SRF2021 - List of Authors (Liepe, M.)

Paper	Title	Page
SUPCAV003	Dynamic Temperature Mapping of Nb₃Sn Cavities	6
	R.D. Porter, N. Banerjee, M. Liepe 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 niobium for SRF accelerator cavities. The material promises nearly twice the potential accelerating gradients (~100 MV/m in TESLA elliptical cavities), increased quality factors, and 4.2 K operation. Current state of the art Nb₃Sn cavities reach quality factors of 2 x 10¹⁰ at 4.2 K and have reached 24 MV/m. Determining the cause of the premature field limitation is the topic of ongoing research. Cornell University has recently developed a high-speed temperature mapping system that can examine cavity quench mechanisms in never before achieved ways. Here we present high-speed temperature map results of Nb₃Sn cavities and examine the quench mechanism and dynamic heating. We show an initial multipacting quench and sudden temperature jumps at multiple locations on the cavity.
DOI •	reference for this paper ※ doi:10.18429/JACoW-SRF2021-SUPCAV003
About •	Received ※ 09 July 2021 — Accepted ※ 12 August 2021 — Issue date ※ 31 August 2021
Cite •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

SUPFDV009	Thermal Annealing of Sputtered Nb₃Sn and V₃Si Thin Films for Superconducting RF Cavities	82
	K. Howard, M. Liepe, Z. Sun Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Funding: U.S. National Science Foundation under Award PHY-1549132, the Center for Bright Beams and Cornell Center for Materials Research Shared Facilities supported through the NSF MRSEC program (DMR-1719875) Nb₃Sn and V₃Si thin films are alternative material candidates for the next-generation of superconducting radio frequency (SRF) cavities. However, past sputtered films suffer from stoichiometry and strain issues during deposition and post annealing. As such, we aim to explore the structural and chemical effects of thermal annealing, both in-situ and post-sputtering, on DC-sputtered Nb₃Sn and V₃Si with varying thickness on Nb or Cu substrates. We successfully enabled recrystallization of 100 nm thin Nb₃Sn films with stoichiometric and strain-free grains at 950 C annealing. For 2 um films, we observed removal of strain and slight increase in grain size with increasing temperature. A phase transformation from unstable to stable structure appeared on thick V₃Si samples, while we observed significant Sn loss in thick Nb₃Sn films at high temperature anneals. For films on Cu substrates, we observed similar Sn and Si loss during annealing likely due to Cu-Sn and Cu-Si phase generation and subsequent Sn and Si evaporation. These results encourage us to refine our process to obtain high quality films for SRF use.
DOI •	reference for this paper ※ doi:10.18429/JACoW-SRF2021-SUPFDV009
About •	Received ※ 22 June 2021 — Revised ※ 06 July 2021 — Accepted ※ 12 August 2021 — Issue date ※ 17 March 2022
Cite •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

SUPTEV007	Development of a System for Coating SRF Cavities Using Remote Plasma CVD	129
	G. Gaitan, P. Bishop, A.T. Holic, G. Kulina, M. Liepe, J. Sears, Z. Sun Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Funding: This work was supported by the National Science Foundation under Grant No. PHY-1549132. Next-generation, thin-film surfaces employing Nb₃Sn, NbN, NbTiN, and other compound superconductors are destined to allow reaching superior RF performance levels in SRF cavities. Optimized, advanced deposition processes are required to enable high-quality films of such materials on large and complex-shaped cavities. For this purpose, Cornell University is developing a remote plasma-enhanced chemical vapor deposition (CVD) system that facilitates coating on complicated geometries with a high deposition rate. This system is based on a high-temperature tube furnace with a clean vacuum and furnace loading system. The use of plasma alongside reacting precursors will significantly reduce the required processing temperature and promote precursor decomposition. A vacuum quality monitor (VQM) is used to characterize the residual gases before coating. The CVD system has been designed and is currently under assembly and commissioning.
DOI •	reference for this paper ※ doi:10.18429/JACoW-SRF2021-SUPTEV007
About •	Received ※ 09 July 2021 — Accepted ※ 21 August 2021 — Issue date ※ 10 February 2022
Cite •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

SUPTEV008	CW Operation of Conduction-Cooled Nb₃Sn SRF Cavity	133
	N.A. Stilin, A.T. Holic, M. Liepe, R.D. Porter, J. Sears, Z. Sun Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Significant progress in the performance of SRF cavities coated with Nb₃Sn films during the last few years has provided an energy efficient alternative to traditional Nb cavities, thereby initiating a fundamental shift in SRF technology. These Nb₃Sn cavities can operate at significantly higher temperatures than Nb cavities while simultaneously requiring less cooling power. This allows for the use of new cryogenic cooling schemes based on conduction cooling with robust, commercialized turn-key style cryocoolers. Cornell University has developed and tested a 2.6 GHz Nb₃Sn cavity assembly which utilizes such cooling methods. These tests have demonstrated stable RF operation at 10 MV/m with measured thermal dynamics which match numerical simulations. These results also serve as a foundation for designing a new standalone SRF cryomodule which will use a pair of cryocoolers to cool a 1.3 GHz Nb₃Sn cavity with an input coupler capable of supporting high beam current operation.
DOI •	reference for this paper ※ doi:10.18429/JACoW-SRF2021-SUPTEV008
About •	Received ※ 22 June 2021 — Accepted ※ 13 August 2021 — Issue date ※ 08 November 2021
Cite •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUOFDV05	Dynamics of RF Dissipation Probed via High-Speed Temperature Mapping	349
	R.D. Porter, N. Banerjee, M. Liepe Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Recently, Cornell University has developed a new high-speed, high-resolution temperature mapping system that can resolve the time dynamics of RF dissipation, i.e., provide high-speed videos of the surface heating across the entire surface of the cavity. This new powerful tool allows to observe rapid changes in the local RF dissipation, as well as to resolve the dynamics of quenches, field emission processing, and other cavity events, giving new insights into these. This contribution presents the development of this new high-speed temperature mapping system, discusses its commissioning and extensive performance testing (e.g., demonstrating micro-Kelvin resolution), as well as show intriguing high-speed temperature mapping results from multiple Nb₃Sn cavities.
DOI •	reference for this paper ※ doi:10.18429/JACoW-SRF2021-TUOFDV05
About •	Received ※ 01 July 2021 — Accepted ※ 21 August 2021 — Issue date ※ 06 February 2022
Cite •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUOFDV07	Sample Test Systems for Next-Gen SRF Surfaces	357
	T.E. Oseroff, M. Liepe, Z. Sun Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	With the increasing worldwide focus on the development of new surfaces for SRF cavities, exploring alternative materials and multilayer structures, test systems that allow measuring the RF performance of simple sample geometries (e.g., flat samples) become increasingly essential. These systems provide RF performance results that are needed to guide the development of these surfaces. This contribution gives an overview of sample test systems currently available, including the improved Cornell sample host cavity. Recent advances in this important technology, performance specifications, and current limitations are discussed. In addition, an overview is given of interesting recent RF performance results on samples coated with non-niobium bulk and multilayer films.
DOI •	reference for this paper ※ doi:10.18429/JACoW-SRF2021-TUOFDV07
About •	Received ※ 08 July 2021 — Accepted ※ 21 August 2021 — Issue date ※ 05 September 2021
Cite •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUPFDV010	New Recipes to Optimize the Niobium Oxide Surface From First-Principles Calculations	426
	N. Sitaraman, T. Arias, Z. Baraissov, M.M. Kelley, D.A. Muller Cornell University, Ithaca, New York, 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 work was supported by the U.S. National Science Foundation under Award No. PHY-1549132, the Center for Bright Beams The properties of niobium oxide are of critical importance for a wide range of topics, from the behavior of nitrogen during infusion treatments, to the nucleation of Nb₃Sn, to the superconducting properties of the surface. However, the modeling of the oxide is often much simplified, ignoring the variety of niobium oxide phases and the extremely different properties of these phases in the presence of impurities and defects. We use density functional theory (DFT) to investigate how electrochemical treatments and gas infusion procedures change the properties of niobium oxide, and to investigate how these properties could be optimized for Nb₃Sn nucleation and for niobium SRF performance.
DOI •	reference for this paper ※ doi:10.18429/JACoW-SRF2021-TUPFDV010
About •	Received ※ 01 July 2021 — Accepted ※ 18 November 2021 — Issue date ※ 22 February 2022
Cite •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

WEPTEV012	Characterization of Atomic-Layer-Deposited NbTiN and NbTiN/AlN Films for SIS Multilayer Structures	662
	Z. Sun, M. Liepe, T.E. Oseroff Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA X. Deng University of Virginia, Charlottesville, Virginia, USA
	SIS (superconductor-insulator-superconductor) mul-tilayer structures are proposed designs to repel early flux penetration and ease the impact of defects in SRF cavities. The demonstration of such device physics is strongly affected by the film qualities ’ material struc-ture and composition. Here, we characterized 100 nm NbTiN / 2 nm AlN / bulk Nb SIS structures and investigated the effect of the presence of the AlN layer on the NbTiN film properties. We find that the hcp-structured AlN layer results in a Nb composition gra-dient as a function of film depth, whereas the Nb con-centration remains constant in the NbTiN/Nb samples, which suggests that interface mismatch could induce significant change in NbTiN composition. The surface composition variation further leads to different oxide structures, which might impact the superconducting performance. Our observations indicate that the choice of the insulating layer in SIS structures is critical, and that interface mismatch together with internal strain could deteriorate the superconducting film.
DOI •	reference for this paper ※ doi:10.18429/JACoW-SRF2021-WEPTEV012
About •	Received ※ 08 July 2021 — Revised ※ 06 August 2021 — Accepted ※ 22 November 2021 — Issue date ※ 02 January 2022
Cite •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

WEOTEV03	Toward Stoichiometric and Low-Surface-Roughness Nb₃Sn Thin Films via Direct Electrochemical Deposition	710
	Z. Sun, G. Gaitan, M. Ge, K. Howard, 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, Z. Baraissov, M.M. Kelley, D.A. Muller, J.P. Sethna, N. Sitaraman Cornell University, Ithaca, New York, USA K.D. Dobson University of Delaware, Newark, Delaware, USA
	Reducing surface roughness and attaining stoichiometry of Nb₃Sn superconducting films are required to push their superheating field to the theoretical limit in SRF cavities. As such, we explore direct electrochemical processes that minimize involving foreign elements to deposit high-quality Sn, Nb, and NbxSn films on Nb and Cu surfaces. These films are then thermally annealed to Nb₃Sn. We find that smooth Sn pre-depositions via electroplating on Nb surfaces significantly reduce the average roughness of resultant Nb₃Sn to 65 nm, with a dramatic reduction in power intensity at medium special frequencies. Structural and superconducting properties demonstrate a Nb₃Sn A15 phase with a stoichiometry of 25 at% Sn. This process is being scaled-up to a 3.9 GHz cavity. Moreover, preliminary results on electroplating on Cu surface show that Nb plating undergoes a slow growth rate while subsequent Sn plating on the plated Nb surface can be controlled with varied thickness. The Nb plating process is currently being optimized.
DOI •	reference for this paper ※ doi:10.18429/JACoW-SRF2021-WEOTEV03
About •	Received ※ 09 July 2021 — Revised ※ 09 August 2021 — Accepted ※ 21 August 2021 — Issue date ※ 16 January 2022
Cite •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

THPFDV005	Superconducting RF Performance of Cornell 500 MHz N-Doped B-Cell SRF Cavitiy	764
	M. Ge, T. Gruber, A.T. Holic, M. Liepe, J. Sears Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	The Cornell SRF group is working on rebuilding a 500 MHz B-cell cryomodule (CRYO-2 BB1-5) as a spared cryomodule for the operation of the CESR ring. To minimize BCS surface resistance, achieve a high quality-factor (Q₀), and increase maximum fields, we prepared the cavity’s surface with electropolishing and performed a 2/6 N2-doping. In this work, we report 4.2 K and 2 K cavity test results with detailed surface resistance analysis, showing improved performance, including significant higher fields.
	Poster THPFDV005 [0.718 MB]
DOI •	reference for this paper ※ doi:10.18429/JACoW-SRF2021-THPFDV005
About •	Received ※ 05 July 2021 — Revised ※ 10 August 2021 — Accepted ※ 21 August 2021 — Issue date ※ 22 April 2022
Cite •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

THPFDV006	Seebeck Coefficient Measurement at Cryogenic Temperatures for the LCLS-II HE Project	768
	M. Ge, A.T. Holic, M. Liepe, J. Sears Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Reducing thermoelectric currents during cooldown is important to maintain high-quality factors (Q₀) of the cavities in the LCLS-II HE cryomodules. The temperature-dependent Seebeck coefficients of the materials used in the cryomodules are needed for quantitative estimations of thermoelectric currents. In this work, we present a setup for cryogenic Seebeck coefficient measurements as well as the measured Seebeck coefficients of high-pure niobium at cryogenic temperatures between 4K and 200K.
	Poster THPFDV006 [0.511 MB]
DOI •	reference for this paper ※ doi:10.18429/JACoW-SRF2021-THPFDV006
About •	Received ※ 29 June 2021 — Revised ※ 10 August 2021 — Accepted ※ 21 August 2021 — Issue date ※ 26 November 2021
Cite •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

THPTEV004	Surface Oxides on Nb and Nb₃Sn Surfaces: Toward a Deeper Understanding	836
	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, Z. Baraissov, D.A. Muller, N. Sitaraman Cornell University, Ithaca, New York, USA C. Dukes University of Virginia, Charlottesville, Virginia, USA D. Johnson-McDaniel, M. Salim CCMR, Ithaca, New York, USA
	Surface oxides on Nb and Nb₃Sn SRF cavities, as a thin ’dirty’ layer, could be critical to their performance as suggested by recent theory. Although these oxides have been studied in the past, we intend here to provide a deeper understanding based on a systematic study on coupon samples that have been processed under the different conditions currently used in SRF cavity treatments. Our aim is to obtain a more complete picture of the oxide evolution. This then might help to explain the observed cavity performance variation, and might allow designing a process to achieve a designed, optimized surface with controlled oxides types and thickness. We find that the surface oxides are in amorphous phase that exhibits normal conducting behaviors, while the pentoxide further degrades with time. Also, we observed a thin hydroxide layer on the outermost surface and possibly Nb(OH)x motifs in the bulk. Moreover, distinctive oxide structures were found in Nb₃Sn samples from vapor diffusion, electroplating, and sputtering. The semiconducting SnOx appeared through the oxide depth in vapor diffused Nb₃Sn, while a ~1 nm SnOx layer merely exists at the outermost surface of electroplated Nb₃Sn.
DOI •	reference for this paper ※ doi:10.18429/JACoW-SRF2021-THPTEV004
About •	Received ※ 09 July 2021 — Revised ※ 11 August 2021 — Accepted ※ 21 August 2021 — Issue date ※ 04 November 2021
Cite •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

Paper

Title

Page

Dynamic Temperature Mapping of Nb₃Sn Cavities

6

R.D. Porter, N. Banerjee, M. Liepe
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 niobium for SRF accelerator cavities. The material promises nearly twice the potential accelerating gradients (~100 MV/m in TESLA elliptical cavities), increased quality factors, and 4.2 K operation. Current state of the art Nb₃Sn cavities reach quality factors of 2 x 10¹⁰ at 4.2 K and have reached 24 MV/m. Determining the cause of the premature field limitation is the topic of ongoing research. Cornell University has recently developed a high-speed temperature mapping system that can examine cavity quench mechanisms in never before achieved ways. Here we present high-speed temperature map results of Nb₃Sn cavities and examine the quench mechanism and dynamic heating. We show an initial multipacting quench and sudden temperature jumps at multiple locations on the cavity.

DOI •

reference for this paper ※ doi:10.18429/JACoW-SRF2021-SUPCAV003

About •

Received ※ 09 July 2021 — Accepted ※ 12 August 2021 — Issue date ※ 31 August 2021

Cite •

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

SUPFDV009

Thermal Annealing of Sputtered Nb₃Sn and V₃Si Thin Films for Superconducting RF Cavities

82

K. Howard, M. Liepe, Z. Sun
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Funding: U.S. National Science Foundation under Award PHY-1549132, the Center for Bright Beams and Cornell Center for Materials Research Shared Facilities supported through the NSF MRSEC program (DMR-1719875)
Nb₃Sn and V₃Si thin films are alternative material candidates for the next-generation of superconducting radio frequency (SRF) cavities. However, past sputtered films suffer from stoichiometry and strain issues during deposition and post annealing. As such, we aim to explore the structural and chemical effects of thermal annealing, both in-situ and post-sputtering, on DC-sputtered Nb₃Sn and V₃Si with varying thickness on Nb or Cu substrates. We successfully enabled recrystallization of 100 nm thin Nb₃Sn films with stoichiometric and strain-free grains at 950 C annealing. For 2 um films, we observed removal of strain and slight increase in grain size with increasing temperature. A phase transformation from unstable to stable structure appeared on thick V₃Si samples, while we observed significant Sn loss in thick Nb₃Sn films at high temperature anneals. For films on Cu substrates, we observed similar Sn and Si loss during annealing likely due to Cu-Sn and Cu-Si phase generation and subsequent Sn and Si evaporation. These results encourage us to refine our process to obtain high quality films for SRF use.

DOI •

reference for this paper ※ doi:10.18429/JACoW-SRF2021-SUPFDV009

About •

Received ※ 22 June 2021 — Revised ※ 06 July 2021 — Accepted ※ 12 August 2021 — Issue date ※ 17 March 2022

Cite •

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

SUPTEV007

Development of a System for Coating SRF Cavities Using Remote Plasma CVD

129

G. Gaitan, P. Bishop, A.T. Holic, G. Kulina, M. Liepe, J. Sears, Z. Sun
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Funding: This work was supported by the National Science Foundation under Grant No. PHY-1549132.
Next-generation, thin-film surfaces employing Nb₃Sn, NbN, NbTiN, and other compound superconductors are destined to allow reaching superior RF performance levels in SRF cavities. Optimized, advanced deposition processes are required to enable high-quality films of such materials on large and complex-shaped cavities. For this purpose, Cornell University is developing a remote plasma-enhanced chemical vapor deposition (CVD) system that facilitates coating on complicated geometries with a high deposition rate. This system is based on a high-temperature tube furnace with a clean vacuum and furnace loading system. The use of plasma alongside reacting precursors will significantly reduce the required processing temperature and promote precursor decomposition. A vacuum quality monitor (VQM) is used to characterize the residual gases before coating. The CVD system has been designed and is currently under assembly and commissioning.

DOI •

reference for this paper ※ doi:10.18429/JACoW-SRF2021-SUPTEV007

About •

Received ※ 09 July 2021 — Accepted ※ 21 August 2021 — Issue date ※ 10 February 2022

Cite •

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

SUPTEV008

CW Operation of Conduction-Cooled Nb₃Sn SRF Cavity

133

N.A. Stilin, A.T. Holic, M. Liepe, R.D. Porter, J. Sears, Z. Sun
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Significant progress in the performance of SRF cavities coated with Nb₃Sn films during the last few years has provided an energy efficient alternative to traditional Nb cavities, thereby initiating a fundamental shift in SRF technology. These Nb₃Sn cavities can operate at significantly higher temperatures than Nb cavities while simultaneously requiring less cooling power. This allows for the use of new cryogenic cooling schemes based on conduction cooling with robust, commercialized turn-key style cryocoolers. Cornell University has developed and tested a 2.6 GHz Nb₃Sn cavity assembly which utilizes such cooling methods. These tests have demonstrated stable RF operation at 10 MV/m with measured thermal dynamics which match numerical simulations. These results also serve as a foundation for designing a new standalone SRF cryomodule which will use a pair of cryocoolers to cool a 1.3 GHz Nb₃Sn cavity with an input coupler capable of supporting high beam current operation.

DOI •

reference for this paper ※ doi:10.18429/JACoW-SRF2021-SUPTEV008

About •

Received ※ 22 June 2021 — Accepted ※ 13 August 2021 — Issue date ※ 08 November 2021

Cite •

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

TUOFDV05

Dynamics of RF Dissipation Probed via High-Speed Temperature Mapping

349

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

Recently, Cornell University has developed a new high-speed, high-resolution temperature mapping system that can resolve the time dynamics of RF dissipation, i.e., provide high-speed videos of the surface heating across the entire surface of the cavity. This new powerful tool allows to observe rapid changes in the local RF dissipation, as well as to resolve the dynamics of quenches, field emission processing, and other cavity events, giving new insights into these. This contribution presents the development of this new high-speed temperature mapping system, discusses its commissioning and extensive performance testing (e.g., demonstrating micro-Kelvin resolution), as well as show intriguing high-speed temperature mapping results from multiple Nb₃Sn cavities.

DOI •

reference for this paper ※ doi:10.18429/JACoW-SRF2021-TUOFDV05

About •

Received ※ 01 July 2021 — Accepted ※ 21 August 2021 — Issue date ※ 06 February 2022

Cite •

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

TUOFDV07

Sample Test Systems for Next-Gen SRF Surfaces

357

T.E. Oseroff, M. Liepe, Z. Sun
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

With the increasing worldwide focus on the development of new surfaces for SRF cavities, exploring alternative materials and multilayer structures, test systems that allow measuring the RF performance of simple sample geometries (e.g., flat samples) become increasingly essential. These systems provide RF performance results that are needed to guide the development of these surfaces. This contribution gives an overview of sample test systems currently available, including the improved Cornell sample host cavity. Recent advances in this important technology, performance specifications, and current limitations are discussed. In addition, an overview is given of interesting recent RF performance results on samples coated with non-niobium bulk and multilayer films.

DOI •

reference for this paper ※ doi:10.18429/JACoW-SRF2021-TUOFDV07

About •

Received ※ 08 July 2021 — Accepted ※ 21 August 2021 — Issue date ※ 05 September 2021

Cite •

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

TUPFDV010

New Recipes to Optimize the Niobium Oxide Surface From First-Principles Calculations

426

N. Sitaraman, T. Arias, Z. Baraissov, M.M. Kelley, D.A. Muller
Cornell University, Ithaca, New York, 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 work was supported by the U.S. National Science Foundation under Award No. PHY-1549132, the Center for Bright Beams
The properties of niobium oxide are of critical importance for a wide range of topics, from the behavior of nitrogen during infusion treatments, to the nucleation of Nb₃Sn, to the superconducting properties of the surface. However, the modeling of the oxide is often much simplified, ignoring the variety of niobium oxide phases and the extremely different properties of these phases in the presence of impurities and defects. We use density functional theory (DFT) to investigate how electrochemical treatments and gas infusion procedures change the properties of niobium oxide, and to investigate how these properties could be optimized for Nb₃Sn nucleation and for niobium SRF performance.

DOI •

reference for this paper ※ doi:10.18429/JACoW-SRF2021-TUPFDV010

About •

Received ※ 01 July 2021 — Accepted ※ 18 November 2021 — Issue date ※ 22 February 2022

Cite •

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

WEPTEV012

Characterization of Atomic-Layer-Deposited NbTiN and NbTiN/AlN Films for SIS Multilayer Structures

662

Z. Sun, M. Liepe, T.E. Oseroff
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
X. Deng
University of Virginia, Charlottesville, Virginia, USA

SIS (superconductor-insulator-superconductor) mul-tilayer structures are proposed designs to repel early flux penetration and ease the impact of defects in SRF cavities. The demonstration of such device physics is strongly affected by the film qualities ’ material struc-ture and composition. Here, we characterized 100 nm NbTiN / 2 nm AlN / bulk Nb SIS structures and investigated the effect of the presence of the AlN layer on the NbTiN film properties. We find that the hcp-structured AlN layer results in a Nb composition gra-dient as a function of film depth, whereas the Nb con-centration remains constant in the NbTiN/Nb samples, which suggests that interface mismatch could induce significant change in NbTiN composition. The surface composition variation further leads to different oxide structures, which might impact the superconducting performance. Our observations indicate that the choice of the insulating layer in SIS structures is critical, and that interface mismatch together with internal strain could deteriorate the superconducting film.

DOI •

reference for this paper ※ doi:10.18429/JACoW-SRF2021-WEPTEV012

About •

Received ※ 08 July 2021 — Revised ※ 06 August 2021 — Accepted ※ 22 November 2021 — Issue date ※ 02 January 2022

Cite •

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

WEOTEV03

Toward Stoichiometric and Low-Surface-Roughness Nb₃Sn Thin Films via Direct Electrochemical Deposition

710

Z. Sun, G. Gaitan, M. Ge, K. Howard, 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, Z. Baraissov, M.M. Kelley, D.A. Muller, J.P. Sethna, N. Sitaraman
Cornell University, Ithaca, New York, USA
K.D. Dobson
University of Delaware, Newark, Delaware, USA

Reducing surface roughness and attaining stoichiometry of Nb₃Sn superconducting films are required to push their superheating field to the theoretical limit in SRF cavities. As such, we explore direct electrochemical processes that minimize involving foreign elements to deposit high-quality Sn, Nb, and NbxSn films on Nb and Cu surfaces. These films are then thermally annealed to Nb₃Sn. We find that smooth Sn pre-depositions via electroplating on Nb surfaces significantly reduce the average roughness of resultant Nb₃Sn to 65 nm, with a dramatic reduction in power intensity at medium special frequencies. Structural and superconducting properties demonstrate a Nb₃Sn A15 phase with a stoichiometry of 25 at% Sn. This process is being scaled-up to a 3.9 GHz cavity. Moreover, preliminary results on electroplating on Cu surface show that Nb plating undergoes a slow growth rate while subsequent Sn plating on the plated Nb surface can be controlled with varied thickness. The Nb plating process is currently being optimized.

DOI •

reference for this paper ※ doi:10.18429/JACoW-SRF2021-WEOTEV03

About •

Received ※ 09 July 2021 — Revised ※ 09 August 2021 — Accepted ※ 21 August 2021 — Issue date ※ 16 January 2022

Cite •

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

THPFDV005

Superconducting RF Performance of Cornell 500 MHz N-Doped B-Cell SRF Cavitiy

764

M. Ge, T. Gruber, A.T. Holic, M. Liepe, J. Sears
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

The Cornell SRF group is working on rebuilding a 500 MHz B-cell cryomodule (CRYO-2 BB1-5) as a spared cryomodule for the operation of the CESR ring. To minimize BCS surface resistance, achieve a high quality-factor (Q₀), and increase maximum fields, we prepared the cavity’s surface with electropolishing and performed a 2/6 N2-doping. In this work, we report 4.2 K and 2 K cavity test results with detailed surface resistance analysis, showing improved performance, including significant higher fields.

Poster THPFDV005 [0.718 MB]

DOI •

reference for this paper ※ doi:10.18429/JACoW-SRF2021-THPFDV005

About •

Received ※ 05 July 2021 — Revised ※ 10 August 2021 — Accepted ※ 21 August 2021 — Issue date ※ 22 April 2022

Cite •

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

THPFDV006

Seebeck Coefficient Measurement at Cryogenic Temperatures for the LCLS-II HE Project

768

M. Ge, A.T. Holic, M. Liepe, J. Sears
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Reducing thermoelectric currents during cooldown is important to maintain high-quality factors (Q₀) of the cavities in the LCLS-II HE cryomodules. The temperature-dependent Seebeck coefficients of the materials used in the cryomodules are needed for quantitative estimations of thermoelectric currents. In this work, we present a setup for cryogenic Seebeck coefficient measurements as well as the measured Seebeck coefficients of high-pure niobium at cryogenic temperatures between 4K and 200K.

Poster THPFDV006 [0.511 MB]

DOI •

reference for this paper ※ doi:10.18429/JACoW-SRF2021-THPFDV006

About •

Received ※ 29 June 2021 — Revised ※ 10 August 2021 — Accepted ※ 21 August 2021 — Issue date ※ 26 November 2021

Cite •

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

THPTEV004

Surface Oxides on Nb and Nb₃Sn Surfaces: Toward a Deeper Understanding

836

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, Z. Baraissov, D.A. Muller, N. Sitaraman
Cornell University, Ithaca, New York, USA
C. Dukes
University of Virginia, Charlottesville, Virginia, USA
D. Johnson-McDaniel, M. Salim
CCMR, Ithaca, New York, USA

Surface oxides on Nb and Nb₃Sn SRF cavities, as a thin ’dirty’ layer, could be critical to their performance as suggested by recent theory. Although these oxides have been studied in the past, we intend here to provide a deeper understanding based on a systematic study on coupon samples that have been processed under the different conditions currently used in SRF cavity treatments. Our aim is to obtain a more complete picture of the oxide evolution. This then might help to explain the observed cavity performance variation, and might allow designing a process to achieve a designed, optimized surface with controlled oxides types and thickness. We find that the surface oxides are in amorphous phase that exhibits normal conducting behaviors, while the pentoxide further degrades with time. Also, we observed a thin hydroxide layer on the outermost surface and possibly Nb(OH)x motifs in the bulk. Moreover, distinctive oxide structures were found in Nb₃Sn samples from vapor diffusion, electroplating, and sputtering. The semiconducting SnOx appeared through the oxide depth in vapor diffused Nb₃Sn, while a ~1 nm SnOx layer merely exists at the outermost surface of electroplated Nb₃Sn.

DOI •

reference for this paper ※ doi:10.18429/JACoW-SRF2021-THPTEV004

About •

Received ※ 09 July 2021 — Revised ※ 11 August 2021 — Accepted ※ 21 August 2021 — Issue date ※ 04 November 2021

Cite •

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