Author: Martinello, M.
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TUPLR023 Impurity Content Optimization to Maximize Q-Factors of Superconducting Resonators 515
SPWR016   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
    Illinois Institute of Technology, Chicago, Illlinois, USA
  • J. Zasadzinski
    IIT, Chicago, Illinois, USA
 
  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 trapped flux, depends on the mean free path. A systematic study of a variety of 1.3 GHz cavities with different surface treatments (EP, 120 C bake and different N-doping) is carried out. A bell shaped trend appears for the range of mean free path studied. Over-doped cavities fall at the maximum of this curve defining the largest values of sensitivity. In addition, we have studied the trend of the BCS surface resistance contribution as a function of mean free path, showing that N-doped cavities follow close to the theoretical minimum. Adding these results together we show that the 2/6 N-doping treatment gives the highest Q-factor values at 2 K and 16 MV/m, as long as the magnetic field fully trapped during the cavity cooldown is lower than 10 mG.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-LINAC2016-TUPLR023  
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TUPLR024 Enhancement of the Accelerating Gradient in Superconducting Microwave Resonators 519
SPWR017   use link to see paper's listing under its alternate paper code  
 
  • M. Checchin, A. Grassellino, M. Martinello, S. Posen, A. Romanenko
    Fermilab, Batavia, Illinois, USA
  • 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. DEAC02-07CH11359 with the United States Department of Energy.
The accelerating gradient of superconducting resonators can be enhanced by engineering the thickness of a dirty layer grown at the cavity's rf surface. In this paper the description of the physics behind the accelerating gradient enhancement by meaning of the dirty layer is carried out by solving numerically the the Ginzburg-Landau (GL) equations for the layered system. The calculation shows that the presence of the dirty layer stabilizes the Meissner state up to the lower critical field of the bulk, increasing the maximum accelerating gradient.
 
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-LINAC2016-TUPLR024  
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WE2A01
N-Doping: The New Breakthrough Technology for SRF Cavities  
 
  • M. Martinello
    Fermilab, Batavia, Illinois, USA
 
  Modern projects of accelerators for High Energy Physics and FEL accelerators (PIP II, LCLS II, etc.) demand operation of the SRF cavities in CW regime. In this situation, low cryogenic losses are essential. Decrease of the losses or, thus, increase of the cavity unloaded quality factor Q0 allows great savings in capital and operational cost. The new N-doping technique for the SRF cavity processing in order to achieve high Q0 is described, which is now a ready-to-use technology for SRF accelerators. The current implementation of this technique for the LCLS-II cavity production, allow us to present how ultra-high Q-factors can be maintained from the vertical to the horizontal test. In particular, efficient cooling and optimization of shielding design will be discussed to address potential Q degradation from the remnant magnetic fields in the cryomodule. The talk will go through the physics and fundamental studies that allowed us a) to define the best nitrogen doping treatment which minimizes the Q sensitivity to trapped magnetic field, b) to maximize magnetic flux expulsion based on cavity treatment.  
slides icon Slides WE2A01 [3.101 MB]  
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