SRF2013 - List of Keywords (superconductivity)

Paper	Title	Other Keywords	Page
MOIOA03	The Challenge and Realization of the Cavity Production and Treatment in Industry for the European XFEL	cavity, niobium, controls, HOM	18
	W. Singer, J. Iversen, A. Matheisen, H. Weise DESY, Hamburg, Germany P. Michelato INFN/LASA, Segrate (MI), Italy
	The main effort in production of 1.3 GHz cavities for the EXFEL was dedicated to transfer the superconducting technology to the industry. These know how transfer is executed by DESY and INFN/LASA team. The preparation phase based on prototype cavities covered: qualification of potential vendors for material and cavity fabrication; work out recipe and strategy for qualification of the infrastructure for cavity surface treatment at industry; definition of the quality management strategy, documentation and electronically data exchange. Production of 800 series cavities on the principle “build to print” is contracted to companies Research Instruments and Ettore Zanon. High purity niobium and NbTi for resonators provides DESY. The principles of the material and cavities production in conformity with European Pressure Equipment Directive are developed together with the notified body. New or upgraded infrastructure has been established at both companies. The first several tens of series cavities have been produced and treated. Most of the cavities handed over to DESY up to now fulfill immediately the EXFEL specifications. The cavity production for EXFEL will be finished mid of 2015.
	Slides MOIOA03 [7.394 MB]

MOIOC02	A New First-Principles Calculation of Field-Dependent RF Surface Impedance of BCS Superconductor	cavity, impedance, electron, niobium	63
	B. P. Xiao, C.E. Reece JLab, Newport News, Virginia, USA B. P. Xiao BNL, Upton, Long Island, New York, USA
	Funding: This manuscript has been authored in part by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. There is a need to understand the intrinsic limit of RF surface impedance that determines the performance of superconducting RF cavities in particle accelerators. Here we present a field-dependent derivation of Mattis-Bardeen theory of the RF surface impedance of BCS superconductors, based on the shifted density of states resulting from coherently moving Cooper pairs. Our theoretical prediction of the effective BCS RF surface resistance of niobium as a function of peak surface magnetic field amplitude agrees well with recently reported record low loss resonant cavity measurements from JLab and Fermi Lab with carefully prepared niobium material. The surprising reduction in resistance with increasing field is explained to be an intrinsic effect.
	Slides MOIOC02 [3.122 MB]

MOP045	Electropolishing for EXFEL Cavities Production at Ettore Zanon SpA	cavity, niobium, controls, cathode	220
	M. Rizzi Ettore Zanon S.p.A., Schio, Italy A. Gresele Ettore Zanon S.p.A., Nuclear Division, Schio, Italy A. Matheisen, N. Steinhau Kühl DESY, Hamburg, Germany P. Michelato INFN/LASA, Segrate (MI), Italy
	A new horizontal electropolishing (EP) facility has been implemented by Ettore Zanon SpA for the series production of the EXFEL cavities produced by the company. According to EXFEL specification a bulk EP of at least 100 micron is the first step of the surface treatment for high performances. Particular attention has been dedicated to find the best configuration during qualification of the system. Correlation between process variables, RF tests at room temperature at Zanon and vertical RF tests at 2 K at DESY have been investigated and the Niobium removal optimized. The facility has been designed for industrial scope, in order to guarantee the required quality and production rate of 4 cavities per week. One of the most important aspects has been the system automation to have complete control of the process.

TUIOC03	Fluorine Free Ionic Liquid Electropolishing of Niobium Cavities	niobium, cavity, cathode, experiment	410
	V.B. Pastushenko, O.V. Malkova, V. Palmieri, A.A. Rossi, F. Stivanello, G. Yu INFN/LNL, Legnaro (PD), Italy G. Yu CIAE, Beijing, People's Republic of China
	Ionic liquids are an emerging breakthrough in green chemistry since the years 2000. In 2006, INFN-LNL was the first to apply a mixture of Choline Chloride and Urea to Niobium electropolishing. It was found that mirror like surfaces could be obtained at temperature higher than 120°C, with high throwing power. Subsequently the process was successfully applied to the electropolishing of a 6 GHz monocell cavity with the addition of sulphamic acid. In this work, we will report an intense investigation of the possible variants of the original recipe. We studied the influence on Niobium surface roughness of several parameters such as: other sulphamic, ammonium and carboxylic containing additives different than sulfamic acid, the possible substitution of Urea with ethylene glycol and malic acid, the current regime; the electrolyte temperature and the cathode shape, rotating horizontal electropolishing versus vertical electropolishing. Due to the cavity hollow cylindrical shape, the electrolyte temperature appeared to be the most crucial parameters among those above mentioned for a uniform dissolution of niobium.
	Slides TUIOC03 [14.464 MB]

TUIOC04	Analysis of Post-Wet-Chemistry Heat Treatment Effects on Nb SRF Surface Resistance	cavity, SRF, niobium, site	414
	P. Dhakal, G. Ciovati, P. Kneisel, G.R. Myneni JLAB, Newport News, Virginia, USA
	Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. Most of the current R&D in SRF is focused on ways to reduce the construction and operating cost of SRF-based accelerators as well as on the development of new or improved cavity processing techniques. The increase in quality factors is the result of the reduction of the surface resistance of the materials. A recent test [] on a 1.5 GHz single cell cavity made from ingot niobium of medium purity and heat treated at 1400 C in a ultra-high vacuum induction furnace resulted in a residual resistance of ~ 1nanoohm and a quality factor increasing with field up to ~ 5×10¹⁰ at a peak magnetic field of 90 mT. In this contribution, we present some results on the investigation of the origin of the extended Q0-increase, obtained by multiple HF rinses, oxypolishing and heat treatment of “all Nb” cavities. [] P. Dhakal et al., Phys. Rev. ST Accel. Beams 16, 042001 (2013).
	Slides TUIOC04 [4.838 MB]

TUP007	Vortex Penetration Field in the Multilayered Coating Model	vacuum, cavity, electromagnetic-fields	430
	T. Kubo, T. Saeki KEK, Ibaraki, Japan Y. Iwashita Kyoto ICR, Uji, Kyoto, Japan
	A multilayered structure with a single superconductor layer and a single insulator layer deposited on a bulk superconductor is studied. General formulae for the vortex penetration-field of the superconductor layer and the magnetic field on the bulk superconductor which is shielded by the superconductor and insulator layers are derived with a rigorous calculation of the magnetic field attenuation in the multilayered structure. The formulae depend not only on the material and the thickness of the superconductor layer but also on the thickness of the insulator layer. The results can be applied to superconducting accelerating cavities with the multilayered structure. Using the formulae, a combination of the thicknesses of superconductor and insulator layers to enhance the RF breakdown field limits can be found for any given materials. (Submitted on 25 Apr 2013) T. Kubo, Y. Iwashita and T. Saeki, arXiv:1304.6876 [physics.acc-ph](Submitted on 25 Apr 2013); arXiv:1306.4823 [physics.acc-ph](Proc. of IPAC13); arXiv:1307.0583 [physics.acc-ph](Proc. of SRF2013)

TUP009	Magnetic Dependence of the Energy Gap: a Good Model to Fit Q Slope of Low Beta Cavities	cavity, experiment, niobium, simulation	438
	D. Longuevergne IPN, Orsay, France
	The reasons why the intrinsic quality factor (noted Qo) of a superconducting cavity drops with the accelerating field (noted Eacc) are still not well understood. In an effort to explain this phenomenon, mainly for high beta cavities, many models have been developed in the community but few of them could fit experimental data whatever the material treatment or surface conditioning. In the specific case of low beta cavities made of bulk Niobium (i.e Spiral 2 Quarter Wave Resonator), a model based on a magnetic field dependence of the energy gap has been developed to fit experimental data. The evolutions of the model input parameters depending on the cavity treatment or test conditions are consistent with the changes described in the literature. The model will be described and specific examples will be given.

TUP011	A Parametric Study of BCS RF Surface Impedance with Magnetic Field Using Xiao Code	impedance, niobium, SRF, survey	444
	C.E. Reece JLab, Newport News, Virginia, USA B. P. Xiao JLAB, Newport News, Virginia, USA B. P. Xiao BNL, Upton, Long Island, New York, USA
	Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. A recent new analysis of field-dependent BCS RF surface impedance based on moving Cooper pairs has been presented.* Using this analysis coded in Mathematica™, survey calculations have been completed which examine the sensitivities of this surface impedance to variation of the BCS material parameters and temperature. The results present a refined description of the “best theoretical” performance available to potential applications with corresponding materials. * Xiao B. P. et al, Physica C: Superconductivity, 490, 2013, pp. 26–31

TUP019	Probing Hot Spot and Cold Spot of SRF Cavities with Tunneling and Raman Spectroscopies	SRF, cavity, laser, electron	466
	C. Cao Illinois Institute of Technology, Chicago, IL, USA G. Ciovati JLAB, Newport News, Virginia, USA L.D. Cooley, A. Grassellino Fermilab, Batavia, USA N. Groll, Th. Proslier ANL, Argonne, USA J. Zasadzinski IIT, Chicago, Illinois, USA
	Point contact tunneling and Raman spectroscopies are presented on high purity Nb samples, including pieces from hot and col spot regions of tested SRF cavities and Nb coupons subject to similar treatment. High quality tunneling spectra were observed on cold spots, revealing the bulk Nb gap, indicating minimal surface contamination. Hot spots exhibit high smearing suggestive of pair breaking along with generally lower superconducting gap. In addition, pronounced zero bias conductance peaks were frequently observed indicative of spin-flip tunneling and thus magnetic impurities in the oxide layer. Optical microscopy reveals higher density of surface blemishes on hot spots. Raman spectra inside those blemishes show clear difference from surrounding areas, exhibiting enhanced intensity peaks identified as either amorphous carbon, hydrocarbons or the ordered NbC phase. The presence of surface NbC is consistent with TEM studies, and these inclusions exhibit enhanced second order phonon response. Such regions with high concentrations of impurities are expected to suppress the local superconductivity and may explain the formation of hot spots.

TUP022	Study of AC/RF Properties of SRF Ingot Niobium	cavity, SRF, niobium, radio-frequency	469
	P. Dhakal, G. Ciovati, G.R. Myneni JLAB, Newport News, Virginia, USA V.M. Genkin, M.I. Tsindlekht The Hebrew University of Jerusalem, The Racah Institute of Physics, Jerusalem, Israel
	Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. In an attempt to correlate the SRF performance of niobium cavities with the superconducting properties, we present the results of the magnetization and ac susceptibility of the niobium used in the superconducting radiofrequency cavity fabrications which were subjected to buffer chemical polishing surface and high temperature heat treatments, typically applied to the SRF cavities fabrications. The analysis of the results show the different surface and bulk ac conductivity for the samples subjected to BCP and HT. Furthermore, the RF surface impedance is measured on the sample using the TE011 microwave cavity for a comparison to the low frequency measurements.

TUP023	Evidence of Magnetic Breakdown on the Defects With Thermally Suppressed Critical Field in High Gradient SRF Cavities	SRF, site, superconducting-RF, niobium	472
	G.V. Eremeev, A.D. Palczewski JLAB, Newport News, Virginia, USA
	Funding: Work supported by DOE. Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. At SRF 2011 we presented the study of quenches in high gradient SRF cavities with dual mode excitation technique[]. The data differed from measurements done in 80’s that indicated thermal breakdown nature of quenches in SRF cavities. In this contribution we present analysis of the data that indicates that our recent data for high gradient quenches is consistent with the magnetic breakdown on the defects with thermally suppressed critical field. From the parametric fits derived within the model we estimate the critical breakdown fields and RF resistances at the breakdown site. [] G. Eremeev et al.,. In Proceedings of the 15th Superconducting RF conference,pp. 746-749, July 2011.

TUP026	Performance of a FNAL Nitrogen Treated Superconducting Niobium Cavity at Cornell	cavity, niobium, SRF, linac	475
	D. Gonnella, M. Liepe Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA A. Grassellino Fermilab, Batavia, USA
	Funding: NSF In many tests of superconducting cavities, the performance of the cavity in the medium field region will be limited by medium field Q slope. For projects such as the proposed Cornell Energy Recovery Linac, high Q operation at medium fields is necessary to meet specifications for efficient CW cavity operation. A single cell cavity was prepared by Fermilab by electropolishing it and baking it at 1000°C with 1x10^-2 Torr of Nitrogen, and subsequently tested at Cornell. The cavity displayed an increase in Q at medium fields between 5 and 20 MV/m at 2.0 K, opposite of the usual medium field Q slope. The material properties of this cavity were studied and correlated with performance. This analysis helps to better understand how to overcome medium field Q slope and improve cavity performance in future CW SRF machines such as the Cornell ERL.

TUP028	Investigation of Spatial Variation of the Surface Resistance of a Superconducting RF Cavity	cavity, resonance, superconducting-RF, SRF	483
	D. Gonnella, M. Liepe Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA R.E. French Corning Community College, Corning, USA
	Funding: NSF Cornell has recently completed a single cell temperature mapping system with a resolution of a few tenths of a millikelvin, corresponding to a surface resistance resolution of 1 nOhm. A superconducting RF cavity was tested using temperature mapping and the surface resistance was extracted from the temperature mapping data as function of position on the cavity surface. The surface resistance was profiled across the surface of the cavity between 5 and 35 MV/m and at different temperatures between 1.6 and 2.1 K. From BCS fitting of the local surface resistance, the spatial variation and the field dependence of the mean free path, energy gap, and residual resistance was found. These studies give interesting new insight into the degree of variation of the properties of the superconductor over the surface of the cavity.

TUP044	Surface Processing Facilities for Spoke Cavities at IHEP	cavity, linac, recirculation, LLRF	508
	J.P. Dai, H. Li, L.H. Li, Q.Y. Wang, J. Zhang IHEP, Beijing, People's Republic of China P. Sha Institute of High Energy Physics (IHEP), Chinese Academy of Sciences, Beijing, People's Republic of China
	Funding: Work supported by the "Strategic Priority Research Program" of CAS, under Grant No. XDA03020600 The China ADS injector I program is building a CW 10MeV Superconducting proton linac at IHEP. To develop the superconducting spoke-type cavities incorporated in this linac, a set of new surface processing facilities were built and successfully used to treat the Spoke012 prototype cavities. In this paper, we present the design, fabrication and operation of these facilities, including BCP, HPR and UPW, etc.

TUP046	Vertical Electropolishing of SRF Cavities and its Parameters Investigation	cavity, cathode, SRF, experiment	514
	F. Eozénou, F. Ballester, Y. Boudigou, P. Carbonnier, J.-P. Charrier, Y. Gasser, D. Roudier, C. Servouin CEA/DSM/IRFU, France K. Muller Grenoble-INP Phelma, Grenoble, France
	Funding: We acknowledge the support of the European Community-Research Infrastructure Activity under the FP7 program (EuCARD, Contract No. 227579),and the support of the ‘‘Conseil General de l’Essonne’’(ASTRE) An advanced set-up for vertical electropolishing (VEP) of SRF niobium elliptical cavities is operating at CEA Saclay. Cavities are VEP’ed with circulating standard HF-H2SO4 electrolyte. Parameters such as voltage, cathode shape, acid flow and temperature were investigated. Low-voltage (<7V), high acid flow (25L/min) and low acid temperature (20°C) are considered as promising parameters. Such recipe was tested on single-cell and 9-cell ILC cavities with nice surface finishing. After 60 μm VEP on a HEP'ed single-cell, the cavity show similar performance at 1.6K compared to previous Horizontal EP: (Eacc > 41MV/m) limited by quench. Another cavity reaches 36MV/m after 300μm removal by VEP in spite of a pitted surface due to initial VEP treatment at higher temperature (> 30°C). The baking effect after HEP/VEP is similar. An asymmetric niobium removal is observed with faster polishing in the upper cell. Nice surface finishing as well as standard Q0 value are obtained at low/medium field on 9Cell but achieved performance is limited by Field Emission. F. Eozenou et al., PRST-AB, 15, 083501 (2012)

TUP053	Estimation of Small Geometry Deviation for TESLA-Shape Cavities Due to Inner Surface Polishing	cavity, coupling, factory, data-analysis	537
	A.A. Sulimov, G. Kreps, J.K. Sekutowicz DESY, Hamburg, Germany
	Two well know polishing methods are used for the inner surface cleaning of superconducting TESLA-shape cavities: electro-polishing (EP) or buffered chemical polishing (BCP). The amount of removed material is relatively small and varies from 10 till 140 um. The cavity after polishing is closed to prevent the scratches or dust appearing on its inner surface. The estimation of the removed material amount is possible by different criteria, for example by comparison of weight before and after cleaning, or by the time - cleaning procedure duration. Both calculations could give us only approximate average value of the removed material amount. We describe the method for estimation of small geometry deviation basing on RF frequency measurements, which allows calculating the different influence of surface treatment on the iris and equator areas.
	Poster TUP053 [0.785 MB]

TUP064	Exploration of Material Removal Rate of SRF Elliptical Cavities as a Function of Media Type and Cavity Shape on Niobium and Copper Using Centrifugal Barrel Polishing (CBP)	cavity, niobium, SRF, status	579
	A.D. Palczewski, G. Ciovati, R.L. Geng, Y.M. Li JLAB, Newport News, Virginia, USA
	Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. Centrifugal barrel polishing (CBP) for SRF application is becoming more wide spread as the technique for cavity surface preparation. CBP is now being used in some form at SRF laboratories around the world. Before the process can become as mature as wet chemistry like eletro-polishing (EP) and buffered chemical polishing (BCP) there are many questions which remain unanswered. One of these topics includes the uniformity of removal as a function of cavity shape and material type. In this presentation we show CBP removal rates for various media types on 1.3 GHz TESLA and 1.5 GHz CEBAF large grain niobium cavities, 1.3 GHz TESLA fine grain niobium cavity, and 1.3GHz low surface field copper cavity. The data will also include calculated RF frequency shift modeling non-uniform removal as a function of cavity position and comparing them with CBP results.

TUP067	Hydrogen Saturation and the Thermal Conductivity of Superconducting Niobium	niobium, cavity, vacuum, lattice	589
	S.K. Chandrasekaran MSU, East Lansing, USA T.R. Bieler Michigan State University, East Lansing, USA C. Compton FRIB, East Lansing, USA N.T. Wright (MSU), East Lansing, USA
	Funding: This work was supported by the U.S. Department of Energy, Office of High Energy Physics, through Grant No. DE-S0004222 The thermal conductivity k of Nb at less than 3 K is dominated by phonon transport. In Nb with sufficiently few lattice imperfections, a maximum in k occurs at 1.8 K, called the phonon peak (PP). A large PP is desired to reduce potential local hot spots and contributes to an increased Q factor. The magnitude of the PP is sensitive to SRF cavity manufacturing processes. The effect of interstitial hydrogen on the magnitude of the PP is examined by subjecting two bicrystal Nb specimens to 300 C for 1 h in a 75% H2, 25% N2 atmosphere at 0.5 atm. Prior to hydrogen infusion, specimen 1 was heated to 800 C for 2 h, while specimen 2 was heated to 1100 C for 4 h. Both specimens displayed a 25% reduction in the PP due to the additional hydrogen, independent of their crystal orientations and heat treatment histories. An 800 C vacuum heating for 2 h was found to be sufficient to recover the PP in specimen 1, while an 1100 C heating for 4 h was required to recover the PP in one of the grains of specimen 2. The results suggest that hydrogen trapped in the Nb lattice will degas when the Nb is heated to at least the temperature to which it was heated at prior to the hydrogen infusion step.

TUP100	Medium Field Q-Slope Studies in High Frequency Cavities	cavity, operation, niobium, factory	705
	O.S. Melnychuk, A. Grassellino, A.I. Sukhanov Fermilab, Batavia, USA
	A phenomenon of Medium Field Q-Slope (MFQS) in superconducting RF cavities is of high importance because it occurs in the field range (5-20MV/m) that includes designed operation fields of future CW accelerators. MFQS impacts resistive losses in the cavity and, consequently, directly affects accelerator operation costs. We present studies of MFQS based on vertical test data for 1.3GHz nine-cell cavities and make comparisons of vertical test data from different laboratories.

TUP102	Quench Detection Diagnostics on 3.9 GHz XFEL Cavities	cavity, site, detector, diagnostics	710
	M. Bertucci, A. Bosotti, L. Garolfi, P. Michelato, L. Monaco, D. Sertore INFN/LASA, Segrate (MI), Italy C. Pagani Università degli Studi di Milano & INFN, Segrate, Italy
	This paper presents results of quench localization on 3.9 GHz XFEL prototype cavities tested at LASA vertical test facility. Cavities have been equipped with OST second sound detectors and thermometry sensors. A first guess for quench position has been obtained from modal analysis. Second sound sensors confirmed the quench position resolving also the symmetry degeneracy given by the RF mode pattern analysis. In a subsequent vertical test, second sound and temperature sensors have been installed nearby the suspect quench position. From Thermometry mapping, a sudden increase in cavity temperature within a small region is evident, therefore confirming that a local thermal breakdown due to defect heating occurs in the predicted quench point. The quench region deduced with the mentioned techniques is eventually compared with results of optical inspection.

WEIOB01	Status of MgB2 Coating Studies for SRF Applications	cavity, SRF, controls, background	777
	T. Tajima, L. Civale, D.J. Devlin, G.C. Martinez, R.K. Schulze LANL, Los Alamos, New Mexico, USA
	Funding: DOE Office of Science/Nuclear Physics MgB2 has shown promising results on small samples and its coating development is entering into the stage to coat large samples and elliptical cavities. Several coating techniques that seem to be appropriate for cavity coating and their status will be shown together with some cavity measurement results with either 6 GHz or 1.3 GHz single-cell cavities. Other data such as RF surface resistance at low temperatures and vortex penetration fields with small samples will also be shown.
	Slides WEIOB01 [2.183 MB]

WEIOC01	High Resolution Surface Resistance Studies	quadrupole, niobium, cavity, shielding	785
	S. Aull, S. Döbert, T. Junginger CERN, Geneva, Switzerland S. Aull, J. Knobloch University of Siegen, Siegen, Germany J. Knobloch HZB, Berlin, Germany
	Funding: Work supported by the German Doctoral Students program of the Federal Ministry of Education and Research (BMBF). The attempt to reach quality factors beyond 10¹¹ and pushing the accelerating gradients of SRF cavities to the theoretical limit, the treatment depending loss mechanisms in niobium need better understanding. CERNs Quadrupole Resonator enables sub-nΩ-resolution measurements of the surface resistance. The available parameters cover resonant modes at 400, 800 and 1200 MHz, any temperature up to 15 K and rf fields up to 60 mT. Recently the setup has been extended with a coil creating a dc magnetic field for trapped flux studies. Overall, much more information about the rf performance is accessible compared to regular cavity measurements. Since the samples are flat disks of 75 mm diameter geometric fabrication issues are simplified which makes the Quadrupole Resonator also the perfect tool to study alternative materials or new coating techniques. In this contribution in depth studies of a heat treated bulk niobium sample exploiting the complete parameter range of the setup are presented.
	Slides WEIOC01 [2.724 MB]

WEIOC02	Multilayers Activities at Saclay / Orsay	niobium, cavity, SRF, vacuum	789
	C. B. Baumier, G. Martinet IPN, Orsay, France C.Z. Antoine CEA/IRFU, Gif-sur-Yvette, France F. F. Fortuna CSNSM, ORSAY CAMPUS, France J.C. Villegier CEA/INAC, Grenoble Cedex 9, France
	In the investigations on the high gradient SRF cavities, the superconducting multilayer is a promising alternative. The predictions show that an SIS (Superconductor/Isolator/Superconductor) nano-composite could improve the efficiency limited by the bulk Nb it-self used today for accelerating cavities. We start, at the IPNO lab in collaboration with the CSNSM lab (CNRS) and Irfu lab (CEA), an experimental study to test the screening effect on multilayer assemblies. Based on 3rd harmonic magnetometer and a TE011 SRF cavity, measurements of first critical magnetic field HC1 and surface resistance of samples have been performed. Along with these first results, we are starting the development of a MBE deposition. This set-up is devoted to optimise the best organisation of the multilayer to produce the model sample, and to find, in a close future, a realistic solution to apply this technique on an accelerating SRF cavity. Labex P2IO funding
	Slides WEIOC02 [3.035 MB]

WEIOC04	Theoretical Field Limits for Multi-Layer Superconductors	SRF, niobium, cavity, experiment	794
	S. Posen, M. Liepe Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA G. Catelani Forschungszentrum Jülich, Peter Gruenberg Institut (PGI-2), Jülich, Germany J.P. Sethna Cornell University, Ithaca, New York, USA M.K. Transtrum M.D.A.C.C., Houston, Texas, USA
	With modern cavity preparation techniques, niobium SRF cavities reach surface magnetic fields very close to the fundamental limit of the superheating field of the material, and researchers are looking to alternative superconductors to sustain even higher fields. However, these materials may have an increased vulnerability to flux penetration at defects, even small ones, as a result of their short coherence lengths. A. Gurevich has proposed [1] a method of mitigating this vulnerability: coating a bulk superconducting cavity with a series of very thin insulating and superconducting films. In this work, we present a thorough mathematical description of the SIS thin films proposed by Gurevich in the language of the SRF community, to help researchers to optimize cavities made from alternative superconductors. [1] A. Gurevich, Appl. Phys. Lett. 88, 012511 (2006)
	Slides WEIOC04 [4.116 MB]

THP005	Characteristics and Fabrication of Spoke Cavities for High-Velocity Applications	cavity, vacuum, operation, target	902
	C.S. Hopper, J.R. Delayen, H. Park ODU, Norfolk, Virginia, USA J.R. Delayen, H. Park JLAB, Newport News, Virginia, USA
	A 500 MHz, velocity-of-light, two-spoke cavity has been designed and optimized for possible use in a compact light source [1]. Here we present the mechanical analysis and steps taken in fabrication of this cavity at Jefferson Lab. *[1] T. Satogata et al, “Compact Accelerator Design for a Compact Light Source,” IPAC13, Shanghai, China, May 2013.

THP047	Performance Degradation of a Superconducting Cavity Quenching in Magnetic Field	cavity, cryomodule, RF-structure, superconducting-RF	1013
	I. Terechkine, T.N. Khabiboulline, D.A. Sergatskov Fermilab, Batavia, USA
	Although degradation of a superconducting RF (SRF) cavity performance induced by magnetic field trapped in its walls is a well understood phenomenon, a criterion for an acceptable level of magnetic field existing in the vicinity of an SRF cavity and generated after the cavity is cooled down has not been agreed upon. The bulk of superconducting Nb should protect the RF surface of the cavity from the magnetic field on the outside; nevertheless a failure mode exists when the cavity quenches while the external field is applied. The amount of trapped magnetic flux in this case depends on the size of normally conducting zone developed in walls of the cavity during quenching. Although propagation of the normally conducting zone in walls of a cavity can be modeled, no dedicated studies of this process that would include experimental verifications of its impact on the cavity performance could be found. We tried to address his issue in a special study by using as an example a superconducting coil mounted near a quenching cavity; the method and some results of the study can be applied to any RF structure and magnetic system.

THP058	Update on the European XFEL RF Power Input Coupler	simulation, cryogenics, linac, electron	1047
	D. Kostin, W.-D. Möller DESY, Hamburg, Germany W. Kaabi LAL, Orsay, France
	European XFEL project is being currently realized in Hamburg, Germany. The 1.5 km 17.5 GeV linear electron accelerator is based on the 1.3GHz 9-cell TESLA type SRF cavity. The RF power input coupler design for the E-XFEL is based on well known TTF3 coupler design, used in FLASH accelerator. Coupler design was adapted for the industrial production with some parameters optimisation revisited and simulations done. Results are presented and discussed.

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Other Keywords

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MOIOA03

The Challenge and Realization of the Cavity Production and Treatment in Industry for the European XFEL

cavity, niobium, controls, HOM

18

W. Singer, J. Iversen, A. Matheisen, H. Weise
DESY, Hamburg, Germany
P. Michelato
INFN/LASA, Segrate (MI), Italy

The main effort in production of 1.3 GHz cavities for the EXFEL was dedicated to transfer the superconducting technology to the industry. These know how transfer is executed by DESY and INFN/LASA team. The preparation phase based on prototype cavities covered: qualification of potential vendors for material and cavity fabrication; work out recipe and strategy for qualification of the infrastructure for cavity surface treatment at industry; definition of the quality management strategy, documentation and electronically data exchange. Production of 800 series cavities on the principle “build to print” is contracted to companies Research Instruments and Ettore Zanon. High purity niobium and NbTi for resonators provides DESY. The principles of the material and cavities production in conformity with European Pressure Equipment Directive are developed together with the notified body. New or upgraded infrastructure has been established at both companies. The first several tens of series cavities have been produced and treated. Most of the cavities handed over to DESY up to now fulfill immediately the EXFEL specifications. The cavity production for EXFEL will be finished mid of 2015.

Slides MOIOA03 [7.394 MB]

MOIOC02

A New First-Principles Calculation of Field-Dependent RF Surface Impedance of BCS Superconductor

cavity, impedance, electron, niobium

63

B. P. Xiao, C.E. Reece
JLab, Newport News, Virginia, USA
B. P. Xiao
BNL, Upton, Long Island, New York, USA

Funding: This manuscript has been authored in part by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
There is a need to understand the intrinsic limit of RF surface impedance that determines the performance of superconducting RF cavities in particle accelerators. Here we present a field-dependent derivation of Mattis-Bardeen theory of the RF surface impedance of BCS superconductors, based on the shifted density of states resulting from coherently moving Cooper pairs. Our theoretical prediction of the effective BCS RF surface resistance of niobium as a function of peak surface magnetic field amplitude agrees well with recently reported record low loss resonant cavity measurements from JLab and Fermi Lab with carefully prepared niobium material. The surprising reduction in resistance with increasing field is explained to be an intrinsic effect.

Slides MOIOC02 [3.122 MB]

MOP045

Electropolishing for EXFEL Cavities Production at Ettore Zanon SpA

cavity, niobium, controls, cathode

220

M. Rizzi
Ettore Zanon S.p.A., Schio, Italy
A. Gresele
Ettore Zanon S.p.A., Nuclear Division, Schio, Italy
A. Matheisen, N. Steinhau Kühl
DESY, Hamburg, Germany
P. Michelato
INFN/LASA, Segrate (MI), Italy

A new horizontal electropolishing (EP) facility has been implemented by Ettore Zanon SpA for the series production of the EXFEL cavities produced by the company. According to EXFEL specification a bulk EP of at least 100 micron is the first step of the surface treatment for high performances. Particular attention has been dedicated to find the best configuration during qualification of the system. Correlation between process variables, RF tests at room temperature at Zanon and vertical RF tests at 2 K at DESY have been investigated and the Niobium removal optimized. The facility has been designed for industrial scope, in order to guarantee the required quality and production rate of 4 cavities per week. One of the most important aspects has been the system automation to have complete control of the process.

TUIOC03

Fluorine Free Ionic Liquid Electropolishing of Niobium Cavities

niobium, cavity, cathode, experiment

410

V.B. Pastushenko, O.V. Malkova, V. Palmieri, A.A. Rossi, F. Stivanello, G. Yu
INFN/LNL, Legnaro (PD), Italy
G. Yu
CIAE, Beijing, People's Republic of China

Ionic liquids are an emerging breakthrough in green chemistry since the years 2000. In 2006, INFN-LNL was the first to apply a mixture of Choline Chloride and Urea to Niobium electropolishing. It was found that mirror like surfaces could be obtained at temperature higher than 120°C, with high throwing power. Subsequently the process was successfully applied to the electropolishing of a 6 GHz monocell cavity with the addition of sulphamic acid. In this work, we will report an intense investigation of the possible variants of the original recipe. We studied the influence on Niobium surface roughness of several parameters such as: other sulphamic, ammonium and carboxylic containing additives different than sulfamic acid, the possible substitution of Urea with ethylene glycol and malic acid, the current regime; the electrolyte temperature and the cathode shape, rotating horizontal electropolishing versus vertical electropolishing. Due to the cavity hollow cylindrical shape, the electrolyte temperature appeared to be the most crucial parameters among those above mentioned for a uniform dissolution of niobium.

Slides TUIOC03 [14.464 MB]

TUIOC04

Analysis of Post-Wet-Chemistry Heat Treatment Effects on Nb SRF Surface Resistance

cavity, SRF, niobium, site

414

P. Dhakal, G. Ciovati, P. Kneisel, G.R. Myneni
JLAB, Newport News, Virginia, USA

Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
Most of the current R&D in SRF is focused on ways to reduce the construction and operating cost of SRF-based accelerators as well as on the development of new or improved cavity processing techniques. The increase in quality factors is the result of the reduction of the surface resistance of the materials. A recent test [*] on a 1.5 GHz single cell cavity made from ingot niobium of medium purity and heat treated at 1400 C in a ultra-high vacuum induction furnace resulted in a residual resistance of ~ 1nanoohm and a quality factor increasing with field up to ~ 5×10¹⁰ at a peak magnetic field of 90 mT. In this contribution, we present some results on the investigation of the origin of the extended Q0-increase, obtained by multiple HF rinses, oxypolishing and heat treatment of “all Nb” cavities.
[*] P. Dhakal et al., Phys. Rev. ST Accel. Beams 16, 042001 (2013).

Slides TUIOC04 [4.838 MB]

TUP007

Vortex Penetration Field in the Multilayered Coating Model

vacuum, cavity, electromagnetic-fields

430

T. Kubo, T. Saeki
KEK, Ibaraki, Japan
Y. Iwashita
Kyoto ICR, Uji, Kyoto, Japan

A multilayered structure with a single superconductor layer and a single insulator layer deposited on a bulk superconductor is studied. General formulae for the vortex penetration-field of the superconductor layer and the magnetic field on the bulk superconductor which is shielded by the superconductor and insulator layers are derived with a rigorous calculation of the magnetic field attenuation in the multilayered structure. The formulae depend not only on the material and the thickness of the superconductor layer but also on the thickness of the insulator layer. The results can be applied to superconducting accelerating cavities with the multilayered structure. Using the formulae, a combination of the thicknesses of superconductor and insulator layers to enhance the RF breakdown field limits can be found for any given materials. (Submitted on 25 Apr 2013)
T. Kubo, Y. Iwashita and T. Saeki, arXiv:1304.6876 [physics.acc-ph](Submitted on 25 Apr 2013); arXiv:1306.4823 [physics.acc-ph](Proc. of IPAC13); arXiv:1307.0583 [physics.acc-ph](Proc. of SRF2013)

TUP009

Magnetic Dependence of the Energy Gap: a Good Model to Fit Q Slope of Low Beta Cavities

cavity, experiment, niobium, simulation

438

D. Longuevergne
IPN, Orsay, France

The reasons why the intrinsic quality factor (noted Qo) of a superconducting cavity drops with the accelerating field (noted Eacc) are still not well understood. In an effort to explain this phenomenon, mainly for high beta cavities, many models have been developed in the community but few of them could fit experimental data whatever the material treatment or surface conditioning. In the specific case of low beta cavities made of bulk Niobium (i.e Spiral 2 Quarter Wave Resonator), a model based on a magnetic field dependence of the energy gap has been developed to fit experimental data. The evolutions of the model input parameters depending on the cavity treatment or test conditions are consistent with the changes described in the literature. The model will be described and specific examples will be given.

TUP011

A Parametric Study of BCS RF Surface Impedance with Magnetic Field Using Xiao Code

impedance, niobium, SRF, survey

444

C.E. Reece
JLab, Newport News, Virginia, USA
B. P. Xiao
JLAB, Newport News, Virginia, USA
B. P. Xiao
BNL, Upton, Long Island, New York, USA

Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
A recent new analysis of field-dependent BCS RF surface impedance based on moving Cooper pairs has been presented.* Using this analysis coded in Mathematica™, survey calculations have been completed which examine the sensitivities of this surface impedance to variation of the BCS material parameters and temperature. The results present a refined description of the “best theoretical” performance available to potential applications with corresponding materials.
* Xiao B. P. et al, Physica C: Superconductivity, 490, 2013, pp. 26–31

TUP019

Probing Hot Spot and Cold Spot of SRF Cavities with Tunneling and Raman Spectroscopies

SRF, cavity, laser, electron

466

C. Cao
Illinois Institute of Technology, Chicago, IL, USA
G. Ciovati
JLAB, Newport News, Virginia, USA
L.D. Cooley, A. Grassellino
Fermilab, Batavia, USA
N. Groll, Th. Proslier
ANL, Argonne, USA
J. Zasadzinski
IIT, Chicago, Illinois, USA

Point contact tunneling and Raman spectroscopies are presented on high purity Nb samples, including pieces from hot and col spot regions of tested SRF cavities and Nb coupons subject to similar treatment. High quality tunneling spectra were observed on cold spots, revealing the bulk Nb gap, indicating minimal surface contamination. Hot spots exhibit high smearing suggestive of pair breaking along with generally lower superconducting gap. In addition, pronounced zero bias conductance peaks were frequently observed indicative of spin-flip tunneling and thus magnetic impurities in the oxide layer. Optical microscopy reveals higher density of surface blemishes on hot spots. Raman spectra inside those blemishes show clear difference from surrounding areas, exhibiting enhanced intensity peaks identified as either amorphous carbon, hydrocarbons or the ordered NbC phase. The presence of surface NbC is consistent with TEM studies, and these inclusions exhibit enhanced second order phonon response. Such regions with high concentrations of impurities are expected to suppress the local superconductivity and may explain the formation of hot spots.

TUP022

Study of AC/RF Properties of SRF Ingot Niobium

cavity, SRF, niobium, radio-frequency

469

P. Dhakal, G. Ciovati, G.R. Myneni
JLAB, Newport News, Virginia, USA
V.M. Genkin, M.I. Tsindlekht
The Hebrew University of Jerusalem, The Racah Institute of Physics, Jerusalem, Israel

Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
In an attempt to correlate the SRF performance of niobium cavities with the superconducting properties, we present the results of the magnetization and ac susceptibility of the niobium used in the superconducting radiofrequency cavity fabrications which were subjected to buffer chemical polishing surface and high temperature heat treatments, typically applied to the SRF cavities fabrications. The analysis of the results show the different surface and bulk ac conductivity for the samples subjected to BCP and HT. Furthermore, the RF surface impedance is measured on the sample using the TE011 microwave cavity for a comparison to the low frequency measurements.

TUP023

Evidence of Magnetic Breakdown on the Defects With Thermally Suppressed Critical Field in High Gradient SRF Cavities

SRF, site, superconducting-RF, niobium

472

G.V. Eremeev, A.D. Palczewski
JLAB, Newport News, Virginia, USA

Funding: Work supported by DOE. Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
At SRF 2011 we presented the study of quenches in high gradient SRF cavities with dual mode excitation technique[*]. The data differed from measurements done in 80’s that indicated thermal breakdown nature of quenches in SRF cavities. In this contribution we present analysis of the data that indicates that our recent data for high gradient quenches is consistent with the magnetic breakdown on the defects with thermally suppressed critical field. From the parametric fits derived within the model we estimate the critical breakdown fields and RF resistances at the breakdown site.
[*] G. Eremeev et al.,. In Proceedings of the 15th Superconducting RF conference,pp. 746-749, July 2011.

TUP026

Performance of a FNAL Nitrogen Treated Superconducting Niobium Cavity at Cornell

cavity, niobium, SRF, linac

475

D. Gonnella, M. Liepe
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
A. Grassellino
Fermilab, Batavia, USA

Funding: NSF
In many tests of superconducting cavities, the performance of the cavity in the medium field region will be limited by medium field Q slope. For projects such as the proposed Cornell Energy Recovery Linac, high Q operation at medium fields is necessary to meet specifications for efficient CW cavity operation. A single cell cavity was prepared by Fermilab by electropolishing it and baking it at 1000°C with 1x10^-2 Torr of Nitrogen, and subsequently tested at Cornell. The cavity displayed an increase in Q at medium fields between 5 and 20 MV/m at 2.0 K, opposite of the usual medium field Q slope. The material properties of this cavity were studied and correlated with performance. This analysis helps to better understand how to overcome medium field Q slope and improve cavity performance in future CW SRF machines such as the Cornell ERL.

TUP028

Investigation of Spatial Variation of the Surface Resistance of a Superconducting RF Cavity

cavity, resonance, superconducting-RF, SRF

483

D. Gonnella, M. Liepe
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
R.E. French
Corning Community College, Corning, USA

Funding: NSF
Cornell has recently completed a single cell temperature mapping system with a resolution of a few tenths of a millikelvin, corresponding to a surface resistance resolution of 1 nOhm. A superconducting RF cavity was tested using temperature mapping and the surface resistance was extracted from the temperature mapping data as function of position on the cavity surface. The surface resistance was profiled across the surface of the cavity between 5 and 35 MV/m and at different temperatures between 1.6 and 2.1 K. From BCS fitting of the local surface resistance, the spatial variation and the field dependence of the mean free path, energy gap, and residual resistance was found. These studies give interesting new insight into the degree of variation of the properties of the superconductor over the surface of the cavity.

TUP044

Surface Processing Facilities for Spoke Cavities at IHEP

cavity, linac, recirculation, LLRF

508

J.P. Dai, H. Li, L.H. Li, Q.Y. Wang, J. Zhang
IHEP, Beijing, People's Republic of China
P. Sha
Institute of High Energy Physics (IHEP), Chinese Academy of Sciences, Beijing, People's Republic of China

Funding: Work supported by the "Strategic Priority Research Program" of CAS, under Grant No. XDA03020600
The China ADS injector I program is building a CW 10MeV Superconducting proton linac at IHEP. To develop the superconducting spoke-type cavities incorporated in this linac, a set of new surface processing facilities were built and successfully used to treat the Spoke012 prototype cavities. In this paper, we present the design, fabrication and operation of these facilities, including BCP, HPR and UPW, etc.

TUP046

Vertical Electropolishing of SRF Cavities and its Parameters Investigation

cavity, cathode, SRF, experiment

514

F. Eozénou, F. Ballester, Y. Boudigou, P. Carbonnier, J.-P. Charrier, Y. Gasser, D. Roudier, C. Servouin
CEA/DSM/IRFU, France
K. Muller
Grenoble-INP Phelma, Grenoble, France

Funding: We acknowledge the support of the European Community-Research Infrastructure Activity under the FP7 program (EuCARD, Contract No. 227579),and the support of the ‘‘Conseil General de l’Essonne’’(ASTRE)
An advanced set-up for vertical electropolishing (VEP) of SRF niobium elliptical cavities is operating at CEA Saclay*. Cavities are VEP’ed with circulating standard HF-H2SO4 electrolyte. Parameters such as voltage, cathode shape, acid flow and temperature were investigated. Low-voltage (<7V), high acid flow (25L/min) and low acid temperature (20°C) are considered as promising parameters. Such recipe was tested on single-cell and 9-cell ILC cavities with nice surface finishing. After 60 μm VEP on a HEP'ed single-cell, the cavity show similar performance at 1.6K compared to previous Horizontal EP: (Eacc > 41MV/m) limited by quench. Another cavity reaches 36MV/m after 300μm removal by VEP in spite of a pitted surface due to initial VEP treatment at higher temperature (> 30°C). The baking effect after HEP/VEP is similar. An asymmetric niobium removal is observed with faster polishing in the upper cell. Nice surface finishing as well as standard Q0 value are obtained at low/medium field on 9Cell but achieved performance is limited by Field Emission.
*F. Eozenou et al., PRST-AB, 15, 083501 (2012)

TUP053

Estimation of Small Geometry Deviation for TESLA-Shape Cavities Due to Inner Surface Polishing

cavity, coupling, factory, data-analysis

537

A.A. Sulimov, G. Kreps, J.K. Sekutowicz
DESY, Hamburg, Germany

Two well know polishing methods are used for the inner surface cleaning of superconducting TESLA-shape cavities: electro-polishing (EP) or buffered chemical polishing (BCP). The amount of removed material is relatively small and varies from 10 till 140 um. The cavity after polishing is closed to prevent the scratches or dust appearing on its inner surface. The estimation of the removed material amount is possible by different criteria, for example by comparison of weight before and after cleaning, or by the time - cleaning procedure duration. Both calculations could give us only approximate average value of the removed material amount. We describe the method for estimation of small geometry deviation basing on RF frequency measurements, which allows calculating the different influence of surface treatment on the iris and equator areas.

Poster TUP053 [0.785 MB]

TUP064

Exploration of Material Removal Rate of SRF Elliptical Cavities as a Function of Media Type and Cavity Shape on Niobium and Copper Using Centrifugal Barrel Polishing (CBP)

cavity, niobium, SRF, status

579

A.D. Palczewski, G. Ciovati, R.L. Geng, Y.M. Li
JLAB, Newport News, Virginia, USA

Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
Centrifugal barrel polishing (CBP) for SRF application is becoming more wide spread as the technique for cavity surface preparation. CBP is now being used in some form at SRF laboratories around the world. Before the process can become as mature as wet chemistry like eletro-polishing (EP) and buffered chemical polishing (BCP) there are many questions which remain unanswered. One of these topics includes the uniformity of removal as a function of cavity shape and material type. In this presentation we show CBP removal rates for various media types on 1.3 GHz TESLA and 1.5 GHz CEBAF large grain niobium cavities, 1.3 GHz TESLA fine grain niobium cavity, and 1.3GHz low surface field copper cavity. The data will also include calculated RF frequency shift modeling non-uniform removal as a function of cavity position and comparing them with CBP results.

TUP067

Hydrogen Saturation and the Thermal Conductivity of Superconducting Niobium

niobium, cavity, vacuum, lattice

589

S.K. Chandrasekaran
MSU, East Lansing, USA
T.R. Bieler
Michigan State University, East Lansing, USA
C. Compton
FRIB, East Lansing, USA
N.T. Wright
(MSU), East Lansing, USA

Funding: This work was supported by the U.S. Department of Energy, Office of High Energy Physics, through Grant No. DE-S0004222
The thermal conductivity k of Nb at less than 3 K is dominated by phonon transport. In Nb with sufficiently few lattice imperfections, a maximum in k occurs at 1.8 K, called the phonon peak (PP). A large PP is desired to reduce potential local hot spots and contributes to an increased Q factor. The magnitude of the PP is sensitive to SRF cavity manufacturing processes. The effect of interstitial hydrogen on the magnitude of the PP is examined by subjecting two bicrystal Nb specimens to 300 C for 1 h in a 75% H2, 25% N2 atmosphere at 0.5 atm. Prior to hydrogen infusion, specimen 1 was heated to 800 C for 2 h, while specimen 2 was heated to 1100 C for 4 h. Both specimens displayed a 25% reduction in the PP due to the additional hydrogen, independent of their crystal orientations and heat treatment histories. An 800 C vacuum heating for 2 h was found to be sufficient to recover the PP in specimen 1, while an 1100 C heating for 4 h was required to recover the PP in one of the grains of specimen 2. The results suggest that hydrogen trapped in the Nb lattice will degas when the Nb is heated to at least the temperature to which it was heated at prior to the hydrogen infusion step.

TUP100

Medium Field Q-Slope Studies in High Frequency Cavities

cavity, operation, niobium, factory

705

O.S. Melnychuk, A. Grassellino, A.I. Sukhanov
Fermilab, Batavia, USA

A phenomenon of Medium Field Q-Slope (MFQS) in superconducting RF cavities is of high importance because it occurs in the field range (5-20MV/m) that includes designed operation fields of future CW accelerators. MFQS impacts resistive losses in the cavity and, consequently, directly affects accelerator operation costs. We present studies of MFQS based on vertical test data for 1.3GHz nine-cell cavities and make comparisons of vertical test data from different laboratories.

TUP102

Quench Detection Diagnostics on 3.9 GHz XFEL Cavities

cavity, site, detector, diagnostics

710

M. Bertucci, A. Bosotti, L. Garolfi, P. Michelato, L. Monaco, D. Sertore
INFN/LASA, Segrate (MI), Italy
C. Pagani
Università degli Studi di Milano & INFN, Segrate, Italy

This paper presents results of quench localization on 3.9 GHz XFEL prototype cavities tested at LASA vertical test facility. Cavities have been equipped with OST second sound detectors and thermometry sensors. A first guess for quench position has been obtained from modal analysis. Second sound sensors confirmed the quench position resolving also the symmetry degeneracy given by the RF mode pattern analysis. In a subsequent vertical test, second sound and temperature sensors have been installed nearby the suspect quench position. From Thermometry mapping, a sudden increase in cavity temperature within a small region is evident, therefore confirming that a local thermal breakdown due to defect heating occurs in the predicted quench point. The quench region deduced with the mentioned techniques is eventually compared with results of optical inspection.

WEIOB01

Status of MgB2 Coating Studies for SRF Applications

cavity, SRF, controls, background

777

T. Tajima, L. Civale, D.J. Devlin, G.C. Martinez, R.K. Schulze
LANL, Los Alamos, New Mexico, USA

Funding: DOE Office of Science/Nuclear Physics
MgB2 has shown promising results on small samples and its coating development is entering into the stage to coat large samples and elliptical cavities. Several coating techniques that seem to be appropriate for cavity coating and their status will be shown together with some cavity measurement results with either 6 GHz or 1.3 GHz single-cell cavities. Other data such as RF surface resistance at low temperatures and vortex penetration fields with small samples will also be shown.

Slides WEIOB01 [2.183 MB]

WEIOC01

High Resolution Surface Resistance Studies

quadrupole, niobium, cavity, shielding

785

S. Aull, S. Döbert, T. Junginger
CERN, Geneva, Switzerland
S. Aull, J. Knobloch
University of Siegen, Siegen, Germany
J. Knobloch
HZB, Berlin, Germany

Funding: Work supported by the German Doctoral Students program of the Federal Ministry of Education and Research (BMBF).
The attempt to reach quality factors beyond 10¹¹ and pushing the accelerating gradients of SRF cavities to the theoretical limit, the treatment depending loss mechanisms in niobium need better understanding. CERNs Quadrupole Resonator enables sub-nΩ-resolution measurements of the surface resistance. The available parameters cover resonant modes at 400, 800 and 1200 MHz, any temperature up to 15 K and rf fields up to 60 mT. Recently the setup has been extended with a coil creating a dc magnetic field for trapped flux studies. Overall, much more information about the rf performance is accessible compared to regular cavity measurements. Since the samples are flat disks of 75 mm diameter geometric fabrication issues are simplified which makes the Quadrupole Resonator also the perfect tool to study alternative materials or new coating techniques. In this contribution in depth studies of a heat treated bulk niobium sample exploiting the complete parameter range of the setup are presented.

Slides WEIOC01 [2.724 MB]

WEIOC02

Multilayers Activities at Saclay / Orsay

niobium, cavity, SRF, vacuum

789

C. B. Baumier, G. Martinet
IPN, Orsay, France
C.Z. Antoine
CEA/IRFU, Gif-sur-Yvette, France
F. F. Fortuna
CSNSM, ORSAY CAMPUS, France
J.C. Villegier
CEA/INAC, Grenoble Cedex 9, France

In the investigations on the high gradient SRF cavities, the superconducting multilayer is a promising alternative. The predictions show that an SIS (Superconductor/Isolator/Superconductor) nano-composite could improve the efficiency limited by the bulk Nb it-self used today for accelerating cavities. We start, at the IPNO lab in collaboration with the CSNSM lab (CNRS) and Irfu lab (CEA), an experimental study to test the screening effect on multilayer assemblies. Based on 3rd harmonic magnetometer and a TE011 SRF cavity, measurements of first critical magnetic field HC1 and surface resistance of samples have been performed. Along with these first results, we are starting the development of a MBE deposition. This set-up is devoted to optimise the best organisation of the multilayer to produce the model sample, and to find, in a close future, a realistic solution to apply this technique on an accelerating SRF cavity.
Labex P2IO funding

Slides WEIOC02 [3.035 MB]

WEIOC04

Theoretical Field Limits for Multi-Layer Superconductors

SRF, niobium, cavity, experiment

794

S. Posen, M. Liepe
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
G. Catelani
Forschungszentrum Jülich, Peter Gruenberg Institut (PGI-2), Jülich, Germany
J.P. Sethna
Cornell University, Ithaca, New York, USA
M.K. Transtrum
M.D.A.C.C., Houston, Texas, USA

With modern cavity preparation techniques, niobium SRF cavities reach surface magnetic fields very close to the fundamental limit of the superheating field of the material, and researchers are looking to alternative superconductors to sustain even higher fields. However, these materials may have an increased vulnerability to flux penetration at defects, even small ones, as a result of their short coherence lengths. A. Gurevich has proposed [1] a method of mitigating this vulnerability: coating a bulk superconducting cavity with a series of very thin insulating and superconducting films. In this work, we present a thorough mathematical description of the SIS thin films proposed by Gurevich in the language of the SRF community, to help researchers to optimize cavities made from alternative superconductors.
[1] A. Gurevich, Appl. Phys. Lett. 88, 012511 (2006)

Slides WEIOC04 [4.116 MB]

THP005

Characteristics and Fabrication of Spoke Cavities for High-Velocity Applications

cavity, vacuum, operation, target

902

C.S. Hopper, J.R. Delayen, H. Park
ODU, Norfolk, Virginia, USA
J.R. Delayen, H. Park
JLAB, Newport News, Virginia, USA

A 500 MHz, velocity-of-light, two-spoke cavity has been designed and optimized for possible use in a compact light source [1]. Here we present the mechanical analysis and steps taken in fabrication of this cavity at Jefferson Lab.
*[1] T. Satogata et al, “Compact Accelerator Design for a Compact Light Source,” IPAC13, Shanghai, China, May 2013.

THP047

Performance Degradation of a Superconducting Cavity Quenching in Magnetic Field

cavity, cryomodule, RF-structure, superconducting-RF

1013

I. Terechkine, T.N. Khabiboulline, D.A. Sergatskov
Fermilab, Batavia, USA

Although degradation of a superconducting RF (SRF) cavity performance induced by magnetic field trapped in its walls is a well understood phenomenon, a criterion for an acceptable level of magnetic field existing in the vicinity of an SRF cavity and generated after the cavity is cooled down has not been agreed upon. The bulk of superconducting Nb should protect the RF surface of the cavity from the magnetic field on the outside; nevertheless a failure mode exists when the cavity quenches while the external field is applied. The amount of trapped magnetic flux in this case depends on the size of normally conducting zone developed in walls of the cavity during quenching. Although propagation of the normally conducting zone in walls of a cavity can be modeled, no dedicated studies of this process that would include experimental verifications of its impact on the cavity performance could be found. We tried to address his issue in a special study by using as an example a superconducting coil mounted near a quenching cavity; the method and some results of the study can be applied to any RF structure and magnetic system.

THP058

Update on the European XFEL RF Power Input Coupler

simulation, cryogenics, linac, electron

1047

D. Kostin, W.-D. Möller
DESY, Hamburg, Germany
W. Kaabi
LAL, Orsay, France

European XFEL project is being currently realized in Hamburg, Germany. The 1.5 km 17.5 GeV linear electron accelerator is based on the 1.3GHz 9-cell TESLA type SRF cavity. The RF power input coupler design for the E-XFEL is based on well known TTF3 coupler design, used in FLASH accelerator. Coupler design was adapted for the industrial production with some parameters optimisation revisited and simulations done. Results are presented and discussed.