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

MOP011

High Frequency Nb₃Sn Cavities

44

SUSP020

use link to see paper's listing under its alternate paper code

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

Niobium-3 Tin (Nb₃Sn) is an alternative material to Nb for SRF cavities. This material is capable of higher temperature operation and has high theoretical maximum accelerating gradients. Cornell University is a leader in the development of this material for SRF applications, and current Nb₃Sn 1.3 GHz single cells produced at Cornell achieve quality factors above 10^zEhNZeHn at 4.2 K at medium fields, far above what can be reached with niobium. Most of the recent Nb₃Sn cavity development has been done at 1.3 GHz. In this paper, we present new results from Nb₃Sn cavities at 2.6 GHz and 3.9 GHz. We compare relative cavity performance and flux trapping sensitivities, and extract frequency dependencies. Results show that the frequency can be increased without degrading the performance of the cavities, opening the path towards a new generation of compact and efficient SRF cavities for a wide range of future applications.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP011

About •

paper received ※ 05 July 2019 paper accepted ※ 12 July 2019 issue date ※ 14 August 2019

Export •

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

MOP013

Reducing Surface Roughness of Nb₃Sn Through Chemical Polishing Treatments

48

H. Hu, M. Liepe, R.D. Porter
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Niobium-3 tin (Nb₃Sn) is a promising alternative material for SRF cavities, with theoretical limits for critical temperatures and superheating fields reaching twice that of conventional Nb cavities. However, currently achievable accelerating gradients in Nb₃Sn cavities are much lower than their theoretical limit. One limitation to the maximum accelerating gradient is surface magnetic field enhancement caused by the surface roughness of Nb₃Sn. However, there are currently no standard techniques used to reduce Nb₃Sn surface roughness. Since Nb₃Sn is only 2-3 microns thick, it is difficult to selectively polish Nb₃Sn without removing the entire layer. Here, we investigate reducing the surface roughness of Nb₃Sn through applying chemical polishing treatments, including modified versions of standard techniques such as Buffered Chemical Polishing (BCP) and Electropolishing (EP). Through data acquired from Atomic Force Microscope (AFM) scans, SEM scans, and SEM-EDS analysis, we show the effects of these chemical treatments in reducing surface roughness and consider the changes in the chemical composition of Nb₃Sn that may occur through the etching process. We find that BCP with a 1:1:8 solution is ineffective while EP halves the surface roughness of Nb₃Sn.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP013

About •

paper received ※ 01 July 2019 paper accepted ※ 04 July 2019 issue date ※ 14 August 2019

Export •

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

MOP014

Electroplating of Sn Film on Nb Substrate for Generating Nb₃Sn Thin Films and Post Laser Annealing

51

SUSP036

use link to see paper's listing under its alternate paper code

Z. Sun, M. Liepe, T.E. Oseroff, R.D. Porter
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
T. Arias, A.B. Connolly, J.M. Scholtz, N. Sitaraman, M.O. Thompson
Cornell University, Ithaca, USA
X. Deng
University of Virginia, Charlottesville, Virginia, USA
K.D. Dobson
University of Delaware, Newark, Delaware, USA

Controlling film quality of Nb₃Sn is critical to its SRF cavity performance. The state-of-the-art vapor diffusion approach for Nb₃Sn deposition observed surface roughness, thin grain regions, and misfit dislocations which negatively affect the RF performance. The Sn deficiency and non-uniformity at the nucleation stage of vapor deposition is believed to be the fundamental reason to cause these roughness and defects issues. Thus, we propose to pre-deposit a uniform Sn film on the Nb substrate, which is able to provide sufficient Sn source during the following heat treatment for Nb₃Sn nucleation and growth. Here, we demonstrated successful electrodeposition of a low-roughness, dendrite-free, excellent-adhesion Sn film on the Nb substrate. More importantly, we further achieved a uniform, low-roughness (Ra = 66 nm), pure-stoichiometric Nb₃Sn film through thermal treatment of this electroplated Sn film in the furnace. Additionally, we provide preliminary results of laser annealing as a post treatment for epitaxial grain growth and roughness reduction.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP014

About •

paper received ※ 22 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019

Export •

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

TUFUB8

CVD Coated Copper Substrate SRF Cavity Research at Cornell University

381

M. Ge, T. Gruber, J.J. Kaufman, M. Liepe, J.T. Maniscalco, T.E. Oseroff, R.D. Porter, Z. Sun
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
V.M. Arrieta, S.R. McNeal
Ultramet, Pacoima, California, USA

Chemical vapor deposition (CVD) is a promising alternative to conventional sputter techniques for coating copper substrate cavities with high-quality superconducting films. Through multiple SRF-related DOE SBIR projects, Ultramet has developed CVD processes and CVD reactor designs for SRF cavities, and Cornell University has conducted extensive RF testing of CVD coated surfaces. Here we report results from thin-film CVD Nb₃Sn coated copper test plates, and for thick-film CVD niobium on copper including full-scale single cell 1.3 GHz copper substrate cavities. Detailed optical inspection and surface characterization show high-quality and well-adhered coatings. No copper contamination is found. The Nb₃Sn coated plates have a uniform Nb₃Sn coating with a slightly low tin concentration (19 -22%), but a BCS resistance well in agreement with predictions. The CVD Nb coatings on copper plates demonstrate excellent adhesion characteristics and exceeded surface fields of 50 mT without showing signs of a strong Q-slope that is frequently observed in sputtered Nb cavities. Multiple single-cell 1.3 GHz copper cavities have been coated to date at Ultramet, and results from RF testing of these are presented and discussed.

Slides TUFUB8 [12.488 MB]

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUFUB8

About •

paper received ※ 01 July 2019 paper accepted ※ 05 July 2019 issue date ※ 14 August 2019

Export •

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

THFUA5

Field Limitation in Nb₃Sn Cavities

R.D. Porter
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Niobium-3 Tin (Nb₃Sn) is the most promising alternative material to Nb for SRF cavities, allowing for twice the operating temperature and potentially twice the accelerating gradient compared to Nb. According to the superheating field, an elliptical Nb₃Sn cavity could reach 96 MV/m. However, current Cornell Nb₃Sn cavities quench between 14 and 18 MV/m in CW operation. Previous work has shown that cavity quench occurs at a thermal surface defect, but the details of the defect are not yet understood. Here we present further studies of the defect/quench mechanism conducted at Cornell and with collaborators. These studies suggest new quench mechanisms and rule out older hypotheses.

Slides THFUA5 [12.734 MB]

Export •

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