Author: Posen, S.
Paper Title Page
TUXBA02
 Flux Expulsion Studies on Niobium  
 
  • S. Posen
    Fermilab, Batavia, Illinois, USA
  
  Increased requirements for high Q0 in SRF linac applications have made it crucial to avoid Q0 degradation resulting from trapped flux. In this contribution, we show results from a new systematic study of cavities at Fermilab that show a strong correlation between the temperature gradient required to expel flux during cooldown and the material history, including 1) niobium vendor processing and 2) heat treatment after purchase from the vendor. Early results from these studies indicated that it would be important to evaluate the niobium purchased for production of high Q0 cavities for the LCLS-II project. We present measurements of both expulsion and Q0 on single-cell and bare 9-cell cavities made from this material, which led to the decision to increase the heat treatment temperature in production. We present data showing the significant improvement in flux expulsion behavior of production cavities after this modification was introduced. In addition, we present new understanding of the physics of flux expulsion, and we discuss the ramifications for procurement of niobium for future applications requiring high Q0.  
slides icon Slides TUXBA02 [3.340 MB]  
Export • reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)  
 
TUYAA02 Advancement in the Understanding of the Field and Frequency Dependent Microwave Surface Resistance of Niobium 364
 
  • M. Martinello, S. Aderhold, S.K. Chandrasekaran, M. Checchin, A. Grassellino, O.S. Melnychuk, S. Posen, A. Romanenko, D.A. Sergatskov
    Fermilab, Batavia, Illinois, USA
  
  The radio-frequency surface resistance of niobium resonators is incredibly reduced when nitrogen impurities are dissolved as interstitial in the material, conferring ultra-high Q-factors at medium values of accelerating field. This effect has been observed in both high and low temperature nitrogen treatments. As a matter of fact, the peculiar anti Q-slope observed in nitrogen doped cavities, i.e. the decreasing of the Q-factor with the increasing of the radio-frequency field, come from the decreasing of the BCS surface resistance component as a function of the field. Such peculiar behavior has been considered consequence of the interstitial nitrogen present in the niobium lattice after the doping treatment. The study here presented show the field dependence of the BCS surface resistance surface of cavities with different resonant frequencies, such as: 650 MHz, 1.3 GHz, 2.6 GHz and 3.9 GHz, and processed with different state-of-the-art surface treatments. These findings show for the first time that the anti Q-slope might be seen at high frequency even for clean Niobium cavities, revealing useful suggestion on the physics underneath the anti Q-slope effect.
*M. Martinello, A. Grassellino, M. Checchin, A. Romanenko, O. Melnychuk, D.A. Sergatskov, S. Posen, J. Zasadzinski App. Phys. Lett. 109, 6 (2016) 		 
slides icon Slides TUYAA02 [4.342 MB]  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUYAA02  
Export • reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)  
 
WEXA05
 Dirty Layers, Bi-layers and Multi-layers: Insights from Muon Spin Rotation Experiments  
 
  • T. Junginger, R.E. Laxdal, D.W. Storey, E. Thoeng
    TRIUMF, Vancouver, Canada
  • D.L. Hall, M. Liepe
    Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
  • T. Junginger
    HZB, Berlin, Germany
  • S. Posen
    Fermilab, Batavia, Illinois, USA
  • T. Prokscha, Z. Salman, A. Suter
    PSI, Villigen PSI, Switzerland
  • D.W. Storey
    Victoria University, Victoria, B.C., Canada
  • T. Tan, W.K. Withanage, M.A. Wolak, X. Xi
    Temple University, Philadelphia, USA
  • E. Thoeng
    UBC & TRIUMF, Vancouver, British Columbia, Canada
  • A-M. Valente-Feliciano
    JLab, Newport News, Virginia, USA
  • W.W. Wasserman
    UBC, Vancouver, B.C., Canada
  
  Funding: This research was supported by a Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007-2013).
The multilayer approach is being investigated for SRF applications since 2006 "*". More recently the option of using a bilayer system of two superconductors has been considered as an alternative approach to reach accelerating gradients beyond bulk niobium or to explain the gradient enhancement from a 120°C bake by introduction of a 'dirty layer "**"'. In this talk results are presented from two muon spin rotation experiments at TRIUMF and PSI. The former measures the field of first entry Hentry. It will be shown that MgB2 and Nb3Sn on top of Nb both push Hentry above Hc1 to a value consistent with Hsh, independent of the layer thickness. 120°C baking increases Hentry slightly but significantly above Hc1. Using the low energy muon beam at PSI we show that there is a long range proximity effect in a bilayer system of NbTiN on Nb. This effect yields a stronger decay of the RF field with depth as expected for pure NbTiN, opposite to what has been predicted for a bi-layer system due to counter current flow at the superconductor-superconductor interface "***". An insulating layer suppresses this proximity effect.
* Gurevich, A. APL 88.1 (2006)
** Checchin, M. Diss. Illinois Institute of Technology, 2016.
Kubo, T. Superconductor Science and Technology 30.2 (2016)
*** Kubo, T et al. APL 104.3 (2014) 		 
slides icon Slides WEXA05 [3.716 MB]  
Export • reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)  
 
WEXA07
 Theoretical Estimates of Maximum Fields in Superconducting Resonant Radio Frequency Cavities: Stability Theory, Disorder, and Laminates  
 
  • D. Liarte, M. Liepe, J.P. Sethna
    Cornell University, Ithaca, New York, USA
  • G. Catelani
    Forschungszentrum Jülich, Peter Gruenberg Institut, Jülich, Germany
  • D.L. Hall
    Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
  • S. Posen
    Fermilab, Batavia, Illinois, USA
  • M.K. Transtrum
    Brigham Young University, Provo, USA
  
  Funding: This work was supported by the US National Science Foundation under Award OIA-1549132, the Center for Bright Beams.
Theoretical limits to the performance of superconductors in high magnetic fields parallel to their surfaces are of key relevance to current and future accelerating cavities. We present intuitive arguments and simple estimates for Hsh, and combine them with rigorous calculations. We explore the effects of materials anisotropy and the danger of disorder in nucleating vortex entry. Will we need to control surface orientation in the layered compound MgB2? Can we estimate theoretically whether dirt and defects make these new materials fundamentally more challenging to optimize than niobium? We discuss and analyze recent proposals to use thin superconducting layers or laminates to enhance the performance of superconducting cavities. Flux entering a laminate can lead to so-called pancake vortices; we consider the physics of the dislocation motion and potential re-annihilation or stabilization of these vortices after their entry. 		 
slides icon Slides WEXA07 [2.975 MB]  
Export • reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)