Fundamental SRF research and development
High quality factors/high gradients
Paper Title Page
MOPMB020 A Comprehensive Picture of Hydride Formation and Dissipation 119
 
  • N. Sitaraman, T. Arias
    Cornell University, Ithaca, New York, USA
  • A.V. Harbick, M.K. Transtrum
    Brigham Young University, Provo, USA
  • M. Liepe
    Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
  
  Funding: This work was supported by the U.S. National Science Foundation under Award PHY-1549132, the Center for Bright Beams.
Research linking surface hydrides to Q-disease, and the subsequent development of methods to eliminate surface hydrides, is one of the great successes of SRF cavity R\&D. We use time-dependent Ginzburg-Landau to extend the theory of hydride dissipation to sub-surface hydrides. Just as surface hydrides cause Q-disease behavior, we show that sub-surface hydrides cause high-field Q-slope (HFQS) behavior. We find that the abrupt onset of HFQS is due to a transition from a vortex-free state to a vortex-penetration state. We show that controlling hydride size and depth through impurity doping can eliminate HFQS. 		 
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-MOPMB020  
About • Received ※ 30 June 2023 — Revised ※ 18 July 2023 — Accepted ※ 19 August 2023 — Issue date ※ 19 August 2023
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MOPMB022 Recent mid-T Single-Cell Treatments R&D at DESY 129
 
  • C. Bate, D. Reschke, J. Schaffran, L. Steder, L. Trelle, H. Weise
    DESY, Hamburg, Germany
  
  The challenge of improving the performance of SRF cavities is being faced worldwide. One approach is to modify the superconducting surface properties through certain baking procedures. Recently a niobium retort furnace placed directly under an ISO4 clean room has been refurbished at DESY. Thanks to an inter-vacuum chamber and cryopumps, with high purity values in the mass spectrum it is working in the UHV range of 2·10-8 mbar. The medium temperature (mid-T) heat treatments around 300°C are promising and successfully deliver reproducible very high Q₀ values of 2-5·1010 at medium field strengths of 16 MV/m. Since the first DESY and ZRI mid-T campaign yielded promising results, further results of 1.3 GHz single-cell cavities are presented here after several modified treatments of the mid-T recipe. In addition, samples were added to each treatment, the RRR value change was examined, and surface analyses were subsequently performed. The main focus of the sample study is the precise role of the changes in the concentration of impurities on the surface. In particular, the change in oxygen content due to diffusion processes is suspected to be the cause of enhancing the performance.  
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-MOPMB022  
About • Received ※ 18 June 2023 — Revised ※ 23 June 2023 — Accepted ※ 26 June 2023 — Issue date ※ 01 July 2023
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MOPMB023 Magnetic Flux Expulsion in TRIUMF’s Multi-Mode Coaxial Cavities 135
SUSPB011   use link to see paper's listing under its alternate paper code  
 
  • R.R. Gregory, T. Junginger, M.W. McMullin
    UVIC, Victoria, Canada
  • T. Junginger, P. Kolb, R.E. Laxdal, M.W. McMullin, Z.Y. Yao
    TRIUMF, Vancouver, Canada
  
  The external magnetic flux sensitivity of SRF cavities is an important characteristic of SRF accelerator design. Previous studies have shown that n-doped elliptical cavities are very sensitive to external fields, resulting in stringent requirements for residual field and cavity cool-down speed. Few such studies have been done on HWRs and QWRs. The impact of applied field direction and cool-down speed of flux expulsion for these cavities is poorly understood. This study explores the effect of these cool-down characteristics on TRIUMF¿s QWR using COMSOL ® simulations and experimental results. This study seeks to maximize the flux expulsion that occurs when a cavity is cooled down through its superconducting temperature. Flux expulsion is affected by the cool-down speed, temperature gradient, and orientation of the cavity relative to an applied magnetic field. It was found that for a vertically applied magnetic field the cool-down speed and temperature gradient did not have a significant effect on flux expulsion. Contrarily, a horizontal magnetic field can be nearly completely expelled by a fast, high temperature gradient cool-down.  
poster icon Poster MOPMB023 [2.191 MB]  
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-MOPMB023  
About • Received ※ 17 June 2023 — Revised ※ 22 June 2023 — Accepted ※ 27 June 2023 — Issue date ※ 30 July 2023
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MOPMB024 Flux Expulsion Studies of Niobium Material of 650 MHz Cavities for PIP-II 141
TUPTB003   use link to see paper's listing under its alternate paper code  
 
  • K.E. McGee
    FRIB, East Lansing, Michigan, USA
  • F. Furuta, M. Martinello, O.S. Melnychuk, A.V. Netepenko, G. Wu, Y. Xie
    Fermilab, Batavia, Illinois, USA
  
  Funding: This manuscript has been authored by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the U.S. Department of Energy, Office of Science, Office of High Energy Physics.
Two different vendors supplied the niobium sheet material for PIP-II 5-cell 650 MHz cavities, which was characterized by multiple different ASTM sizes. Cavities subsequently fabricated from these sheets were heat-treated at various temperatures, then the cavities’ flux-expulsion performance was measured. Where the initial measurements of vendor O materials showed that nearly all flux remained trapped despite a high thermal gradient, 900C heat treatment subsequently improved the flux expulsion to an acceptable rate. Understanding and characterizing vendor O materials in this way is key for upcoming and future projects planning to employ niobium sheet from this supplier. 		 
poster icon Poster MOPMB024 [4.064 MB]  
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-MOPMB024  
About • Received ※ 26 June 2023 — Revised ※ 27 June 2023 — Accepted ※ 19 August 2023 — Issue date ※ 21 August 2023
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MOPMB026 Development of Transformative Cavity Processing - Superiority of Electropolishing on High Gradient Performance over Buffered Chemical Polishing at Low Frequency (322 MHz) 145
 
  • K. Saito, C. Compton, K. Elliott, W. Hartung, S.H. Kim, T.K. Konomi, E.S. Metzgar, S.J. Miller, L. Popielarski, A.T. Taylor, T. Xu
    FRIB, East Lansing, Michigan, USA
  
  Funding: The work is supported by DOE Awards DE-SC0022994.
A DOE grant R&D titled ¿Development of Transformative Preparation Technology to Push up High Q/G Performance of FRIB Spare HWR Cryomodule Cavities¿ is ongoing at FRIB. This R&D is for 2 years since September 2022. This project proposes four objectives: 1) Superiority on high gradient performance of electropolishing (EP) over buffered chemical polishing at low frequency (322 MHz), 2) High Qo performance by the local magnetic shield, 3) Development of HFQS-free BCP and, 4) Wet N-doping method. This paper will report the result of first object, and a local magnetic shield design and simulation to reduce the residual magnetic field < 0.1 mG in the vertical test Dewar, for the object 2. 		 
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-MOPMB026  
About • Received ※ 14 June 2023 — Revised ※ 23 June 2023 — Accepted ※ 26 June 2023 — Issue date ※ 08 July 2023
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MOPMB027 Successful Superheating Field Formulas from an Intuitive Model 151
 
  • K. Saito, T. Konomi
    FRIB, East Lansing, Michigan, USA
  
  Funding: Work supported by the U.S. Department of Energy Office of Science DE-S0000661 and the National Science Foundation under Cooperative Agreement PHY-1102511
To date, many theoretical formulas for superheating field on SRF cavity are already proposed based rather complicated calculations. This paper proposes the formulas by a very intuitive simple model: energy balance between RF magnetic energy and superconducting condensed one, and a condition of vanishing the mirror vortex line image. The penetration of a single vortex determines the superheating field for a type II superconductor. On the other hand, for type I superconductors, the surface flux penetration determines it. The formula fits very well quantitatively the results of niobium cavity and Nb₃Sn one. In addition, it gives a nice guideline for new material beyond niobium.
male, senior 		 
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-MOPMB027  
About • Received ※ 23 June 2023 — Revised ※ 24 June 2023 — Accepted ※ 28 June 2023 — Issue date ※ 15 July 2023
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MOPMB029 Exploring the Dynamics of Transverse Inter-Planar Coupling in the Superconducting Section of the PIP-II Linac 155
 
  • A. Pathak
    Fermilab, Batavia, Illinois, USA
  • E. Pozdeyev
    JLab, Newport News, USA
  
  This study investigates the crucial role that an accurate understanding of inter-planar coupling in the transverse plane plays in regulating charged particle dynamics in a high-intensity linear accelerator and minimizing foil/septum impacts during injection from the linac to a ring. We in-depth analyze the emergence and evolution of transverse inter-planar coupling through multiple active lattice elements, taking into account space charge and field nonlinearities in the superconducting section of the PIP-II linac. The article compares various analytical, numerical, and experimental techniques for measuring transverse coupling using beam and lattice matrices and provides insight into effective strategies for its mitigation prior to ring injectio  
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-MOPMB029  
About • Received ※ 21 June 2023 — Revised ※ 24 June 2023 — Accepted ※ 27 June 2023 — Issue date ※ 05 July 2023
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MOPMB030 Medium Temperature Furnace Baking of Low-beta 650 MHz Five-cell Cavities 158
 
  • G. Wu, S.K. Chandrasekaran, V. Chouhan, G.V. Eremeev, F. Furuta, K.E. McGee, A.A. Murthy, A.V. Netepenko, J.P. Ozelis, H. Park, S. Posen
    Fermilab, Batavia, Illinois, USA
  
  Funding: This manuscript has been authored by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the U.S. Department of Energy, Office of Science, Office of High Energy Physics.
Medium Temperature baking of low beta 650 MHz cavities was conducted in a UHV furnace. A systematic study of cavity surface resistance components, residual and BCS, was conducted, including analyzing surface resistance due to trapped magnetic flux. Cavities showed an average 4.5 nano-ohm surface resistance at 17 MV/m under 2 K, which meets PIP-II specifications with a 40% margin. The results provided helpful information for the PIP-II project to optimize the cavity processing recipe for cryomodule application. The results were compared to the 1.3 GHz cavity that received a similar furnace baking. 		 
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-MOPMB030  
About • Received ※ 19 June 2023 — Revised ※ 23 June 2023 — Accepted ※ 26 June 2023 — Issue date ※ 18 July 2023
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MOPMB032 The Collaborative Effects of Intrinsic and Extrinsic Impurities in Low RRR SRF Cavities 162
SUSPB012   use link to see paper's listing under its alternate paper code  
 
  • K. Howard, Y.K. Kim
    University of Chicago, Chicago, Illinois, USA
  • D. Bafia, A. Grassellino
    Fermilab, Batavia, Illinois, USA
  
  Funding: This manuscript has been authored by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the U.S. Department of Energy, Office of Science, Office of High Energy Physics.
The SRF community has shown that introducing certain impurities into high-purity niobium can improve quality factors and accelerating gradients. We question why some impurities improve RF performance while others hinder it. The purpose of this study is to characterize the impurity profile of niobium coupons with a low residual resistance ratio (RRR) and correlate these impurities with the RF performance of low RRR cavities so that the mechanism of impurity-based improvements can be better understood and improved upon. The combination of RF testing and material analysis reveals a microscopic picture of why low RRR cavities experience low BCS resistance behavior more prominently than their high RRR counterparts. We performed surface treatments, low temperature baking and nitrogen-doping, on low RRR cavities to evaluate how the intentional addition of oxygen and nitrogen to the RF layer further improves performance through changes in the mean free path and impurity profile. The results of this study have the potential to unlock a new understanding on SRF materials and enable the next generation of high Q/high gradient surface treatments. 		 
poster icon Poster MOPMB032 [1.444 MB]  
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-MOPMB032  
About • Received ※ 21 June 2023 — Revised ※ 22 June 2023 — Accepted ※ 26 June 2023 — Issue date ※ 23 July 2023
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MOPMB033 Efforts to Suppress Field Emission in SRF Cavities at KEK 167
 
  • M. Omet, H. Araki, T. Dohmae, H. Ito, R. Katayama, K. Umemori, Y. Yamamoto
    KEK, Ibaraki, Japan
  
  Our main objective is to achieve as high as possible quality factors Q₀ and maximal accelerating voltages Eacc within 1.3 GHz superconducting radio frequency (SRF) cavities. Beside an adequate surface treatment, key to achieve good performance is a proper assembly in the clean room prior cavity testing or operation. In this contribution we present the methods and results of our efforts to get a better understanding of our clean room environment and the particulate generation caused during the assembly work. Furthermore, we present the measures taken to suppress filed emission, followed by an analysis of vertical test results of the last six years.  
poster icon Poster MOPMB033 [1.532 MB]  
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-MOPMB033  
About • Received ※ 14 June 2023 — Revised ※ 25 June 2023 — Accepted ※ 02 September 2023 — Issue date ※ 02 September 2023
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MOPMB036 Magnetic Field Mapping of a Large-Grain 1.3 GHz Single-Cell Cavity 172
 
  • I.P. Parajuli, J.R. Delayen, A.V. Gurevich
    ODU, Norfolk, Virginia, USA
  • G. Ciovati
    JLab, Newport News, Virginia, USA
  
  Funding: This work was supported by the National Science Foundation under Grant No. PHY 100614-010. G.C. is supported by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
A new magnetic field mapping system for 1.3 GHz single-cell cavities was developed in order to reveal the impact of ambient magnetic field and temperature gradients during cool-down on the flux trapping phenomenon. Measurements were done at 2 K for different cool-down conditions of a large-grain cavity before and after 120 °C bake. The fraction of applied magnetic field trapped in the cavity walls was ~ 50% after slow cool-down and ~20% after fast cool-down. The results showed a weak correlation between between trapped flux locations and hot-spots causing the high-field Q-slope. The results also showed an increase of the trapped flux at the quench location, after quenching, and a local redistribution of trapped flux with increasing RF field. 		 
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-MOPMB036  
About • Received ※ 15 June 2023 — Revised ※ 23 June 2023 — Accepted ※ 26 June 2023 — Issue date ※ 05 July 2023
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MOPMB037 Exploration of Parameters that Affect High Field Q-Slope 178
SUSPB013   use link to see paper's listing under its alternate paper code  
 
  • K. Howard, Y.K. Kim
    University of Chicago, Chicago, Illinois, USA
  • D. Bafia, A. Grassellino
    Fermilab, Batavia, Illinois, USA
  
  Funding: This manuscript has been authored by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the U.S. Department of Energy, Office of Science, Office of High Energy Physics.
The onset of high field Q-slope (HFQS) around 25 MV/m prevents cavities in electropolished (EP) condition from reaching high quality factors at high gradients due to the precipitation of niobium hydrides during cooldown. These hydrides are non-superconducting at 2 K, and contribute to losses such as Q disease and HFQS. We are interested in exploring the parameters that affect the behavior of HFQS. We study a high RRR cavity that received an 800 C by 3 hour bake and EP treatment to observe HFQS. First, we explore the effect of trapped magnetic flux. The cavity is tested after cooling slowly through Tc while applying various levels of ambient field. We observe the onset of the HFQS and correlate this behavior with the amount of trapped flux. Next, we investigate the effect of the size/concentration of hydrides. The cavity is tested after holding the temperature at 100 K for 12 hours during the cooldown to promote the growth of hydrides. We can correlate the behavior of the HFQS with the increased hydride concentration. Our results will help further the understanding of the mechanism of HFQS. 		 
poster icon Poster MOPMB037 [1.648 MB]  
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-MOPMB037  
About • Received ※ 12 June 2023 — Revised ※ 22 June 2023 — Accepted ※ 26 June 2023 — Issue date ※ 19 August 2023
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MOPMB038 Temperature Mapping for Coaxial Cavities at TRIUMF 183
 
  • P. Kolb, T. Junginger, J.J. Keir, R.E. Laxdal, B. Matheson, Z.Y. Yao
    TRIUMF, Vancouver, Canada
  • H. Al Hassini, T. Junginger
    UVIC, Victoria, Canada
  • L. Fearn
    UW/Physics, Waterloo, Ontario, Canada
  
  Temperature mapping (T-map) on superconducting radio-frequency (SRF) cavities has been shown as a useful tool to identify defects and other abnormal sources of losses. So far T-map systems have only been realized for elliptical cavities that have an easily accessible outer surface. TEM mode cavities such as quarterwave and halfwave resonators (QWR, HWR) dissipate most of their power on the inner conductor of the coaxial structure. The limited access and constrained space are a challenge for the design of a temperature mapping system. This paper describes the mechanical and electrical design including the data acquisition of a T-map system for the TRIUMF multi-mode coaxial cavities, and first results are shown.  
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-MOPMB038  
About • Received ※ 20 June 2023 — Revised ※ 23 June 2023 — Accepted ※ 26 June 2023 — Issue date ※ 30 June 2023
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MOPMB040 Comparing the Effectiveness of Low Temperature Bake in EP and BCP Cavities 187
SUSPB014   use link to see paper's listing under its alternate paper code  
 
  • H. Hu, Y.K. Kim
    University of Chicago, Chicago, Illinois, USA
  • D. Bafia
    Fermilab, Batavia, Illinois, USA
  
  Funding: This manuscript has been authored by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the U.S. Department of Energy, Office of Science, Office of High Energy Physics.
Electropolishing (EP) and buffered chemical polishing (BCP) are conventional surface preparation techniques for superconducting radiofrequency (SRF) cavities. Both EP and BCP treated SRF cavities display high field Q-slope (HFQS) which degrades performance at high gradients. While high gradient performance in EP cavities can be improved by introducing oxygen via a low temperature bake (LTB) of 120°C by 48 hours, LTB does not consistently remove HFQS in BCP cavities. There is no consensus as to why LTB is not effective on BCP prepared cavities. We examine quench in EP, BCP, EP+LTB, and BCP+LTB treated 1.3 GHz single-cell Nb cavities by studying the heating behavior with field using a temperature mapping system. Cavity performance is correlated to characterizations of surface impurity profile obtained via time of flight secondary ion mass spectrometry studies. We observe a difference in near surface hydrogen concentration following BCP compared to EP that may suggest that the causes of quench in EP and BCP cavities are different. 		 
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-MOPMB040  
About • Received ※ 14 June 2023 — Revised ※ 28 June 2023 — Accepted ※ 29 June 2023 — Issue date ※ 03 July 2023
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MOPMB041 Microstructure Development in a Cold Worked SRF Niobium Sheet After Heat Treatments 191
 
  • S. Balachandran, P. Dhakal, A-M. Valente-Feliciano
    JLab, Newport News, Virginia, USA
  • T.R. Bieler
    Michigan State University, East Lansing, Michigan, USA
  • S. Chetri, P.J. Lee
    NHMFL, Tallahassee, Florida, USA
  • Z.L. Thune
    MSU, East Lansing, USA
  
  Funding: Jefferson Science Associates, LLC under U.S. DOE Grant DEAC05-06OR23177, U.S. DOE, Office of HEP under Grant DE-SC0009960, and NHMFL through NSF Grant DMR-1644779 and the State of Florida.
Bulk Nb for TESLA shaped SRF cavities is a mature technology. Significant advances are in order to push Q0’s to 1010-11(T= 2K), and involve modifications to the sub-surface Nb layers by impurity doping. In order to achieve the lowest surface resistance any trapped flux needs to be expelled for cavities to reach high Q0’s. There is clear evidence that cavities fabricated from polycrystalline sheets meeting current specifications require higher temperatures beyond 800 °C leads to better flux expulsion, and hence improves Q0. Recently, cavities fabricated with a non-traditional Nb sheet with initial cold work due to cold rolling expelled flux better after 800 °C/3h heat treatment than cavities fabricated using fine-grain poly-crystalline Nb sheets. Here, we analyze the microstructure development in Nb from the vendor supplied cold work non- annealed sheet that was fabricated into an SRF cavity as a function of heat treatment building upon the methodology development to analyze microstructure being developed by the FSU-MSU-UT, Austin-JLAB collaboration. The results indicate correlation between full recrystallization and better flux expulsion. 		 
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-MOPMB041  
About • Received ※ 19 June 2023 — Revised ※ 23 June 2023 — Accepted ※ 26 June 2023 — Issue date ※ 09 July 2023
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MOPMB043 Characterization of Dissipative Regions of an N-Doped SRF Cavity 202
 
  • E.M. Lechner, G. Ciovati
    JLab, Newport News, Virginia, USA
  • G. Ciovati, A.V. Gurevich, J. Makita
    ODU, Norfolk, Virginia, USA
  • M. Iavarone, E.M. Lechner, B.D. Oli
    Temple University, Philadelphia, USA
  
  Funding: DE-AC05-06OR23177 NSF Award No. 1734075 W911NF-16-2-0189
We report scanning tunneling microscopy measurements on N-doped cavity hot and cold spot cutouts. Analysis of the electron tunneling spectra using a proximity effect theory shows that hot spots have a reduced superconducting gap and a wider distribution of the contact resistance. Alone, these degraded superconducting properties account for a much weaker excess dissipation as compared with the vortex contribution. Based on the correlation between the quasiparticle density of states and temperature mapping, we suggest that degraded superconducting properties may facilitate vortex nucleation or settling of trapped flux during cooling the cavity through the critical temperature. 		 
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-MOPMB043  
About • Received ※ 19 June 2023 — Revised ※ 23 June 2023 — Accepted ※ 29 June 2023 — Issue date ※ 13 July 2023
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MOPMB044 Topographic Evolution of Nitrogen Doped Nb Subjected to Electropolishing 207
 
  • E.M. Lechner, C.G. Baxley, M.J. Kelley, C.E. Reece
    JLab, Newport News, Virginia, USA
  • J.W. Angle, M.J. Kelley
    Virginia Polytechnic Institute and State University, Blacksburg, USA
  
  Funding: DE-AC05-06OR23177 DE-SC-0014475
Surface quality is paramount in facilitating high perfor-mance SRF cavity operation. Here, we investigate the topographic evolution of samples subjected to N-doping and 600 °C vacuum anneal. We show that in N-doped Nb, niobium nitrides may grow continuously along grain boundaries. Upon electropolishing high slope angle grooves are revealed which sets up a condition that may facilitate a supression of the superheating field. 		 
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-MOPMB044  
About • Received ※ 19 June 2023 — Revised ※ 23 June 2023 — Accepted ※ 27 June 2023 — Issue date ※ 17 July 2023
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MOPMB050 Thermal Feedback in Coaxial SRF Cavities 224
SUSPB020   use link to see paper's listing under its alternate paper code  
 
  • M.W. McMullin, P. Kolb, R.E. Laxdal, Z.Y. Yao
    TRIUMF, Vancouver, Canada
  • T. Junginger
    UVIC, Victoria, Canada
  
  Funding: Natural Sciences and Engineering Research Council of Canada
The phenomenon of Q-slope in SRF cavities is caused by a combination of thermal feedback and field-dependent surface resistance. There is currently no commonly accepted model of field-dependent surface resistance, and studies of Q-slope generally treat thermal feedback as a correction to whichever surface resistance model is being used. In the present study, we treat thermal feedback as a distinct physical effect whose effect on Q-slope is calculated using a novel finite-element code. We performed direct measurements of liquid helium pool boiling from niobium surfaces to obtain input parameters for the finite-element code. This code was used to analyze data from TRIUMF’s coaxial test cavity program, which has provided a rich dataset of Q-curves at temperatures between 1.7 K and 4.4 K at five different frequencies. Preliminary results show that thermal feedback makes only a small contribution to Q-slope at temperatures near 4.2 K, but has stronger effects as the bath temperature is lowered. 		 
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-MOPMB050  
About • Received ※ 17 June 2023 — Revised ※ 22 June 2023 — Accepted ※ 26 June 2023 — Issue date ※ 09 August 2023
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MOPMB076 Surface Characterization Studies of Gold-Plated Niobium 290
SUSPB024   use link to see paper's listing under its alternate paper code  
 
  • S.G. Seddon-Stettler, M. Liepe, T.E. Oseroff, Z. Sun
    Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
  • N. Sitaraman
    Cornell University, Ithaca, New York, USA
  
  Funding: The National Science Foundation, Grant No. PHY-1549132
The native niobium oxide layer present on niobium has been shown to affect the performace of superconducting RF cavities. Extremely thin layers of gold on the surface of niobium have the potential to suppress surface oxidation and improve cavity performance. However, depositing uniform layers of gold at the desired thickness (sub-nm) is difficult, and different deposition methods may have different effects on the gold surface, on the niobium surface, and on the interface between the two. In particular, the question of whether gold deposition actually passivates the niobium oxide is extremely relevant for assessing the potential of gold deposition to improve RF performance. This work builds on previous research studying the RF performance of gold/niobium bilayers with different gold layer thicknesses. We here consider alternative methods to characterize the composition and chemical properties of gold/niobium bilayers to supplement the previous RF study. 		 
poster icon Poster MOPMB076 [1.536 MB]  
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-MOPMB076  
About • Received ※ 25 June 2023 — Revised ※ 27 June 2023 — Accepted ※ 29 June 2023 — Issue date ※ 03 July 2023
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TUIBA01 A Three-Fluid Model of Dissipation at Surfaces in Superconducting Radiofrequency Cavities 361
 
  • M.M. Kelley, T. Arias, S. Deyo, D. Liarte, J.P. Sethna, N. Sitaraman
    Cornell University, Ithaca, New York, USA
  • M. Liepe, T.E. Oseroff
    Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
  
  Funding: This work was supported by the U.S. National Science Foundation under Award PHY-1549132, the Center for Bright Beams.
Experiments on superconducting cavities have found that under large RF fields the quality factor can improve with increasing field amplitude, a so-called anti-Q slope. We numerically solve the Bogoliubov-de Gennes equations at a superconducting surface in a parallel magnetic field, finding at large fields there are surface quasiparticle states with energies below the bulk superconducting gap that emerge and disappear as the field cycles. Modifying the standard two-fluid model, we introduce a ‘‘three’’-fluid model where we partition the normal fluid to consider continuum and surface quasiparticle states separately. We compute dissipation in a semi-classical theory of conductivity, where we provide physical estimates of elastic scattering times of Bogoliubov quasiparticles with point-like impurities having potential strengths informed from complementary ab initio calculations of impurities in bulk niobium. We show, in this simple yet effective framework, how the relative scattering rates of surface and continuum quasiparticle states can play a role in producing an anti-Q slope while demonstrating how this model naturally includes a mechanism for turning the anti-Q slope on and off.
S. Deyo, M. Kelley et al. Phys. Rev. B 106, 104502 (2022) 		 
slides icon Slides TUIBA01 [2.019 MB]  
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-TUIBA01  
About • Received ※ 19 June 2023 — Revised ※ 23 June 2023 — Accepted ※ 28 June 2023 — Issue date ※ 08 July 2023
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TUIBA02
 Vacancy Dynamics in Niobium and Its Native Oxides and Their Potential Implications for Quantum Computing and Superconducting Accelerators  
 
  • M. Wenskat
    DESY, Hamburg, Germany
  
  Funding: This work was supported by the BMBF under the research grants 05K19GUB and 05H2021.
In recent years, superconducting radio-frequency (SRF) cavities have been considered as candidates for qubits in quantum computing, showing longer photon lifetimes and, therefore, longer decoherence times of a cavity stored qubit compared to many other realizations. In modern particle accelerators, SRF cavities are the workhorse. Continuous research and development efforts are being undertaken to improve their properties, i.e., to increase the accelerating field and lower the surface resistance, which in turn increase the energy reach and duty cycle of accelerators. While some experimental milestones have been achieved, the mechanisms behind the still observed losses remain not fully understood. This talk will show that a recently reported temperature treatment of Nb SRF cavities in the temperature range of 573-673 K, which reduces the residual surface resistance to unprecedented values, is linked to a reorganization of the niobium oxide and near-surface vacancy structure and that this reorganization can explain the observed improved performance in both applications, quantum computing and SRF cavities. 		 
slides icon Slides TUIBA02 [2.352 MB]  
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TUCBA01 Measurements of the Amplitude-Dependent Microwave Surface Resistance of a Proximity-Coupled Au/Nb Bilayer 369
 
  • T.E. Oseroff, M. Liepe, Z. Sun
    Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
  
  A sample host cavity is used to measure the surface resistance of a niobium substrate with a gold film deposited in place of its surface oxide. This talk will report about this measurement result. The film thickness of the gold layer was increased from 0.1 nm to 2.0 nm in five steps to study the impact of the normal layer thickness. The 0.1 nm film was found to reduce the surface resistance below its value with the surface oxide present and to enhance the quench field. The magnitude of the surface resistance increased substantially with gold film thickness. The surface resistance field-dependence appeared to be independent from the normal layer thickness. The observations reported in this work have profound implications for both low-field and high-field S.C. microwave devices. By controlling or eliminating the niobium oxide using a gold layer to passivate the niobium surface, it may be possible to improve the performance of SRF cavities used for particle acceleration. This method to reduce surface oxidation while maintaining low surface resistance could also be relevant for minimizing dissipation due to two-level systems observed in low-field low-temperature devices.  
slides icon Slides TUCBA01 [2.292 MB]  
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-TUCBA01  
About • Received ※ 19 June 2023 — Revised ※ 29 June 2023 — Accepted ※ 30 June 2023 — Issue date ※ 26 July 2023
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TUIXA01
 Understanding the Field and Frequency Dependence of Rf Loss in SRF Cavities  
 
  • P. Dhakal, G. Ciovati
    JLab, Newport News, Virginia, USA
  • G. Ciovati, A.V. Gurevich, B.D. Khanal
    ODU, Norfolk, Virginia, USA
  
  Funding: This is authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05- 06OR23177.
SRF cavities subjected to heat treatment below 200 °C in the presence of nitrogen showed an improvement in quality factor while maintaining an accelerating gradient above 25 MV/m. Here, we report the rf performance of several single-cell superconducting radio frequency cavities with frequency ranging from 0.75 - 3.0 GHz subjected to low temperature heat treatment in nitrogen environment. The cavities were treated at temperature 120 - 175 oC for 24 - 48 hours in low partial pressure of ultra-pure nitrogen gas. The improvement in Q₀ with Q-rise was observed when nitrogen gas was injected ~300 °C during the furnace treatment. The surface modification was confirmed by the change in electronic mean free path and near surface elemental analysis by SIMS. The field dependence of the rf losses is strongly correlated to the cavity frequency. The analysis of experimental data with available theoretical models as well as comparison with similar study on high temperature nitrogen doped cavities will be presented. 		 
slides icon Slides TUIXA01 [4.416 MB]  
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TUIXA02
 The Role of Nitrogen and Other Impurities in SRF Cavity Performance  
 
  • D. Bafia
    Fermilab, Batavia, Illinois, USA
  
  Funding: This manuscript has been authored by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the U.S. Department of Energy, Office of Science, Office of High Energy Physics.
Bulk niobium SRF cavities remain as a leading technology in the realization of the next generation of particle accelerators and serve as the highest Q₀ platform for 3-D quantum computing architectures. Whether utilized in kilometer long accelerators or 10 cm long quantum processors, the performance of these cavities is largely determined by the material properties within the 100 nm from the inner RF surface. This talk will discuss advancements made in the development and understanding of surface engineering techniques (doping with O or N, N-infusion, and low/mid temperature baking) on niobium SRF cavities in different regimes: mK and single photon levels for quantum computing and high Q/high G accelerator applications. By coupling material science and resonator measurements, we delineate the role of different impurities in enabling excellent performance in each of these regimes. 		 
slides icon Slides TUIXA02 [6.669 MB]  
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TUIXA03
 Surface Resistance and Trapped Flux Sensitivity as Function of Baking Temperature  
 
  • H. Ito, H. Araki, K. Umemori
    KEK, Ibaraki, Japan
  
  We have investigated the influence of furnace baking at various baking temperatures on Q-value and trapped flux sensitivity. We find that mid-temperature baking is a promising process for obtaining a high Q-value, but it results in a high flux sensitivity. In particular, 300°C baking results in extremely high Q-value and sensitivity. Instead, 250°C baking is found to be a more effective process than 300°C baking for accelerator applications, as it can reach a higher accelerating gradient while keeping a high Q-value and a lower sensitivity. In addition, we find that 200°C baking can reach a higher Q-value at a high accelerating gradient e.g. 35 MV/m compared to 120°C 48 h baking that is applied to the cavity normally.  
slides icon Slides TUIXA03 [32.219 MB]  
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TUIXA04 First Results from beta-SRF- Testing SRF Samples at High Parallel Field 374
 
  • E. Thoeng, R. Kiefl, W.A. MacFarlane, J.O. Ticknor
    UBC & TRIUMF, Vancouver, British Columbia, Canada
  • M. Asaduzzaman, S.R. Dunsiger, D. Fujimoto, T. Junginger, V.L. Karner, P. Kolb, R.E. Laxdal, R. Li, R.M.L. McFadden, G. Morris, M. Stachura
    TRIUMF, Vancouver, Canada
  • T. Junginger, R.M.L. McFadden
    UVIC, Victoria, Canada
  
  The new ¿-SRF facility at TRIUMF has recently been commissioned. A new 1 m extension has been added to an existing ¿-NMR beamline with a large Helmholtz coil to produce fields up to 200 mT parallel to sample surfaces. The ¿-NMR technique allows depth dependent characterization of the local magnetic field in the first 100 nm of the sample surface making the probe ideal for studying Meissner screen- ing in heat treated Niobium or layered SRF materials. First measurements of Meissner screening at fields up to 200 mT have been analyzed. The results show clear differences in the Meissner screening of baseline treatments compared to mid-T baked (O-doped) Niobium.  
slides icon Slides TUIXA04 [1.644 MB]  
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-TUIXA04  
About • Received ※ 24 June 2023 — Revised ※ 28 June 2023 — Accepted ※ 06 July 2023 — Issue date ※ 09 July 2023
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TUCXA01 Study of the Dynamics of Flux Trapping in Different SRF Materials 380
 
  • F. Kramer, S. Keckert, J. Knobloch, O. Kugeler
    HZB, Berlin, Germany
  • J. Knobloch
    University of Siegen, Siegen, Germany
  • T. Kubo
    KEK, Ibaraki, Japan
  
  A dedicated experimental setup to measure magnetic flux dynamics and trapped flux in samples is used to precisely map out how trapped flux is influenced by different parameters. The setup allows for rapid thermal cycling of the sample so that effects of cooldown parameters can be investigated in detail. We show how temperature gradient, cooldown rate, and the magnitude of external field influence trapped flux in large grain, fine grain and coated niobium samples. The detailed measurements show unexpected results, namely that too fast cooldowns increase trapped flux, large grain material traps flux only when the external field is larger than a temperature gradient dependent threshold field, and the measured dependence of trapped flux on temperature gradient does not agree with an existing model. Therefore, a new model is presented which agrees better with the measured results.  
slides icon Slides TUCXA01 [3.180 MB]  
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-TUCXA01  
About • Received ※ 17 June 2023 — Revised ※ 23 June 2023 — Accepted ※ 26 June 2023 — Issue date ※ 26 June 2023
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TUPTB002 Modelling Trapped Flux in Niobium 393
SUSPB017   use link to see paper's listing under its alternate paper code  
 
  • F. Kramer, S. Keckert, J. Knobloch, O. Kugeler
    HZB, Berlin, Germany
  • J. Knobloch
    University of Siegen, Siegen, Germany
  • T. Kubo
    KEK, Ibaraki, Japan
  
  Detailed measurements of magnetic flux dynamics and trapped magnetic flux in niobium samples were conducted with a new experimental setup that permits precise control of the cooldown parameters. With this setup the dependency of trapped flux on the temperature gradient, external magnetic field, and cooldown rate can be mapped out in more detail compared to cavity measurements. We have obtained unexpected results, and an existing model describing trapped flux in dependence of temperature gradient does not agree with the measured data. Therefore, a new model is developed which describes the magnitude of trapped flux in dependence of the temperature gradient across the sample during cooldown. The model describes the amount of trapped flux lines with help of a density distribution function of the pinning forces of pinning centers and the thermal force which can de-pin flux lines from pinning centers. The model shows good agreement with the measured data and correctly predicts trapped flux at different external flux densities.  
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-TUPTB002  
About • Received ※ 17 June 2023 — Revised ※ 22 June 2023 — Accepted ※ 26 June 2023 — Issue date ※ 13 July 2023
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WEIAA01
 The Frequency Shift and Q of Disordered Superconducting RF Cavities  
 
  • H. Ueki, J.A. Sauls, M. Zarea
    LSU, Baton Rouge, USA
  
  Funding: U.S. Department of Energy, Office of Science, National Quantum Information Science Research Centers, Superconducting Quantum Materials and Systems Center (SQMS) under contract No. DE-AC02-07CH11359.
Superconducting RF (SRF) cavity resonators with ultrahigh-Q, originally developed for particle accelerator technology, are a key technology platform for detectors of rare events, e.g. light by light scattering mediated by virtual electron-positron pairs, axions [1] and high-frequency gravitational waves [2]. The mechanism(s) leading to current limits in Q are not fully understood. We developed a numerical method to calculate Q and cavity resonant frequency shifts based on nonequilibrium theory of superconductivity, including the role of impurity disorder, combined with Slater’s method for solving Maxwell’s equations for the EM field confined in a cavity [3]. Our results for the frequency shift and Q are in excellent agreement with experimental data reported by the SRF group at Fermilab [4]. As a measure of the predictive capability of the theory we are able to quantitatively account for changes in the resonant frequency of order 10 Hz for GHz SRF cavities over temperature ranges of 0.001 Tc. This level of predictive theory is essential for further improvements in performance of superconducting resonators and devices for quantum sensing and quantum processors.
[1] Z. Bogorad et al., Phys. Rev. Lett. 123, 021801 (2019).
[2] A. Berlin et al., arXiv:2303.0151.
[3] H. Ueki, M. Zarea, and J. A. Sauls, arXiv:2207.14236.
[4] D. Bafia et al., arXiv:2103.10601. 		 
slides icon Slides WEIAA01 [4.722 MB]  
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WEIAA02
 Temperature Responses of Superconducting Niobium Properties in Experiment and Simulation  
 
  • Z.T. Yang, J.K. Hao, H. Liu, K.X. Liu, S.W. Quan
    PKU, Beijing, People’s Republic of China
  
  Mild, medium, and high temperature baking has been researched to obtain high-Q₀ SRF niobium cavities in past decades. It suggests that niobium has different properties when treated at different temperatures. We conducted various experiments on SRF-cavity-class niobium samples, and the systematic measurements included not only impurity analysis via TOF-SIMS, in-situ XPS, in-situ ESEM, HRTEM, but also superconductor measurements via in-situ ARPES. We also performed quantitative atomic simulation of the impurities in niobium bulks at zero temperature, and found interstitial carbon had similar trapping effect on interstitial hydrogen as interstitial nitrogen and oxygen did. We found the mildly increased interstitial carbons and oxygens during medium temperature baking not only suppressed the hydrogen accumulation and hydride precipitation during cooling down, but also reduced the electron mean free path to the optimal range which yielded declined BCS resistance. Therefore, the surface resistances of the cavities were reduced and the Q₀ values were improved correspondingly.  
slides icon Slides WEIAA02 [15.615 MB]  
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WEPWB044 Realization of Accelerating Gradient Larger than 25 MV/m on High-Q 1.3 GHz 9-Cell Cavities for SHINE 658
SUSPB039   use link to see paper's listing under its alternate paper code  
 
  • Y. Zong, Q.X. Chen, X. Huang, Z. Wang
    SINAP, Shanghai, People’s Republic of China
  • J.F. Chen, P.C. Dong, H.T. Hou, X.Y. Pu, J. Shi, S. Sun, D. Wang, J.N. Wu, S. Xing, S.J. Zhao, Y.L. Zhao
    SARI-CAS, Pudong, Shanghai, People’s Republic of China
  • Y.W. Huang
    ShanghaiTech University, Shanghai, People’s Republic of China
  • X.W. Wu
    Zhangjiang Lab, Shanghai, People’s Republic of China
  
  Funding: This work was supported by Shanghai Municipal Science and Technology Major Project (No. 2017SHZDZX02).
We present our studies on the optimized nitrogen-doping and medium-temperature baking recipes applied on 1.3GHz SRF cavities, aiming at meeting the requirements of the SHINE project. The optimized nitrogen-doping process resulted in achieving a Q₀ of over 4.0×1010 at medium field and a maximum accelerating gradient exceeding 35 MV/m on single cell cavities, and a Q₀ of over 2.8×1010 at medium field and a maximum accelerating gradient exceeding 26 MV/m in 9-cell cavities. For 1.3 GHz 9-cell cavities subjected to medium-temperature baking, Q₀ values exceeding 3.5×1010 at 16 MV/m and maximum accelerating gradients surpassing 25 MV/m were achieved. These studies provide two options of high-Q recipes for SHINE cavities. The treatment processes of cavities and their vertical test results are described in this paper.
*chenjinfang@sari.ac.cn
 		 
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-WEPWB044  
About • Received ※ 19 June 2023 — Revised ※ 22 June 2023 — Accepted ※ 26 June 2023 — Issue date ※ 26 June 2023
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WEPWB045 The Oxidizing Responses of Baked Niobium Exposed to Various Gases via In-situ NAXPS 662
SUSPB040   use link to see paper's listing under its alternate paper code  
 
  • Z.T. Yang, J.K. Hao, K.X. Liu, S.W. Quan
    PKU, Beijing, People’s Republic of China
  
  We carried out in-situ NAXPS (Near-atmospheric X-ray Photoelectron Spectroscopy) on SRF-cavity class niobium to observe its oxidizing responses when exposed to various gases. The niobium samples were baked at 800°C until the peaks of niobium oxides disappeared in the spectrum. Then the revealed pure niobium samples were exposed to the air-proportion mixture of nitrogen and oxygen, pure oxygen, and pure water vapor respectively. And for the pure oxygen and water vapor group, we also carried out TOF-SIMS (Time-of-Flight Secondary Ion Mass Spectroscopy) measurements before and after the baking and oxidation experiments. We found that pure oxygen and water vapor could oxidize niobium at similar rate which was faster than the N2/O2 mixture. After re-oxidized by pure oxygen and water vapor, the niobium sample presented a significant increase of interstitial carbon and a moderate increase of interstitial oxygen in the magnetic penetration depth, while it showed a mild decrease of interstitial hydrogen.  
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-WEPWB045  
About • Received ※ 15 June 2023 — Revised ※ 23 June 2023 — Accepted ※ 25 June 2023 — Issue date ※ 31 July 2023
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WEPWB084 The Interaction among Interstitial C/N/O/H and Vacancy in Niobium via First-Principles Calculation 778
 
  • H. Liu, J.K. Hao, Z.T. Yang
    PKU, Beijing, People’s Republic of China
  
  We calculate the interaction among zero dimensional defects in niobium lattice through first-principles calculation. And we compare the trapping effect of hydrogen among carbon, nitrogen, and oxygen as well as the trapping effect of interstitial atoms by vacancy. We find that the interstitial C/N/O have similar effect of trapping interstitial hydrogen in niobium lattice, and the vacancy can trap interstitial C/N/O/H in adjacent protocells and strengthen their chemical bond with Nb. These calculations give some explanation for improving superconducting performance of niobium cavities through medium temperature baking.  
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2023-WEPWB084  
About • Received ※ 15 June 2023 — Revised ※ 25 June 2023 — Accepted ※ 29 June 2023 — Issue date ※ 03 July 2023
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