SRF2017 - List of Keywords (electron)

Paper	Title	Other Keywords	Page
MOXA03	The 30MeV Stage of the ARIEL e-linac	ion, cavity, linac, cryomodule	6
	R.E. Laxdal, Z.T. Ang, T. Au, K. Fong, O.K. Kester, S.R. Koscielniak, A.N. Koveshnikov, M.P. Laverty, Y. Ma, D.W. Storey, E. Thoeng, Z.Y. Yao, Q. Zheng, V. Zvyagintsev TRIUMF, Vancouver, Canada
	A MW class cw superconducting electron linac (e-Linac) is being installed at TRIUMF as a driver for radioactive beam production as part of the ARIEL project. The e-linac final configuration is planned to consist of five 1.3GHz nine-cell cavities housed in three cryomodules with one single cavity injector cryomodule (EINJ) and two double cavity accelerating cryomodules (EACA, EACB) to accelerate in continuous-wave (cw) up to 10mA of electrons to 50MeV. The e-Linac is being installed in stages. A demonstrator phase (2014) consisting of a 300kV electron gun, EINJ, and a partially outfitted EACA with just one accelerating cavity was installed for initial technical and beam tests to 22.9MeV. A Stage 2 upgrade now installed has a completed EACA to reach an operational goal of 3mA of electrons to 30MeV for first science from the ARIEL ISOL targets. A single 290kW klystron is used to feed the two EACA cavities in vector-sum closed-loop control. The paper is focused on the SRF challenges: systems design, cavity and cryomodule performance, rf ancillaries preparation and performance, LLRF and RF system performance and final beam test results.
	Slides MOXA03 [13.981 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-MOXA03
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MOPB002	eRHIC Crab Cavity Choice for Ring-ring Design	ion, cavity, luminosity, impedance	43
	Q. Wu, I. Ben-Zvi, Y. Hao, S. Verdú-Andrés, B. P. Xiao BNL, Upton, Long Island, New York, USA
	Funding: This work was supported by Brookhaven Science Associates, LLC under Contract No. DE-AC02-98CH10886 with the U.S. DOE. The future electron ion collider eRHIC adopts large crossing angle (22 mrad) to allow fast separation of two beams in the ring-ring scheme. Crab cavities are required to recover the luminosity from geometric losses. Initial calculation shows that the frequency of the cavities for the ion beam is no more than 336MHz. In this paper, we discuss the crab cavity related lattice parameters for both ion and electron beams in ring-ring design, the frequency choice, and the cavity design considerations.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-MOPB002
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MOPB054	Design of Fundamental Power Coupler for High Intensity Heavy-ion Accelerator	ion, cavity, simulation, multipactoring	183
	B. Bing, T.M. Huang, Q. Ma, F. Meng, W.M. Pan IHEP, Beijing, People's Republic of China K.X. Gu Institute of High Energy Physics (IHEP), Chinese Academy of Sciences, Beijing, People's Republic of China
	A single-window coaxial coupler at warm has been designed for high intensity heavy-ion accelerator. The coupler is designed to handle 100 kW CW power of 325 MHz and is currently being fabricated. T-bend transition and doorknob have been taken into account. The length of the T-bend short circuit is sensitive to S parameters and contributes to the online adjustment of VSWR in RF conditioning. The doorknob type is adopted to realize the transition from a half-height WR 2300 waveguide to a coaxial line ended with a coupling antenna. This paper describes the RF design, thermal stress and heat load analysis of the coupler as well as multipacting simula-tions.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-MOPB054
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MOPB064	High Power Test for Plug-compatible STF-type Power Coupler for ILC	ion, vacuum, simulation, GUI	199
	Y. Yamamoto, E. Kako, T. Matsumoto, S. Michizono, A. Yamamoto KEK, Ibaraki, Japan C. Julie, E. Montesinos CERN, Geneva, Switzerland
	From the view point of plug-compatibility for the power coupler in the ILC, recommended by Linear Collider Collaboration in 2013, new STF-type power couplers with 40mm of input port diameter were re-designed, fabricated and successfully high-power-tested. Moreover, from the view point of the cost reduction for the ILC, another type of power couplers with TiN coating-free ceramic were also fabricated and high-power-tested by the collaboration between CERN and KEK. In this paper, the detailed results for the both power couplers will be presented.
	Poster MOPB064 [6.671 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-MOPB064
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MOPB065	Development of Hybrid Superconducting Photocathodes on Niobium Using High QE Coatings	ion, cathode, site, SRF	205
	M. Warren, A.R. Denchfield, N. Samuelson, J. Zasadzinski IIT, Chicago, Illinois, USA W. Gai, J.H. Shao, E.E. Wisniewski ANL, Argonne, Illinois, USA L.K. Spentzouris, Z.M. Yusof Illinois Institute of Technology, Chicago, Illinois, USA
	High power, low emittance electron beams require superconducting RF photoinjectors, typically made of pure Nb, and a superconducting photocathode is desired. However, superconductivity and high photocathode quantum efficiency (QE) are not compatible, e.g. QE for pure Nb is only 10^-5 at 260 nm wavelength. Here is presented the current status of the development of hybrid superconducting photocathodes by the deposition of thin films of a high QE metal or semiconductor on Nb. Nb plugs coated with 10-100 nm of Mg have been tested for adhesion and dark current under RF fields as high as 60MV/m. QE measurements show significant enhancements over Nb. In another test, ultra thin films of the high QE material Cs2Te deposited on Nb are reported. Using the standard deposition procedure, QE ~12% is found for films ~ 200Å. As the thickness is reduced QE maintains a high value ~ 6% for films as thin as 2.0 nm. These results are quite promising for future superconducting photocathodes.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-MOPB065
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MOPB070	The Improvement of the Power Coupler for CADS SC Spoke Cavities	ion, cavity, experiment, cryomodule	220
	T.M. Huang, B. Bing, X. Chen, H.Y. Lin, Q. Ma, F. Meng, W.M. Pan, G.W. Wang IHEP, Beijing, People's Republic of China K.X. Gu Institute of High Energy Physics (IHEP), Chinese Academy of Sciences, Beijing, People's Republic of China
	Twenty superconducting spoke cavities mounted in three cryomodules (CM1, CM2 and CM4) were in-stalled in the CADS, a test facility of 10 mA, 25 MeV CW proton linac. Each cavity was equipped with one coaxial type fundamental power coupler (FPC). Fatal window crack was observed during the test cryomod-ule (TCM) commissioning. A series of experiments were subsequently implemented and eventually at-tributed the window crack to the electron bombard-ment from cavity field emission (FE). Improvements covering the coupler cleaning and assembly proce-dure, the structure and position modifications were thus implemented, aiming to reduce the cavity contam-ination and avoid the window damaged by cavity FE electrons. This paper will describe how the coupler window damaged by cavity field emission and the improve-ments for cure. In addition, the performances of FPCs for CM1, CM2 and CM4 were compared.
	Poster MOPB070 [0.613 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-MOPB070
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MOPB080	The Stable Operation of MPG and Measurement of Output	ion, experiment, cavity, cathode	254
	B.T. Li, X.Y. Lu, L. Xiao, D.Y. Yang, Y. Yang, Z.Q. Yang, J. Zhao PKU, Beijing, People's Republic of China
	The concept of micro-pulse electron gun(MPG) was proposed decades ago. It can provide electron beam with high current, short pulse and low emittance. But it is still not put into practical use as electron source because of its unsteady operating state. This paper presents an experimental result of the steady running of MPG which can operate stably for more than ten hours. The energy spread of the electron beam is also measured, the peak is located at near 20eV and half width is less than 15eV .
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-MOPB080
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MOPB082	A Preliminary Scheme for X-ray Emission Based on Micro-pulse Electron Gun	ion, target, radiation, simulation	259
	Y. Yang, B.T. Li, X.Y. Lu, W.W. Tan, L. Xiao, D.Y. Yang, Z.Q. Yang, J. Zhao PKU, Beijing, People's Republic of China
	X-ray is now widely used in many areas of physics, biology, chemistry and materials. And how to achieve emission, monochrome, and focusing of x-ray is of great significance to study. Micro-pulse electron gun (MPG) is a new type of electron source, with characteristics of high repetition frequency, short-pulse and low cost. Generating x-ray with better monochromaticity is one of the potential applications of MPG. And a preliminary scheme of X-ray based on MPG is proposed in this paper. The scheme is designed by comparing different anode materials and the thickness of filters. The simulation results based on the software MCNP5 show that the proposed scheme can effectively improve the monotonicity of the generated X-rays.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-MOPB082
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TUYAA01	The Importance of the Electron Mean Free Path for Superconducting RF Cavities	cavity, ion, niobium, 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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TUPB001	338 MHz Crab Cavity Design for the eRHIC Hadron Beam	cavity, ion, proton, HOM	382
	S. Verdú-Andrés, I. Ben-Zvi, Q. Wu, B. P. Xiao BNL, Upton, Long Island, New York, USA I. Ben-Zvi Stony Brook University, Stony Brook, USA
	Funding: Work supported by the US Department of Energy via Brookhaven Science Associates LLC under contract no. DE-AC02-98CH10886. Crab crossing is an essential mechanism to restore high luminosity and avoid synchro-betatron resonances in the electron-hadron collider eRHIC. The current ring-ring eRHIC design envisages a set of crab cavities operating at 338 MHz. This set of cavities will provide the crabbing kick to the hadron beam of eRHIC. Double-Quarter Wave (DQW) cavities are compact, superconducting RF deflecting cavities appropriate for crab crossing. This paper summarizes the main design requirements and presents an optimized RF design of a DQW cavity for the crabbing system of the ring-ring eRHIC hadron beam.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB001
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TUPB010	Multipactor Study in the Coupler Region of the Diamond SCRF Cavities	ion, cavity, pick-up, GUI	405
	S.A. Pande, C. Christou, P. Gu DLS, Oxfordshire, United Kingdom G. Burt Cockcroft Institute, Lancaster University, Lancaster, United Kingdom
	The Diamond storage ring operates with two CESR-B type Superconducting RF cavities. The cavities suffer from trips with a sudden loss of accelerating field if operated above a certain voltage. Consequently the cavities are operated at voltages up to 1.4 MV for better reliability. These cavities are iris coupled and have fixed Qext. At these lower operating voltages, the optimum condition for beam loading is satisfied at powers around 100 kW. For operation at 300 mA with two cavities, the power needed per system exceeds 200 kW. Therefore 3 stub tuners are used to lower the Qext to move the optimum condition close to 200kW. Additionally, the step due to the difference in the height of the coupling waveguide on the cavity and that of the vacuum side waveguide on the window results in a standing wave between the cavity and the window even at matched operation. The 3 stub tuner further enhances this standing wave. Numerical simulation reveals that the standing wave field from the cavity penetrates into the coupling waveguide increasing the probability of multipactor and breakdown in the coupler region. The results of multipactor simulations in this region with CST Studio are discussed.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB010
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TUPB046	Experience on Design, Fabrication and Testing of a Large Grain ESS Medium Beta Prototype Cavity	ion, cavity, radiation, niobium	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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TUPB047	Passband Modes Excitation Triggered by Field Emission in ESS Medium Beta Cavity Prototype	ion, cavity, site, simulation	489
	J.F. Chen, M. Bertucci, A. Bosotti, P. Michelato, L. Monaco, R. Paparella, D. Sertore INFN/LASA, Segrate (MI), Italy M. Eshraqi, M. Lindroos, S. Pirani ESS, Lund, Sweden C. Pagani Università degli Studi di Milano & INFN, Segrate, Italy T.P.Å. Åkesson Lund University, Department of Physics, Lund, Sweden
	During the first vertical test of ESS Medium Beta large-grain prototype cavity in INFN-LASA, a phenomenon of coexisting two passband-modes was observed – 4π/6 mode was excited spontaneously during the power rise of 3π/6 mode. This phenomenon is most likely due to the field-emission electrons that transfer their energy gained from the 3π/6 mode to the 4π/6 mode. In this paper, we present the experimental results, the excitation mechanism and the related simulation results.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB047
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TUPB071	Test Result of 650 MHz, Beta 0.61 Single-cell Niobium Cavity	cavity, ion, niobium, accelerating-gradient	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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TUPB087	Low Temperature and Low Pressure Plasma for the HWR Superconducting Cavity In-situ Cleaning	ion, plasma, cavity, experiment	595
	A.D. Wu, W. Chang, H. Guo, Y. He, C.L. Li, Y.M. Li, P.R. Xiong, L. Yang, W.M. Yue, S.H. Zhang, H.W. Zhao IMP/CAS, Lanzhou, People's Republic of China F. Gou Sichuan University, Chengdu, People's Republic of China
	The glow discharge for low temperature and low pressures plasma were utilized for the half-wave resonator (HWR) superconducting cavity in-situ cleaning. The plasma was on ignition of the Argon/Oxygen mixture atmosphere, which was under the low pressure of 0.5 to 5.0 Pascal. Driven by the RF power with the frequency of the cavity fundamental mode, the plasma showed the typical characteristic of the typical RF glow discharge, which the temperature of the electrons about 1eV that diagnosed by the optical emission spectrum. The experimental parameters for the discharge were optimized to obtain the uniform plasma distribution on the HWR cavity, including the RF power, the atmospheric pressure and the oxygen proportion. At last, the vertical cryogenic test was completed to investigate the impact of active oxygen plasma cleaning on the HWR cavity performance recovery, which contaminated by hydrocarbons. The test proves that the glow plasma clean can relieve the x-ray radiation which caused by the field electron emission effect.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB087
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TUPB108	Genesis of Topography in Buffered Chemical Polishing of Niobium for Application to Superconducting Radiofrequency Accelerator Cavities	ion, niobium, cavity, 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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TUPB113	Surface Characterization of Nitrided Niobium Surfaces	ion, niobium, cavity, 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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WEXA03	High-performance Thin-film Niobium Produced via Chemical Vapor Deposition (CVD)	niobium, ion, cavity, SRF	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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THPB002	Role of Nitrogen on Hydride Nucleation in Pure Niobium by First Principles Calculations	ion, niobium, site, cavity	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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THPB003	An Innovative Design of a Flexible Temperature-mapping System	cavity, ion, SRF, superconducting-cavity	746
	M. Ge, F. Furuta, M. Liepe, P.J. Pamel Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	A temperature-mapping (T-Map) system is an essential tool for fundamental SRF research as it provides spatial information of RF power dissipation and so allows localizing hot-spots on a cavity surface at cryogenic temperatures. However, the temperature sensors are mounted on rigid boards in most current systems, so each can only work for one specific cavity size and shape. In this paper, we proposed a flexible design, which allows this temperature mapping system to work for different cavity shapes.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB003
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THPB024	Investigation on Depth Profiling of Niobium Surface Composition and Work Function of SRF Cavities	ion, niobium, cavity, 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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THPB027	Characterization of Microstructural Defects in SRF Cavity Niobium using Electron Channeling Contrast Imaging	ion, cavity, niobium, 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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THPB029	XPS Studies of Nitrogen Doping Nb Samples before and after GCIB Etching	ion, niobium, cavity, SRF	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, niobium, experiment, cavity	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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THPB034	Development of High Purity Niobium Components and Cavities for SRF Accelerator	cavity, ion, SRF, niobium	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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THPB041	Cavity Quench Studies in Nb3Sn Using Temperature Mapping and Surface Analysis of Cavity Cut-outs	ion, cavity, detector, niobium	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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THPB061	Effect Of Dislocations On the Thermal Conductivity Of Superconducting Nb	ion, scattering, ECR, cavity	886
	P. Xu, N.T. Wright MSU, East Lansing, Michigan, USA T.R. Bieler Michigan State University, East Lansing, Michigan, USA
	Funding: This work is funded by DOE and OHEP through grant number DE-FOA-0001438. The thermal conductivity of Niobium (Nb) often experiences a local maximum (a phonon peak) at a temperature between 1.8 and 3 K. While the magnitude of the phonon peak has been shown to be related to the dislocation density and may be influenced by manufacturing processes, little has been discussed as to the temperature at which the peak occurs. In examining these phenomena, it has been determined that more explicit accounting of phonon–dislocation scattering in a popular model better represents the thermal conductivity at temperatures colder than 3 K. Scaled sensitivity coefficients show this term to have similar influence as the phonon-electron and phonon-boundary scattering terms. Results using the enhanced model also show an apparent threshold of dislocation density below which there is little contribution to the thermal conductivity of Nb.
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THPB075	GaN-based Photocathodes for High Brightness Electron Beams	ion, cathode, target, experiment	906
	M. Schumacher, X. Jiang, M. Vogel University Siegen, Siegen, Germany
	Funding: This research is funded by the Federal Ministry of Education and Research of Germany in the framework of HOPE II (project number 05K16PS1). Prospective light sources requires photocathodes with high quantum efficiency (QE), long lifetime and minimized thermal emittance. One promising candidate meeting the aforementioned specifications is GaN. Due to its wide band gap (Eg = 3,4 eV), GaN can be excited by UV-light sources. Its thermal and chemical stability are added bonuses. In the framework of the present activity, the synthesis of GaN films on Si, Cu, Mo and Nb by means of rf magnetron sputtering is proposed. In this context, Ga, GaAs and GaN are suitable source material candidates, which are sputtered in a nitrogen/argon plasma discharge. The conductivity as well as the band-gap of the corresponding films can be modified by dopants like Mg and In, respectively. Standard materials science characterization techniques such as SEM, EDX, XRD or XPS are used to explore the growth mechanism of GaN alongside with a morphological and chemical examination. To assess and optimize the performance of the photocathode the abovementioned requirements are tested in an in-situ setup. In addition to the project outline, first experimental results of GaN coatings synthesized based on a GaAs source sputtered in pure N2 are presented.
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THPB076	Carbon-based Coatings for Electron Cloud Mitigation in SRF Photocathodes	ion, SRF, gun, cathode	910
	M. Vogel, X. Jiang, C. Schlemper University Siegen, Siegen, Germany
	Funding: This research has been funded by the Federal Ministry of Education and Research of Germany in the framework of HOPE I (project number 05K13PS2). During the past three years, we developed a coating along with a corresponding in-situ characterization process in order to realize SRF-gun surfaces featuring low secondary electron yield (SEY). Important aspects that have been accounted for are the homogeneity and adhesion of the coatings deposited on the cylindrical SRF-gun mantle. Furthermore, the correlation between SEY and crystallinity, morphology, and contamination was studied in detail. The SEY maximum can be tuned between 1.5 and less than 0.7 depending on the deposition conditions. In this work, we recap the results and present a general strategy for the effective mitigation of electron cloud multiplication.
	Poster THPB076 [5.583 MB]
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Paper

Title

Other Keywords

Page

MOXA03

The 30MeV Stage of the ARIEL e-linac

ion, cavity, linac, cryomodule

6

R.E. Laxdal, Z.T. Ang, T. Au, K. Fong, O.K. Kester, S.R. Koscielniak, A.N. Koveshnikov, M.P. Laverty, Y. Ma, D.W. Storey, E. Thoeng, Z.Y. Yao, Q. Zheng, V. Zvyagintsev
TRIUMF, Vancouver, Canada

A MW class cw superconducting electron linac (e-Linac) is being installed at TRIUMF as a driver for radioactive beam production as part of the ARIEL project. The e-linac final configuration is planned to consist of five 1.3GHz nine-cell cavities housed in three cryomodules with one single cavity injector cryomodule (EINJ) and two double cavity accelerating cryomodules (EACA, EACB) to accelerate in continuous-wave (cw) up to 10mA of electrons to 50MeV. The e-Linac is being installed in stages. A demonstrator phase (2014) consisting of a 300kV electron gun, EINJ, and a partially outfitted EACA with just one accelerating cavity was installed for initial technical and beam tests to 22.9MeV. A Stage 2 upgrade now installed has a completed EACA to reach an operational goal of 3mA of electrons to 30MeV for first science from the ARIEL ISOL targets. A single 290kW klystron is used to feed the two EACA cavities in vector-sum closed-loop control. The paper is focused on the SRF challenges: systems design, cavity and cryomodule performance, rf ancillaries preparation and performance, LLRF and RF system performance and final beam test results.

Slides MOXA03 [13.981 MB]

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MOPB002

eRHIC Crab Cavity Choice for Ring-ring Design

ion, cavity, luminosity, impedance

43

Q. Wu, I. Ben-Zvi, Y. Hao, S. Verdú-Andrés, B. P. Xiao
BNL, Upton, Long Island, New York, USA

Funding: This work was supported by Brookhaven Science Associates, LLC under Contract No. DE-AC02-98CH10886 with the U.S. DOE.
The future electron ion collider eRHIC adopts large crossing angle (22 mrad) to allow fast separation of two beams in the ring-ring scheme. Crab cavities are required to recover the luminosity from geometric losses. Initial calculation shows that the frequency of the cavities for the ion beam is no more than 336MHz. In this paper, we discuss the crab cavity related lattice parameters for both ion and electron beams in ring-ring design, the frequency choice, and the cavity design considerations.

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MOPB054

Design of Fundamental Power Coupler for High Intensity Heavy-ion Accelerator

ion, cavity, simulation, multipactoring

183

B. Bing, T.M. Huang, Q. Ma, F. Meng, W.M. Pan
IHEP, Beijing, People's Republic of China
K.X. Gu
Institute of High Energy Physics (IHEP), Chinese Academy of Sciences, Beijing, People's Republic of China

A single-window coaxial coupler at warm has been designed for high intensity heavy-ion accelerator. The coupler is designed to handle 100 kW CW power of 325 MHz and is currently being fabricated. T-bend transition and doorknob have been taken into account. The length of the T-bend short circuit is sensitive to S parameters and contributes to the online adjustment of VSWR in RF conditioning. The doorknob type is adopted to realize the transition from a half-height WR 2300 waveguide to a coaxial line ended with a coupling antenna. This paper describes the RF design, thermal stress and heat load analysis of the coupler as well as multipacting simula-tions.

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MOPB064

High Power Test for Plug-compatible STF-type Power Coupler for ILC

ion, vacuum, simulation, GUI

199

Y. Yamamoto, E. Kako, T. Matsumoto, S. Michizono, A. Yamamoto
KEK, Ibaraki, Japan
C. Julie, E. Montesinos
CERN, Geneva, Switzerland

From the view point of plug-compatibility for the power coupler in the ILC, recommended by Linear Collider Collaboration in 2013, new STF-type power couplers with 40mm of input port diameter were re-designed, fabricated and successfully high-power-tested. Moreover, from the view point of the cost reduction for the ILC, another type of power couplers with TiN coating-free ceramic were also fabricated and high-power-tested by the collaboration between CERN and KEK. In this paper, the detailed results for the both power couplers will be presented.

Poster MOPB064 [6.671 MB]

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MOPB065

Development of Hybrid Superconducting Photocathodes on Niobium Using High QE Coatings

ion, cathode, site, SRF

205

M. Warren, A.R. Denchfield, N. Samuelson, J. Zasadzinski
IIT, Chicago, Illinois, USA
W. Gai, J.H. Shao, E.E. Wisniewski
ANL, Argonne, Illinois, USA
L.K. Spentzouris, Z.M. Yusof
Illinois Institute of Technology, Chicago, Illinois, USA

High power, low emittance electron beams require superconducting RF photoinjectors, typically made of pure Nb, and a superconducting photocathode is desired. However, superconductivity and high photocathode quantum efficiency (QE) are not compatible, e.g. QE for pure Nb is only 10^-5 at 260 nm wavelength. Here is presented the current status of the development of hybrid superconducting photocathodes by the deposition of thin films of a high QE metal or semiconductor on Nb. Nb plugs coated with 10-100 nm of Mg have been tested for adhesion and dark current under RF fields as high as 60MV/m. QE measurements show significant enhancements over Nb. In another test, ultra thin films of the high QE material Cs2Te deposited on Nb are reported. Using the standard deposition procedure, QE ~12% is found for films ~ 200Å. As the thickness is reduced QE maintains a high value ~ 6% for films as thin as 2.0 nm. These results are quite promising for future superconducting photocathodes.

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MOPB070

The Improvement of the Power Coupler for CADS SC Spoke Cavities

ion, cavity, experiment, cryomodule

220

T.M. Huang, B. Bing, X. Chen, H.Y. Lin, Q. Ma, F. Meng, W.M. Pan, G.W. Wang
IHEP, Beijing, People's Republic of China
K.X. Gu
Institute of High Energy Physics (IHEP), Chinese Academy of Sciences, Beijing, People's Republic of China

Twenty superconducting spoke cavities mounted in three cryomodules (CM1, CM2 and CM4) were in-stalled in the CADS, a test facility of 10 mA, 25 MeV CW proton linac. Each cavity was equipped with one coaxial type fundamental power coupler (FPC). Fatal window crack was observed during the test cryomod-ule (TCM) commissioning. A series of experiments were subsequently implemented and eventually at-tributed the window crack to the electron bombard-ment from cavity field emission (FE). Improvements covering the coupler cleaning and assembly proce-dure, the structure and position modifications were thus implemented, aiming to reduce the cavity contam-ination and avoid the window damaged by cavity FE electrons. This paper will describe how the coupler window damaged by cavity field emission and the improve-ments for cure. In addition, the performances of FPCs for CM1, CM2 and CM4 were compared.

Poster MOPB070 [0.613 MB]

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MOPB080

The Stable Operation of MPG and Measurement of Output

ion, experiment, cavity, cathode

254

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

The concept of micro-pulse electron gun(MPG) was proposed decades ago. It can provide electron beam with high current, short pulse and low emittance. But it is still not put into practical use as electron source because of its unsteady operating state. This paper presents an experimental result of the steady running of MPG which can operate stably for more than ten hours. The energy spread of the electron beam is also measured, the peak is located at near 20eV and half width is less than 15eV .

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MOPB082

A Preliminary Scheme for X-ray Emission Based on Micro-pulse Electron Gun

ion, target, radiation, simulation

259

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

X-ray is now widely used in many areas of physics, biology, chemistry and materials. And how to achieve emission, monochrome, and focusing of x-ray is of great significance to study. Micro-pulse electron gun (MPG) is a new type of electron source, with characteristics of high repetition frequency, short-pulse and low cost. Generating x-ray with better monochromaticity is one of the potential applications of MPG. And a preliminary scheme of X-ray based on MPG is proposed in this paper. The scheme is designed by comparing different anode materials and the thickness of filters. The simulation results based on the software MCNP5 show that the proposed scheme can effectively improve the monotonicity of the generated X-rays.

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TUYAA01

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

cavity, ion, niobium, 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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TUPB001

338 MHz Crab Cavity Design for the eRHIC Hadron Beam

cavity, ion, proton, HOM

382

S. Verdú-Andrés, I. Ben-Zvi, Q. Wu, B. P. Xiao
BNL, Upton, Long Island, New York, USA
I. Ben-Zvi
Stony Brook University, Stony Brook, USA

Funding: Work supported by the US Department of Energy via Brookhaven Science Associates LLC under contract no. DE-AC02-98CH10886.
Crab crossing is an essential mechanism to restore high luminosity and avoid synchro-betatron resonances in the electron-hadron collider eRHIC. The current ring-ring eRHIC design envisages a set of crab cavities operating at 338 MHz. This set of cavities will provide the crabbing kick to the hadron beam of eRHIC. Double-Quarter Wave (DQW) cavities are compact, superconducting RF deflecting cavities appropriate for crab crossing. This paper summarizes the main design requirements and presents an optimized RF design of a DQW cavity for the crabbing system of the ring-ring eRHIC hadron beam.

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TUPB010

Multipactor Study in the Coupler Region of the Diamond SCRF Cavities

ion, cavity, pick-up, GUI

405

S.A. Pande, C. Christou, P. Gu
DLS, Oxfordshire, United Kingdom
G. Burt
Cockcroft Institute, Lancaster University, Lancaster, United Kingdom

The Diamond storage ring operates with two CESR-B type Superconducting RF cavities. The cavities suffer from trips with a sudden loss of accelerating field if operated above a certain voltage. Consequently the cavities are operated at voltages up to 1.4 MV for better reliability. These cavities are iris coupled and have fixed Qext. At these lower operating voltages, the optimum condition for beam loading is satisfied at powers around 100 kW. For operation at 300 mA with two cavities, the power needed per system exceeds 200 kW. Therefore 3 stub tuners are used to lower the Qext to move the optimum condition close to 200kW. Additionally, the step due to the difference in the height of the coupling waveguide on the cavity and that of the vacuum side waveguide on the window results in a standing wave between the cavity and the window even at matched operation. The 3 stub tuner further enhances this standing wave. Numerical simulation reveals that the standing wave field from the cavity penetrates into the coupling waveguide increasing the probability of multipactor and breakdown in the coupler region. The results of multipactor simulations in this region with CST Studio are discussed.

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TUPB046

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

ion, cavity, radiation, niobium

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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TUPB047

Passband Modes Excitation Triggered by Field Emission in ESS Medium Beta Cavity Prototype

ion, cavity, site, simulation

489

J.F. Chen, M. Bertucci, A. Bosotti, P. Michelato, L. Monaco, R. Paparella, D. Sertore
INFN/LASA, Segrate (MI), Italy
M. Eshraqi, M. Lindroos, S. Pirani
ESS, Lund, Sweden
C. Pagani
Università degli Studi di Milano & INFN, Segrate, Italy
T.P.Å. Åkesson
Lund University, Department of Physics, Lund, Sweden

During the first vertical test of ESS Medium Beta large-grain prototype cavity in INFN-LASA, a phenomenon of coexisting two passband-modes was observed – 4π/6 mode was excited spontaneously during the power rise of 3π/6 mode. This phenomenon is most likely due to the field-emission electrons that transfer their energy gained from the 3π/6 mode to the 4π/6 mode. In this paper, we present the experimental results, the excitation mechanism and the related simulation results.

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TUPB071

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

cavity, ion, niobium, accelerating-gradient

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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TUPB087

Low Temperature and Low Pressure Plasma for the HWR Superconducting Cavity In-situ Cleaning

ion, plasma, cavity, experiment

595

A.D. Wu, W. Chang, H. Guo, Y. He, C.L. Li, Y.M. Li, P.R. Xiong, L. Yang, W.M. Yue, S.H. Zhang, H.W. Zhao
IMP/CAS, Lanzhou, People's Republic of China
F. Gou
Sichuan University, Chengdu, People's Republic of China

The glow discharge for low temperature and low pressures plasma were utilized for the half-wave resonator (HWR) superconducting cavity in-situ cleaning. The plasma was on ignition of the Argon/Oxygen mixture atmosphere, which was under the low pressure of 0.5 to 5.0 Pascal. Driven by the RF power with the frequency of the cavity fundamental mode, the plasma showed the typical characteristic of the typical RF glow discharge, which the temperature of the electrons about 1eV that diagnosed by the optical emission spectrum. The experimental parameters for the discharge were optimized to obtain the uniform plasma distribution on the HWR cavity, including the RF power, the atmospheric pressure and the oxygen proportion. At last, the vertical cryogenic test was completed to investigate the impact of active oxygen plasma cleaning on the HWR cavity performance recovery, which contaminated by hydrocarbons. The test proves that the glow plasma clean can relieve the x-ray radiation which caused by the field electron emission effect.

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TUPB108

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

ion, niobium, cavity, 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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TUPB113

Surface Characterization of Nitrided Niobium Surfaces

ion, niobium, cavity, 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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WEXA03

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

niobium, ion, cavity, SRF

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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THPB002

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

ion, niobium, site, cavity

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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THPB003

An Innovative Design of a Flexible Temperature-mapping System

cavity, ion, SRF, superconducting-cavity

746

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

A temperature-mapping (T-Map) system is an essential tool for fundamental SRF research as it provides spatial information of RF power dissipation and so allows localizing hot-spots on a cavity surface at cryogenic temperatures. However, the temperature sensors are mounted on rigid boards in most current systems, so each can only work for one specific cavity size and shape. In this paper, we proposed a flexible design, which allows this temperature mapping system to work for different cavity shapes.

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THPB024

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

ion, niobium, cavity, 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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THPB027

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

ion, cavity, niobium, 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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THPB029

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

ion, niobium, cavity, SRF

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, niobium, experiment, cavity

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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THPB034

Development of High Purity Niobium Components and Cavities for SRF Accelerator

cavity, ion, SRF, niobium

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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THPB041

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

ion, cavity, detector, niobium

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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THPB061

Effect Of Dislocations On the Thermal Conductivity Of Superconducting Nb

ion, scattering, ECR, cavity

886

P. Xu, N.T. Wright
MSU, East Lansing, Michigan, USA
T.R. Bieler
Michigan State University, East Lansing, Michigan, USA

Funding: This work is funded by DOE and OHEP through grant number DE-FOA-0001438.
The thermal conductivity of Niobium (Nb) often experiences a local maximum (a phonon peak) at a temperature between 1.8 and 3 K. While the magnitude of the phonon peak has been shown to be related to the dislocation density and may be influenced by manufacturing processes, little has been discussed as to the temperature at which the peak occurs. In examining these phenomena, it has been determined that more explicit accounting of phonon–dislocation scattering in a popular model better represents the thermal conductivity at temperatures colder than 3 K. Scaled sensitivity coefficients show this term to have similar influence as the phonon-electron and phonon-boundary scattering terms. Results using the enhanced model also show an apparent threshold of dislocation density below which there is little contribution to the thermal conductivity of Nb.

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THPB075

GaN-based Photocathodes for High Brightness Electron Beams

ion, cathode, target, experiment

906

M. Schumacher, X. Jiang, M. Vogel
University Siegen, Siegen, Germany

Funding: This research is funded by the Federal Ministry of Education and Research of Germany in the framework of HOPE II (project number 05K16PS1).
Prospective light sources requires photocathodes with high quantum efficiency (QE), long lifetime and minimized thermal emittance. One promising candidate meeting the aforementioned specifications is GaN. Due to its wide band gap (Eg = 3,4 eV), GaN can be excited by UV-light sources. Its thermal and chemical stability are added bonuses. In the framework of the present activity, the synthesis of GaN films on Si, Cu, Mo and Nb by means of rf magnetron sputtering is proposed. In this context, Ga, GaAs and GaN are suitable source material candidates, which are sputtered in a nitrogen/argon plasma discharge. The conductivity as well as the band-gap of the corresponding films can be modified by dopants like Mg and In, respectively. Standard materials science characterization techniques such as SEM, EDX, XRD or XPS are used to explore the growth mechanism of GaN alongside with a morphological and chemical examination. To assess and optimize the performance of the photocathode the abovementioned requirements are tested in an in-situ setup. In addition to the project outline, first experimental results of GaN coatings synthesized based on a GaAs source sputtered in pure N2 are presented.

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THPB076

Carbon-based Coatings for Electron Cloud Mitigation in SRF Photocathodes

ion, SRF, gun, cathode

910

M. Vogel, X. Jiang, C. Schlemper
University Siegen, Siegen, Germany

Funding: This research has been funded by the Federal Ministry of Education and Research of Germany in the framework of HOPE I (project number 05K13PS2).
During the past three years, we developed a coating along with a corresponding in-situ characterization process in order to realize SRF-gun surfaces featuring low secondary electron yield (SEY). Important aspects that have been accounted for are the homogeneity and adhesion of the coatings deposited on the cylindrical SRF-gun mantle. Furthermore, the correlation between SEY and crystallinity, morphology, and contamination was studied in detail. The SEY maximum can be tuned between 1.5 and less than 0.7 depending on the deposition conditions. In this work, we recap the results and present a general strategy for the effective mitigation of electron cloud multiplication.

Poster THPB076 [5.583 MB]

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