SRF2019 - List of Keywords (ECR)

Paper	Title	Other Keywords	Page
MOP019	Surface Preparation and Optimization of SC CH Cavities	cavity, operation, linac, coupling	71
	P. Müller, M. Basten, M. Busch, T. Conrad, H. Podlech IAP, Frankfurt am Main, Germany K. Aulenbacher, F.D. Dziuba, M. Miski-Oglu HIM, Mainz, Germany W.A. Barth GSI, Darmstadt, Germany
	The Institute of Applied Physics (IAP) introduced the superconducting multi-gap CH-structure, which is mainly designed for low beta hadron acceleration. In 2017, a 217 MHz sc CH-structure was successfully tested with beam at GSI and multiple CH-structures are currently under development for the GSI cw linac. RF performance of all sc cavities are limited by the surface properties of the used material. Therefore, sufficient surface preparation and optimization is necessary to achieve optimal performance. Presently as standard procedure BCP and HPR is used for CH-cavities. Several surface treatments will be applied to the very first CH-prototype, a 360 MHz, 19-cell cavity. Prior to the first treatment, the status of the cavity was examined, including leak tests and performance tests at 4 and 2 K. This paper presents the performance development of a sc CH cavity depending on different preparation methods.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP019
About •	paper received ※ 23 June 2019 paper accepted ※ 05 July 2019 issue date ※ 14 August 2019
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MOP029	N-Doping Studies With Single-Cell Cavities for the SHINE Project	cavity, niobium, SRF, FEL	102
	J.F. Chen, H.T. Hou, Y.F. Liu, D. Wang, Y. Wang SARI-CAS, Pudong, Shanghai, People’s Republic of China Y.W. Huang ShanghaiTech University, Shanghai, People’s Republic of China Z. Wang SINAP, Shanghai, People’s Republic of China
	The SHINE SRF accelerator is designed to operate in CW mode with more than six hundred superconducting cavities. In order to reduce the high cost of construction and operation of the cryogenic system, high-Q cavities with nitrogen-doping technology together with tradition-ally treated large-grain cavities have been considered as two possible options. In this paper, we present N-doping studies on single-cell cavities fabricated with fine-grain and large-grain niobium.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP029
About •	paper received ※ 23 June 2019 paper accepted ※ 29 June 2019 issue date ※ 14 August 2019
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MOP031	Investigation of Frequency Behavior Near Tc of Niobium Superconducting Radio-Frequency Cavities	cavity, SRF, niobium, electron	112
	D. Bafia, J. Zasadzinski IIT, Chicago, Illinois, USA D. Bafia, M. Checchin, A. Grassellino, O.S. Melnychuk, A.S. Romanenko, D.A. Sergatskov Fermilab, Batavia, Illinois, USA
	This paper will present a systematic investigation of the resonant frequency behavior of niobium SRF cavities subject to different surface processing (nitrogen doping, nitrogen infusion, 120°C bake, EP, etc.) near the critical transition temperature. We find features occurring in frequency versus temperature (FvsT) data near Tc that seem to vary with surface processing. Emphasis is placed on one of the observed features: a dip in the superconducting resonant frequency below the normal conducting value which is prominent in nitrogen doped cavities and appears to be a signature of nitrogen doping. This gives further insights on the mechanisms responsible for the large increase in performance of cavities subject to this surface treatment. The magnitude of this dip in frequency is studied and related to possible physical parameters such as the concentration of impurities near the surface and the design resonant frequency of the cavity. A possible explanation for the meaning of this dip is discussed, namely, that it is a result of strong coupling between electrons and phonons within the resonator.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP031
About •	paper received ※ 23 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019
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MOP032	Effect of Low Temperature Infusion Heat Treatments and "2/0" Doping on Superconducting Cavity Performance	cavity, niobium, SRF, background	118
	P.N. Koufalis, M. Ge, M. Liepe, J.T. Maniscalco Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Under specific circumstances, low temperature infusion heat treatments of niobium cavities have resulted in the ubiquitous "Q-rise". This is an increase in quality factor with increasing field strength or equivalently a decrease in the temperature-dependent component of the surface resistance. We investigate the results of various infusion conditions with infusion bake time as a free parameter. To study the very near surface effects of infusion, we employ HF rinsing, light VEP, and oxypolishing to remove several or tens of nm at a time. We present results from RF performance tests of low temperature infusion heat treated niobium cavities, and correlate these with SIMS impurity depth profiles obtained from witness samples. We also present results of a cavity doped at 800 C with the "2/0" recipe.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP032
About •	paper received ※ 26 June 2019 paper accepted ※ 02 July 2019 issue date ※ 14 August 2019
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MOP039	Nitrogen Doping Studies of Superconducting Cavities at Peking University	cavity, niobium, accelerating-gradient, superconducting-cavity	141
	S. Chen, M. Chen, L.W. Feng, J.K. Hao, L. Lin, K.X. Liu, S.W. Quan, F. Wang, F. Zhu PKU, Beijing, People’s Republic of China
	Nitrogen doping studies with 1.3 GHz superconducting cavities were carried out at Peking University in recent years. We have realized 4×10¹⁰ of high quality factor at 12 MV/m and 2.0 K with large grain single cell cavities by heavy doping. To improve the accelerating gradient of high Q cavities, light doping recipe is adopted. Accelerating gradient is improved to 20 MV/m and the quality factor is larger than 3×10¹⁰ at 16 MV/m and 2.0 K for light doped cavities. The nitrogen treatment, test and analysis are presented in this paper.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP039
About •	paper received ※ 21 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019
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MOP083	R&D of Copper Electroplating Process for Power Couplers: Effect of Microstructures on RRR	SRF, experiment, target, electron	278
	Y. Okii, J. Taguchi Nomura Plating Co, Ltd., Osaka, Japan E. Kako, S. Michizono, Y. Yamamoto KEK, Ibaraki, Japan H. Takahashi, H. Yasutake CETD, Tochigi, Japan
	Power couplers for superconducting cavities are required to have both low-thermal conductivity and high-electrical conductivity, because high-thermal conductivity and low-electrical conductivity could generate unexpected increase for heat load. In order to combine these contrary properties, power couplers are made of stainless steel and plated with copper plating. As electrical conductivity of copper layer affects dynamic heat load, it is crucial to optimize plating processes. In this study, we investigated influences of plating parameters (i.e., thickness of copper layer, plating bath composition, bath temperature, heat-treatment conditions) on RRR by collaborative work among Nomura plating, CETD, and KEK. As a result, we obtained high-RRR samples with conditions noted below; (1) electroformed copper plate, (2) copper layer thickness of over 50 µm, and (3) heat-treatment at 200deg-1h, (4) other plating bath composition. In addition, we observed microstructures of several samples, then found that microstructures of copper layer are strongly related to RRR. In this paper, we will present the recent results for this investigation.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP083
About •	paper received ※ 23 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019
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TUFUA1	The Field-Dependent Surface Resistance of Doped Niobium: New Experimental and Theoretical Results	cavity, SRF, niobium, electron	340
	J.T. Maniscalco, M. Ge, P.N. Koufalis, M. Liepe Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA T. Arias, D. Liarte, J.P. Sethna, N. Sitaraman Cornell University, Ithaca, New York, USA
	We present systematic work investigating how different doping and post-doping treatments affect the BCS surface resistance at 1.3~GHz and higher frequencies. We examine the field-dependent BCS resistance at many temperatures as well as the field-dependent residual resistance and use the results to reveal how impurity species and concentration levels affect the field-dependent RF properties. We further demonstrate the importance of thermal effects and their direct dependence on doping level. We use the tools of Density Functional Theory to work towards an {\em ab initio} model of electron overheating to theoretically confirm the impact of doping, create a full model that includes thermal effects to predict the field dependent resistance, and show that the predictions of the model agree with results from doped and non-doped cavities ({\em e.g.} the strength of the anti-Q-slope and the high-field Q slope). Finally, we use our experimental results to systematically assess and compare theories of the field-dependent BCS resistance, showing that the current theory on smearing of the density of states is incomplete.
	Slides TUFUA1 [6.780 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUFUA1
About •	paper received ※ 01 July 2019 paper accepted ※ 02 July 2019 issue date ※ 14 August 2019
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TUFUA4	New Insights on Nitrogen Doping	cavity, electron, SRF, niobium	347
	D. Bafia, J. Zasadzinski IIT, Chicago, Illinois, USA D. Bafia, M. Checchin, A. Grassellino, O.S. Melnychuk, A.S. Romanenko, D.A. Sergatskov Fermilab, Batavia, Illinois, USA
	This paper covers a systematic study of the quench in nitrogen doped cavities: a cavity was sequentially treated/reset with different N-doping recipes which are known to produce different levels of quench field. Analysis of cavity heating profiles using TMAP are used to gain insight on the origins of quench; new recipes demonstrate the possibility to increase quench fields well beyond 30 MV/m. In addition, a new signature of nitrogen doping is explored, namely, a dip in the superconducting resonant frequency below the normal conducting value just below the critical transition temperature, giving further insights on the mechanisms responsible for the large increase in performance of cavities subject to this surface treatment.
	Slides TUFUA4 [3.097 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUFUA4
About •	paper received ※ 23 June 2019 paper accepted ※ 03 July 2019 issue date ※ 14 August 2019
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TUFUB7	Measurement of Surface Resistance Properties with Coaxial Resonators - Review	cavity, multipactoring, experiment, SRF	374
	H. Park, S.U. De Silva, J.R. Delayen ODU, Norfolk, Virginia, USA
	Achieving ever decreasing surface resistance at higher field in superconducting RF accelerating structures is one of most outstanding developments in modern accelerators. The BCS theory has been used widely to estimate the surface resistance and to direct the technology. However, recent research results show that the behavior of the surface resistance further deviates from the BCS theory. So far the study on surface resistance was performed usually with cavities of single frequency which limited the study of frequency dependent surface resistance. The Center for Accelerator Science at Old Dominion University has designed and built several half wave coaxial cavities to study the frequency, temperature, and RF field dependence of surface resistance. TRIUMF in Canada also joined this line of research using such multi frequency quarter wave and half wave coaxial cavities. This type of multi mode cavity will allow us to systematically study the parameters affecting surface resistance on the same cavity surface. In this paper, we review the results ODU and TRIUMF collected so far and proper analysis methods.
	Slides TUFUB7 [3.551 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUFUB7
About •	paper received ※ 01 July 2019 paper accepted ※ 12 July 2019 issue date ※ 14 August 2019
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TUP031	Heat Treatment for Jacketed Half-wave Resonator Cavity	cavity, cryomodule, vacuum, SRF	482
	Y. Jung, B.H. Choi, D.H. Gil, M.O. Hyun, H. Kim, J.W. Kim, M.S. Kim, D.Y. Lee, J. Lee, S. Lee IBS, Daejeon, Republic of Korea
	Vertical tests of a prototype half-wave resonator cavity are being tested. The performance of the cavities, such as quality factor and accelerating electrical field, are measured and compared to the target design value. In previous study, we reported the effect of the heat treatment on a prototype bare HWR cavity. We baked a jacketed HWR cavity to improve a performance for 10 h at 650°C. In this study, we will report the effect of the heat treatment on the jacketed HWR cavity.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP031
About •	paper received ※ 23 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019
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TUP042	Measurement of Mechanical Vibration of SRILAC Cavities	cavity, niobium, linac, cryomodule	513
	O. Kamigaito, K. Ozeki, N. Sakamoto, K. Suda, K. Yamada RIKEN Nishina Center, Wako, Japan
	Mechanical vibration of quarter-wavelength resonators of SRILAC, the superconducting booster of the RIKEN heavy-ion linac, was measured during a vertical cold test. The measurements were performed for fully assembled cavities as well as for bare niobium cavities without the titanium jacket. In the procedure, the instantaneous resonant frequencies were measured for 10 seconds at a time interval of 1 ms and were recorded as a time series. The frequencies were analyzed by means of conventional signal analysis. The power spectrum was deduced from the autocorrelation function calculated with the fluctuation of resonant frequencies. Although the vibration amplitudes were smaller in the cavities assembled with the titanium jacket, we could not find a clear reason for this.
	Poster TUP042 [6.957 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP042
About •	paper received ※ 27 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019
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TUP050	A Multi-layered SRF Cavity for Conduction Cooling Applications	cavity, SRF, cryogenics, niobium	538
	G. Ciovati, G. Cheng, E. Daly, G.V. Eremeev, J. Henry, R.A. Rimmer JLab, Newport News, Virginia, USA I.P. Parajuli ODU, Norfolk, Virginia, USA U. Pudasaini The College of William and Mary, Williamsburg, Virginia, USA
	Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. Some of the work was supported by the 2008 PECASE Award of G. Ciovati. I. Parajuli is supported by NSF Grant PHYS-100614-010 Industrial application of SRF technology would favor the use of cryocoolers to conductively cool SRF cavities for particle accelerators, operating at or above 4.3 K. In order to achieve a lower surface resistance than Nb at 4.3 K, a superconductor with higher critical temperature should be used, whereas a metal with higher thermal conductivity than Nb should be used to conduct the heat to the cryocoolers. A standard 1.5 GHz bulk Nb single-cell cavity has been coated with a ~2 µm thick layer of Nb₃Sn on the inner surface and with a 5 mm thick Cu layer on the outer surface for conduction cooled applications. The cavity performance has been measured at 4.3 K and 2.0 K in liquid He. The cavity reached a peak surface magnetic field of ~40 mT with a quality factor of 6×10⁹ and 3.5×10⁹ at 4.3 K, before and after applying the thick Cu layer, respectively.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP050
About •	paper received ※ 21 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019
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TUP055	Nonlinear Dynamics and Dissipation of Vortex Lines Driven by Strong RF Fields	cavity, radio-frequency, niobium, electron	560
	M.R.P. Walive Pathiranage, A.V. Gurevich ODU, Norfolk, Virginia, USA
	Trapped vortices can contribute significantly to a residual surface resistance of superconducting radio frequency (SRF) cavities but the nonlinear dynamics of flexible vortex lines driven by strong rf currents has not been well understood. Here we report extensive numerical simulations of large-amplitude oscillations of a trapped vortex line under the strong rf magnetic field. The rf power dissipated by an oscillating vortex segment driven by the rf Meissner currents was calculated by taking into account the nonlinear vortex line tension, vortex mass and a nonlinear Larkin-Ovchinnikov and overheating viscous drag force. We calculated the field dependence of the surface resistance Rs and showed that at low frequencies Rs(H) increases with H but as the frequency increases, Rs(H) becomes a non-monotonic function of H which decreases with H at higher fields. These results suggest that trapped vortices can contribute to the extended Q(H) rise observed on the SRF cavities.
	Poster TUP055 [1.744 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP055
About •	paper received ※ 23 June 2019 paper accepted ※ 05 July 2019 issue date ※ 14 August 2019
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TUP059	Investigation of Trapped Flux Dynamics via DC-Magnetic Quenching	cavity, experiment, superconducting-cavity, SRF	580
	P. Nuñez von Voigt, J. Knobloch, O. Kugeler HZB, Berlin, Germany J. Knobloch University of Siegen, Siegen, Germany
	Trapped magnetic flux increases the surface resistance in superconducting radio-frequency cavities. A better understanding of its behaviour could help to develop a method of expelling trapped flux from the superconducting surface. Using a superconducting coil with ferrite core attached to a 3 GHz sample Niobium cavity fully immersed in liquid Helium, we were able to subject the cavity walls to unusually large magnetic fields (estimated > 150 mT) and create magnetic quenches. With Fluxgate sensors attached in three spatial directions inside the cavity, we were able to monitor the quench dynamics and extract parameters of the flux dynamics from the hysteretic behaviour of the measured fields resulting from the applied coil current. First results of manipulation of the trapped flux with high magnetic fields are presented.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP059
About •	paper received ※ 24 June 2019 paper accepted ※ 05 July 2019 issue date ※ 14 August 2019
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TUP061	Gradients of 50 MV/m in TESLA Shaped Cavities via Modified Low Temperature Bake	cavity, niobium, SRF, site	586
	D. Bafia, J. Zasadzinski IIT, Chicago, Illinois, USA D. Bafia, A. Grassellino, O.S. Melnychuk, A.S. Romanenko, Z-H. Sung Fermilab, Batavia, Illinois, USA
	This paper will discuss the 75/120 C modified low temperature bake capable of giving unprecedented accel-erating gradients of above 50 MV/m for 1.3 GHz TESLA-shaped niobium SRF cavities in CW operation. A bifurca-tion in the Q₀ vs E_acc curve is observed after retesting cavities without disassembly in between, yielding per-formance that ranges from exceptional to above state-of-the-art. Atomic Force Microscopy studies on cavity cut-outs gives a possible mechanism responsible for this branching in performance, namely, the dissociation and growth of room temperature niobium nano-hydrides that exist near the RF surface, which are made superconduct-ing only through the proximity effect. In-situ low temper-ature baking of cavity cutouts reveals a dissociation of these room temperature nano-hydrides, which could ex-plain the higher performance of cavities subject to similar in-situ heating in the dewar.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP061
About •	paper received ※ 23 June 2019 paper accepted ※ 03 July 2019 issue date ※ 14 August 2019
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THFUA1	RF Characterization of an S-I-S’ Multilayer Sample	niobium, quadrupole, SRF, cavity	800
	S. Keckert, J. Knobloch, O. Kugeler, D.B. Tikhonov HZB, Berlin, Germany A-M. Valente-Feliciano JLab, Newport News, Virginia, USA
	S-I-S’ multilayers promise to boost the performance of bulk superconductors in terms of maximum field and surface resistance. At HZB, RF-surface resistance measurements were performed with a Quadrupole Resonator (QPR) and an S-I-S’ sample (75 nm NbTiN on 15 nm AlN insulator on bulk Nb) prepared at JLab. Measurements were performed at 414, 845, and 1286 MHz at sample temperatures from 2 K up to well above the transition temperature of NbTiN of ~17.3 K. The Rs exhibits an unexpected temperature dependence: Rather than rising monotonically, as expected from BCS theory, a local maximum is observed. There is a temperature range where Rs decreases with increasing temperature. Such behavior indicates that an additional interaction between the superconducting layers may have to be included in the surface resistance model. Measurements of the baseline Nb sample prior to coating exhibited no such behavior; hence systematic measurement errors can be excluded as the explanation. The maximum field was limited by a hard magnetic quench near 20 mT, close to Hc1 of NbTiN, suggesting that the sample is limited by early flux penetration.
	Slides THFUA1 [1.004 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THFUA1
About •	paper received ※ 22 June 2019 paper accepted ※ 03 July 2019 issue date ※ 14 August 2019
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THP010	The Mechanism of Electropolishing of Niobium from Choline Chloride-based Deep Eutectic Solvents	polarization, impedance, niobium, cavity	852
	Q.W. Chu, H. Guo, S.C. Huang, A.D. Wu, Z.M. You IMP/CAS, Lanzhou, People’s Republic of China
	Funding: National Natural Science Foundation (11705252) The mechanism of electropolishing of niobium (Nb) from choline chloride-based deep eutectic solvent (DES) was studied by anodic polarization tests and electrochemical impedance spectroscopy (EIS) using a Nb rotating disk electrode (RDE). Based on the results of an anodic polarisation test, the electropolishing of Nb is mass transport controlled. EIS results are consistent with the compact salt film mechanism for niobium electropolishing in this electrolyte. The influence of rotation rate, applied potential and electrolyte temperature on the electropolishing mechanism of Nb was investigated. As the applied potential positively shift, Rct, Rp and L increase, CPE decrease and Rs unchanged. The increase in rotation rate and electrolyte temperature leads to a decrease of Rs, Rct, Rp and L, and an increase of CPE.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP010
About •	paper received ※ 18 June 2019 paper accepted ※ 02 July 2019 issue date ※ 14 August 2019
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THP011	Niobium Semiproducts for the Superconducting Strands and SRF Cavities in Russia	niobium, SRF, cavity, target	857
	M.V. Alekseev, I.M. Abdyukhanov, V.A. Drobyshev, M.V. Kravtsova, M.V. Krylova, P.A. Lykianov, K.A. Mareev, V.I. Pantsyrny, M.V. Polikarpova, M.M. Potapenko, A.G. Silaev, A.S. Tsapleva SC A A Bochvar High-Technology Research Institute of Inorganic Materials, Moscow, Russia M.Y. Shlyakhov, S.M. Zernov JSC - TVEL, Moscow, Russia
	The melting regimes of the niobium ingots with high chemical purity and low hardness for the Nb₃Sn, NbTi and other superconducting materials manufacture have been developed at SC "VNIINM". Using this niobium material and by the SC "VNIINM" manufacture regimes at the SC "Chepetsky Mechanical Plant" 220 tons of Nb₃Sn and NbTi strands for ITER and 12 km of Nb₃Sn strands for HL-LHC (CERN) with the required characteristics have been successfully produced. The review of the characteristics of the different semiproducts (sheets, tubes, rods), made in Russia from the special grade niobium, and of the superconducting strands, manufactured with the use of them, is presented in the paper. The ways of the further improvement of the niobium ingots melting regimes and niobium sheets deformation and annealing regimes with the target of achieving RRR > 300 for the SRF cavities application are discussed.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP011
About •	paper received ※ 23 June 2019 paper accepted ※ 04 July 2019 issue date ※ 14 August 2019
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THP012	Assessment of the Mechanical Properties of Ultra-High Purity Niobium After Cold Work and Heat Treatment With the HL-LHC Crab Cavities as Benchmark	cavity, operation, niobium, simulation	860
	A. Gallifa Terricabras, A. Amorim Carvalho, I. Aviles Santillana, S. Barrière, R. Calaga, E. Cano-Pleite, O. Capatina, M.D. Crouvizier, L. Dassa, M.S. Meyer, N. Valverde Alonso CERN, Geneva, Switzerland M. Benke, A.B. Palotas, G. Szabó, M. Szűcs University of Miskolc, Faculty of Materials Science and Engineering, Miskolc-Egyetemváros, Hungary A. Hlavács, G.J. Krallics, V. Mertinger, M. Sepsi University of Miskolc, Miskolc, Hungary
	The High Luminosity Large Hadron Collider (HL-LHC) is the upgrade of the world’s largest particle collider; it will allow the full exploitation of the LHC potential and its operation beyond 2025. An essential part of the HL-LHC project are the Crab Cavities, that are particle deflecting SRF cavities of non-axisymmetric shape made of bulk ultra-high purity Nb. Since the cavities are produced by complex metal sheet forming processes, followed by a heat treatment (HT) for H outgassing (650 °C, 24 h), there is uncertainty on their mechanical properties after manufacturing and in service conditions (2 K). Mechanical tests at room temperature have been conducted on RRR300 pure Nb samples. The samples were previously submitted, by cold cross-rolling, to different levels of plastic deformation representative of the effective plastic strain seen by the Nb sheets during forming operations. Moreover, a comparison of the mechanical properties of cold cross-rolled samples before and after HT has been established. Results of evolution of the microstructure and hardness are also presented. This study can be of interest for Nb cavities to be sub-mitted to HT at 650 °C, and may help to push the design of novel SRF cavities.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP012
About •	paper received ※ 22 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019
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THP042	Initial Results from Investigations into Different Surface Peparation Techniques of OFHC Copper for SRF Applications	site, cavity, experiment, SRF	941
	S.B. Leith, X. Jiang, Z. Khalil, A.S.H. Mohamed, M. Vogel University Siegen, Siegen, Germany
	Funding: This work forms part of the EASITrain research programme. This Marie Sklodowska-Curie Action (MSCA) Innovative Training Networks (ITN) has received funding from the European Union¿s H₂020 Framework Programme under Grant Agreement no. 764879 As part of efforts to improve the performance of thin film coated accelerating cavities, improvement of the topography of the surface of copper is being pursued. This is known to strongly affect the properties of the deposited superconducting thin film. This study focuses on determining the optimal procedure to enhance homogeneity and smoothness of the copper surface. OFHC copper substrates have been processed using mechanical polishing (MP), chemical polishing (CP) and electropolishing (EP) procedures as well as a combina-tion thereof. The parameters of each of the procedures have been tested and optimised to produce the smoothest surface possible. The resulting samples have been analysed using a scanning electron microscope, a laser profilometer and a confocal microscope. Results indicate the superior per-formance of electrochemical polishing over chemical polishing in terms of planarization efficiency, while a combination of mechanical polishing followed by electropolishing provides the most homogeneous and smooth surface when utilising the critical current density of the electrolyte.
	Poster THP042 [1.190 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP042
About •	paper received ※ 22 June 2019 paper accepted ※ 29 June 2019 issue date ※ 14 August 2019
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THP043	Deposition Parameter Effects on Niobium Nitride (NbN) Thin Films Deposited Onto Copper Substrates with DC Magnetron Sputtering	site, cathode, cavity, niobium	945
	S.B. Leith, X. Jiang, M. Vogel University Siegen, Siegen, Germany R. Ries, E. Seiler Slovak Academy of Sciences, Institute of Electrical Engineering, Bratislava, Slovak Republic
	Funding: The EASITrain project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant No 764879. As part of efforts to improve the performance of SRF cavities, to that prescribed by future operating requirements, alternative materials are currently being investigated. NbN is one of the alternatives under investigation to provide these better performance figures. In this contribution, a summary of results from an investigation into DC magnetron sputtered NbN thin films deposited onto copper substrates is presented. The copper substrates were prepared using a mechanical polishing process, followed by a chemical etching process. The NbN films were prepared in a large scale commercial coating system. A high and low value for the substrate temperature, process pressure, bias voltage, cathode power, nitrogen gas percentage, and the working gas type, using either Argon or Krypton, constitute the parameters of this study. The base pressure of the system prior to deposition was 5x10⁷ hPa for all coatings. The resulting films have been characterised using various surface characterisation methods to determine the effects of the deposition parameters during the film growth process. The deposition parameters have been optimised based on the characterisation results.
	Poster THP043 [1.169 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP043
About •	paper received ※ 23 June 2019 paper accepted ※ 29 June 2019 issue date ※ 14 August 2019
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MOP019

Surface Preparation and Optimization of SC CH Cavities

cavity, operation, linac, coupling

71

P. Müller, M. Basten, M. Busch, T. Conrad, H. Podlech
IAP, Frankfurt am Main, Germany
K. Aulenbacher, F.D. Dziuba, M. Miski-Oglu
HIM, Mainz, Germany
W.A. Barth
GSI, Darmstadt, Germany

The Institute of Applied Physics (IAP) introduced the superconducting multi-gap CH-structure, which is mainly designed for low beta hadron acceleration. In 2017, a 217 MHz sc CH-structure was successfully tested with beam at GSI and multiple CH-structures are currently under development for the GSI cw linac. RF performance of all sc cavities are limited by the surface properties of the used material. Therefore, sufficient surface preparation and optimization is necessary to achieve optimal performance. Presently as standard procedure BCP and HPR is used for CH-cavities. Several surface treatments will be applied to the very first CH-prototype, a 360 MHz, 19-cell cavity. Prior to the first treatment, the status of the cavity was examined, including leak tests and performance tests at 4 and 2 K. This paper presents the performance development of a sc CH cavity depending on different preparation methods.

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-MOP019

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paper received ※ 23 June 2019 paper accepted ※ 05 July 2019 issue date ※ 14 August 2019

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MOP029

N-Doping Studies With Single-Cell Cavities for the SHINE Project

cavity, niobium, SRF, FEL

102

J.F. Chen, H.T. Hou, Y.F. Liu, D. Wang, Y. Wang
SARI-CAS, Pudong, Shanghai, People’s Republic of China
Y.W. Huang
ShanghaiTech University, Shanghai, People’s Republic of China
Z. Wang
SINAP, Shanghai, People’s Republic of China

The SHINE SRF accelerator is designed to operate in CW mode with more than six hundred superconducting cavities. In order to reduce the high cost of construction and operation of the cryogenic system, high-Q cavities with nitrogen-doping technology together with tradition-ally treated large-grain cavities have been considered as two possible options. In this paper, we present N-doping studies on single-cell cavities fabricated with fine-grain and large-grain niobium.

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

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MOP031

Investigation of Frequency Behavior Near Tc of Niobium Superconducting Radio-Frequency Cavities

cavity, SRF, niobium, electron

112

D. Bafia, J. Zasadzinski
IIT, Chicago, Illinois, USA
D. Bafia, M. Checchin, A. Grassellino, O.S. Melnychuk, A.S. Romanenko, D.A. Sergatskov
Fermilab, Batavia, Illinois, USA

This paper will present a systematic investigation of the resonant frequency behavior of niobium SRF cavities subject to different surface processing (nitrogen doping, nitrogen infusion, 120°C bake, EP, etc.) near the critical transition temperature. We find features occurring in frequency versus temperature (FvsT) data near Tc that seem to vary with surface processing. Emphasis is placed on one of the observed features: a dip in the superconducting resonant frequency below the normal conducting value which is prominent in nitrogen doped cavities and appears to be a signature of nitrogen doping. This gives further insights on the mechanisms responsible for the large increase in performance of cavities subject to this surface treatment. The magnitude of this dip in frequency is studied and related to possible physical parameters such as the concentration of impurities near the surface and the design resonant frequency of the cavity. A possible explanation for the meaning of this dip is discussed, namely, that it is a result of strong coupling between electrons and phonons within the resonator.

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

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MOP032

Effect of Low Temperature Infusion Heat Treatments and "2/0" Doping on Superconducting Cavity Performance

cavity, niobium, SRF, background

118

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

Under specific circumstances, low temperature infusion heat treatments of niobium cavities have resulted in the ubiquitous "Q-rise". This is an increase in quality factor with increasing field strength or equivalently a decrease in the temperature-dependent component of the surface resistance. We investigate the results of various infusion conditions with infusion bake time as a free parameter. To study the very near surface effects of infusion, we employ HF rinsing, light VEP, and oxypolishing to remove several or tens of nm at a time. We present results from RF performance tests of low temperature infusion heat treated niobium cavities, and correlate these with SIMS impurity depth profiles obtained from witness samples. We also present results of a cavity doped at 800 C with the "2/0" recipe.

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paper received ※ 26 June 2019 paper accepted ※ 02 July 2019 issue date ※ 14 August 2019

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MOP039

Nitrogen Doping Studies of Superconducting Cavities at Peking University

cavity, niobium, accelerating-gradient, superconducting-cavity

141

S. Chen, M. Chen, L.W. Feng, J.K. Hao, L. Lin, K.X. Liu, S.W. Quan, F. Wang, F. Zhu
PKU, Beijing, People’s Republic of China

Nitrogen doping studies with 1.3 GHz superconducting cavities were carried out at Peking University in recent years. We have realized 4×10¹⁰ of high quality factor at 12 MV/m and 2.0 K with large grain single cell cavities by heavy doping. To improve the accelerating gradient of high Q cavities, light doping recipe is adopted. Accelerating gradient is improved to 20 MV/m and the quality factor is larger than 3×10¹⁰ at 16 MV/m and 2.0 K for light doped cavities. The nitrogen treatment, test and analysis are presented in this paper.

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paper received ※ 21 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019

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MOP083

R&D of Copper Electroplating Process for Power Couplers: Effect of Microstructures on RRR

SRF, experiment, target, electron

278

Y. Okii, J. Taguchi
Nomura Plating Co, Ltd., Osaka, Japan
E. Kako, S. Michizono, Y. Yamamoto
KEK, Ibaraki, Japan
H. Takahashi, H. Yasutake
CETD, Tochigi, Japan

Power couplers for superconducting cavities are required to have both low-thermal conductivity and high-electrical conductivity, because high-thermal conductivity and low-electrical conductivity could generate unexpected increase for heat load. In order to combine these contrary properties, power couplers are made of stainless steel and plated with copper plating. As electrical conductivity of copper layer affects dynamic heat load, it is crucial to optimize plating processes. In this study, we investigated influences of plating parameters (i.e., thickness of copper layer, plating bath composition, bath temperature, heat-treatment conditions) on RRR by collaborative work among Nomura plating, CETD, and KEK. As a result, we obtained high-RRR samples with conditions noted below; (1) electroformed copper plate, (2) copper layer thickness of over 50 µm, and (3) heat-treatment at 200deg-1h, (4) other plating bath composition. In addition, we observed microstructures of several samples, then found that microstructures of copper layer are strongly related to RRR. In this paper, we will present the recent results for this investigation.

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

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TUFUA1

The Field-Dependent Surface Resistance of Doped Niobium: New Experimental and Theoretical Results

cavity, SRF, niobium, electron

340

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

We present systematic work investigating how different doping and post-doping treatments affect the BCS surface resistance at 1.3~GHz and higher frequencies. We examine the field-dependent BCS resistance at many temperatures as well as the field-dependent residual resistance and use the results to reveal how impurity species and concentration levels affect the field-dependent RF properties. We further demonstrate the importance of thermal effects and their direct dependence on doping level. We use the tools of Density Functional Theory to work towards an {\em ab initio} model of electron overheating to theoretically confirm the impact of doping, create a full model that includes thermal effects to predict the field dependent resistance, and show that the predictions of the model agree with results from doped and non-doped cavities ({\em e.g.} the strength of the anti-Q-slope and the high-field Q slope). Finally, we use our experimental results to systematically assess and compare theories of the field-dependent BCS resistance, showing that the current theory on smearing of the density of states is incomplete.

Slides TUFUA1 [6.780 MB]

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paper received ※ 01 July 2019 paper accepted ※ 02 July 2019 issue date ※ 14 August 2019

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TUFUA4

New Insights on Nitrogen Doping

cavity, electron, SRF, niobium

347

D. Bafia, J. Zasadzinski
IIT, Chicago, Illinois, USA
D. Bafia, M. Checchin, A. Grassellino, O.S. Melnychuk, A.S. Romanenko, D.A. Sergatskov
Fermilab, Batavia, Illinois, USA

This paper covers a systematic study of the quench in nitrogen doped cavities: a cavity was sequentially treated/reset with different N-doping recipes which are known to produce different levels of quench field. Analysis of cavity heating profiles using TMAP are used to gain insight on the origins of quench; new recipes demonstrate the possibility to increase quench fields well beyond 30 MV/m. In addition, a new signature of nitrogen doping is explored, namely, a dip in the superconducting resonant frequency below the normal conducting value just below the critical transition temperature, giving further insights on the mechanisms responsible for the large increase in performance of cavities subject to this surface treatment.

Slides TUFUA4 [3.097 MB]

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

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TUFUB7

Measurement of Surface Resistance Properties with Coaxial Resonators - Review

cavity, multipactoring, experiment, SRF

374

H. Park, S.U. De Silva, J.R. Delayen
ODU, Norfolk, Virginia, USA

Achieving ever decreasing surface resistance at higher field in superconducting RF accelerating structures is one of most outstanding developments in modern accelerators. The BCS theory has been used widely to estimate the surface resistance and to direct the technology. However, recent research results show that the behavior of the surface resistance further deviates from the BCS theory. So far the study on surface resistance was performed usually with cavities of single frequency which limited the study of frequency dependent surface resistance. The Center for Accelerator Science at Old Dominion University has designed and built several half wave coaxial cavities to study the frequency, temperature, and RF field dependence of surface resistance. TRIUMF in Canada also joined this line of research using such multi frequency quarter wave and half wave coaxial cavities. This type of multi mode cavity will allow us to systematically study the parameters affecting surface resistance on the same cavity surface. In this paper, we review the results ODU and TRIUMF collected so far and proper analysis methods.

Slides TUFUB7 [3.551 MB]

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paper received ※ 01 July 2019 paper accepted ※ 12 July 2019 issue date ※ 14 August 2019

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TUP031

Heat Treatment for Jacketed Half-wave Resonator Cavity

cavity, cryomodule, vacuum, SRF

482

Y. Jung, B.H. Choi, D.H. Gil, M.O. Hyun, H. Kim, J.W. Kim, M.S. Kim, D.Y. Lee, J. Lee, S. Lee
IBS, Daejeon, Republic of Korea

Vertical tests of a prototype half-wave resonator cavity are being tested. The performance of the cavities, such as quality factor and accelerating electrical field, are measured and compared to the target design value. In previous study, we reported the effect of the heat treatment on a prototype bare HWR cavity. We baked a jacketed HWR cavity to improve a performance for 10 h at 650°C. In this study, we will report the effect of the heat treatment on the jacketed HWR cavity.

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

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TUP042

Measurement of Mechanical Vibration of SRILAC Cavities

cavity, niobium, linac, cryomodule

513

O. Kamigaito, K. Ozeki, N. Sakamoto, K. Suda, K. Yamada
RIKEN Nishina Center, Wako, Japan

Mechanical vibration of quarter-wavelength resonators of SRILAC, the superconducting booster of the RIKEN heavy-ion linac, was measured during a vertical cold test. The measurements were performed for fully assembled cavities as well as for bare niobium cavities without the titanium jacket. In the procedure, the instantaneous resonant frequencies were measured for 10 seconds at a time interval of 1 ms and were recorded as a time series. The frequencies were analyzed by means of conventional signal analysis. The power spectrum was deduced from the autocorrelation function calculated with the fluctuation of resonant frequencies. Although the vibration amplitudes were smaller in the cavities assembled with the titanium jacket, we could not find a clear reason for this.

Poster TUP042 [6.957 MB]

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paper received ※ 27 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019

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TUP050

A Multi-layered SRF Cavity for Conduction Cooling Applications

cavity, SRF, cryogenics, niobium

538

G. Ciovati, G. Cheng, E. Daly, G.V. Eremeev, J. Henry, R.A. Rimmer
JLab, Newport News, Virginia, USA
I.P. Parajuli
ODU, Norfolk, Virginia, USA
U. Pudasaini
The College of William and Mary, Williamsburg, Virginia, USA

Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. Some of the work was supported by the 2008 PECASE Award of G. Ciovati. I. Parajuli is supported by NSF Grant PHYS-100614-010
Industrial application of SRF technology would favor the use of cryocoolers to conductively cool SRF cavities for particle accelerators, operating at or above 4.3 K. In order to achieve a lower surface resistance than Nb at 4.3 K, a superconductor with higher critical temperature should be used, whereas a metal with higher thermal conductivity than Nb should be used to conduct the heat to the cryocoolers. A standard 1.5 GHz bulk Nb single-cell cavity has been coated with a ~2 µm thick layer of Nb₃Sn on the inner surface and with a 5 mm thick Cu layer on the outer surface for conduction cooled applications. The cavity performance has been measured at 4.3 K and 2.0 K in liquid He. The cavity reached a peak surface magnetic field of ~40 mT with a quality factor of 6×10⁹ and 3.5×10⁹ at 4.3 K, before and after applying the thick Cu layer, respectively.

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paper received ※ 21 June 2019 paper accepted ※ 30 June 2019 issue date ※ 14 August 2019

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TUP055

Nonlinear Dynamics and Dissipation of Vortex Lines Driven by Strong RF Fields

cavity, radio-frequency, niobium, electron

560

M.R.P. Walive Pathiranage, A.V. Gurevich
ODU, Norfolk, Virginia, USA

Trapped vortices can contribute significantly to a residual surface resistance of superconducting radio frequency (SRF) cavities but the nonlinear dynamics of flexible vortex lines driven by strong rf currents has not been well understood. Here we report extensive numerical simulations of large-amplitude oscillations of a trapped vortex line under the strong rf magnetic field. The rf power dissipated by an oscillating vortex segment driven by the rf Meissner currents was calculated by taking into account the nonlinear vortex line tension, vortex mass and a nonlinear Larkin-Ovchinnikov and overheating viscous drag force. We calculated the field dependence of the surface resistance Rs and showed that at low frequencies Rs(H) increases with H but as the frequency increases, Rs(H) becomes a non-monotonic function of H which decreases with H at higher fields. These results suggest that trapped vortices can contribute to the extended Q(H) rise observed on the SRF cavities.

Poster TUP055 [1.744 MB]

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paper received ※ 23 June 2019 paper accepted ※ 05 July 2019 issue date ※ 14 August 2019

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TUP059

Investigation of Trapped Flux Dynamics via DC-Magnetic Quenching

cavity, experiment, superconducting-cavity, SRF

580

P. Nuñez von Voigt, J. Knobloch, O. Kugeler
HZB, Berlin, Germany
J. Knobloch
University of Siegen, Siegen, Germany

Trapped magnetic flux increases the surface resistance in superconducting radio-frequency cavities. A better understanding of its behaviour could help to develop a method of expelling trapped flux from the superconducting surface. Using a superconducting coil with ferrite core attached to a 3 GHz sample Niobium cavity fully immersed in liquid Helium, we were able to subject the cavity walls to unusually large magnetic fields (estimated > 150 mT) and create magnetic quenches. With Fluxgate sensors attached in three spatial directions inside the cavity, we were able to monitor the quench dynamics and extract parameters of the flux dynamics from the hysteretic behaviour of the measured fields resulting from the applied coil current. First results of manipulation of the trapped flux with high magnetic fields are presented.

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paper received ※ 24 June 2019 paper accepted ※ 05 July 2019 issue date ※ 14 August 2019

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TUP061

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

cavity, niobium, SRF, site

586

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

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

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

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

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THFUA1

RF Characterization of an S-I-S’ Multilayer Sample

niobium, quadrupole, SRF, cavity

800

S. Keckert, J. Knobloch, O. Kugeler, D.B. Tikhonov
HZB, Berlin, Germany
A-M. Valente-Feliciano
JLab, Newport News, Virginia, USA

S-I-S’ multilayers promise to boost the performance of bulk superconductors in terms of maximum field and surface resistance. At HZB, RF-surface resistance measurements were performed with a Quadrupole Resonator (QPR) and an S-I-S’ sample (75 nm NbTiN on 15 nm AlN insulator on bulk Nb) prepared at JLab. Measurements were performed at 414, 845, and 1286 MHz at sample temperatures from 2 K up to well above the transition temperature of NbTiN of ~17.3 K. The Rs exhibits an unexpected temperature dependence: Rather than rising monotonically, as expected from BCS theory, a local maximum is observed. There is a temperature range where Rs decreases with increasing temperature. Such behavior indicates that an additional interaction between the superconducting layers may have to be included in the surface resistance model. Measurements of the baseline Nb sample prior to coating exhibited no such behavior; hence systematic measurement errors can be excluded as the explanation. The maximum field was limited by a hard magnetic quench near 20 mT, close to Hc1 of NbTiN, suggesting that the sample is limited by early flux penetration.

Slides THFUA1 [1.004 MB]

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THFUA1

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paper received ※ 22 June 2019 paper accepted ※ 03 July 2019 issue date ※ 14 August 2019

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THP010

The Mechanism of Electropolishing of Niobium from Choline Chloride-based Deep Eutectic Solvents

polarization, impedance, niobium, cavity

852

Q.W. Chu, H. Guo, S.C. Huang, A.D. Wu, Z.M. You
IMP/CAS, Lanzhou, People’s Republic of China

Funding: National Natural Science Foundation (11705252)
The mechanism of electropolishing of niobium (Nb) from choline chloride-based deep eutectic solvent (DES) was studied by anodic polarization tests and electrochemical impedance spectroscopy (EIS) using a Nb rotating disk electrode (RDE). Based on the results of an anodic polarisation test, the electropolishing of Nb is mass transport controlled. EIS results are consistent with the compact salt film mechanism for niobium electropolishing in this electrolyte. The influence of rotation rate, applied potential and electrolyte temperature on the electropolishing mechanism of Nb was investigated. As the applied potential positively shift, Rct, Rp and L increase, CPE decrease and Rs unchanged. The increase in rotation rate and electrolyte temperature leads to a decrease of Rs, Rct, Rp and L, and an increase of CPE.

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paper received ※ 18 June 2019 paper accepted ※ 02 July 2019 issue date ※ 14 August 2019

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THP011

Niobium Semiproducts for the Superconducting Strands and SRF Cavities in Russia

niobium, SRF, cavity, target

857

M.V. Alekseev, I.M. Abdyukhanov, V.A. Drobyshev, M.V. Kravtsova, M.V. Krylova, P.A. Lykianov, K.A. Mareev, V.I. Pantsyrny, M.V. Polikarpova, M.M. Potapenko, A.G. Silaev, A.S. Tsapleva
SC A A Bochvar High-Technology Research Institute of Inorganic Materials, Moscow, Russia
M.Y. Shlyakhov, S.M. Zernov
JSC - TVEL, Moscow, Russia

The melting regimes of the niobium ingots with high chemical purity and low hardness for the Nb₃Sn, NbTi and other superconducting materials manufacture have been developed at SC "VNIINM". Using this niobium material and by the SC "VNIINM" manufacture regimes at the SC "Chepetsky Mechanical Plant" 220 tons of Nb₃Sn and NbTi strands for ITER and 12 km of Nb₃Sn strands for HL-LHC (CERN) with the required characteristics have been successfully produced. The review of the characteristics of the different semiproducts (sheets, tubes, rods), made in Russia from the special grade niobium, and of the superconducting strands, manufactured with the use of them, is presented in the paper. The ways of the further improvement of the niobium ingots melting regimes and niobium sheets deformation and annealing regimes with the target of achieving RRR > 300 for the SRF cavities application are discussed.

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reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP011

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paper received ※ 23 June 2019 paper accepted ※ 04 July 2019 issue date ※ 14 August 2019

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THP012

Assessment of the Mechanical Properties of Ultra-High Purity Niobium After Cold Work and Heat Treatment With the HL-LHC Crab Cavities as Benchmark

cavity, operation, niobium, simulation

860

A. Gallifa Terricabras, A. Amorim Carvalho, I. Aviles Santillana, S. Barrière, R. Calaga, E. Cano-Pleite, O. Capatina, M.D. Crouvizier, L. Dassa, M.S. Meyer, N. Valverde Alonso
CERN, Geneva, Switzerland
M. Benke, A.B. Palotas, G. Szabó, M. Szűcs
University of Miskolc, Faculty of Materials Science and Engineering, Miskolc-Egyetemváros, Hungary
A. Hlavács, G.J. Krallics, V. Mertinger, M. Sepsi
University of Miskolc, Miskolc, Hungary

The High Luminosity Large Hadron Collider (HL-LHC) is the upgrade of the world’s largest particle collider; it will allow the full exploitation of the LHC potential and its operation beyond 2025. An essential part of the HL-LHC project are the Crab Cavities, that are particle deflecting SRF cavities of non-axisymmetric shape made of bulk ultra-high purity Nb. Since the cavities are produced by complex metal sheet forming processes, followed by a heat treatment (HT) for H outgassing (650 °C, 24 h), there is uncertainty on their mechanical properties after manufacturing and in service conditions (2 K). Mechanical tests at room temperature have been conducted on RRR300 pure Nb samples. The samples were previously submitted, by cold cross-rolling, to different levels of plastic deformation representative of the effective plastic strain seen by the Nb sheets during forming operations. Moreover, a comparison of the mechanical properties of cold cross-rolled samples before and after HT has been established. Results of evolution of the microstructure and hardness are also presented. This study can be of interest for Nb cavities to be sub-mitted to HT at 650 °C, and may help to push the design of novel SRF cavities.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP012

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

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THP042

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

site, cavity, experiment, SRF

941

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

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

Poster THP042 [1.190 MB]

DOI •

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

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

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THP043

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

site, cathode, cavity, niobium

945

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

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

Poster THP043 [1.169 MB]

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

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

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