SRF2017 - List of Keywords (niobium)

Paper	Title	Other Keywords	Page
MOPB035	Cryogenic Probe Station at Old Dominion University Center for Accelerator Science	ion, cavity, SRF, cryogenics	128
	J. Makita, J.R. Delayen, A.V. Gurevich ODU, Norfolk, Virginia, USA G. Ciovati JLab, Newport News, Virginia, USA
	With a growing effort in research and development of an alternative material to bulk Nb for a superconducting radiofrequency (SRF) cavity, it is important to have a cost effective method to benchmark new materials of choice. At Old Dominion University's Center for Accelerator Science, a cryogenic probe station (CPS) will be used to measure the response of superconductor samples under RF fields. The setup consists of a closed-cycle refrigerator for cooling a sample wafer to a cryogenic temperature, a superconducting magnet providing a field parallel to the sample, and DC probes in addition to RF probes. The RF probes will extract a quality factor from a sample patterned in a coplanar waveguide resonator structure on a 2' wafer. From the measured quality factor, the surface resistance and the penetration depth as a function of temperature and magnetic field will be calculated. This paper will discuss the design and measurement procedures of the current CPS setup.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-MOPB035
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MOPB036	The Study of Deposition Method of Nb3Sn Film on Cu Substrate	ion, SRF, cavity, superconductivity	131
	L. Xiao, X.Y. Lu, W.W. Tan, D. Xie, D.Y. Yang, Y. Yang, Z.Q. Yang, J. Zhao PKU, Beijing, People's Republic of China
	Our work is mainly focused on the fabrication methods of Nb3Sn films on Cu substrates and film's properties. There are diffraction peaks of Nb3Sn in the X-ray diffraction patterns in which without diffraction peaks of copper compounds. Scanning electron microstructures of Nb3Sn film reflect its nice compactness and binding force between film and substrate.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-MOPB036
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MOPB040	ESS High-beta Cavity Test Preparations at Daresbury Laboratory	ion, cavity, LLRF, SRF	137
	P.A. Smith, L. Bizel-Bizellot, K.D. Dumbell, M. Ellis, P. Goudket, A.J. Moss, E.F. Palade, S.M. Pattalwar, M.D. Pendleton, A.E. Wheelhouse STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom
	Science and Technology Facility Council is responsible for supplying, and testing 84 High beta elliptical SRF cavities, as part of the UK In Kind Contribution to the European Spallation Source (ESS). The High-β=0.86, cavities have been designed by CEA- Saclay and are a five cell Niobium cavity operating at 704.42 MHz. They are required to provide an accelerating gradient of 19.9 MV/m at an unloaded Q of 5x10⁹. Preparations are underway to upgrade the cryogenic and RF facilities at Daresbury laboratory prior to the arrival of the first cavities. As part of these arrangements, a niobium coaxial resonator has been manufactured, to validate the test facility. The design considerations, for the coaxial resonator are presented, along with preliminary results. The RF measurement system to perform the cavity conditioning and testing is also presented.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-MOPB040
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TUXBA01	Low Temperature Doping of Niobium Cavities: What is Really Going on?	ion, cavity, vacuum, background	353
	P.N. Koufalis, M. Liepe, J.T. Maniscalco Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Initial work, first at Fermilab and subsequently at Cornell, has shown that low temperature heat treatments (120 - 160 C) in a low pressure atmosphere can lead to a 'Q-rise' and high quality factors similar to that of cavities nitrogen-doped at high temperatures (~800 C). It was suggested that the low-temperature baking effect is a result of nitrogen doping or 'infusion'. We conducted a systematic study of this effect, using both RF measurements of cavities treated at different doping temperatures as well as detailed SIMS analysis of the surface layer. We match RF performance and extracted material parameters (especially electron mean free path) to the measured doping concentration profiles. We conclusively show that the low-temperature baking is drastically lowering the mean free path in the penetration layer, and that this is not the result of nitrogen doping or infusion. Instead, other interstitial impurities (specifically oxygen and carbon) are diffused into the surface in the low temperature heat treatment and are the source of lowering of the mean free path and, thus, of the observed Q-rise.
	Slides TUXBA01 [4.153 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUXBA01
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TUYAA01	The Importance of the Electron Mean Free Path for Superconducting RF Cavities	cavity, ion, electron, SRF	359
	J.T. Maniscalco, P.N. Koufalis, M. Liepe Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Theoretical results offer a potential explanation for the anti-Q-slope, the phenomenon of decreasing microwave surface resistance with increasing radiofrequency electromagnetic field strength. This effect has been observed in niobium doped with impurities, chiefly nitrogen, and has been put to use in the Linac Coherent Light Source II (LCLS-II) accelerator currently under construction. Our work, presented here, finds a strong link between the electron mean free path, the main measure of impurity doping, to the overheating of quasiparticles in the RF penetration layer. This is an important effect that adjusts the magnitude of the theoretical anti-Q-slope by providing a mechanism to counteract it and introduce a surface resistance that increases with field strength. We discuss our findings in a study of niobium cavities doped at high temperature (800-990 °C) as well as new analysis of low-temperature-doped cavities.
	Slides TUYAA01 [6.988 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUYAA01
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TUYAA02	Advancement in the Understanding of the Field and Frequency Dependent Microwave Surface Resistance of Niobium	ion, cavity, ECR, resonance	364
	M. Martinello, S. Aderhold, S.K. Chandrasekaran, M. Checchin, A. Grassellino, O.S. Melnychuk, S. Posen, A. Romanenko, D.A. Sergatskov Fermilab, Batavia, Illinois, USA
	The radio-frequency surface resistance of niobium resonators is incredibly reduced when nitrogen impurities are dissolved as interstitial in the material, conferring ultra-high Q-factors at medium values of accelerating field. This effect has been observed in both high and low temperature nitrogen treatments. As a matter of fact, the peculiar anti Q-slope observed in nitrogen doped cavities, i.e. the decreasing of the Q-factor with the increasing of the radio-frequency field, come from the decreasing of the BCS surface resistance component as a function of the field. Such peculiar behavior has been considered consequence of the interstitial nitrogen present in the niobium lattice after the doping treatment. The study here presented show the field dependence of the BCS surface resistance surface of cavities with different resonant frequencies, such as: 650 MHz, 1.3 GHz, 2.6 GHz and 3.9 GHz, and processed with different state-of-the-art surface treatments. These findings show for the first time that the anti Q-slope might be seen at high frequency even for clean Niobium cavities, revealing useful suggestion on the physics underneath the anti Q-slope effect. *M. Martinello, A. Grassellino, M. Checchin, A. Romanenko, O. Melnychuk, D.A. Sergatskov, S. Posen, J. Zasadzinski App. Phys. Lett. 109, 6 (2016)
	Slides TUYAA02 [4.342 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUYAA02
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TUYBA01	Progress on Characterization and Optimization of Multilayers	ion, SRF, cavity, site	368
	C.Z. Antoine CEA/DSM/IRFU, France M. Aburas, A. Four CEA/DRF/IRFU, Gif-sur-Yvette, France H. Hayano, S. Kato, T. Kubo, T. Saeki KEK, Ibaraki, Japan Y. Iwashita Kyoto ICR, Uji, Kyoto, Japan
	Multilayers MgO/NbN/MgO/Nb with several thicknesses are being tested by local magnetometry, Scanning tunneling and various standard structural techniques experiment providing usefull information to compare experiments and recent theoretical advances proposed by A. Gurevich or T. Kubo.
	Slides TUYBA01 [5.819 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUYBA01
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TUYBA03	The Way of Thick Films toward a Flat Q-curve in Sputtered Cavities	ion, cavity, interface, site	378
	V. Palmieri INFN/LNL, Legnaro (PD), Italy
	Thick films have bulk like properties. In this paper it is explored the possibility to sputter 70 micron thick films in order to get rid of the Q-slope in Niobium sputtered Cop-per Cavities. An innovative method based on the multi-layer deposition of zero-stress single layers is reported. The deposition of zero-stress thick films into 6 GHz Cop-per seamless cavities, has shown the possibility to obtain straight curves for the Q-factor versus accelerating fields Work supported by the INFN V group experiment ISIDE and performed under the CERN-INFN-FTSC Agreement N. KE2722/BE/FCC
	Slides TUYBA03 [4.935 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUYBA03
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TUPB005	Developed Spoke Cavity Module for Main Linac of China ADS HOM Simulations and Damping Scheme for CEPC Cavities	cavity, ion, linac, SRF	393
	Z.Q. Li, J.S. Cao, Y.L. Chi, F.S. He, S.P. Li, H.Y. Lin, Q. Ma, Z.H. Mi, W.M. Pan, P. Sha, B. Xu, J.Y. Zhai, X.Y. Zhang IHEP, Beijing, People's Republic of China
	During past five year, two kind of spoke of Beta equal 0.21 and 0.40 were developed at IHEP CAS, the spoke cavity of beta 0.21 was adopted to accelerate proton from 10 to 32MeV, and 32 to 160MeV for beta 0.40 spoke cavity. Up to now, two kind of naked spoke cavities have been test in vertical, also the module of beta 0.21 spoke cavity, which equipped the liquid helium jacket, magnetic shield layer and frequency tuner has been fulfilled and test, the performance of all of components reach the design requirements.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB005
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TUPB013	Advanced Manufacturing Techniques for the Fabrication of Hl-LHC Crab Cavities at CERN	ion, cavity, simulation, MMI	409
	M. Garlaschè CERN, Geneva, Switzerland
	RF Crab Cavities are an essential part of the HL-LHC upgrade at CERN. Two concepts of such systems are being developed: the Double Quarter Wave (DQW) and the RF Dipole (RFD). The following paper describes the advanced manufacturing techniques developed for the fabrication of the DQW cavity prototype with an outlook on the upcoming RFD prototype production.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB013
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TUPB014	In-situ Bulk Residual Resistivity Ratio Measurement on Double Quarter Wave Crab Cavities	cavity, ion, luminosity, cryomodule	415
	N.C. Shipman, A. Castilla, K.G. Hernández-Chahín, A. Macpherson CERN, Geneva, Switzerland I. Ben-Zvi BNL, Upton, Long Island, New York, USA G. Burt, N.C. Shipman Cockcroft Institute, Lancaster University, Lancaster, United Kingdom K.G. Hernández-Chahín Universidad de Guanajuato, División de Ciencias e Ingenierías, León, Mexico J.A. Mitchell Lancaster University, Lancaster, United Kingdom N.C. Shipman UMAN, Manchester, United Kingdom
	A four wire measurement was used to measure the bulk RRR on two DQW Crab Cavities. The measurement procedure is explained and the values obtained for each cavity are compared together with the values obtained from Niobium samples of the same stock from which the cavities were manufactured. Measurement errors and carefully analysed and further improvements to the measurement procedure are suggested.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB014
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TUPB039	Electropolishing of Niobium from Deep Eutectic Solvents Based on Choline Chloride	ion, ECR, cavity, SRF	475
	Q.W. Chu, H. Guo, Y. He, L. Li, P.R. Xiong, Z.M. You, S.H. Zhang IMP/CAS, Lanzhou, People's Republic of China
	Niobium (Nb) was successfully electropolished from a green ionic liquid, choline chloride/urea deep eutectic solvent (DES). This paper was to investigate the influence of various electropolishing parameters, including electropolishing time, temperature and voltage, on the electropolishing rate, surface roughness, glossiness and microstructure of Nb. The result showed that the electropolishing parameters had a significant impact on the performance of Nb. Based on surface analysis by scanning electron microscope (SEM) and atomic force microscope (AFM), smooth Nb can be achieved under properly controlled conditions.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB039
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TUPB046	Experience on Design, Fabrication and Testing of a Large Grain ESS Medium Beta Prototype Cavity	ion, cavity, radiation, electron	484
	M. Bertucci, A. Bignami, A. Bosotti, J.F. Chen, P. Michelato, L. Monaco, R. Paparella, D. Sertore INFN/LASA, Segrate (MI), Italy C. Pagani Università degli Studi di Milano & INFN, Segrate, Italy S. Pirani ESS, Lund, Sweden
	INFN-LASA built a complete Medium Beta cavity, based on the ESS prototype design, with novel large-grain material sliced in sheets from an ingot provided by CBMM manufacturing experience. Design and fabrication are reported as well as results on the physical and chemical analyses performed on samples at different cavity production stages. Results from the cold tests performed are also summarized and critically discussed in view of future R&D activities
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB046
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TUPB048	INFN- LASA Medium Beta Cavity Prototypes for ESS Linac	cavity, ion, controls, vacuum	494
	D. Sertore, M. Bertucci, A. Bignami, A. Bosotti, J.F. Chen, P. Michelato, L. Monaco, R. Paparella INFN/LASA, Segrate (MI), Italy C. Pagani Università degli Studi di Milano & INFN, Segrate, Italy S. Pirani ESS, Lund, Sweden
	INFN-LASA, in the framework of INFN contribution to the European Spallation Source, has developed, produced and tested 704.4 MHz Medium Beta (β = 0.67) cavities. Mode separation and avoidance of HOM excitation by machine line frequencies have driven the cavity design. The production at the industry, also in view of the INFN in-kind contribution of series cavities, has been done "build-to-print" and we have implemented our own quality control process, based on our XFEL experience, from raw material to cavity ready for test. The cavities have been then cold tested in our upgraded Vertical Test Facility. In this paper, we report on our experience on the different phases of the cavity production and test processes.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB048
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TUPB055	Nb3Sn Thin Film Deposition On Copper By DC Magnetron Sputtering	ion, target, SRF, cavity	512
	W.W. Tan, B.T. Li, X.Y. Lu, L. Xiao, D. Xie, D.Y. Yang, Y. Yang, Z.Q. Yang, J. Zhao PKU, Beijing, People's Republic of China
	Nb3Sn for SRF cavities has been coated on copper samples by DC magnetron Sputtering. Pure Nb target and pure Sn target were installed separately in the magnetron sputtering device. Nb3Sn precursor was coated on copper in the Ar atmosphere of 0.5 Pa. The Nb3Sn precursor was annealed in the vacuum furnace whose pressure is 10^-4 Pa. The XRD results demonstrate the exist of Nb3Sn crystal, and MPMS results show superconductivity of Nb3Sn. The highest critical temperature obtained is 15K.
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TUPB063	Fabrication of a SRF Deflecting Cavity for the ARIEL-Linac	ion, cavity, linac, TRIUMF	524
	D.W. Storey, R.E. Laxdal, B. Matheson, N. Muller TRIUMF, Vancouver, Canada D.W. Storey Victoria University, Victoria, B.C., Canada
	A superconducting RF deflecting cavity has been designed and is being fabricated at TRIUMF to allow simultaneous beam delivery to both rare isotope production and an energy recovery linac. The 650 MHz cavity will operate in a TE-like mode in CW. The design has been optimised for high shunt impedance and minimal longitudinal footprint, reaching roughly 50% higher shunt impedance with 50% less length than comparable non-TM mode cavity geometries. Due to low power dissipation at 4K at the maximum required deflecting voltage of 0.6 MV, low cost manufacturing techniques have been employed in the construction of the cavity. These include the use of reactor grade Niobium and TIG welding in an inert atmosphere. Development of the manufacturing processes will be presented along with the status of fabrication.
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TUPB065	Design of Multi-frequency Coaxial Test Resonators	ion, cavity, SRF, operation	531
	Z.Y. Yao, T. Junginger, R.E. Laxdal, B. Matheson, B.S. Waraich, V. Zvyagintsev TRIUMF, Vancouver, Canada
	A significant issue in low beta resonators is medium field Q-slope (MFQS) at 4K. To study the MFQS and the field dependence of surface resistance in low beta resonators, a quarter-wave resonator (QWR) and a half-wave resonator (HWR) were designed to be tested at integer harmonic frequencies of 200MHz, and up to 1.2GHz. A series of chemistry and heat treatments will be applied to these cavities so that a systemic study on the surface resistance of the coaxial resonators associating with post-processing, RF field, and frequency can be done. The detail design of these cavities and the status of cavity fabrication will be reported in this paper.
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TUPB067	Insights into Formation of Nb3Sn Film During the Vapor Diffusion Process	ion, experiment, cavity, SRF	539
	U. Pudasaini, M.J. Kelley The College of William and Mary, Williamsburg, Virginia, USA G.V. Eremeev, M.J. Kelley, C.E. Reece JLab, Newport News, Virginia, USA M.J. Kelley, J. Tuggle Virginia Polytechnic Institute and State University, Blacksburg, USA
	Funding: Supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics under contract DE-AC05-06OR23177 and Office of High Energy Physics under grant DE-SC-0014475. The potential of Nb3Sn for SRF cavities is widely recognized and renewed R&D efforts continue to bring new insights about material structure and its properties. We have systematically coated niobium with Nb3Sn using "vapor diffusion" under varying coating conditions to elucidate the reaction of tin with niobium at the temperatures of interest. The analysis of the coated samples is revealing new understanding about the two-stage nucleation/deposition ("vapor diffusion") process that allows us to form a hypothesis regarding Nb3Sn formation mechanism. The essential aspect of nucleation is the deposition of a high coverage, nanoscale thin tin film with particle assemblage by decomposition of tin chloride on the niobium surface at temperatures sufficient for reduction of the thick niobium oxide film, usually at about 500°C. The deposition is followed by the reaction of tin from tin vapor with the niobium surface to form Nb3Sn at about 1200°C, where the surface and grain boundaries start to play key role in the formation process initiation and progression. These findings improve understanding of the Nb3Sn growth in the typical vapor diffusion process used for accelerator cavity coatings.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB067
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TUPB071	Test Result of 650 MHz, Beta 0.61 Single-cell Niobium Cavity	cavity, ion, accelerating-gradient, electron	553
	S. Seth, P. Bhattacharyya, A. Dutta Gupta, S. Ghosh, S. Ghosh, A. Mandal, S. Som VECC, Kolkata, India A. Grassellino, T.N. Khabiboulline, O.S. Melnychuk, C.S. Mishra, T.H. Nicol, A.M. Rowe, D.A. Sergatskov Fermilab, Batavia, Illinois, USA M.P. Kelly, T. Reid ANL, Argonne, Illinois, USA K.K. Mistri, P.N. Prakash IUAC, New Delhi, India
	VECC has been involved in the design, analysis and development of 650 MHz, beta 0.61 (LB650), elliptical Superconducting RF linac cavity, as part of research and development activities on SRF cavities and associated technologies under Indian Institutions Fermilab Collaboration (IIFC). A single-cell niobium cavity has been indigenously designed and developed at VECC, with the help of Electron Beam Welding (EBW) facility at IUAC, New Delhi. Various measurements, processing and testing at 2K in Vertical Test Stand (VTS) of the single-cell cavity was carried out at ANL and Fermilab, USA, with active participation of VECC engineers. It achieved a maximum accelerating gradient(Eacc) of 34.5 MV/m with Quality Factor of 2·10⁹ and 30 MV/m with Quality Factor of 1.5·10¹⁰. This is probably the highest accelerating gradient achieved so far in the world for LB650 cavities. This paper describes the design, fabrication and measurement of the single cell niobium cavity. Cavity processing and test results of Vertical Test of the single-cell niobium cavity are also presented.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB071
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TUPB072	Investigation of BCP Parameters for Mastery of SRF Cavity Treatment	ion, cavity, experiment, SRF	558
	F. Éozénou CEA/DSM/IRFU, France E. Cenni, G. Devanz, T. Percerou, Th. Proslier, C. Servouin CEA/DRF/IRFU, Gif-sur-Yvette, France M.L.L. Nghiem UPMC, Paris, France
	Mastery of Standard Buffered Chemical Polishing (with mixture of hydrofluoric, nitric and phosphoric acids) is of paramount importance for the treatment of SRF resonators with complex geometry has IFMIF half-wave resonators, in order to control accurately their frequency evolution. Furthermore, strong and unexpected asymmetry in removals has recently been observed after BCP treatment of ESS-medium beta resonators. The goal of this study is to investigate accurately influence of parameters such as surface geometry and orientation, acid temperature, agitation and their coupling on the removal rate. We will also focus on the influence of by-products such has NOx on kinetics. The mixture used is HF(40%)- HNO3(65%)-H3PO4(85%) with ratio 1-1-2.4.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB072
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TUPB074	RF Performance of Multi-cell Scale Niobium SRF Cavities Prepared with HF Free Bipolar Electro-polishing at Faraday Technology	cavity, ion, SRF, MMI	567
	F. Furuta, M. Ge, T. Gruber, J.J. Kaufman, M. Liepe, J. Sears Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA T.D. Hall, M.E. Inman, R. Radhakrishnan, S.T. Snyder, E.J. Taylor Faraday Technology, Inc., Clayton, Ohio, USA
	Cornell's SRF group and Faraday Technology, Inc. have been collaborating on two phase-II SBIR projects. One of them is the development and commissioning of a 9-cell scale HF free Bipolar Electro-Polishing (BEP) system. Faraday Technology had completed the proof of principle on BEP with single cell scale prior to the work reported here, and has now developed a new 9-cell scale BEP system. Cornell has fabricated three single cell cavities and has assembled them together as a 9-cell scale test string. The 9-cell scale test string has received BEP at Faraday Technology and RF testing has been performed on the three single cell cavities one-by-one at Cornell. Here we give a status update on the new 9-cell scale BEP system commissioning and on results from RF tests of the BEP cavities.
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TUPB082	Setup of a Spatially Resolving Vector Magnetometry System for the Investigation of Flux Trapping in Superconducting Cavities	ion, cavity, SRF, superconducting-cavity	580
	B. Schmitz, K. Alomari, J. Knobloch, O. Kugeler, J.M. Köszegi, Y. Tamashevich HZB, Berlin, Germany
	Flux trapping is the major contribution to the residual resistance of superconducting cavities. In order to gain a better understanding of the mechanisms involved and aiming at an eventual minimization of trapped flux, a measurement setup based on AMR sensors was devised that allows for monitoring the magnetic field vector at various positions near the cavity surface. First results of the efforts are presented.
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TUPB086	Study on Local Chemical Treatment for Recovery From Surface Oxidation by HPR Process on SRF Cavities	cavity, ion, experiment, SRF	592
	H. Guo, Y. He, Y.M. Li, T. Tan, A.D. Wu, P.R. Xiong, Z.M. You, W.M. Yue, S.H. Zhang IMP/CAS, Lanzhou, People's Republic of China
	High pressure rinsing (HPR) with ultra-pure water (UPW) is the last step which is commonly used for SRF cavities cleaning. The serious surface damage will be caused due to the failure of the distance control between the jet and cavity surface or the breakdown of the jet rotation. The surface of taper HWR cavities which are used for CIADS project was damaged in HPR process. Two methods were used for surface recovery and the result will be presented in this paper.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB086
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TUPB090	Investigation of High Temperature Baking of Jacketed Quarter Wave Resonators	ion, cavity, linac, SRF	598
	A. Rai, D. Kanjilal, K.K. Mistri, P. Patra, P.N. Potukuchi, S.K. Sonti IUAC, New Delhi, India
	The Superconducting booster Linac at IUAC has been delivering accelerated beams for scheduled experiments since 2013. It has three accelerating modules with 8 Quarter Wave Resonators (QWR)in each. The QWRs for the first module were built at Argonne National Laboratory while those for the second and third modules have been built in-house. During the electropolishing of one of the indigenously built resonators (QWR # I03) the RF surface got spoiled due to a wrong acid mixture that was being used for etching. In subsequent cold tests of the cavity, its performance was poor (2.6 MV/m @ 4W). There was evidence of Q disease also, as the performance deteriorated further (~20%) when the cavity was held at 100-120K for ~8 hours .In an attempt to recover the cavity it was baked at 650 °C for 10 hours along with its stainless steel jacket. A series of tests were conducted thereafter wherein, a substantial improvement (factor of two) in the performance was observed. Encouraged with the results another QWR designed for a lower beta (β=0.05) was also heat treated identically. This paper presents the different treatments followed to enhance the cavity performance vis-à-vis the test results.
	Poster TUPB090 [1.240 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB090
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TUPB091	Study on Vertical Electropolising of 9-cell Niobium Coupon Cavity	ion, cavity, cathode, polarization	602
	V. Chouhan, Y.I. Ida, K.N. Nii, T.Y. Yamaguchi MGH, Hyogo-ken, Japan H. Hayano, S. Kato, H. Monjushiro, T. Saeki, M. Sawabe KEK, Ibaraki, Japan
	Authors report a study on vertical electropolishing (VEP) carried out for a 1.3 GHz 9-cell niobium (Nb) coupon cavity using a unique cathode namely 'Ninja Cathode'. The design of the cathode for VEP of a 9-cell cavity was based on the Ninja cathode used for 1-cell cavity since the 1-cell Ninja cathode was found effective to reduce longitudinal asymmetry in material removal and to obtain a smooth surface of a 1-cell cavity. Moreover, 1-cell Nb cavities after being treated in VEP using the Ninja cathode showed good performance in vertical RF tests. The 9-cell coupon cavity used in this study was designed to have totally nine coupons set on the iris and equator positions of the first, fifth and ninth cells. These three cells contain viewports as well at their upper and lower iris positions. Measurement of currents from the individual coupons and in-situ observation are possible using the cavity to understand EP phenomenon at different locations of the cavity. VEP results, which include removal thicknesses at different positions of the cavity and surface study of the coupons, are discussed.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB091
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TUPB092	Analysis of Niobium Surface and Generated Particles in Vertical Electropolishing of Single-cell Coupon Cavity	ion, cavity, experiment, cathode	607
	V. Chouhan, Y.I. Ida, K.N. Nii, T.Y. Yamaguchi MGH, Hyogo-ken, Japan H. Hayano, S. Kato, H. Monjushiro, T. Saeki, M. Sawabe KEK, Ibaraki, Japan
	In our previous studies, we have reported parameter investigation for vertical electropolishing (VEP) of 1-cell niobium (Nb) tesla/ILC type cavities using a Ninja cathode. A 1-cell coupon cavity containing six Nb disk coupons at its different positions was found effective to reduce time and cost to establish an optimized VEP recipe. In this work, we present surface analyses of VEPed Nb coupon surfaces using scanning electron microscope (SEM), energy dispersive x-ray spectroscopy (EDX) and x-ray photoelectron spectroscopy (XPS). Surfaces contained micro- and nano-sized particles which were found with random distributions and different number densities on the beam pipe and iris coupons. Surfaces of equator coupons were found to have relatively less number of particles or almost clean. To analyze particles, a few particles were picked-up from a coupon surface using a tungsten tip under SEM and analyzed with EDX while the coupon was moved out from the SEM chamber to avoid its effect in EDX spectra. The particles were confirmed as oxygen-rich niobium and contained fluorine and carbon also. XPS analysis of the coupon surfaces was also carried out for further study of surface chemistry.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB092
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TUPB095	Modeling the Hydroforming of a Large Grain Niobium Tube With Crystal Plasticity	ion, cavity, experiment, SRF	616
	A. Mapar MSU, East Lansing, Michigan, USA T.R. Bieler Michigan State University, East Lansing, Michigan, USA J.E. Murphy University of Nevada, Reno, Reno, Nevada, USA F. Pourboghrat Ohio State University, Columbus, Ohio, USA
	Current SRF cavities are made from fine grained polycrystalline niobium half-cells welded together. Hot spots are commonly found in the heat-affected zone, making seamless hydroformed cavities attractive. Large grain cavities usually perform as well as fine grain cavities, often having a higher Q, presumably due to fewer grain boundaries. Large grain Nb forms non-uniformly, which introduces problems in manufacturing. A model that could realistically predict the deformation response of large grain Nb could facilitate the design of large grain hydroformed tubes. To this end, a crystal plasticity model was developed and calibrated with tensile stress-strain data of Nb single crystals. A seamless large grain tube was made from rolling a fine grain sheet into a tube, welded, and heat treated to grow large grains. The heat treatment resulted in a large grain tube with a single grain orientation in the center. The tube was hydroformed until it cracked. The hydroforming process was simulated with the crystal plasticity model, which was able to predict the deformed shape of the tube, the location of the crack and other localized areas with heterogeneous strain.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB095
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TUPB103	DC Magnetism of Niobium Thin Films	ion, site, cavity, power-supply	640
	S. Wilde, B. Chesca Loughborough University, Loughborough, Leicestershire, United Kingdom A.N. Hannah, O.B. Malyshev, N. Pattalwar, S.M. Pattalwar, B.S. Sian, R. Valizadeh, S. Wilde STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom B.S. Sian UMAN, Manchester, United Kingdom G.B.G. Stenning STFC/RAL/ISIS, Chilton, Didcot, Oxon, United Kingdom
	Niobium thin films were deposited onto a-plane sapphire with varying kinetic energy and varying substrate temperature. There were no consistent trends which related the particle energy or substrate temperature to RRR. The sample which displayed the largest RRR of 229 was then compared to both a thin film deposited with similar conditions onto copper substrate and to bulk niobium. DC magnetometry measurements suggest that the mechanism of flux entry into thin film niobium and bulk niobium may vary due to differences in the volumes of both defects and impurities located within the grains. Results also suggest that magnetic flux may penetrate thin films at small fields due to the sample geometry.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB103
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TUPB104	First Full Cryogenic Test of the SRF Thin Film Test Cavity	ion, cavity, SRF, cryogenics	644
	R. Valizadeh, L. Bizel-Bizellot, P. Goudket, L. Gurran, O.B. Malyshev, N. Pattalwar, S.M. Pattalwar STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom G. Burt, L. Gurran Lancaster University, Lancaster, United Kingdom G. Burt, P. Goudket, O.B. Malyshev, S.M. Pattalwar, R. Valizadeh Cockcroft Institute, Warrington, Cheshire, United Kingdom L. Gurran Cockcroft Institute, Lancaster University, Lancaster, United Kingdom
	A test cavity that uses RF chokes, rather than a physical seal, to contain the field is a promising method of SRF sample testing, especially in thin films research where the rate of sample production far outstrips that of full SRF characterisation. Having the sample and cavity physically separate reduces the complexity involved in changing samples - major causes of low throughput rate and high running costs for other test cavities - and also allows direct measurement of the RF power dissipated in the sample via power calorimetry. Choked test cavities operating at 7.8 GHz with three RF chokes have been designed and tested at Daresbury Laboratory. As part of the commissioning of this system, we performed the first full SRF test with a bulk Nb sample and we verified that the system would perform as required for future superconducting thin film sample tests.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB104
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TUPB108	Genesis of Topography in Buffered Chemical Polishing of Niobium for Application to Superconducting Radiofrequency Accelerator Cavities	ion, cavity, electron, SRF	651
	L. Zhao, C.E. Reece JLab, Newport News, Virginia, USA M.J. Kelley The College of William and Mary, Williamsburg, Virginia, USA
	Funding: Supported by Office of High Energy Physics, U.S. Department of Energy, Grant SC0007907 to the College of William & Mary and by U.S. DOE Contract No. DE-AC05-06OR23177 to Jefferson Science Associates Topography arising from the final etch step in preparing niobium superconducting radiofrequency (SRF) accelerator cavities is understood to significantly impact cavity performance at high field levels. This study investigated the effect of process temperature and time on the etch rate and topography arising from the widely-used buffered chemical polishing (BCP). This study aims to understand more thoroughly the genesis of topography in BCP of polycrystalline niobium, with the ultimate aim of finding a path to surface smoothness comparable to that obtained by electropolishing (EP). It was found that the etch process is controlled by the surface reaction; and that the etch rate varies with crystallographic orientation. The familiar micron-scale roughening necessarily results. Gas evolution has an impact, but is secondary. The major outcome is that surface smoothness comparable to EP appears to be inherently unachievable for polycrystalline niobium using BCP, setting an upper limit to the gradient for which it is useful.
	Poster TUPB108 [3.782 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB108
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TUPB111	R&D Activities on Centrifugal Barrel Polishing of 1.3 GHz Niobium Cavities at DESY/University of Hamburg	ion, cavity, ECR, laser	655
	A.L. Prudnikava, B. Foster, Y. Tamashevich University of Hamburg, Institut für Experimentalphysik, Hamburg, Germany A. Ermakov, B. Foster DESY, Hamburg, Germany Y. Tamashevich HZB, Berlin, Germany
	In this paper the status of research activities at ILC-HiGrade Lab (DESY/University of Hamburg) on Centrifugal Barrel Polishing (CBP) of 1.3 GHz Niobium Cavities is presented. We focus on CBP based on the polishing recipe reported by Fermi National Laboratory and Jefferson Lab. The aim is to gain a better understanding of the limitations of this technique, detailed characterization of the treated surface after each polishing step using a "coupon" single cell cavity. Plastic deformations upon initial CBP steps, embedded polishing media and residual damage upon final polishing were investigated at different areas of the cavity. C. A. Cooper, L. D. Cooley, Supercond. Sci. Technol. 26 (2013) 015011
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB111
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TUPB113	Surface Characterization of Nitrided Niobium Surfaces	ion, cavity, electron, experiment	663
	A.L. Prudnikava, B. Foster University of Hamburg, Institut für Experimentalphysik, Hamburg, Germany B. Foster DESY, Hamburg, Germany Y. Tamashevich HZB, Berlin, Germany Y. Tamashevich University of Hamburg, Hamburg, Germany
	Thermal treatment of niobium radio frequency cavities in nitrogen atmosphere is employed in ILCLS-II Project in order to improve the quality factor of Nb cavities. A so called "N-infusion" thermal treatment is applied without any post processing, , whereas "N-doping" requires the removal of the upper layer of 5-30 um. For better understanding the mechanism of such an improvement, a detailed characterization of the nitrided surface is necessary. Our studies are focused on characterization of the niobium surface subjected to such treatments (surface morphology, nitrogen concentration profile, hardness, phase composition). The sample preparation technique for studying the hydride precipitation in N-Nb system is presented, and current activities on studying of N-infused Nb samples by SQUID and PPMS are briefly discussed. A. Grassellino, et al, Supercond. Sci. Technol. 26 (2013) 102001. ** A. Grassellino, arXiv:1701.06077
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB113
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WEXA01	High Performance Nb3Sn Cavities	ion, cavity, SRF, site	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)	ion, cavity, SRF, electron	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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THXA08	Review of Heat Treatments for Low Beta Cavities : What's So Different from Elliptical Cavities	ion, cavity, SRF, accelerating-gradient	708
	D. Longuevergne IPN, Orsay, France
	Heat treatments done for low beta (low frequency) cavities are usually, due to the lack of feedback, inspired from elliptical (high frequency) cavity results. Is that still relevant now that experimental data are available thanks to the florishing business of low beta structures (Spiral2, ESS, FRIB, C-ADS, MYRRHA, PROJECTX, …). These 2 families are moreover not usually operating in the same resistance regime (BCS and residual). The paper will review procedures applied and results obtained on different type of cavities (Quarter-Wave resonator, Half-Wave resonator and Spoke) and different temperature treatments (low temperature baking, hydrogen degassing, nitrogen doping, …) and compare these to elliptical cavities.
	Slides THXA08 [5.567 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THXA08
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THPB002	Role of Nitrogen on Hydride Nucleation in Pure Niobium by First Principles Calculations	ion, site, cavity, electron	741
	P. Garg, I. Adlakha, K.N. Solanki Arizona State University, Tempe, USA S. Balachandran, P.J. Lee NHMFL, Tallahassee, Florida, USA T.R. Bieler Michigan State University, East Lansing, Michigan, USA
	It is known that formation and growth of Nb hydride degrades superconducting radio frequency (SRF) properties of Nb cavities and the treatments that reduce H concentration improve quality factor. Recently it is has also been shown that addition of N through doping or infusion improves the quality factor. Thus, we probe role of N addition in Nb on hydride precipitation and stability through first principles calculations & compared with coupon samples. In presence of N, energetic preference for H to occupy interstitial sites in the vicinity of N is reduced. Furthermore, presence of N forces H to occupy interstitial octahedral site instead of a tetrahedral site. The thermodynamic stability of hydride is decreased in the presence of N in Nb.The quantum insights using charge transfer and density of states show a strong tendency of N to accumulate charge, thereby decreasing the bond strength of neighboring Nb and H atoms. These atomic scale results explain the lesser tendency of surface hydride formation in SRF Nb cavities in presence of N. These results are consistent with metallographic examination of N-treated Nb coupons, which show suppressed hydride formation near N-treated surface.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB002
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THPB004	Impact of the Duration of Low Temperature Doping on Superconducting Cavity Performance	cavity, ion, vacuum, superconducting-cavity	750
	P.N. Koufalis, F. Furuta, J.J. Kaufman, M. Liepe Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Low temperature treatments of superconducting cavities in a low pressure ambient atmosphere have been shown to introduce a 'Q-rise' up to moderate surface fields and an overall increase in quality factor. However, the effect of varying the doping time at a fixed temperature on cavity performance has not been systematically examined. We present results of such an investigation for cavities prepared at 120 and 160 C in a continuously flowing low pressure atmosphere for various amounts of time. We show that the introduction of impurities to the RF penetration layer can improve cavity performance and investigate the relationship between electron mean free path and the temperature-dependent component of the surface resistance.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB004
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THPB005	Design Updates on Cavity to Measure Suppression of Microwave Surface Resistance by DC Magnetic Fields	ion, cavity, SRF, experiment	754
	J.T. Maniscalco, M. Liepe, R.D. Porter Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Our research has shown good agreement between experimental measurements of the anti-Q-slope in niobium SRF cavities and predictions from a recent theoretical model of the suppression of the microwave surface resistance with applied RF field. To confirm that this mechanism is indeed what causes the anti-Q-slope in impurity-doped niobium, it will be necessary to measure the theory's prediction that the same effect may be achieved by applying a constant (i.e. DC) magnetic field parallel to the RF surface. This will also allow for systematic studies of the proposed fundamental effect of the anti-Q-slope and of the behavior of the anti-Q-slope for many surface preparations and alternative materials, since it provides a cleaner measurement by eliminating the counteracting quasiparticle overheating and the complexifying oscillation of the screening currents. In this report we give an update on work at Cornell to design and build a coaxial cavity to measure this effect.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB005
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THPB007	Nitrogen Infusion R&D on Single Cell Cavities at DESY	ion, cavity, SRF, injection	759
	M. Wenskat, D. Reschke, J. Schaffran DESY, Hamburg, Germany A.L. Prudnikava University of Hamburg, Institut für Experimentalphysik, Hamburg, Germany
	A first series of single cell cavities underwent the "Nitrogen Infusion" treatment at DESY. Samples, which were in the furnace together with the cavities, underwent a series of SEM/EDX measurements and showed some unexpected structures. In parallel, the cavity performance deteriorated after the treatment. The furnace pressure and temperature and the residual gases during the treatment were analyzed and the possible cause for the deterioration has been found. Steps to prevent this deterioration in following treatments are discussed and first results are shown.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB007
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THPB017	Investigation of Trapped Magnetic Flux in Superconducting Niobium Samples with Neutron Radiography	ion, neutron, experiment, detector	762
	O. Kugeler, T. Junginger, J. Knobloch, M.M. Krzyzagorski, J.M. Köszegi, L. Riik, W. Treimer, R.F. Ziesche HZB, Berlin, Germany
	The dynamics of flux expulsion in Nb samples during superconducting transition has been investigated with neutron radiography. Aiming at a reduction of the trapped flux with respect to obtaining a small residual resistance it was attempted to influence the expulsion by applying external AC magnetic fields. The results of these experiments are presented.
	Poster THPB017 [1.528 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB017
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THPB018	Towards the Perfect Meissner State: A Magneto-Optical Study on Competing Pinning Centers in Niobium	ion, cavity, SRF, background	766
	J.M. Köszegi, J. Knobloch, O. Kugeler HZB, Berlin, Germany
	Over the past years trapped magnetic flux has emerged as a main limiting factor of high quality factors in SRF cavities. Several studies investigated how the ambient magnetic field can be minimized or how the flux expulsion during the phase transition can be improved. We now present a study that targets the pinning centers which allow for the flux to remain inside the superconductor in the first place. Using magneto-optical imaging we were able to not only measure the amount of trapped flux but in addition we managed to image its distribution with a resolution below 10μm and correlate it with electron backscatter diffraction maps. As a result we found that the grain boundaries did not play a major role as pinning centers nor did the crystal orientation influence the amount of trapped flux signifi-cantly. Niobium hydrides which formed during the cool down to cryogenic temperatures however were found to enhance trapping.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB018
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THPB021	Trial of Nitrogen Infusion and Nitrogen Doping by Using J-PARC Furnace	ion, cavity, controls, SRF	775
	T. Konomi, T. Dohmae, Y. Hori, E. Kako, T. Kubo, G.-T. Park, H. Sakai, K. Umemori KEK, Ibaraki, Japan J. Kamiya, K. Takeishi JAEA/J-PARC, Tokai-mura, Japan T. Nagata ULVAC, Inc., Tsukuba, Japan
	KEK has been carrying out SRF cavity developments toward higher Q-values and higher accelerating gradients. In the past nitrogen-doping was tested using the KEK furnaces, but it did not succeed. This time nitrogen infusion and nitrogen doping are tested using the J-PARC's furnace, which has an oil-free pumping system and is mainly pumped by a 10000 L/s cryopump and three 3000 L/s turbo pumps. Nitrogen pressure is controlled by a variable leak valve and an additional turbo pump. To avoid performance degradation during heat treatment, flanges of cavities are covered by Nb caps and foils. Nitrogen infusion at 120 degrees was applied to a single cell cavity and cavity performance was measured by vertical tests after HPR and assembly. Nitrogen doping at 800 degrees is also applied to another single cell cavity. After applying EP and HPR, vertical tests were carried out. Nb samples were also installed into the furnace during heat treatment. Surfaces are analyzed by SIMS and XPS. In this presentation, we report procedure of nitrogen infusion and doping, vertical test results and results of surface analysis.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB021
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THPB024	Investigation on Depth Profiling of Niobium Surface Composition and Work Function of SRF Cavities	ion, cavity, electron, SRF	779
	A.D. Wu, L. Chen, Q.W. Chu, H. Guo, Y. He, Y.M. Li, F. Pan, L. Yang, S.H. Zhang, H.W. Zhao IMP/CAS, Lanzhou, People's Republic of China
	The niobium samples were prepared by different surface treatments that commonly applied for the superconducting RF cavities preparation, as the following of electrochemical polishing, the buffered chemical polishing and high temperature annealing. In order to understand the property of niobium surface, especially the relationship between the composition and the work function value, the X-ray and ultraviolet photoelectron spectra depth profiling has been studied. The intensity photoelectrons signals of O1s, C1s and the Nb3d were identified for composition of the niobium oxide and the hydrocarbon contamination. And the work function of sample surface was measured via the means of the ultraviolet photoelectron spectra band width. To make a depth profiling, the sputtering of Argon ions was used to remove surface material gradually under by control the sputtering times. The results shown that the value of work function strongly depends on the chemical composition.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB024
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THPB025	A Crystal Plasticity Study on Influence of Dislocation Mean Free Path on Stage II Hardening in Nb Single Crystals	ion, experiment, simulation, cavity	783
	T. Maiti, A. Chakrabarty, P. Eisenlohr MSU, East Lansing, USA T.R. Bieler, D. Kang Michigan State University, East Lansing, Michigan, USA
	Funding: Financial support from the Department of Energy through grant DE-SC0009962 is gratefully acknowledged. This work was supported in part by MSU through computational resources provided by the ICER. Constitutive models based on thermally-activated stress-assisted dislocation kinetics have been successful in predicting deformation behavior of crystalline materials, particularly in face-centered cubic (fcc) metals. In body-centered cubic (bcc) metals, success has been more or less limited, owing to ill-defined nature of slip planes and non-planar spreading of 1/2\hkl<111> screw dislocation cores. As a direct consequence of this, bcc metals show a strong dependence of flow stress on temperature and strain rate, and violation of Schmid law. We present high-resolution full-field crystal plasticity simulations of single crystal Niobium under tensile loading with an emphasis on multi-stage hardening, orientation dependence, and non-Schmid behavior. A dislocation density-based constitutive model with storage and recovery rates derived from Discrete Dislocation Dynamics is used to model strain hardening in stage II. The influence of dislocation mean free path and initial dislocation content on stage II hardening is simulated and compared with in-situ tensile experiments.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB025
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THPB026	Investigation of the Effect of Strategically Selected Grain Boundaries on Superconducting Properties of SRF Cavity Niobium	ion, cavity, SRF, cryogenics	787
	M. Wang, T.R. Bieler Michigan State University, East Lansing, Michigan, USA S. Balachandran, P.J. Lee NHMFL, Tallahassee, Florida, USA S. Chetri, A. Polyanskii ASC, Tallahassee, Florida, USA C. Compton FRIB, East Lansing, Michigan, USA
	Funding: Research supported by DOE/OHEP contracts DE-SC0009962, DE-SC0009960, NSF-DMR-1157490, and the State of Florida. High purity Nb is commonly used for fabricating SRF cavities due to its high critical temperature and its formability. However, microstructural defects such as dislocations and grain boundaries in niobium can serve as favorable sites for pinning centers of magnetic flux that can degrade SRF cavity performance. In this study, two bi-crystal niobium samples extracted from strategically selected grain boundaries were investigated for the effect of grain misorientation on magnetic flux behavior. Laue X-ray and EBSD-OIM crystallographic analyses were used to characterize grain orientations and orientation gradients. Cryogenic Magneto-Optical Imaging (MOI) was used to directly observe magnetic flux penetration at about 5-8 K. Flux penetration was observed along one of the grain boundaries, as well as along a low angle boundary that was not detected prior to MOI imaging. Hydride scars on the sample surface after MOI were examined using atomic force microscopy (AFM) analysis. The relationships between dislocation content, cryo-cooling, flux penetration and grain boundaries are examined.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB026
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THPB027	Characterization of Microstructural Defects in SRF Cavity Niobium using Electron Channeling Contrast Imaging	ion, cavity, electron, SRF	792
	M. Wang, T.R. Bieler, D. Kang Michigan State University, East Lansing, Michigan, USA C. Compton FRIB, East Lansing, Michigan, USA
	Funding: Research supported by DOE/OHEP contract DE-SC0009962 Although the quality factor of niobium cavities has improved, performance variability arises from microstructural defects such as dislocations and grain boundaries that can trap magnetic flux, block heat transfer, and perturb superconducting currents. Microstructural defect evolution is compared in four samples extracted from a 2.8 mm thick large-grain niobium slice, with tensile axes chosen to generate desired dislocation structures during deformation. The four samples are 1) as-extracted, 2) extracted and annealed, 3) extracted and then deformed to 40% strain, and 4) extracted, annealed at 800 °C 2 hours, and deformed to 40% strain. Electron Channeling Contrast Imaging (ECCI) was performed on all samples to characterize initial dislocation density, dislocation structure evolution due to annealing and deformation, and related to the mechanical behavior observed in stress-strain curves. The orientation evolution and geometrically necessary dislocation (GND) density were characterized with electron backscattered diffraction (EBSD) maps. Fundamental understanding of dislocation evolution in niobium is necessary to develop models for computational cavity design.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB027
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THPB028	Flux Pinning Study of OTIC Niobium Material	ion, site, cavity, superconducting-cavity	797
	S. Chen, J.K. Hao, L. Lin, K.X. Liu, S.W. Quan PKU, Beijing, People's Republic of China
	The performance of superconducting cavities is influenced by the trapped flux during the cooling down through critical temperature, especially for nitrogen doped cavities which are more sensitive to flux trapping. We have investigated the flux trapping of OTIC niobium samples with different grain size. Samples were prepared and heat treated at 800°C and 900°C, followed with different surface removal by BCP. A series of measurements, including MPMS, TOF-SIMS, were carried out on the niobium samples. The results and analysis will be presented.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB028
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THPB029	XPS Studies of Nitrogen Doping Nb Samples before and after GCIB Etching	ion, cavity, SRF, electron	802
	Z.Q. Yang, X.Y. Lu, W.W. Tan, D.Y. Yang, Y. Yang, J. Zhao PKU, Beijing, People's Republic of China
	The surface chemical composition of nitrogen doping Nb samples used for the fabrication of superconducting radio frequency (SRF) cavities, followed by the subsequent successive EP with different amounts of material removal, has been studied by XPS. The chemical composition of Nb, O, C and N was presented before and after Gas Cluster Ion Beams (GCIB) etching. No signals of bad superconducting nitrides NbNx was found in any doped and un-doped samples before etching. However, in the depth range greater than 30nm, the content of N elements is below the XPS detection precision scope even in the samples directly after nitrogen doping treatment.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB029
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THPB030	Direct Observation of Hydrides Formation of Nitrogen Doping Nb Samples	ion, experiment, cavity, electron	805
	Z.Q. Yang, X.Y. Lu, W.W. Tan, D.Y. Yang, Y. Yang, J. Zhao PKU, Beijing, People's Republic of China
	Direct observation of hydrides precipitates formation on both nitrogen doped and un-doped Nb samples at 80K has been carried out using Scanning Electron Microscope (SEM) with Cold Stand. We have found that, under our experimental conditions, when the subsequent EP removal is less than 7μm, the amounts of hydrides formed on the surface of doped samples can be effectively reduced. When the subsequent material removal is larger than 9μm, the amounts of precipitated hydrides increased with the EP removal. When the EP removal is 7-9μm, the amounts of hydrides can still be effectively reduced. Also, more hydrides were precipitated on the surface of un-doped samples. The amounts of hydrides of doped samples may be reduced to varying degrees with different amounts of material removal.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB030
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THPB031	Magnetic Properties of Nitrogen Doping Niobium Samples	ion, cavity, SRF, experiment	809
	Z.Q. Yang, X.Y. Lu, W.W. Tan, D.Y. Yang, Y. Yang, J. Zhao PKU, Beijing, People's Republic of China
	Nitrogen doping study on Niobium samples used for the fabrication of superconducting radio frequency (SRF) cavities was carried out. The samples' surface treatments were attempted to replicate that of the cavities, which included heavy electropolishing (EP), nitrogen doping and the subsequent successive EP with different amounts of material removal. The magnetization curves of both doped and un-doped samples have been measured, from which the lower critical field Hffp (First Flux Penetration, ffp) and upper critical field Hc2 was extracted. The thermodynamic critical field Hc, superheating field Hsh and superconducting parameters of samples with different treatments was calculated from the determined reversible magnetization curves. Hsh of doped samples is obviously smaller than that of un-doped samples, which may be a possible reason for the reduction of achievable accelerating gradient in SRF niobium cavities after nitrogen doping treatments.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB031
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THPB034	Development of High Purity Niobium Components and Cavities for SRF Accelerator	cavity, ion, electron, SRF	814
	T. Nagata, H. Masui, S. Shinozawa ULVAC, Inc, Chiba, Japan H. Inoue, E. Kako, M. Yamanaka KEK, Ibaraki, Japan M. Murakami ULVAC, Inc., Tsukuba, Japan
	Comprehensive cavity fabrication process from Niobium ingot was investigated. In order to purify ingots, A 600 kW electron beam furnace was introduced in ULVAC. It makes possible the stable quality of Niobium sheets and tubes. In evaluation of chemical components, mechanical properties, and RRR of our materials, all the value satisfies the ASTM Type 5 (superconducting grade) specification. In this study, we performed the trial manufacturing of welding-type and seamless-type cavities were made of our high purity Niobium ingots (RRR > 300). Accelerating gradient over 40 MV/m was shown in both cavities. We also tried to manufacture a 3-cell seamless cavity as scale up study. A seamless tube with a length of 830 mm, an inner diameter of 131 mm and a thickness of 3.5 mm was prepared. We succeed in direct forming from tube to cavity shape by using a hydroforming process. Cavity surface could be smoother than that of single cell cavity caused by small crystal grain size.
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THPB036	Fundamental SIMS Analyses for Nitrogen-enriched Niobium	ion, cavity, interface, instrumentation	821
	J. Tuggle, M.J. Kelley Virginia Polytechnic Institute and State University, Blacksburg, USA M.J. Kelley, U. Pudasaini The College of William and Mary, Williamsburg, Virginia, USA M.J. Kelley, A.D. Palczewski, C.E. Reece JLab, Newport News, Virginia, USA F.A. Stevie NCSU AIF, Raleigh, North Carolina, USA
	Funding: Co-Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. W&M and VT work supported by the Office of High Energy Physics, U.S. DOE under grant DE-SC-0014475 In order to fully understand nitrogen addition techniques it is vital to have a full understanding of the material, including the content, location, and speciation of nitrogen contained in the treated Nb. In this work Secondary Ion Mass Spectrometry (SIMS) is used to elucidate content and location. Dynamic SIMS nitrogen analysis is reported, for the first time, for "as-received" cavity grade niobium from three separate suppliers. In addition, a number of method and instrumental issues are discussed including depth resolution, detection limit, and quantification.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB036
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THPB041	Cavity Quench Studies in Nb3Sn Using Temperature Mapping and Surface Analysis of Cavity Cut-outs	ion, cavity, detector, electron	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	ion, cavity, site, SRF	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	ion, cavity, SRF, linac	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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THPB044	Update on Sample Host Cavity Design Work for Measuring Flux Entry and Quench Field	cavity, ion, SRF, radio-frequency	851
	R.D. Porter, M. Liepe, J.T. Maniscalco, R.A. Strauss Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Current state-of-the-art Niobium superconducting radio-frequency (SRF) accelerator cavities have reached surface magnetic field close to the theoretical maximum set by the superheating field. Further increasing accelerating gradients will require new superconducting materials for accelerator cavities that are capable of supporting higher surface magnetic fields. This necessitates measuring the quench fields of new materials in high power RF fields. Previous work at Cornell University has used electromagnetic simulations to optimize the shape of a dipole mode sample host cavity such that the surface magnetic fields on the sample are high compared to the energy inside the cavity and the surface magnetic field on the rest of the cavity. In this paper we present an update of the design that includes how to mount samples in the cavity and the addition of a low field chamber.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB044
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THPB053	Surface Resistance Characterization of Nb3Sn Using the HZB Quadrupole Resonator	ion, SRF, quadrupole, cavity	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]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB053
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THPB055	Plasma-enhanced ALD System for SRF Cavity	ion, cavity, plasma, SRF	870
	S. Kato, H. Hayano KEK, Ibaraki, Japan
	Funding: This work was supported by Photon and Quantum Basic Research Coordinated Development Program by MEXT and partly by JST-CREST. A remote PEALD (Plasma-enhanced Atomic Layer Deposition) system which would offer a high conformality and a low deposition temperature has been being developed for deposition of NbN on an SRF cavity. The deposition equipment consists of a deposition chamber, a remote plasma exciter, a precursor supply system, vacuum pumps, a quartz crystal microbalance (QCM) as a film growth rate meter, a detoxifying system and a safety system. An RF frequency of 13.56MHz was used for the inductively coupled plasma exciter of a reactant gas. The whole equipment is in a draft booth for operation safety. For ALD of an SRF cavity, the ALD system allows us to easily replace the deposition chamber with a single cell Nb cavity. The prepared precursors are tris[ethylmethylamido][tert-butylimido] niobium (TBTEMN) and trimethylaluminium (TMA). Ammonia, hydrogen and water are also prepared as reactants. We will report the ALD system design and result of the NbN deposition on sample coupons which are analysed with SEM, EDX and XPS.
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THPB057	Investigation of Nucleation Stage in Diffusion Coating of Nb3Sn on Nb	ion, experiment, SRF, cavity	873
	F. Pan IMP/CAS, Lanzhou, People's Republic of China
	Nb3Sn has the potential to improve properties of SRF cavities, such as the gradients and the working tempera-tures. Institute of Modern Physics has launched its Nb3Sn thin film coated SRF cavity project in 2016. Samples have been successfully coated to study the process of tin vapor diffusion. The main part of the deposition system is a tube furnace, which working temperature can reach 1100°C. Basic material characterization of the Sn-Nb film will be presented in this work.
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THPB066	Introducing the Vertical High-temperature UHV Furnace of the S-DALINAC for Future Cavity Material Studies	ion, vacuum, cavity, linac	891
	R. Grewe, L. Alff, M. Arnold, J. Conrad, S. Flege, M. Major, N. Pietralla TU Darmstadt, Darmstadt, Germany F. Hug IKP, Mainz, Germany
	Funding: Work supported by the Federal Ministry of Education and Research through grant No. 05H15RDRBA. Since 2005 the Institute for Nuclear Physics in Darmstadt operates a high temperature UHV furnace for temperatures of up to 1750°C. It has been used several times for hydrogen bake-out of the SRF cavities of the S-DALINAC with proven success. In 2013, studies at FNAL have shown that cavities treated with nitrogen reached an up to four times higher q-factor. The cavities are exposed to N2 at 850°C at the end of the H2 bake-out. A thin layer of normalconducting hexagonal niobium nitride (NbN) forms at the surface which is removed by electropolishing while the higher quality factors are attributed to the N2 diffusion into the bulk Nb. At temperatures from 1300°C to 1700°C a thin layer of the superconducting cubic phase of NbN can be observed, e.g. delta-phase NbN, which has a higher critical field and higher critical temperature and thus is very intereresting for applications for SRF cavities*. The UHV furnace has been prepared for future treatments of Nb samples and cavities in a N2 atmosphere at high temperatures for research on cubic NbN. The material properties of the samples will be analyzed at the ATFT group at the Department for Material Sciences of TU Darmstadt. Grasselino et al., Superconducting Science and Technology, 2013 Hennessey et al., Oxidation of Metals, 1992 *Martienssen et al., Springer Handbook of Condensed Matter and Materials Data, 2005
	Poster THPB066 [3.024 MB]
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THPB070	Electrochemical Finishing Treatment of Nb3Sn Diffusion-coated Niobium	ion, cavity, experiment, SRF	900
	U. Pudasaini, M.J. Kelley The College of William and Mary, Williamsburg, Virginia, USA G.V. Eremeev, M.J. Kelley, C.E. Reece, H. Tian JLab, Newport News, Virginia, USA
	Funding: Supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics under contract DEAC0506OR23177 and Office of High Energy Physics under grant DESC0014475. Nb3Sn cavities are now routinely prepared by depositing few micron thick Nb3Sn coatings on Nb cavities using tin vapor diffusion process. For the¬¬ case of niobium there is a significant improvement after electropolishing (EP), but electrochemical finishing treatment on Nb3Sn coatings has not been studied. Controlled removal of the first few layers could lead to a smoother and cleaner surface that is conducive to better RF performance. Several samples, which were coated with Nb3Sn by vapor diffusion process in a JLab sample chamber, were used to explore polishing parameters, such as I-V characteristics, removal rate, topography, etc. Preliminary results from the first runs are discussed here.
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THPB078	Fabrication of Large-area MgB2 Films on Copper Substrates	ion, cavity, SRF, radio-frequency	917
	X. Guo, K.X. Liu, Z. Ni PKU, Beijing, People's Republic of China Q. Feng, C. Yang Peking University, Beijing, People's Republic of China
	Magnesium diboride (MgB2) is a promising candidate material for SRF cavities because of its higher transition temperature and critical field compared with niobium. To meet the demand of RF test devices, the fabrication of large-area MgB2 films on metal substrates is needed. In this work, MgB2 films with 50-mm diameter were fabri-cated on Cu substrates by using an improved HPCVD setups at Peking University. The transition temperatures of MgB2 film on Cu substrate and with Mo buffer layer on Cu substrate are 36.2 K and 36.5 K, respectively. The fabrication processes, surface morphology, superconduct-ing properties of these large-area MgB2 films are present-ed.
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THPB079	Simulations of RF Field-induced Thermal Feedback in Niobium and Nb3Sn Cavities	ion, cavity, simulation, feedback	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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MOPB035

Cryogenic Probe Station at Old Dominion University Center for Accelerator Science

ion, cavity, SRF, cryogenics

128

J. Makita, J.R. Delayen, A.V. Gurevich
ODU, Norfolk, Virginia, USA
G. Ciovati
JLab, Newport News, Virginia, USA

With a growing effort in research and development of an alternative material to bulk Nb for a superconducting radiofrequency (SRF) cavity, it is important to have a cost effective method to benchmark new materials of choice. At Old Dominion University's Center for Accelerator Science, a cryogenic probe station (CPS) will be used to measure the response of superconductor samples under RF fields. The setup consists of a closed-cycle refrigerator for cooling a sample wafer to a cryogenic temperature, a superconducting magnet providing a field parallel to the sample, and DC probes in addition to RF probes. The RF probes will extract a quality factor from a sample patterned in a coplanar waveguide resonator structure on a 2' wafer. From the measured quality factor, the surface resistance and the penetration depth as a function of temperature and magnetic field will be calculated. This paper will discuss the design and measurement procedures of the current CPS setup.

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MOPB036

The Study of Deposition Method of Nb3Sn Film on Cu Substrate

ion, SRF, cavity, superconductivity

131

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

Our work is mainly focused on the fabrication methods of Nb3Sn films on Cu substrates and film's properties. There are diffraction peaks of Nb3Sn in the X-ray diffraction patterns in which without diffraction peaks of copper compounds. Scanning electron microstructures of Nb3Sn film reflect its nice compactness and binding force between film and substrate.

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MOPB040

ESS High-beta Cavity Test Preparations at Daresbury Laboratory

ion, cavity, LLRF, SRF

137

P.A. Smith, L. Bizel-Bizellot, K.D. Dumbell, M. Ellis, P. Goudket, A.J. Moss, E.F. Palade, S.M. Pattalwar, M.D. Pendleton, A.E. Wheelhouse
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom

Science and Technology Facility Council is responsible for supplying, and testing 84 High beta elliptical SRF cavities, as part of the UK In Kind Contribution to the European Spallation Source (ESS). The High-β=0.86, cavities have been designed by CEA- Saclay and are a five cell Niobium cavity operating at 704.42 MHz. They are required to provide an accelerating gradient of 19.9 MV/m at an unloaded Q of 5x10⁹. Preparations are underway to upgrade the cryogenic and RF facilities at Daresbury laboratory prior to the arrival of the first cavities. As part of these arrangements, a niobium coaxial resonator has been manufactured, to validate the test facility. The design considerations, for the coaxial resonator are presented, along with preliminary results. The RF measurement system to perform the cavity conditioning and testing is also presented.

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TUXBA01

Low Temperature Doping of Niobium Cavities: What is Really Going on?

ion, cavity, vacuum, background

353

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

Initial work, first at Fermilab and subsequently at Cornell, has shown that low temperature heat treatments (120 - 160 C) in a low pressure atmosphere can lead to a 'Q-rise' and high quality factors similar to that of cavities nitrogen-doped at high temperatures (~800 C). It was suggested that the low-temperature baking effect is a result of nitrogen doping or 'infusion'. We conducted a systematic study of this effect, using both RF measurements of cavities treated at different doping temperatures as well as detailed SIMS analysis of the surface layer. We match RF performance and extracted material parameters (especially electron mean free path) to the measured doping concentration profiles. We conclusively show that the low-temperature baking is drastically lowering the mean free path in the penetration layer, and that this is not the result of nitrogen doping or infusion. Instead, other interstitial impurities (specifically oxygen and carbon) are diffused into the surface in the low temperature heat treatment and are the source of lowering of the mean free path and, thus, of the observed Q-rise.

Slides TUXBA01 [4.153 MB]

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TUYAA01

The Importance of the Electron Mean Free Path for Superconducting RF Cavities

cavity, ion, electron, SRF

359

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

Theoretical results offer a potential explanation for the anti-Q-slope, the phenomenon of decreasing microwave surface resistance with increasing radiofrequency electromagnetic field strength. This effect has been observed in niobium doped with impurities, chiefly nitrogen, and has been put to use in the Linac Coherent Light Source II (LCLS-II) accelerator currently under construction. Our work, presented here, finds a strong link between the electron mean free path, the main measure of impurity doping, to the overheating of quasiparticles in the RF penetration layer. This is an important effect that adjusts the magnitude of the theoretical anti-Q-slope by providing a mechanism to counteract it and introduce a surface resistance that increases with field strength. We discuss our findings in a study of niobium cavities doped at high temperature (800-990 °C) as well as new analysis of low-temperature-doped cavities.

Slides TUYAA01 [6.988 MB]

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TUYAA02

Advancement in the Understanding of the Field and Frequency Dependent Microwave Surface Resistance of Niobium

ion, cavity, ECR, resonance

364

M. Martinello, S. Aderhold, S.K. Chandrasekaran, M. Checchin, A. Grassellino, O.S. Melnychuk, S. Posen, A. Romanenko, D.A. Sergatskov
Fermilab, Batavia, Illinois, USA

The radio-frequency surface resistance of niobium resonators is incredibly reduced when nitrogen impurities are dissolved as interstitial in the material, conferring ultra-high Q-factors at medium values of accelerating field. This effect has been observed in both high and low temperature nitrogen treatments. As a matter of fact, the peculiar anti Q-slope observed in nitrogen doped cavities, i.e. the decreasing of the Q-factor with the increasing of the radio-frequency field, come from the decreasing of the BCS surface resistance component as a function of the field. Such peculiar behavior has been considered consequence of the interstitial nitrogen present in the niobium lattice after the doping treatment. The study here presented show the field dependence of the BCS surface resistance surface of cavities with different resonant frequencies, such as: 650 MHz, 1.3 GHz, 2.6 GHz and 3.9 GHz, and processed with different state-of-the-art surface treatments. These findings show for the first time that the anti Q-slope might be seen at high frequency even for clean Niobium cavities, revealing useful suggestion on the physics underneath the anti Q-slope effect.
*M. Martinello, A. Grassellino, M. Checchin, A. Romanenko, O. Melnychuk, D.A. Sergatskov, S. Posen, J. Zasadzinski App. Phys. Lett. 109, 6 (2016)

Slides TUYAA02 [4.342 MB]

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TUYBA01

Progress on Characterization and Optimization of Multilayers

ion, SRF, cavity, site

368

C.Z. Antoine
CEA/DSM/IRFU, France
M. Aburas, A. Four
CEA/DRF/IRFU, Gif-sur-Yvette, France
H. Hayano, S. Kato, T. Kubo, T. Saeki
KEK, Ibaraki, Japan
Y. Iwashita
Kyoto ICR, Uji, Kyoto, Japan

Multilayers MgO/NbN/MgO/Nb with several thicknesses are being tested by local magnetometry, Scanning tunneling and various standard structural techniques experiment providing usefull information to compare experiments and recent theoretical advances proposed by A. Gurevich or T. Kubo.

Slides TUYBA01 [5.819 MB]

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TUYBA03

The Way of Thick Films toward a Flat Q-curve in Sputtered Cavities

ion, cavity, interface, site

378

V. Palmieri
INFN/LNL, Legnaro (PD), Italy

Thick films have bulk like properties. In this paper it is explored the possibility to sputter 70 micron thick films in order to get rid of the Q-slope in Niobium sputtered Cop-per Cavities. An innovative method based on the multi-layer deposition of zero-stress single layers is reported. The deposition of zero-stress thick films into 6 GHz Cop-per seamless cavities, has shown the possibility to obtain straight curves for the Q-factor versus accelerating fields
Work supported by the INFN V group experiment ISIDE and performed under the CERN-INFN-FTSC Agreement N. KE2722/BE/FCC

Slides TUYBA03 [4.935 MB]

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TUPB005

Developed Spoke Cavity Module for Main Linac of China ADS HOM Simulations and Damping Scheme for CEPC Cavities

cavity, ion, linac, SRF

393

Z.Q. Li, J.S. Cao, Y.L. Chi, F.S. He, S.P. Li, H.Y. Lin, Q. Ma, Z.H. Mi, W.M. Pan, P. Sha, B. Xu, J.Y. Zhai, X.Y. Zhang
IHEP, Beijing, People's Republic of China

During past five year, two kind of spoke of Beta equal 0.21 and 0.40 were developed at IHEP CAS, the spoke cavity of beta 0.21 was adopted to accelerate proton from 10 to 32MeV, and 32 to 160MeV for beta 0.40 spoke cavity. Up to now, two kind of naked spoke cavities have been test in vertical, also the module of beta 0.21 spoke cavity, which equipped the liquid helium jacket, magnetic shield layer and frequency tuner has been fulfilled and test, the performance of all of components reach the design requirements.

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TUPB013

Advanced Manufacturing Techniques for the Fabrication of Hl-LHC Crab Cavities at CERN

ion, cavity, simulation, MMI

409

M. Garlaschè
CERN, Geneva, Switzerland

RF Crab Cavities are an essential part of the HL-LHC upgrade at CERN. Two concepts of such systems are being developed: the Double Quarter Wave (DQW) and the RF Dipole (RFD). The following paper describes the advanced manufacturing techniques developed for the fabrication of the DQW cavity prototype with an outlook on the upcoming RFD prototype production.

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TUPB014

In-situ Bulk Residual Resistivity Ratio Measurement on Double Quarter Wave Crab Cavities

cavity, ion, luminosity, cryomodule

415

N.C. Shipman, A. Castilla, K.G. Hernández-Chahín, A. Macpherson
CERN, Geneva, Switzerland
I. Ben-Zvi
BNL, Upton, Long Island, New York, USA
G. Burt, N.C. Shipman
Cockcroft Institute, Lancaster University, Lancaster, United Kingdom
K.G. Hernández-Chahín
Universidad de Guanajuato, División de Ciencias e Ingenierías, León, Mexico
J.A. Mitchell
Lancaster University, Lancaster, United Kingdom
N.C. Shipman
UMAN, Manchester, United Kingdom

A four wire measurement was used to measure the bulk RRR on two DQW Crab Cavities. The measurement procedure is explained and the values obtained for each cavity are compared together with the values obtained from Niobium samples of the same stock from which the cavities were manufactured. Measurement errors and carefully analysed and further improvements to the measurement procedure are suggested.

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TUPB039

Electropolishing of Niobium from Deep Eutectic Solvents Based on Choline Chloride

ion, ECR, cavity, SRF

475

Q.W. Chu, H. Guo, Y. He, L. Li, P.R. Xiong, Z.M. You, S.H. Zhang
IMP/CAS, Lanzhou, People's Republic of China

Niobium (Nb) was successfully electropolished from a green ionic liquid, choline chloride/urea deep eutectic solvent (DES). This paper was to investigate the influence of various electropolishing parameters, including electropolishing time, temperature and voltage, on the electropolishing rate, surface roughness, glossiness and microstructure of Nb. The result showed that the electropolishing parameters had a significant impact on the performance of Nb. Based on surface analysis by scanning electron microscope (SEM) and atomic force microscope (AFM), smooth Nb can be achieved under properly controlled conditions.

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TUPB046

Experience on Design, Fabrication and Testing of a Large Grain ESS Medium Beta Prototype Cavity

ion, cavity, radiation, electron

484

M. Bertucci, A. Bignami, A. Bosotti, J.F. Chen, P. Michelato, L. Monaco, R. Paparella, D. Sertore
INFN/LASA, Segrate (MI), Italy
C. Pagani
Università degli Studi di Milano & INFN, Segrate, Italy
S. Pirani
ESS, Lund, Sweden

INFN-LASA built a complete Medium Beta cavity, based on the ESS prototype design, with novel large-grain material sliced in sheets from an ingot provided by CBMM manufacturing experience. Design and fabrication are reported as well as results on the physical and chemical analyses performed on samples at different cavity production stages. Results from the cold tests performed are also summarized and critically discussed in view of future R&D activities

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TUPB048

INFN- LASA Medium Beta Cavity Prototypes for ESS Linac

cavity, ion, controls, vacuum

494

D. Sertore, M. Bertucci, A. Bignami, A. Bosotti, J.F. Chen, P. Michelato, L. Monaco, R. Paparella
INFN/LASA, Segrate (MI), Italy
C. Pagani
Università degli Studi di Milano & INFN, Segrate, Italy
S. Pirani
ESS, Lund, Sweden

INFN-LASA, in the framework of INFN contribution to the European Spallation Source, has developed, produced and tested 704.4 MHz Medium Beta (β = 0.67) cavities. Mode separation and avoidance of HOM excitation by machine line frequencies have driven the cavity design. The production at the industry, also in view of the INFN in-kind contribution of series cavities, has been done "build-to-print" and we have implemented our own quality control process, based on our XFEL experience, from raw material to cavity ready for test. The cavities have been then cold tested in our upgraded Vertical Test Facility. In this paper, we report on our experience on the different phases of the cavity production and test processes.

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TUPB055

Nb3Sn Thin Film Deposition On Copper By DC Magnetron Sputtering

ion, target, SRF, cavity

512

W.W. Tan, B.T. Li, X.Y. Lu, L. Xiao, D. Xie, D.Y. Yang, Y. Yang, Z.Q. Yang, J. Zhao
PKU, Beijing, People's Republic of China

Nb3Sn for SRF cavities has been coated on copper samples by DC magnetron Sputtering. Pure Nb target and pure Sn target were installed separately in the magnetron sputtering device. Nb3Sn precursor was coated on copper in the Ar atmosphere of 0.5 Pa. The Nb3Sn precursor was annealed in the vacuum furnace whose pressure is 10^-4 Pa. The XRD results demonstrate the exist of Nb3Sn crystal, and MPMS results show superconductivity of Nb3Sn. The highest critical temperature obtained is 15K.

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TUPB063

Fabrication of a SRF Deflecting Cavity for the ARIEL-Linac

ion, cavity, linac, TRIUMF

524

D.W. Storey, R.E. Laxdal, B. Matheson, N. Muller
TRIUMF, Vancouver, Canada
D.W. Storey
Victoria University, Victoria, B.C., Canada

A superconducting RF deflecting cavity has been designed and is being fabricated at TRIUMF to allow simultaneous beam delivery to both rare isotope production and an energy recovery linac. The 650 MHz cavity will operate in a TE-like mode in CW. The design has been optimised for high shunt impedance and minimal longitudinal footprint, reaching roughly 50% higher shunt impedance with 50% less length than comparable non-TM mode cavity geometries. Due to low power dissipation at 4K at the maximum required deflecting voltage of 0.6 MV, low cost manufacturing techniques have been employed in the construction of the cavity. These include the use of reactor grade Niobium and TIG welding in an inert atmosphere. Development of the manufacturing processes will be presented along with the status of fabrication.

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TUPB065

Design of Multi-frequency Coaxial Test Resonators

ion, cavity, SRF, operation

531

Z.Y. Yao, T. Junginger, R.E. Laxdal, B. Matheson, B.S. Waraich, V. Zvyagintsev
TRIUMF, Vancouver, Canada

A significant issue in low beta resonators is medium field Q-slope (MFQS) at 4K. To study the MFQS and the field dependence of surface resistance in low beta resonators, a quarter-wave resonator (QWR) and a half-wave resonator (HWR) were designed to be tested at integer harmonic frequencies of 200MHz, and up to 1.2GHz. A series of chemistry and heat treatments will be applied to these cavities so that a systemic study on the surface resistance of the coaxial resonators associating with post-processing, RF field, and frequency can be done. The detail design of these cavities and the status of cavity fabrication will be reported in this paper.

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TUPB067

Insights into Formation of Nb3Sn Film During the Vapor Diffusion Process

ion, experiment, cavity, SRF

539

U. Pudasaini, M.J. Kelley
The College of William and Mary, Williamsburg, Virginia, USA
G.V. Eremeev, M.J. Kelley, C.E. Reece
JLab, Newport News, Virginia, USA
M.J. Kelley, J. Tuggle
Virginia Polytechnic Institute and State University, Blacksburg, USA

Funding: Supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics under contract DE-AC05-06OR23177 and Office of High Energy Physics under grant DE-SC-0014475.
The potential of Nb3Sn for SRF cavities is widely recognized and renewed R&D efforts continue to bring new insights about material structure and its properties. We have systematically coated niobium with Nb3Sn using "vapor diffusion" under varying coating conditions to elucidate the reaction of tin with niobium at the temperatures of interest. The analysis of the coated samples is revealing new understanding about the two-stage nucleation/deposition ("vapor diffusion") process that allows us to form a hypothesis regarding Nb3Sn formation mechanism. The essential aspect of nucleation is the deposition of a high coverage, nanoscale thin tin film with particle assemblage by decomposition of tin chloride on the niobium surface at temperatures sufficient for reduction of the thick niobium oxide film, usually at about 500°C. The deposition is followed by the reaction of tin from tin vapor with the niobium surface to form Nb3Sn at about 1200°C, where the surface and grain boundaries start to play key role in the formation process initiation and progression. These findings improve understanding of the Nb3Sn growth in the typical vapor diffusion process used for accelerator cavity coatings.

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TUPB071

Test Result of 650 MHz, Beta 0.61 Single-cell Niobium Cavity

cavity, ion, accelerating-gradient, electron

553

S. Seth, P. Bhattacharyya, A. Dutta Gupta, S. Ghosh, S. Ghosh, A. Mandal, S. Som
VECC, Kolkata, India
A. Grassellino, T.N. Khabiboulline, O.S. Melnychuk, C.S. Mishra, T.H. Nicol, A.M. Rowe, D.A. Sergatskov
Fermilab, Batavia, Illinois, USA
M.P. Kelly, T. Reid
ANL, Argonne, Illinois, USA
K.K. Mistri, P.N. Prakash
IUAC, New Delhi, India

VECC has been involved in the design, analysis and development of 650 MHz, beta 0.61 (LB650), elliptical Superconducting RF linac cavity, as part of research and development activities on SRF cavities and associated technologies under Indian Institutions Fermilab Collaboration (IIFC). A single-cell niobium cavity has been indigenously designed and developed at VECC, with the help of Electron Beam Welding (EBW) facility at IUAC, New Delhi. Various measurements, processing and testing at 2K in Vertical Test Stand (VTS) of the single-cell cavity was carried out at ANL and Fermilab, USA, with active participation of VECC engineers. It achieved a maximum accelerating gradient(Eacc) of 34.5 MV/m with Quality Factor of 2·10⁹ and 30 MV/m with Quality Factor of 1.5·10¹⁰. This is probably the highest accelerating gradient achieved so far in the world for LB650 cavities. This paper describes the design, fabrication and measurement of the single cell niobium cavity. Cavity processing and test results of Vertical Test of the single-cell niobium cavity are also presented.

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TUPB072

Investigation of BCP Parameters for Mastery of SRF Cavity Treatment

ion, cavity, experiment, SRF

558

F. Éozénou
CEA/DSM/IRFU, France
E. Cenni, G. Devanz, T. Percerou, Th. Proslier, C. Servouin
CEA/DRF/IRFU, Gif-sur-Yvette, France
M.L.L. Nghiem
UPMC, Paris, France

Mastery of Standard Buffered Chemical Polishing (with mixture of hydrofluoric, nitric and phosphoric acids) is of paramount importance for the treatment of SRF resonators with complex geometry has IFMIF half-wave resonators, in order to control accurately their frequency evolution. Furthermore, strong and unexpected asymmetry in removals has recently been observed after BCP treatment of ESS-medium beta resonators. The goal of this study is to investigate accurately influence of parameters such as surface geometry and orientation, acid temperature, agitation and their coupling on the removal rate. We will also focus on the influence of by-products such has NOx on kinetics. The mixture used is HF(40%)- HNO3(65%)-H3PO4(85%) with ratio 1-1-2.4.

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TUPB074

RF Performance of Multi-cell Scale Niobium SRF Cavities Prepared with HF Free Bipolar Electro-polishing at Faraday Technology

cavity, ion, SRF, MMI

567

F. Furuta, M. Ge, T. Gruber, J.J. Kaufman, M. Liepe, J. Sears
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
T.D. Hall, M.E. Inman, R. Radhakrishnan, S.T. Snyder, E.J. Taylor
Faraday Technology, Inc., Clayton, Ohio, USA

Cornell's SRF group and Faraday Technology, Inc. have been collaborating on two phase-II SBIR projects. One of them is the development and commissioning of a 9-cell scale HF free Bipolar Electro-Polishing (BEP) system. Faraday Technology had completed the proof of principle on BEP with single cell scale prior to the work reported here, and has now developed a new 9-cell scale BEP system. Cornell has fabricated three single cell cavities and has assembled them together as a 9-cell scale test string. The 9-cell scale test string has received BEP at Faraday Technology and RF testing has been performed on the three single cell cavities one-by-one at Cornell. Here we give a status update on the new 9-cell scale BEP system commissioning and on results from RF tests of the BEP cavities.

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TUPB082

Setup of a Spatially Resolving Vector Magnetometry System for the Investigation of Flux Trapping in Superconducting Cavities

ion, cavity, SRF, superconducting-cavity

580

B. Schmitz, K. Alomari, J. Knobloch, O. Kugeler, J.M. Köszegi, Y. Tamashevich
HZB, Berlin, Germany

Flux trapping is the major contribution to the residual resistance of superconducting cavities. In order to gain a better understanding of the mechanisms involved and aiming at an eventual minimization of trapped flux, a measurement setup based on AMR sensors was devised that allows for monitoring the magnetic field vector at various positions near the cavity surface. First results of the efforts are presented.

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TUPB086

Study on Local Chemical Treatment for Recovery From Surface Oxidation by HPR Process on SRF Cavities

cavity, ion, experiment, SRF

592

H. Guo, Y. He, Y.M. Li, T. Tan, A.D. Wu, P.R. Xiong, Z.M. You, W.M. Yue, S.H. Zhang
IMP/CAS, Lanzhou, People's Republic of China

High pressure rinsing (HPR) with ultra-pure water (UPW) is the last step which is commonly used for SRF cavities cleaning. The serious surface damage will be caused due to the failure of the distance control between the jet and cavity surface or the breakdown of the jet rotation. The surface of taper HWR cavities which are used for CIADS project was damaged in HPR process. Two methods were used for surface recovery and the result will be presented in this paper.

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TUPB090

Investigation of High Temperature Baking of Jacketed Quarter Wave Resonators

ion, cavity, linac, SRF

598

A. Rai, D. Kanjilal, K.K. Mistri, P. Patra, P.N. Potukuchi, S.K. Sonti
IUAC, New Delhi, India

The Superconducting booster Linac at IUAC has been delivering accelerated beams for scheduled experiments since 2013. It has three accelerating modules with 8 Quarter Wave Resonators (QWR)in each. The QWRs for the first module were built at Argonne National Laboratory while those for the second and third modules have been built in-house. During the electropolishing of one of the indigenously built resonators (QWR # I03) the RF surface got spoiled due to a wrong acid mixture that was being used for etching. In subsequent cold tests of the cavity, its performance was poor (2.6 MV/m @ 4W). There was evidence of Q disease also, as the performance deteriorated further (~20%) when the cavity was held at 100-120K for ~8 hours .In an attempt to recover the cavity it was baked at 650 °C for 10 hours along with its stainless steel jacket. A series of tests were conducted thereafter wherein, a substantial improvement (factor of two) in the performance was observed. Encouraged with the results another QWR designed for a lower beta (β=0.05) was also heat treated identically. This paper presents the different treatments followed to enhance the cavity performance vis-à-vis the test results.

Poster TUPB090 [1.240 MB]

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TUPB091

Study on Vertical Electropolising of 9-cell Niobium Coupon Cavity

ion, cavity, cathode, polarization

602

V. Chouhan, Y.I. Ida, K.N. Nii, T.Y. Yamaguchi
MGH, Hyogo-ken, Japan
H. Hayano, S. Kato, H. Monjushiro, T. Saeki, M. Sawabe
KEK, Ibaraki, Japan

Authors report a study on vertical electropolishing (VEP) carried out for a 1.3 GHz 9-cell niobium (Nb) coupon cavity using a unique cathode namely 'Ninja Cathode'. The design of the cathode for VEP of a 9-cell cavity was based on the Ninja cathode used for 1-cell cavity since the 1-cell Ninja cathode was found effective to reduce longitudinal asymmetry in material removal and to obtain a smooth surface of a 1-cell cavity. Moreover, 1-cell Nb cavities after being treated in VEP using the Ninja cathode showed good performance in vertical RF tests. The 9-cell coupon cavity used in this study was designed to have totally nine coupons set on the iris and equator positions of the first, fifth and ninth cells. These three cells contain viewports as well at their upper and lower iris positions. Measurement of currents from the individual coupons and in-situ observation are possible using the cavity to understand EP phenomenon at different locations of the cavity. VEP results, which include removal thicknesses at different positions of the cavity and surface study of the coupons, are discussed.

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TUPB092

Analysis of Niobium Surface and Generated Particles in Vertical Electropolishing of Single-cell Coupon Cavity

ion, cavity, experiment, cathode

607

V. Chouhan, Y.I. Ida, K.N. Nii, T.Y. Yamaguchi
MGH, Hyogo-ken, Japan
H. Hayano, S. Kato, H. Monjushiro, T. Saeki, M. Sawabe
KEK, Ibaraki, Japan

In our previous studies, we have reported parameter investigation for vertical electropolishing (VEP) of 1-cell niobium (Nb) tesla/ILC type cavities using a Ninja cathode. A 1-cell coupon cavity containing six Nb disk coupons at its different positions was found effective to reduce time and cost to establish an optimized VEP recipe. In this work, we present surface analyses of VEPed Nb coupon surfaces using scanning electron microscope (SEM), energy dispersive x-ray spectroscopy (EDX) and x-ray photoelectron spectroscopy (XPS). Surfaces contained micro- and nano-sized particles which were found with random distributions and different number densities on the beam pipe and iris coupons. Surfaces of equator coupons were found to have relatively less number of particles or almost clean. To analyze particles, a few particles were picked-up from a coupon surface using a tungsten tip under SEM and analyzed with EDX while the coupon was moved out from the SEM chamber to avoid its effect in EDX spectra. The particles were confirmed as oxygen-rich niobium and contained fluorine and carbon also. XPS analysis of the coupon surfaces was also carried out for further study of surface chemistry.

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TUPB095

Modeling the Hydroforming of a Large Grain Niobium Tube With Crystal Plasticity

ion, cavity, experiment, SRF

616

A. Mapar
MSU, East Lansing, Michigan, USA
T.R. Bieler
Michigan State University, East Lansing, Michigan, USA
J.E. Murphy
University of Nevada, Reno, Reno, Nevada, USA
F. Pourboghrat
Ohio State University, Columbus, Ohio, USA

Current SRF cavities are made from fine grained polycrystalline niobium half-cells welded together. Hot spots are commonly found in the heat-affected zone, making seamless hydroformed cavities attractive. Large grain cavities usually perform as well as fine grain cavities, often having a higher Q, presumably due to fewer grain boundaries. Large grain Nb forms non-uniformly, which introduces problems in manufacturing. A model that could realistically predict the deformation response of large grain Nb could facilitate the design of large grain hydroformed tubes. To this end, a crystal plasticity model was developed and calibrated with tensile stress-strain data of Nb single crystals. A seamless large grain tube was made from rolling a fine grain sheet into a tube, welded, and heat treated to grow large grains. The heat treatment resulted in a large grain tube with a single grain orientation in the center. The tube was hydroformed until it cracked. The hydroforming process was simulated with the crystal plasticity model, which was able to predict the deformed shape of the tube, the location of the crack and other localized areas with heterogeneous strain.

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TUPB103

DC Magnetism of Niobium Thin Films

ion, site, cavity, power-supply

640

S. Wilde, B. Chesca
Loughborough University, Loughborough, Leicestershire, United Kingdom
A.N. Hannah, O.B. Malyshev, N. Pattalwar, S.M. Pattalwar, B.S. Sian, R. Valizadeh, S. Wilde
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom
B.S. Sian
UMAN, Manchester, United Kingdom
G.B.G. Stenning
STFC/RAL/ISIS, Chilton, Didcot, Oxon, United Kingdom

Niobium thin films were deposited onto a-plane sapphire with varying kinetic energy and varying substrate temperature. There were no consistent trends which related the particle energy or substrate temperature to RRR. The sample which displayed the largest RRR of 229 was then compared to both a thin film deposited with similar conditions onto copper substrate and to bulk niobium. DC magnetometry measurements suggest that the mechanism of flux entry into thin film niobium and bulk niobium may vary due to differences in the volumes of both defects and impurities located within the grains. Results also suggest that magnetic flux may penetrate thin films at small fields due to the sample geometry.

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TUPB104

First Full Cryogenic Test of the SRF Thin Film Test Cavity

ion, cavity, SRF, cryogenics

644

R. Valizadeh, L. Bizel-Bizellot, P. Goudket, L. Gurran, O.B. Malyshev, N. Pattalwar, S.M. Pattalwar
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom
G. Burt, L. Gurran
Lancaster University, Lancaster, United Kingdom
G. Burt, P. Goudket, O.B. Malyshev, S.M. Pattalwar, R. Valizadeh
Cockcroft Institute, Warrington, Cheshire, United Kingdom
L. Gurran
Cockcroft Institute, Lancaster University, Lancaster, United Kingdom

A test cavity that uses RF chokes, rather than a physical seal, to contain the field is a promising method of SRF sample testing, especially in thin films research where the rate of sample production far outstrips that of full SRF characterisation. Having the sample and cavity physically separate reduces the complexity involved in changing samples - major causes of low throughput rate and high running costs for other test cavities - and also allows direct measurement of the RF power dissipated in the sample via power calorimetry. Choked test cavities operating at 7.8 GHz with three RF chokes have been designed and tested at Daresbury Laboratory. As part of the commissioning of this system, we performed the first full SRF test with a bulk Nb sample and we verified that the system would perform as required for future superconducting thin film sample tests.

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TUPB108

Genesis of Topography in Buffered Chemical Polishing of Niobium for Application to Superconducting Radiofrequency Accelerator Cavities

ion, cavity, electron, SRF

651

L. Zhao, C.E. Reece
JLab, Newport News, Virginia, USA
M.J. Kelley
The College of William and Mary, Williamsburg, Virginia, USA

Funding: Supported by Office of High Energy Physics, U.S. Department of Energy, Grant SC0007907 to the College of William & Mary and by U.S. DOE Contract No. DE-AC05-06OR23177 to Jefferson Science Associates
Topography arising from the final etch step in preparing niobium superconducting radiofrequency (SRF) accelerator cavities is understood to significantly impact cavity performance at high field levels. This study investigated the effect of process temperature and time on the etch rate and topography arising from the widely-used buffered chemical polishing (BCP). This study aims to understand more thoroughly the genesis of topography in BCP of polycrystalline niobium, with the ultimate aim of finding a path to surface smoothness comparable to that obtained by electropolishing (EP). It was found that the etch process is controlled by the surface reaction; and that the etch rate varies with crystallographic orientation. The familiar micron-scale roughening necessarily results. Gas evolution has an impact, but is secondary. The major outcome is that surface smoothness comparable to EP appears to be inherently unachievable for polycrystalline niobium using BCP, setting an upper limit to the gradient for which it is useful.

Poster TUPB108 [3.782 MB]

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TUPB111

R&D Activities on Centrifugal Barrel Polishing of 1.3 GHz Niobium Cavities at DESY/University of Hamburg

ion, cavity, ECR, laser

655

A.L. Prudnikava, B. Foster, Y. Tamashevich
University of Hamburg, Institut für Experimentalphysik, Hamburg, Germany
A. Ermakov, B. Foster
DESY, Hamburg, Germany
Y. Tamashevich
HZB, Berlin, Germany

In this paper the status of research activities at ILC-HiGrade Lab (DESY/University of Hamburg) on Centrifugal Barrel Polishing (CBP) of 1.3 GHz Niobium Cavities is presented. We focus on CBP based on the polishing recipe reported by Fermi National Laboratory and Jefferson Lab*. The aim is to gain a better understanding of the limitations of this technique, detailed characterization of the treated surface after each polishing step using a "coupon" single cell cavity. Plastic deformations upon initial CBP steps, embedded polishing media and residual damage upon final polishing were investigated at different areas of the cavity.
* C. A. Cooper, L. D. Cooley, Supercond. Sci. Technol. 26 (2013) 015011

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TUPB113

Surface Characterization of Nitrided Niobium Surfaces

ion, cavity, electron, experiment

663

A.L. Prudnikava, B. Foster
University of Hamburg, Institut für Experimentalphysik, Hamburg, Germany
B. Foster
DESY, Hamburg, Germany
Y. Tamashevich
HZB, Berlin, Germany
Y. Tamashevich
University of Hamburg, Hamburg, Germany

Thermal treatment of niobium radio frequency cavities in nitrogen atmosphere is employed in ILCLS-II Project in order to improve the quality factor of Nb cavities. A so called "N-infusion" thermal treatment is applied without any post processing*, **, whereas "N-doping" requires the removal of the upper layer of 5-30 um. For better understanding the mechanism of such an improvement, a detailed characterization of the nitrided surface is necessary. Our studies are focused on characterization of the niobium surface subjected to such treatments (surface morphology, nitrogen concentration profile, hardness, phase composition). The sample preparation technique for studying the hydride precipitation in N-Nb system is presented, and current activities on studying of N-infused Nb samples by SQUID and PPMS are briefly discussed.
* A. Grassellino, et al, Supercond. Sci. Technol. 26 (2013) 102001.
** A. Grassellino, arXiv:1701.06077

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WEXA01

High Performance Nb3Sn Cavities

ion, cavity, SRF, site

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)

ion, cavity, SRF, electron

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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THXA08

Review of Heat Treatments for Low Beta Cavities : What's So Different from Elliptical Cavities

ion, cavity, SRF, accelerating-gradient

708

D. Longuevergne
IPN, Orsay, France

Heat treatments done for low beta (low frequency) cavities are usually, due to the lack of feedback, inspired from elliptical (high frequency) cavity results. Is that still relevant now that experimental data are available thanks to the florishing business of low beta structures (Spiral2, ESS, FRIB, C-ADS, MYRRHA, PROJECTX, …). These 2 families are moreover not usually operating in the same resistance regime (BCS and residual). The paper will review procedures applied and results obtained on different type of cavities (Quarter-Wave resonator, Half-Wave resonator and Spoke) and different temperature treatments (low temperature baking, hydrogen degassing, nitrogen doping, …) and compare these to elliptical cavities.

Slides THXA08 [5.567 MB]

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THPB002

Role of Nitrogen on Hydride Nucleation in Pure Niobium by First Principles Calculations

ion, site, cavity, electron

741

P. Garg, I. Adlakha, K.N. Solanki
Arizona State University, Tempe, USA
S. Balachandran, P.J. Lee
NHMFL, Tallahassee, Florida, USA
T.R. Bieler
Michigan State University, East Lansing, Michigan, USA

It is known that formation and growth of Nb hydride degrades superconducting radio frequency (SRF) properties of Nb cavities and the treatments that reduce H concentration improve quality factor. Recently it is has also been shown that addition of N through doping or infusion improves the quality factor. Thus, we probe role of N addition in Nb on hydride precipitation and stability through first principles calculations & compared with coupon samples. In presence of N, energetic preference for H to occupy interstitial sites in the vicinity of N is reduced. Furthermore, presence of N forces H to occupy interstitial octahedral site instead of a tetrahedral site. The thermodynamic stability of hydride is decreased in the presence of N in Nb.The quantum insights using charge transfer and density of states show a strong tendency of N to accumulate charge, thereby decreasing the bond strength of neighboring Nb and H atoms. These atomic scale results explain the lesser tendency of surface hydride formation in SRF Nb cavities in presence of N. These results are consistent with metallographic examination of N-treated Nb coupons, which show suppressed hydride formation near N-treated surface.

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THPB004

Impact of the Duration of Low Temperature Doping on Superconducting Cavity Performance

cavity, ion, vacuum, superconducting-cavity

750

P.N. Koufalis, F. Furuta, J.J. Kaufman, M. Liepe
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Low temperature treatments of superconducting cavities in a low pressure ambient atmosphere have been shown to introduce a 'Q-rise' up to moderate surface fields and an overall increase in quality factor. However, the effect of varying the doping time at a fixed temperature on cavity performance has not been systematically examined. We present results of such an investigation for cavities prepared at 120 and 160 C in a continuously flowing low pressure atmosphere for various amounts of time. We show that the introduction of impurities to the RF penetration layer can improve cavity performance and investigate the relationship between electron mean free path and the temperature-dependent component of the surface resistance.

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THPB005

Design Updates on Cavity to Measure Suppression of Microwave Surface Resistance by DC Magnetic Fields

ion, cavity, SRF, experiment

754

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

Our research has shown good agreement between experimental measurements of the anti-Q-slope in niobium SRF cavities and predictions from a recent theoretical model of the suppression of the microwave surface resistance with applied RF field. To confirm that this mechanism is indeed what causes the anti-Q-slope in impurity-doped niobium, it will be necessary to measure the theory's prediction that the same effect may be achieved by applying a constant (i.e. DC) magnetic field parallel to the RF surface. This will also allow for systematic studies of the proposed fundamental effect of the anti-Q-slope and of the behavior of the anti-Q-slope for many surface preparations and alternative materials, since it provides a cleaner measurement by eliminating the counteracting quasiparticle overheating and the complexifying oscillation of the screening currents. In this report we give an update on work at Cornell to design and build a coaxial cavity to measure this effect.

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THPB007

Nitrogen Infusion R&D on Single Cell Cavities at DESY

ion, cavity, SRF, injection

759

M. Wenskat, D. Reschke, J. Schaffran
DESY, Hamburg, Germany
A.L. Prudnikava
University of Hamburg, Institut für Experimentalphysik, Hamburg, Germany

A first series of single cell cavities underwent the "Nitrogen Infusion" treatment at DESY. Samples, which were in the furnace together with the cavities, underwent a series of SEM/EDX measurements and showed some unexpected structures. In parallel, the cavity performance deteriorated after the treatment. The furnace pressure and temperature and the residual gases during the treatment were analyzed and the possible cause for the deterioration has been found. Steps to prevent this deterioration in following treatments are discussed and first results are shown.

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THPB017

Investigation of Trapped Magnetic Flux in Superconducting Niobium Samples with Neutron Radiography

ion, neutron, experiment, detector

762

O. Kugeler, T. Junginger, J. Knobloch, M.M. Krzyzagorski, J.M. Köszegi, L. Riik, W. Treimer, R.F. Ziesche
HZB, Berlin, Germany

The dynamics of flux expulsion in Nb samples during superconducting transition has been investigated with neutron radiography. Aiming at a reduction of the trapped flux with respect to obtaining a small residual resistance it was attempted to influence the expulsion by applying external AC magnetic fields. The results of these experiments are presented.

Poster THPB017 [1.528 MB]

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THPB018

Towards the Perfect Meissner State: A Magneto-Optical Study on Competing Pinning Centers in Niobium

ion, cavity, SRF, background

766

J.M. Köszegi, J. Knobloch, O. Kugeler
HZB, Berlin, Germany

Over the past years trapped magnetic flux has emerged as a main limiting factor of high quality factors in SRF cavities. Several studies investigated how the ambient magnetic field can be minimized or how the flux expulsion during the phase transition can be improved. We now present a study that targets the pinning centers which allow for the flux to remain inside the superconductor in the first place. Using magneto-optical imaging we were able to not only measure the amount of trapped flux but in addition we managed to image its distribution with a resolution below 10μm and correlate it with electron backscatter diffraction maps. As a result we found that the grain boundaries did not play a major role as pinning centers nor did the crystal orientation influence the amount of trapped flux signifi-cantly. Niobium hydrides which formed during the cool down to cryogenic temperatures however were found to enhance trapping.

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THPB021

Trial of Nitrogen Infusion and Nitrogen Doping by Using J-PARC Furnace

ion, cavity, controls, SRF

775

T. Konomi, T. Dohmae, Y. Hori, E. Kako, T. Kubo, G.-T. Park, H. Sakai, K. Umemori
KEK, Ibaraki, Japan
J. Kamiya, K. Takeishi
JAEA/J-PARC, Tokai-mura, Japan
T. Nagata
ULVAC, Inc., Tsukuba, Japan

KEK has been carrying out SRF cavity developments toward higher Q-values and higher accelerating gradients. In the past nitrogen-doping was tested using the KEK furnaces, but it did not succeed. This time nitrogen infusion and nitrogen doping are tested using the J-PARC's furnace, which has an oil-free pumping system and is mainly pumped by a 10000 L/s cryopump and three 3000 L/s turbo pumps. Nitrogen pressure is controlled by a variable leak valve and an additional turbo pump. To avoid performance degradation during heat treatment, flanges of cavities are covered by Nb caps and foils. Nitrogen infusion at 120 degrees was applied to a single cell cavity and cavity performance was measured by vertical tests after HPR and assembly. Nitrogen doping at 800 degrees is also applied to another single cell cavity. After applying EP and HPR, vertical tests were carried out. Nb samples were also installed into the furnace during heat treatment. Surfaces are analyzed by SIMS and XPS. In this presentation, we report procedure of nitrogen infusion and doping, vertical test results and results of surface analysis.

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THPB024

Investigation on Depth Profiling of Niobium Surface Composition and Work Function of SRF Cavities

ion, cavity, electron, SRF

779

A.D. Wu, L. Chen, Q.W. Chu, H. Guo, Y. He, Y.M. Li, F. Pan, L. Yang, S.H. Zhang, H.W. Zhao
IMP/CAS, Lanzhou, People's Republic of China

The niobium samples were prepared by different surface treatments that commonly applied for the superconducting RF cavities preparation, as the following of electrochemical polishing, the buffered chemical polishing and high temperature annealing. In order to understand the property of niobium surface, especially the relationship between the composition and the work function value, the X-ray and ultraviolet photoelectron spectra depth profiling has been studied. The intensity photoelectrons signals of O1s, C1s and the Nb3d were identified for composition of the niobium oxide and the hydrocarbon contamination. And the work function of sample surface was measured via the means of the ultraviolet photoelectron spectra band width. To make a depth profiling, the sputtering of Argon ions was used to remove surface material gradually under by control the sputtering times. The results shown that the value of work function strongly depends on the chemical composition.

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THPB025

A Crystal Plasticity Study on Influence of Dislocation Mean Free Path on Stage II Hardening in Nb Single Crystals

ion, experiment, simulation, cavity

783

T. Maiti, A. Chakrabarty, P. Eisenlohr
MSU, East Lansing, USA
T.R. Bieler, D. Kang
Michigan State University, East Lansing, Michigan, USA

Funding: Financial support from the Department of Energy through grant DE-SC0009962 is gratefully acknowledged. This work was supported in part by MSU through computational resources provided by the ICER.
Constitutive models based on thermally-activated stress-assisted dislocation kinetics have been successful in predicting deformation behavior of crystalline materials, particularly in face-centered cubic (fcc) metals. In body-centered cubic (bcc) metals, success has been more or less limited, owing to ill-defined nature of slip planes and non-planar spreading of 1/2\hkl<111> screw dislocation cores. As a direct consequence of this, bcc metals show a strong dependence of flow stress on temperature and strain rate, and violation of Schmid law. We present high-resolution full-field crystal plasticity simulations of single crystal Niobium under tensile loading with an emphasis on multi-stage hardening, orientation dependence, and non-Schmid behavior. A dislocation density-based constitutive model with storage and recovery rates derived from Discrete Dislocation Dynamics is used to model strain hardening in stage II. The influence of dislocation mean free path and initial dislocation content on stage II hardening is simulated and compared with in-situ tensile experiments.

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THPB026

Investigation of the Effect of Strategically Selected Grain Boundaries on Superconducting Properties of SRF Cavity Niobium

ion, cavity, SRF, cryogenics

787

M. Wang, T.R. Bieler
Michigan State University, East Lansing, Michigan, USA
S. Balachandran, P.J. Lee
NHMFL, Tallahassee, Florida, USA
S. Chetri, A. Polyanskii
ASC, Tallahassee, Florida, USA
C. Compton
FRIB, East Lansing, Michigan, USA

Funding: Research supported by DOE/OHEP contracts DE-SC0009962, DE-SC0009960, NSF-DMR-1157490, and the State of Florida.
High purity Nb is commonly used for fabricating SRF cavities due to its high critical temperature and its formability. However, microstructural defects such as dislocations and grain boundaries in niobium can serve as favorable sites for pinning centers of magnetic flux that can degrade SRF cavity performance. In this study, two bi-crystal niobium samples extracted from strategically selected grain boundaries were investigated for the effect of grain misorientation on magnetic flux behavior. Laue X-ray and EBSD-OIM crystallographic analyses were used to characterize grain orientations and orientation gradients. Cryogenic Magneto-Optical Imaging (MOI) was used to directly observe magnetic flux penetration at about 5-8 K. Flux penetration was observed along one of the grain boundaries, as well as along a low angle boundary that was not detected prior to MOI imaging. Hydride scars on the sample surface after MOI were examined using atomic force microscopy (AFM) analysis. The relationships between dislocation content, cryo-cooling, flux penetration and grain boundaries are examined.

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THPB027

Characterization of Microstructural Defects in SRF Cavity Niobium using Electron Channeling Contrast Imaging

ion, cavity, electron, SRF

792

M. Wang, T.R. Bieler, D. Kang
Michigan State University, East Lansing, Michigan, USA
C. Compton
FRIB, East Lansing, Michigan, USA

Funding: Research supported by DOE/OHEP contract DE-SC0009962
Although the quality factor of niobium cavities has improved, performance variability arises from microstructural defects such as dislocations and grain boundaries that can trap magnetic flux, block heat transfer, and perturb superconducting currents. Microstructural defect evolution is compared in four samples extracted from a 2.8 mm thick large-grain niobium slice, with tensile axes chosen to generate desired dislocation structures during deformation. The four samples are 1) as-extracted, 2) extracted and annealed, 3) extracted and then deformed to 40% strain, and 4) extracted, annealed at 800 °C 2 hours, and deformed to 40% strain. Electron Channeling Contrast Imaging (ECCI) was performed on all samples to characterize initial dislocation density, dislocation structure evolution due to annealing and deformation, and related to the mechanical behavior observed in stress-strain curves. The orientation evolution and geometrically necessary dislocation (GND) density were characterized with electron backscattered diffraction (EBSD) maps. Fundamental understanding of dislocation evolution in niobium is necessary to develop models for computational cavity design.

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THPB028

Flux Pinning Study of OTIC Niobium Material

ion, site, cavity, superconducting-cavity

797

S. Chen, J.K. Hao, L. Lin, K.X. Liu, S.W. Quan
PKU, Beijing, People's Republic of China

The performance of superconducting cavities is influenced by the trapped flux during the cooling down through critical temperature, especially for nitrogen doped cavities which are more sensitive to flux trapping. We have investigated the flux trapping of OTIC niobium samples with different grain size. Samples were prepared and heat treated at 800°C and 900°C, followed with different surface removal by BCP. A series of measurements, including MPMS, TOF-SIMS, were carried out on the niobium samples. The results and analysis will be presented.

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THPB029

XPS Studies of Nitrogen Doping Nb Samples before and after GCIB Etching

ion, cavity, SRF, electron

802

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

The surface chemical composition of nitrogen doping Nb samples used for the fabrication of superconducting radio frequency (SRF) cavities, followed by the subsequent successive EP with different amounts of material removal, has been studied by XPS. The chemical composition of Nb, O, C and N was presented before and after Gas Cluster Ion Beams (GCIB) etching. No signals of bad superconducting nitrides NbNx was found in any doped and un-doped samples before etching. However, in the depth range greater than 30nm, the content of N elements is below the XPS detection precision scope even in the samples directly after nitrogen doping treatment.

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THPB030

Direct Observation of Hydrides Formation of Nitrogen Doping Nb Samples

ion, experiment, cavity, electron

805

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

Direct observation of hydrides precipitates formation on both nitrogen doped and un-doped Nb samples at 80K has been carried out using Scanning Electron Microscope (SEM) with Cold Stand. We have found that, under our experimental conditions, when the subsequent EP removal is less than 7μm, the amounts of hydrides formed on the surface of doped samples can be effectively reduced. When the subsequent material removal is larger than 9μm, the amounts of precipitated hydrides increased with the EP removal. When the EP removal is 7-9μm, the amounts of hydrides can still be effectively reduced. Also, more hydrides were precipitated on the surface of un-doped samples. The amounts of hydrides of doped samples may be reduced to varying degrees with different amounts of material removal.

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB030

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THPB031

Magnetic Properties of Nitrogen Doping Niobium Samples

ion, cavity, SRF, experiment

809

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

Nitrogen doping study on Niobium samples used for the fabrication of superconducting radio frequency (SRF) cavities was carried out. The samples' surface treatments were attempted to replicate that of the cavities, which included heavy electropolishing (EP), nitrogen doping and the subsequent successive EP with different amounts of material removal. The magnetization curves of both doped and un-doped samples have been measured, from which the lower critical field Hffp (First Flux Penetration, ffp) and upper critical field Hc2 was extracted. The thermodynamic critical field Hc, superheating field Hsh and superconducting parameters of samples with different treatments was calculated from the determined reversible magnetization curves. Hsh of doped samples is obviously smaller than that of un-doped samples, which may be a possible reason for the reduction of achievable accelerating gradient in SRF niobium cavities after nitrogen doping treatments.

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THPB034

Development of High Purity Niobium Components and Cavities for SRF Accelerator

cavity, ion, electron, SRF

814

T. Nagata, H. Masui, S. Shinozawa
ULVAC, Inc, Chiba, Japan
H. Inoue, E. Kako, M. Yamanaka
KEK, Ibaraki, Japan
M. Murakami
ULVAC, Inc., Tsukuba, Japan

Comprehensive cavity fabrication process from Niobium ingot was investigated. In order to purify ingots, A 600 kW electron beam furnace was introduced in ULVAC. It makes possible the stable quality of Niobium sheets and tubes. In evaluation of chemical components, mechanical properties, and RRR of our materials, all the value satisfies the ASTM Type 5 (superconducting grade) specification. In this study, we performed the trial manufacturing of welding-type and seamless-type cavities were made of our high purity Niobium ingots (RRR > 300). Accelerating gradient over 40 MV/m was shown in both cavities. We also tried to manufacture a 3-cell seamless cavity as scale up study. A seamless tube with a length of 830 mm, an inner diameter of 131 mm and a thickness of 3.5 mm was prepared. We succeed in direct forming from tube to cavity shape by using a hydroforming process. Cavity surface could be smoother than that of single cell cavity caused by small crystal grain size.

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB034

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THPB036

Fundamental SIMS Analyses for Nitrogen-enriched Niobium

ion, cavity, interface, instrumentation

821

J. Tuggle, M.J. Kelley
Virginia Polytechnic Institute and State University, Blacksburg, USA
M.J. Kelley, U. Pudasaini
The College of William and Mary, Williamsburg, Virginia, USA
M.J. Kelley, A.D. Palczewski, C.E. Reece
JLab, Newport News, Virginia, USA
F.A. Stevie
NCSU AIF, Raleigh, North Carolina, USA

Funding: Co-Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. W&M and VT work supported by the Office of High Energy Physics, U.S. DOE under grant DE-SC-0014475
In order to fully understand nitrogen addition techniques it is vital to have a full understanding of the material, including the content, location, and speciation of nitrogen contained in the treated Nb. In this work Secondary Ion Mass Spectrometry (SIMS) is used to elucidate content and location. Dynamic SIMS nitrogen analysis is reported, for the first time, for "as-received" cavity grade niobium from three separate suppliers. In addition, a number of method and instrumental issues are discussed including depth resolution, detection limit, and quantification.

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THPB041

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

ion, cavity, detector, electron

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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THPB042

Field-dependence of the Sensitivity to Trapped Flux in Nb3Sn

ion, cavity, site, SRF

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

ion, cavity, SRF, linac

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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THPB044

Update on Sample Host Cavity Design Work for Measuring Flux Entry and Quench Field

cavity, ion, SRF, radio-frequency

851

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

Current state-of-the-art Niobium superconducting radio-frequency (SRF) accelerator cavities have reached surface magnetic field close to the theoretical maximum set by the superheating field. Further increasing accelerating gradients will require new superconducting materials for accelerator cavities that are capable of supporting higher surface magnetic fields. This necessitates measuring the quench fields of new materials in high power RF fields. Previous work at Cornell University has used electromagnetic simulations to optimize the shape of a dipole mode sample host cavity such that the surface magnetic fields on the sample are high compared to the energy inside the cavity and the surface magnetic field on the rest of the cavity. In this paper we present an update of the design that includes how to mount samples in the cavity and the addition of a low field chamber.

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THPB053

Surface Resistance Characterization of Nb3Sn Using the HZB Quadrupole Resonator

ion, SRF, quadrupole, cavity

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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THPB055

Plasma-enhanced ALD System for SRF Cavity

ion, cavity, plasma, SRF

870

S. Kato, H. Hayano
KEK, Ibaraki, Japan

Funding: This work was supported by Photon and Quantum Basic Research Coordinated Development Program by MEXT and partly by JST-CREST.
A remote PEALD (Plasma-enhanced Atomic Layer Deposition) system which would offer a high conformality and a low deposition temperature has been being developed for deposition of NbN on an SRF cavity. The deposition equipment consists of a deposition chamber, a remote plasma exciter, a precursor supply system, vacuum pumps, a quartz crystal microbalance (QCM) as a film growth rate meter, a detoxifying system and a safety system. An RF frequency of 13.56MHz was used for the inductively coupled plasma exciter of a reactant gas. The whole equipment is in a draft booth for operation safety. For ALD of an SRF cavity, the ALD system allows us to easily replace the deposition chamber with a single cell Nb cavity. The prepared precursors are tris[ethylmethylamido][tert-butylimido] niobium (TBTEMN) and trimethylaluminium (TMA). Ammonia, hydrogen and water are also prepared as reactants. We will report the ALD system design and result of the NbN deposition on sample coupons which are analysed with SEM, EDX and XPS.

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THPB057

Investigation of Nucleation Stage in Diffusion Coating of Nb3Sn on Nb

ion, experiment, SRF, cavity

873

F. Pan
IMP/CAS, Lanzhou, People's Republic of China

Nb3Sn has the potential to improve properties of SRF cavities, such as the gradients and the working tempera-tures. Institute of Modern Physics has launched its Nb3Sn thin film coated SRF cavity project in 2016. Samples have been successfully coated to study the process of tin vapor diffusion. The main part of the deposition system is a tube furnace, which working temperature can reach 1100°C. Basic material characterization of the Sn-Nb film will be presented in this work.

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THPB066

Introducing the Vertical High-temperature UHV Furnace of the S-DALINAC for Future Cavity Material Studies

ion, vacuum, cavity, linac

891

R. Grewe, L. Alff, M. Arnold, J. Conrad, S. Flege, M. Major, N. Pietralla
TU Darmstadt, Darmstadt, Germany
F. Hug
IKP, Mainz, Germany

Funding: Work supported by the Federal Ministry of Education and Research through grant No. 05H15RDRBA.
Since 2005 the Institute for Nuclear Physics in Darmstadt operates a high temperature UHV furnace for temperatures of up to 1750°C. It has been used several times for hydrogen bake-out of the SRF cavities of the S-DALINAC with proven success. In 2013, studies at FNAL have shown that cavities treated with nitrogen reached an up to four times higher q-factor*. The cavities are exposed to N2 at 850°C at the end of the H2 bake-out. A thin layer of normalconducting hexagonal niobium nitride (NbN) forms at the surface which is removed by electropolishing while the higher quality factors are attributed to the N2 diffusion into the bulk Nb. At temperatures from 1300°C to 1700°C a thin layer of the superconducting cubic phase of NbN can be observed, e.g. delta-phase NbN**, which has a higher critical field and higher critical temperature and thus is very intereresting for applications for SRF cavities***. The UHV furnace has been prepared for future treatments of Nb samples and cavities in a N2 atmosphere at high temperatures for research on cubic NbN. The material properties of the samples will be analyzed at the ATFT group at the Department for Material Sciences of TU Darmstadt.
*Grasselino et al., Superconducting Science and Technology, 2013
**Hennessey et al., Oxidation of Metals, 1992
***Martienssen et al., Springer Handbook of Condensed Matter and Materials Data, 2005

Poster THPB066 [3.024 MB]

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THPB070

Electrochemical Finishing Treatment of Nb3Sn Diffusion-coated Niobium

ion, cavity, experiment, SRF

900

U. Pudasaini, M.J. Kelley
The College of William and Mary, Williamsburg, Virginia, USA
G.V. Eremeev, M.J. Kelley, C.E. Reece, H. Tian
JLab, Newport News, Virginia, USA

Funding: Supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics under contract DEAC0506OR23177 and Office of High Energy Physics under grant DESC0014475.
Nb3Sn cavities are now routinely prepared by depositing few micron thick Nb3Sn coatings on Nb cavities using tin vapor diffusion process. For the¬¬ case of niobium there is a significant improvement after electropolishing (EP), but electrochemical finishing treatment on Nb3Sn coatings has not been studied. Controlled removal of the first few layers could lead to a smoother and cleaner surface that is conducive to better RF performance. Several samples, which were coated with Nb3Sn by vapor diffusion process in a JLab sample chamber, were used to explore polishing parameters, such as I-V characteristics, removal rate, topography, etc. Preliminary results from the first runs are discussed here.

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THPB078

Fabrication of Large-area MgB2 Films on Copper Substrates

ion, cavity, SRF, radio-frequency

917

X. Guo, K.X. Liu, Z. Ni
PKU, Beijing, People's Republic of China
Q. Feng, C. Yang
Peking University, Beijing, People's Republic of China

Magnesium diboride (MgB2) is a promising candidate material for SRF cavities because of its higher transition temperature and critical field compared with niobium. To meet the demand of RF test devices, the fabrication of large-area MgB2 films on metal substrates is needed. In this work, MgB2 films with 50-mm diameter were fabri-cated on Cu substrates by using an improved HPCVD setups at Peking University. The transition temperatures of MgB2 film on Cu substrate and with Mo buffer layer on Cu substrate are 36.2 K and 36.5 K, respectively. The fabrication processes, surface morphology, superconduct-ing properties of these large-area MgB2 films are present-ed.

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THPB079

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

ion, cavity, simulation, feedback

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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