Fundamental R&D - Nb
flux trapping
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TUFUB3 Mapping Flux Trapping in SRF Cavities to Analyze the Impact of Geometry 364
 
  • F. Kramer, J. Knobloch, O. Kugeler, J.M. Köszegi
    HZB, Berlin, Germany
  • J. Knobloch
    University of Siegen, Siegen, Germany
 
  A combined temperature and magnetic field mapping system was used to investigate the impact of an ambient field on trapped flux and on the resulting local surface resistance. For this, a 1.3 GHz TESLA single cell cavity was cooled through the superconducting transition at different magnetic field angles with respect to the cavity axis. The measurements suggest, that the field is trapped homogeneously over the cavity volume, without changing its orientation. Flux trapped perpendicular the surface contributed significantly more to the surface resistance, than trapped flux parallel to the surface.  
slides icon Slides TUFUB3 [12.777 MB]  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUFUB3  
About • paper received ※ 21 June 2019       paper accepted ※ 01 July 2019       issue date ※ 14 August 2019  
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TUFUB4
Microscopic Investigation of Flux Trapping Sites in Bulk Nb  
 
  • M. Martinello
    Fermilab, Batavia, Illinois, USA
 
  Flux expulsion efficiency of SRF cavities is strictly dependent on the properties of the material used for cavity fabrication. Some cavities improve their flux expulsion properties after being heath treated at high temperature (usually larger than 900°C), suggesting that flux trapping sites are intimately related to the material microstructure. In this presentation, results of a detailed microstructure analysis that aims to unavailing which features are culprit of pinning flux in bulk niobium cavities, will be presented.  
slides icon Slides TUFUB4 [31.040 MB]  
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TUFUB5 Effects of Static Magnetic Fields on a Low-frequency TEM Class Superconducting Cavity 370
 
  • M.K. Ng, Z.A. Conway, M.P. Kelly, K.W. Shepard
    ANL, Lemont, Illinois, USA
 
  Systematic studies on the effect of magnetic fields on a 330 MHz superconducting (TEM-mode) half-wave cavity are presented. The practical application of the results is for a possible future 2 K operation in the ATLAS heavy-ion accelerator at Argonne. The low frequency and the integral stainless steel jacket, rather than titanium, provide important new data for this full production model low-beta cavity. The studies include multi-axial magnetic field measurements near the cavity surface due to ambient and applied fields. Cavity performance under different conditions is measured at temperatures ranging between 1.6 K and 4.5 K. A residual resistance of approximately 5-7 nΩ at 1.6 K is observed. Data suggest that an appreciable fraction arises from losses that are not due to flux trapping.  
slides icon Slides TUFUB5 [1.195 MB]  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUFUB5  
About • paper received ※ 24 June 2019       paper accepted ※ 14 August 2019       issue date ※ 14 August 2019  
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TUP052 Design and Commissioning of a Magnetic Field Scanning System for SRF Cavities 547
SUSP031   use link to see paper's listing under its alternate paper code  
 
  • I.P. Parajuli, J.R. Delayen, A.V. Gurevich, J. Nice
    ODU, Norfolk, Virginia, USA
  • G. Ciovati, W.A. Clemens, J.R. Delayen
    JLab, Newport News, Virginia, USA
 
  Funding: Work supported by NSF Grant 100614-010. G. C. is supported by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
Trapped magnetic vortices are one of the leading sources of residual losses in SRF cavities. Mechanisms of flux pinning depend on the materials treatment and cool-down conditions. A magnetic field scanning system using flux-gate magnetometers and Hall probes has been designed and built to allow measuring the local magnetic field of trapped vortices normal to the outer surface of 1.3 GHz single-cell SRF cavities at cryogenic temperatures. Such system will allow inferring the key information about the distribution and magnitude of trapped flux in the SRF cavities for different material, surface preparations and cool-down conditions.
 
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP052  
About • paper received ※ 22 June 2019       paper accepted ※ 30 June 2019       issue date ※ 14 August 2019  
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TUP053 Optimal Thermal Gradient for Flux Expulsion in 600°C Heat-treated CEBAF 12 GeV Upgrade Cavities 550
 
  • R.L. Geng, F. Marhauser, P.D. Owen
    JLab, Newport News, Virginia, USA
 
  We will present results on measurement of flux expulsion in CEBAF 12 GeV upgrade cavities and original CEBAF cavities and the search for optimal thermal gradient for reducing the trapped flux in cavities installed in CEBAF linacs. Preliminary measurements of one C100 cavities has shown that a nearly perfect flux expulsion can be achieved at an optimal thermal gradient - a surprising result contrary to the expectation of zero flux expulsion for 600°C heat treated niobium cavities. These results could lead to a cost-effective path for improving the quality factor of cavities installed in CEBAF and ultimately saving accelerator operation cost.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP053  
About • paper received ※ 24 June 2019       paper accepted ※ 29 June 2019       issue date ※ 14 August 2019  
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TUP054 How Is Flux Expulsion Affected by Geometry: Experimental Evidence and Model 555
 
  • D. Longuevergne
    IPN, Orsay, France
 
  Measurements of magnetic sensitivity to trapped flux on several type of cavity geometries have been performed at IPNO showing a clear geometrical effect. Magnetic sensitivity depends not only on material quality but also on the cavity geometry and on the residual magnetic field orientation. A presentation of experimental data will be done. These will be as well compared to the theoretical magnetic sensitivities calculated thanks to a simple Labview routine  
poster icon Poster TUP054 [1.312 MB]  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP054  
About • paper received ※ 03 July 2019       paper accepted ※ 04 July 2019       issue date ※ 14 August 2019  
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TUP055 Nonlinear Dynamics and Dissipation of Vortex Lines Driven by Strong RF Fields 560
SUSP033   use link to see paper's listing under its alternate paper code  
 
  • 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 icon 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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TUP056 A First-Principles Study on Magnetic Flux Trapping at Niobium Grain Boundaries 565
 
  • P. Garg, K.N. Solanki
    Arizona State University, Tempe, USA
  • T.R. Bieler
    Michigan State University, East Lansing, Michigan, USA
  • L.D. Cooley
    NHMFL, Tallahassee, Florida, USA
 
  Niobium is basis for all superconducting radio frequency cavities, a technology that accelerates charged particle beams to energy levels not possible by other means. When cavities are pushed to limits, significant heating appears at extended material defects, like grain boundaries. Therefore, it is crucial to understand how grain boundary (GB) structure and associated properties lead to trapping of magnetic field, and whether GB itself has any unusual magnetic behavior. Using first-principles calculations, external magnetic field along the GB plane was simulated within an all-electron full-potential linearized augmented plane-wave framework. A ground state with non-zero flux, indicative of flux trapping, was obtained at some grain boundaries, this outcome being influenced strongly by GB local structure. Furthermore, electronic density of states and charge-transfer calculations suggested non-zero spin polarization at grain boundaries, which may be consistent with recent observations of unusual paramagnetic magnetization as a function of specimen surface area for cavity-grade niobium.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP056  
About • paper received ※ 23 June 2019       paper accepted ※ 04 July 2019       issue date ※ 14 August 2019  
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TUP057 Study of Flux Trapping Variability between Batches of Tokyo Denkai Niobium used for the LCLS-II Project and Subsequent 9-cell RF Loss Distribution between the Batches 570
 
  • A.D. Palczewski
    JLab, Newport News, Virginia, USA
  • D. Gonnella
    SLAC, Menlo Park, California, USA
  • O.S. Melnychuk, D.A. Sergatskov
    Fermilab, Batavia, Illinois, USA
 
  Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
During the LCLS-II project a second batch of niobium was procured from Tokyo Denkai Co Ltd in order to make additional cavities. The original production material came from Two vendors Tokyo Denkai Co., Ltd. (TD) and Ningxia Orient Tantalum Industry Co., Ltd. (OTIC/NX)). It was found TD niobium required a lower annealing temperature (900°C) to obtain satisfactory flux expulsion characteristics compared to NX which required a slightly higher annealing temperature (950°-975°C). In order to ensure the new TD material performed equivalent to the niobium produced 4 year before after 900°C annealing; each heat lot of niobium had its flux expulsion characteristics parametrized and custom thermal treatments developed for each lot. Subsequent pure heat lot 9 cell cavities were made and tested. We will look at the flux expulsion characteristics of each lot, and RF loss of the 9-cell cavities produced using the individual heat lots.
 
poster icon Poster TUP057 [1.446 MB]  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP057  
About • paper received ※ 25 June 2019       paper accepted ※ 30 June 2019       issue date ※ 14 August 2019  
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TUP058 Characterization of Small AMR Sensors in Liquid Helium to Measure Residual Magnetic Field on Superconducting Samples 576
 
  • G. Martinet
    IPN, Orsay, France
 
  Trapped residual magnetic flux is responsible of residual resistance degradation on superconducting materials used in SRF technologies. To characterize this effect on superconducting samples, compact sensors are required to mount on sample characterization devices. In this paper, we present results on AMR sensors supplied from different manufacturers in the temperature range from 4.2 K up to 300 K.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP058  
About • paper received ※ 23 June 2019       paper accepted ※ 03 July 2019       issue date ※ 14 August 2019  
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TUP059 Investigation of Trapped Flux Dynamics via DC-Magnetic Quenching 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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TUP060 Development of Temperature and Magnetic Field Mapping System for Superconducting Cavities at KEK 583
SUSP019   use link to see paper's listing under its alternate paper code  
 
  • T. Okada, E. Kako, T. Konomi, H. Sakai, K. Umemori
    Sokendai, Ibaraki, Japan
  • E. Kako, T. Konomi, M. Masuzawa, H. Sakai, K. Tsuchiya, R. Ueki, K. Umemori
    KEK, Ibaraki, Japan
  • A. Poudel, T. Tajima
    LANL, Los Alamos, New Mexico, USA
 
  A temperature and magnetic field mapping system for a single cell superconducting cavity is being developed at KEK. The mapping system is used to observe the temperature distribution and the ambient magnetic field distribution around the outer surface of the cavity. A total of 36 boards at every 10 degrees are attached on the cavity. Each board consists of 15 carbon resistors of 100 Ω at room temperature and 3 AMR sensors of X, Y and Z directions at the equator. The calibration of the resisters and AMR sensors were carefully and precisely carried out at low temperature. The data logging system using NI loggers is enabled to measure within 1 ms in the whole cavity surface. The initial test results in the vertical test of the single-cell cavity will be reported in this paper.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-TUP060  
About • paper received ※ 05 July 2019       paper accepted ※ 05 July 2019       issue date ※ 14 August 2019  
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THP046 Magnetic Field Mapping System for Cornell Sample Host Cavity 961
 
  • S.N. Lobo, M. Liepe, T.E. Oseroff
    Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
 
  Funding: Cornell Laboratory for Accelerator-Based Sciences and Education
Dissipation due to flux trapping is a persistent problem experienced in SRF cavity testing and cryomodule operation. This work addresses accurately and cheaply measuring magnetic fields in a cryostat without using delicate and expensive fluxgate magnetometers. Anisotropic Magnetoresistive (AMR) magnetic field sensors were investigated for the detection of small fields in a cryogenic environment. Initial development of instrumentation using 16 AMR sensors is presented for the purpose of measuring magnetic fields perpendicular the normal of a 5" diameter sample plate on the Cornell sample host cavity.
 
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2019-THP046  
About • paper received ※ 29 June 2019       paper accepted ※ 01 July 2019       issue date ※ 14 August 2019  
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