SRF2019 - List of Keywords (site)

Paper	Title	Other Keywords	Page
MOP010	Ab Initio Calculations on the Growth and Superconducting Properties of Nb₃Sn	cavity, niobium, SRF, electron	39
	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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TUP058	Characterization of Small AMR Sensors in Liquid Helium to Measure Residual Magnetic Field on Superconducting Samples	SRF, cavity, experiment, controls	576
	G. Martinet Université Paris-Saclay, CNRS/IN2P3, IJCLab, Orsay, France
	Trapped residual magnetic flux is responsible of residual resistance degradation on superconducting materials used in SRF technologies. To characterize this effect on superconducting samples, compact sensors are required to mount on sample characterization devices. In this paper, we present results on AMR sensors supplied from different manufacturers in the temperature range from 4.2 K up to 300 K.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP058
About •	paper received ※ 23 June 2019 paper accepted ※ 03 July 2019 issue date ※ 14 August 2019
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TUP060	Development of Temperature and Magnetic Field Mapping System for Superconducting Cavities at KEK	cavity, controls, solenoid, radio-frequency	583
	T. Okada, E. Kako, T. Konomi, H. Sakai, K. Umemori Sokendai, Ibaraki, Japan E. Kako, T. Konomi, M. Masuzawa, H. Sakai, K. Tsuchiya, R. Ueki, K. Umemori KEK, Ibaraki, Japan A. Poudel, T. Tajima LANL, Los Alamos, New Mexico, USA
	A temperature and magnetic field mapping system for a single cell superconducting cavity is being developed at KEK. The mapping system is used to observe the temperature distribution and the ambient magnetic field distribution around the outer surface of the cavity. A total of 36 boards at every 10 degrees are attached on the cavity. Each board consists of 15 carbon resistors of 100 Ω at room temperature and 3 AMR sensors of X, Y and Z directions at the equator. The calibration of the resisters and AMR sensors were carefully and precisely carried out at low temperature. The data logging system using NI loggers is enabled to measure within 1 ms in the whole cavity surface. The initial test results in the vertical test of the single-cell cavity will be reported in this paper.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP060
About •	paper received ※ 05 July 2019 paper accepted ※ 05 July 2019 issue date ※ 14 August 2019
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TUP061	Gradients of 50 MV/m in TESLA Shaped Cavities via Modified Low Temperature Bake	cavity, niobium, SRF, ECR	586
	D. Bafia, J. Zasadzinski IIT, Chicago, Illinois, USA D. Bafia, A. Grassellino, O.S. Melnychuk, A.S. Romanenko, Z-H. Sung Fermilab, Batavia, Illinois, USA
	This paper will discuss the 75/120 C modified low temperature bake capable of giving unprecedented accel-erating gradients of above 50 MV/m for 1.3 GHz TESLA-shaped niobium SRF cavities in CW operation. A bifurca-tion in the Q₀ vs E_acc curve is observed after retesting cavities without disassembly in between, yielding per-formance that ranges from exceptional to above state-of-the-art. Atomic Force Microscopy studies on cavity cut-outs gives a possible mechanism responsible for this branching in performance, namely, the dissociation and growth of room temperature niobium nano-hydrides that exist near the RF surface, which are made superconduct-ing only through the proximity effect. In-situ low temper-ature baking of cavity cutouts reveals a dissociation of these room temperature nano-hydrides, which could ex-plain the higher performance of cavities subject to similar in-situ heating in the dewar.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP061
About •	paper received ※ 23 June 2019 paper accepted ※ 03 July 2019 issue date ※ 14 August 2019
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TUP076	Electrochemical Deposition of Nb₃Sn on the Surface of Cu Substrates	niobium, cavity, controls, SRF	624
	M. Lu, Q.W. Chu, Y. He, Z.Q. Lin, F. Pan, T. Tan, Z.Q. Yang IMP/CAS, Lanzhou, People’s Republic of China
	Coating superconducting Nb₃Sn thin film on the inner surface of a superconducting RF cavity is one of the most promising approaches to improve the performance of the accelerating cavity. Compared with traditional evaporation and sputtering, electrochemical coating has the advantages on process simplicity, low cost and mass production. However, the conventional electroplating, because of its low growth temperature and aqueous reaction environment, tends to produce porous, loosely bonded, and often contaminated film. All these properties result in excessive pinning center and deteriorate the superconducting radio frequency cavities performance. In this paper, a new method including multi-layer electroplating and heat treatment is used to deposit Nb₃Sn thin film on top of copper substrates. Important growth parameters, e.g. electrical current density, layer thickness ratio, and annealing temperature are studied. The morphology of the film surfaces was observed by scanning electron microscope (SEM) and the structure of the film was analyzed by X-ray diffraction (XRD). The results showed that a flat and uniform Nb₃Sn layer on copper can be obtained, and the thickness is about 7 micron.
	Poster TUP076 [0.716 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP076
About •	paper received ※ 23 June 2019 paper accepted ※ 01 July 2019 issue date ※ 14 August 2019
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TUP077	Nb₃Sn Thin Film Coating Method for Superconducting Multilayered Structure	SRF, cavity, target, experiment	628
	R. Ito, T. Nagata ULVAC, Inc, Chiba, Japan H. Hayano, R. Katayama, T. Kubo, T. Saeki KEK, Ibaraki, Japan H. Ito Sokendai, Ibaraki, Japan Y. Iwashita Kyoto ICR, Uji, Kyoto, Japan
	S-I-S (superconductor-insulator-superconductor) multilayered structure has been proposed in order to increase the maximum acceleration gradient of SRF cavities. Nb₃Sn is the material most expected as a superconducting layer of the S-I-S multilayered structure because it offers both a large critical temperature and large predicted Hsh. Most important in fabricating Nb₃Sn thin films is the stoichiometry of the material produced, and the lack of tin leads to performance degradation. We have launched a new in-house DC magnetron sputtering apparatus for Nb₃Sn deposition. Nb and Sn layers were alternately and repeatedly deposited on Si wafer while adjusting the film thickness of each layer, so we successfully obtained Nb-Sn films having appropriate composition ratio. The as-deposited films were annealed under the temperature of 600 degree C for 1 hour to generate the Nb₃Sn phase. The characteristics of Nb-Sn films evaluated by XRD, XRF, FE-SEM, and so on. We also measured critical temperature of the annealed films. In this paper, the detail of the Nb₃Sn coating method and the measurement result of the Nb-Sn films will be reported.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP077
About •	paper received ※ 02 July 2019 paper accepted ※ 03 July 2019 issue date ※ 14 August 2019
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TUP079	Deposition of Nb₃Sn Films by Multilayer Sequential Sputtering for SRF Cavity Application	cavity, SRF, FEL, niobium	637
	Md.N. Sayeed, H. Elsayed-Ali ODU, Norfolk, Virginia, USA M.C. Burton, G.V. Eremeev, C.E. Reece, A-M. Valente-Feliciano JLab, Newport News, Virginia, USA U. Pudasaini The College of William and Mary, Williamsburg, Virginia, USA
	Nb₃Sn is considered as an alternative of Nb for SRF accelerator cavity application due to its potential to obtain higher quality factors and higher accelerating gradients at a higher operating temperature. Magnetron sputtering is one of the effective techniques that can be used to fabricate Nb₃Sn on SRF cavity surface. We report on the surface properties of Nb₃Sn films fabricated by sputtering multiple layers of Nb and Sn on sapphire and niobium substrates followed by annealing at 950°C for 3 h. The crystal structure, film microstructure, composition and surface roughness were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and atomic force microscopy (AFM). The RF performance of the Nb₃Sn coated Nb substrates were measured by a surface impedance characterization system. We also report on the design of a multilayer sputter deposition system to coat a single-cell SRF cavity.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP079
About •	paper received ※ 22 June 2019 paper accepted ※ 01 July 2019 issue date ※ 14 August 2019
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THP042	Initial Results from Investigations into Different Surface Peparation Techniques of OFHC Copper for SRF Applications	cavity, experiment, ECR, SRF	941
	S.B. Leith, X. Jiang, Z. Khalil, A.S.H. Mohamed, M. Vogel University Siegen, Siegen, Germany
	Funding: This work forms part of the EASITrain research programme. This Marie Sklodowska-Curie Action (MSCA) Innovative Training Networks (ITN) has received funding from the European Union¿s H₂020 Framework Programme under Grant Agreement no. 764879 As part of efforts to improve the performance of thin film coated accelerating cavities, improvement of the topography of the surface of copper is being pursued. This is known to strongly affect the properties of the deposited superconducting thin film. This study focuses on determining the optimal procedure to enhance homogeneity and smoothness of the copper surface. OFHC copper substrates have been processed using mechanical polishing (MP), chemical polishing (CP) and electropolishing (EP) procedures as well as a combina-tion thereof. The parameters of each of the procedures have been tested and optimised to produce the smoothest surface possible. The resulting samples have been analysed using a scanning electron microscope, a laser profilometer and a confocal microscope. Results indicate the superior per-formance of electrochemical polishing over chemical polishing in terms of planarization efficiency, while a combination of mechanical polishing followed by electropolishing provides the most homogeneous and smooth surface when utilising the critical current density of the electrolyte.
	Poster THP042 [1.190 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP042
About •	paper received ※ 22 June 2019 paper accepted ※ 29 June 2019 issue date ※ 14 August 2019
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THP043	Deposition Parameter Effects on Niobium Nitride (NbN) Thin Films Deposited Onto Copper Substrates with DC Magnetron Sputtering	cathode, ECR, cavity, niobium	945
	S.B. Leith, X. Jiang, M. Vogel University Siegen, Siegen, Germany R. Ries, E. Seiler Slovak Academy of Sciences, Institute of Electrical Engineering, Bratislava, Slovak Republic
	Funding: The EASITrain project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant No 764879. As part of efforts to improve the performance of SRF cavities, to that prescribed by future operating requirements, alternative materials are currently being investigated. NbN is one of the alternatives under investigation to provide these better performance figures. In this contribution, a summary of results from an investigation into DC magnetron sputtered NbN thin films deposited onto copper substrates is presented. The copper substrates were prepared using a mechanical polishing process, followed by a chemical etching process. The NbN films were prepared in a large scale commercial coating system. A high and low value for the substrate temperature, process pressure, bias voltage, cathode power, nitrogen gas percentage, and the working gas type, using either Argon or Krypton, constitute the parameters of this study. The base pressure of the system prior to deposition was 5x10⁷ hPa for all coatings. The resulting films have been characterised using various surface characterisation methods to determine the effects of the deposition parameters during the film growth process. The deposition parameters have been optimised based on the characterisation results.
	Poster THP043 [1.169 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP043
About •	paper received ※ 23 June 2019 paper accepted ※ 29 June 2019 issue date ※ 14 August 2019
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THP044	RF Characterization of Novel Superconducting Materials and Multilayers	cavity, niobium, SRF, plasma	950
	T.E. Oseroff, M. Liepe, Z. Sun Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA B. Moeckly STI, Santa Barbara, California, USA M.J. Sowa Veeco-CNT, Medford, USA
	Cutting edge SRF technology is likely approaching the fundamental limitations of niobium cavities operating in the Meissner state. This combined with the obvious advantages of using higher critical temperature superconductors and thin film depositions leads to interest in the RF characterization of such materials. A TE mode niobium sample host cavity was used to characterize the RF performance of 5" (12.7 cm) diameter sample plates as a function of field and temperature at 4 GHz. Materials studied include MgB₂ and thin film atomic layer deposition (ALD) NbN and NbTiN on Nb substrates. These higher critical temperature superconductors all having coherence lengths on the order of a few nm. It is therefore likely that defects on the order of the coherence lengths will cause early flux penetration well before the theorized superheating field of an ideal superconducting surface. Superconductor-insulator-superconductor (SIS) multilayers have been proposed as a mechanism of arresting these early penetration flux avalanches and are therefore studied here as well, using the same NbN and NbTiN films, but over thin layers of insulating AlN on Nb substrates.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP044
About •	paper received ※ 02 July 2019 paper accepted ※ 03 July 2019 issue date ※ 14 August 2019
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THP048	Characterization of Flat Multilayer Thin Film Superconductors	dipole, superconducting-magnet, experiment, SRF	968
	D. Turner, A.J. May STFC/DL, Daresbury, Warrington, Cheshire, United Kingdom G. Burt, L. Gurran Cockcroft Institute, Lancaster University, Lancaster, United Kingdom K.D. Dumbell, N. Pattalwar, S.M. Pattalwar STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom T. Junginger, O.B. Malyshev Cockcroft Institute, Warrington, Cheshire, United Kingdom
	The maximum accelerating gradient of SRF cavities can be increased by raising the field of initial flux penetration, Hvp. Thin alternating layers of superconductors and insulators (SIS) can potentially increase Hvp. Magnetometry is commercially available but consists of limitations, such as SQUID measurements apply a field over both superconducting layers, so Hvp through the sample cannot be measured. If SIS structures are to be investigated a magnetic field must be applied locally, from one plane of the sample, with no magnetic field on the opposing side to allow Hvp to be measured. A magnetic field penetration experiment has been developed at Daresbury laboratory, where a VTI has been created for a cryostat where Hvp of a sample can be measured. The VTI has been designed to allow flat samples to be measured to reduce limitations such as edge effects by creating a DC magnetic field smaller than the sample. A small, parallel magnetic field is produced on the sample by the use of a ferrite yoke. The field is increased to determine Hvp by using 2 hall probes either side of the sample.
	Poster THP048 [0.327 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP048
About •	paper received ※ 23 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019
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Paper

Title

Other Keywords

Page

MOP010

Ab Initio Calculations on the Growth and Superconducting Properties of Nb₃Sn

cavity, niobium, SRF, electron

39

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

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

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TUP058

Characterization of Small AMR Sensors in Liquid Helium to Measure Residual Magnetic Field on Superconducting Samples

SRF, cavity, experiment, controls

576

G. Martinet
Université Paris-Saclay, CNRS/IN2P3, IJCLab, Orsay, France

Trapped residual magnetic flux is responsible of residual resistance degradation on superconducting materials used in SRF technologies. To characterize this effect on superconducting samples, compact sensors are required to mount on sample characterization devices. In this paper, we present results on AMR sensors supplied from different manufacturers in the temperature range from 4.2 K up to 300 K.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP058

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paper received ※ 23 June 2019 paper accepted ※ 03 July 2019 issue date ※ 14 August 2019

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TUP060

Development of Temperature and Magnetic Field Mapping System for Superconducting Cavities at KEK

cavity, controls, solenoid, radio-frequency

583

T. Okada, E. Kako, T. Konomi, H. Sakai, K. Umemori
Sokendai, Ibaraki, Japan
E. Kako, T. Konomi, M. Masuzawa, H. Sakai, K. Tsuchiya, R. Ueki, K. Umemori
KEK, Ibaraki, Japan
A. Poudel, T. Tajima
LANL, Los Alamos, New Mexico, USA

A temperature and magnetic field mapping system for a single cell superconducting cavity is being developed at KEK. The mapping system is used to observe the temperature distribution and the ambient magnetic field distribution around the outer surface of the cavity. A total of 36 boards at every 10 degrees are attached on the cavity. Each board consists of 15 carbon resistors of 100 Ω at room temperature and 3 AMR sensors of X, Y and Z directions at the equator. The calibration of the resisters and AMR sensors were carefully and precisely carried out at low temperature. The data logging system using NI loggers is enabled to measure within 1 ms in the whole cavity surface. The initial test results in the vertical test of the single-cell cavity will be reported in this paper.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP060

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

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TUP061

Gradients of 50 MV/m in TESLA Shaped Cavities via Modified Low Temperature Bake

cavity, niobium, SRF, ECR

586

D. Bafia, J. Zasadzinski
IIT, Chicago, Illinois, USA
D. Bafia, A. Grassellino, O.S. Melnychuk, A.S. Romanenko, Z-H. Sung
Fermilab, Batavia, Illinois, USA

This paper will discuss the 75/120 C modified low temperature bake capable of giving unprecedented accel-erating gradients of above 50 MV/m for 1.3 GHz TESLA-shaped niobium SRF cavities in CW operation. A bifurca-tion in the Q₀ vs E_acc curve is observed after retesting cavities without disassembly in between, yielding per-formance that ranges from exceptional to above state-of-the-art. Atomic Force Microscopy studies on cavity cut-outs gives a possible mechanism responsible for this branching in performance, namely, the dissociation and growth of room temperature niobium nano-hydrides that exist near the RF surface, which are made superconduct-ing only through the proximity effect. In-situ low temper-ature baking of cavity cutouts reveals a dissociation of these room temperature nano-hydrides, which could ex-plain the higher performance of cavities subject to similar in-situ heating in the dewar.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP061

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paper received ※ 23 June 2019 paper accepted ※ 03 July 2019 issue date ※ 14 August 2019

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TUP076

Electrochemical Deposition of Nb₃Sn on the Surface of Cu Substrates

niobium, cavity, controls, SRF

624

M. Lu, Q.W. Chu, Y. He, Z.Q. Lin, F. Pan, T. Tan, Z.Q. Yang
IMP/CAS, Lanzhou, People’s Republic of China

Coating superconducting Nb₃Sn thin film on the inner surface of a superconducting RF cavity is one of the most promising approaches to improve the performance of the accelerating cavity. Compared with traditional evaporation and sputtering, electrochemical coating has the advantages on process simplicity, low cost and mass production. However, the conventional electroplating, because of its low growth temperature and aqueous reaction environment, tends to produce porous, loosely bonded, and often contaminated film. All these properties result in excessive pinning center and deteriorate the superconducting radio frequency cavities performance. In this paper, a new method including multi-layer electroplating and heat treatment is used to deposit Nb₃Sn thin film on top of copper substrates. Important growth parameters, e.g. electrical current density, layer thickness ratio, and annealing temperature are studied. The morphology of the film surfaces was observed by scanning electron microscope (SEM) and the structure of the film was analyzed by X-ray diffraction (XRD). The results showed that a flat and uniform Nb₃Sn layer on copper can be obtained, and the thickness is about 7 micron.

Poster TUP076 [0.716 MB]

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP076

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paper received ※ 23 June 2019 paper accepted ※ 01 July 2019 issue date ※ 14 August 2019

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TUP077

Nb₃Sn Thin Film Coating Method for Superconducting Multilayered Structure

SRF, cavity, target, experiment

628

R. Ito, T. Nagata
ULVAC, Inc, Chiba, Japan
H. Hayano, R. Katayama, T. Kubo, T. Saeki
KEK, Ibaraki, Japan
H. Ito
Sokendai, Ibaraki, Japan
Y. Iwashita
Kyoto ICR, Uji, Kyoto, Japan

S-I-S (superconductor-insulator-superconductor) multilayered structure has been proposed in order to increase the maximum acceleration gradient of SRF cavities. Nb₃Sn is the material most expected as a superconducting layer of the S-I-S multilayered structure because it offers both a large critical temperature and large predicted Hsh. Most important in fabricating Nb₃Sn thin films is the stoichiometry of the material produced, and the lack of tin leads to performance degradation. We have launched a new in-house DC magnetron sputtering apparatus for Nb₃Sn deposition. Nb and Sn layers were alternately and repeatedly deposited on Si wafer while adjusting the film thickness of each layer, so we successfully obtained Nb-Sn films having appropriate composition ratio. The as-deposited films were annealed under the temperature of 600 degree C for 1 hour to generate the Nb₃Sn phase. The characteristics of Nb-Sn films evaluated by XRD, XRF, FE-SEM, and so on. We also measured critical temperature of the annealed films. In this paper, the detail of the Nb₃Sn coating method and the measurement result of the Nb-Sn films will be reported.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP077

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

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

TUP079

Deposition of Nb₃Sn Films by Multilayer Sequential Sputtering for SRF Cavity Application

cavity, SRF, FEL, niobium

637

Md.N. Sayeed, H. Elsayed-Ali
ODU, Norfolk, Virginia, USA
M.C. Burton, G.V. Eremeev, C.E. Reece, A-M. Valente-Feliciano
JLab, Newport News, Virginia, USA
U. Pudasaini
The College of William and Mary, Williamsburg, Virginia, USA

Nb₃Sn is considered as an alternative of Nb for SRF accelerator cavity application due to its potential to obtain higher quality factors and higher accelerating gradients at a higher operating temperature. Magnetron sputtering is one of the effective techniques that can be used to fabricate Nb₃Sn on SRF cavity surface. We report on the surface properties of Nb₃Sn films fabricated by sputtering multiple layers of Nb and Sn on sapphire and niobium substrates followed by annealing at 950°C for 3 h. The crystal structure, film microstructure, composition and surface roughness were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and atomic force microscopy (AFM). The RF performance of the Nb₃Sn coated Nb substrates were measured by a surface impedance characterization system. We also report on the design of a multilayer sputter deposition system to coat a single-cell SRF cavity.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP079

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paper received ※ 22 June 2019 paper accepted ※ 01 July 2019 issue date ※ 14 August 2019

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THP042

Initial Results from Investigations into Different Surface Peparation Techniques of OFHC Copper for SRF Applications

cavity, experiment, ECR, SRF

941

S.B. Leith, X. Jiang, Z. Khalil, A.S.H. Mohamed, M. Vogel
University Siegen, Siegen, Germany

Funding: This work forms part of the EASITrain research programme. This Marie Sklodowska-Curie Action (MSCA) Innovative Training Networks (ITN) has received funding from the European Union¿s H₂020 Framework Programme under Grant Agreement no. 764879
As part of efforts to improve the performance of thin film coated accelerating cavities, improvement of the topography of the surface of copper is being pursued. This is known to strongly affect the properties of the deposited superconducting thin film. This study focuses on determining the optimal procedure to enhance homogeneity and smoothness of the copper surface. OFHC copper substrates have been processed using mechanical polishing (MP), chemical polishing (CP) and electropolishing (EP) procedures as well as a combina-tion thereof. The parameters of each of the procedures have been tested and optimised to produce the smoothest surface possible. The resulting samples have been analysed using a scanning electron microscope, a laser profilometer and a confocal microscope. Results indicate the superior per-formance of electrochemical polishing over chemical polishing in terms of planarization efficiency, while a combination of mechanical polishing followed by electropolishing provides the most homogeneous and smooth surface when utilising the critical current density of the electrolyte.

Poster THP042 [1.190 MB]

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP042

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paper received ※ 22 June 2019 paper accepted ※ 29 June 2019 issue date ※ 14 August 2019

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THP043

Deposition Parameter Effects on Niobium Nitride (NbN) Thin Films Deposited Onto Copper Substrates with DC Magnetron Sputtering

cathode, ECR, cavity, niobium

945

S.B. Leith, X. Jiang, M. Vogel
University Siegen, Siegen, Germany
R. Ries, E. Seiler
Slovak Academy of Sciences, Institute of Electrical Engineering, Bratislava, Slovak Republic

Funding: The EASITrain project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant No 764879.
As part of efforts to improve the performance of SRF cavities, to that prescribed by future operating requirements, alternative materials are currently being investigated. NbN is one of the alternatives under investigation to provide these better performance figures. In this contribution, a summary of results from an investigation into DC magnetron sputtered NbN thin films deposited onto copper substrates is presented. The copper substrates were prepared using a mechanical polishing process, followed by a chemical etching process. The NbN films were prepared in a large scale commercial coating system. A high and low value for the substrate temperature, process pressure, bias voltage, cathode power, nitrogen gas percentage, and the working gas type, using either Argon or Krypton, constitute the parameters of this study. The base pressure of the system prior to deposition was 5x10⁷ hPa for all coatings. The resulting films have been characterised using various surface characterisation methods to determine the effects of the deposition parameters during the film growth process. The deposition parameters have been optimised based on the characterisation results.

Poster THP043 [1.169 MB]

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP043

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paper received ※ 23 June 2019 paper accepted ※ 29 June 2019 issue date ※ 14 August 2019

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THP044

RF Characterization of Novel Superconducting Materials and Multilayers

cavity, niobium, SRF, plasma

950

T.E. Oseroff, M. Liepe, Z. Sun
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
B. Moeckly
STI, Santa Barbara, California, USA
M.J. Sowa
Veeco-CNT, Medford, USA

Cutting edge SRF technology is likely approaching the fundamental limitations of niobium cavities operating in the Meissner state. This combined with the obvious advantages of using higher critical temperature superconductors and thin film depositions leads to interest in the RF characterization of such materials. A TE mode niobium sample host cavity was used to characterize the RF performance of 5" (12.7 cm) diameter sample plates as a function of field and temperature at 4 GHz. Materials studied include MgB₂ and thin film atomic layer deposition (ALD) NbN and NbTiN on Nb substrates. These higher critical temperature superconductors all having coherence lengths on the order of a few nm. It is therefore likely that defects on the order of the coherence lengths will cause early flux penetration well before the theorized superheating field of an ideal superconducting surface. Superconductor-insulator-superconductor (SIS) multilayers have been proposed as a mechanism of arresting these early penetration flux avalanches and are therefore studied here as well, using the same NbN and NbTiN films, but over thin layers of insulating AlN on Nb substrates.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP044

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

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THP048

Characterization of Flat Multilayer Thin Film Superconductors

dipole, superconducting-magnet, experiment, SRF

968

D. Turner, A.J. May
STFC/DL, Daresbury, Warrington, Cheshire, United Kingdom
G. Burt, L. Gurran
Cockcroft Institute, Lancaster University, Lancaster, United Kingdom
K.D. Dumbell, N. Pattalwar, S.M. Pattalwar
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom
T. Junginger, O.B. Malyshev
Cockcroft Institute, Warrington, Cheshire, United Kingdom

The maximum accelerating gradient of SRF cavities can be increased by raising the field of initial flux penetration, Hvp. Thin alternating layers of superconductors and insulators (SIS) can potentially increase Hvp. Magnetometry is commercially available but consists of limitations, such as SQUID measurements apply a field over both superconducting layers, so Hvp through the sample cannot be measured. If SIS structures are to be investigated a magnetic field must be applied locally, from one plane of the sample, with no magnetic field on the opposing side to allow Hvp to be measured. A magnetic field penetration experiment has been developed at Daresbury laboratory, where a VTI has been created for a cryostat where Hvp of a sample can be measured. The VTI has been designed to allow flat samples to be measured to reduce limitations such as edge effects by creating a DC magnetic field smaller than the sample. A small, parallel magnetic field is produced on the sample by the use of a ferrite yoke. The field is increased to determine Hvp by using 2 hall probes either side of the sample.

Poster THP048 [0.327 MB]

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP048

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paper received ※ 23 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019

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