Author: Melnychuk, O.S.
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WEPMR003 Tailoring Surface Impurity Content to Maximize Q-factors of Superconducting Resonators 2258
SUPSS094   use link to see paper's listing under its alternate paper code  
  • M. Martinello, M. Checchin, A. Grassellino, O.S. Melnychuk, S. Posen, A. Romanenko, D.A. Sergatskov
    Fermilab, Batavia, Illinois, USA
  • M. Checchin, M. Martinello
    Illinois Institute of Technology, Chicago, Illlinois, USA
  • J. Zasadzinski
    IIT, Chicago, Illinois, USA
  Funding: Fermilab is operated by Fermi Research Alliance, LLC under contract No. DE-AC02-07CH11359 with the United States Department of Energy.
Quality factor of superconducting radio-frequency (SRF) cavities is degraded whenever magnetic flux is trapped in the cavity walls during the cooldown. In this contribution we study how the trapped flux sensitivity, defined as the trapped flux surface resistance normalized for the amount of flux trapped, depends on the mean free path. A variety of 1.3 GHz cavities with different surface treatments (EP, 120 C bake and different N-doping) were studied in order to cover the largest range of mean free path nowadays achievable, from few to thousands of nanometers. A bell shaped trend appears for the range of mean free path studied. Over doped cavities falls at the maximum of this curve defining the largest values of sensitivity. In addition, we have also studied the trend of the BCS surface resistance contribution as a function of mean free path, revealing that N-doped cavities follow close to the theoretical minimum of the BCS surface resistance as a function of the mean free path. Adding these results together we unveil that optimal N-doping treatment allows to maximize Q-factor at 2 K and 16 MV/m until the magnetic field fully trapped during the cavity cooldown stays below 10 mG.
DOI • reference for this paper ※ DOI:10.18429/JACoW-IPAC2016-WEPMR003  
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WEPMR009 Magnetic Flux Expulsion Studies in Niobium SRF Cavities 2277
  • S. Posen, M. Checchin, A.C. Crawford, A. Grassellino, M. Martinello, O.S. Melnychuk, A. Romanenko, D.A. Sergatskov, Y. Trenikhina
    Fermilab, Batavia, Illinois, USA
  With the recent discovery of nitrogen doping treatment for SRF cavities, ultra-high quality factors at medium accelerating fields are regularly achieved in vertical RF tests. To preserve these quality factors into the cryomodule, it is important to consider background magnetic fields, which can become trapped in the surface of the cavity during cooldown and cause Q0 degradation. Building on the recent discovery that spatial thermal gradients during cooldown can significantly improve expulsion of magnetic flux, a detailed study was performed of flux expulsion on two cavities with different furnace treatments that are cooled in magnetic fields amplitudes representative of what is expected in a realistic cryomodule. In this contribution, we summarize these cavity results, in order to improve understanding of the impact of flux expulsion on cavity performance.  
DOI • reference for this paper ※ DOI:10.18429/JACoW-IPAC2016-WEPMR009  
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WEPOR031 Field Emission Radiation Characterization of LCLS-II Cavities 2736
  • M. Santana-Leitner, C. Adolphsen, L. Ge, Z. Li, T.O. Raubenheimer, M.C. Ross
    SLAC, Menlo Park, California, USA
  • S. Aderhold, A. Grassellino, O.S. Melnychuk, R.V. Pilipenko, D.A. Sergatskov
    Fermilab, Batavia, Illinois, USA
  Funding: Work supported by U.S. Department of Energy contract DE-AC02-76SF00515
LCLS-II XFEL facility at the SLAC National Accelerator Laboratory will accelerate CW beams of up to 300 uA to 4 GeV using superconducting radio frequency cavities. Before installation, fully assembled cryomodules will be tested at Fermilab and Jefferson Lab. Besides the basic measurements of cavity gradients and cryogenic heat loads, radiation and dark current levels will be recorded. The latter parameters need to be limited to ensure the safety of the machine and the lifetime of radio-sensitive components installed near the cavities. In this paper we describe the simulation studies being done in preparation of tests, where expected radiation measurements in the different detectors are correlated with field emission and with dark currents in Faraday cups at each end of the cryomodule. This work includes simulations using a detailed model of the cryomodules and detectors, where field emission data generated with Track3P is parsed to the FLUKA radiation transport code.

DOI • reference for this paper ※ DOI:10.18429/JACoW-IPAC2016-WEPOR031  
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