Keyword: scattering
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MOP041 Comparison of the Lattice Thermal Conductivity of Superconducting Tantalum and Niobium electron, lattice, simulation, niobium 148
  • P. Xu, N.T. Wright
    MSU, East Lansing, Michigan, USA
  • T.R. Bieler
    Michigan State University, East Lansing, Michigan, USA
  Funding: This work is supported by the U.S. Department of Energy, Office of High Energy Physics through Grant No. DE-FG02-13ER41974.
The thermal conductivity k of superconducting Ta behaves similarly to that of superconducting Nb, albeit at colder temperatures. This shift is due to the superconducting transition temperature of Ta being 4.3 K, versus 9.25 K for Nb. For example, the temperature of the phonon peak of properly treated Ta is about 1 K as opposed to a phonon peak at about 2 K for Nb. The typical value of k of Ta is smaller than Nb with the value at the phonon peak for Ta being O(10) W/ m/ K. Like Nb, k is dominated by phonons at these temperatures. This lattice k can be modeled by the Boltzmann transport equation, solved here by a Monte Carlo method using the relaxation time approximation. Individual scattering mechanisms due to boundaries, dislocations, and residual normal electrons are examined, and the phonon dispersion relation is included. Differences in the thermal response of deformed Ta, as compared with Nb, may be attributed to differences in dislocation densities of the two metals following similar levels of deformation. Boundary scattering dominates at the coldest temperatures. The phonon peak decreases and shifts to warmer temperatures with deformation.
DOI • reference for this paper ※  
About • paper received ※ 19 June 2019       paper accepted ※ 30 June 2019       issue date ※ 14 August 2019  
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TUP028 Development of Vertical Electropolishing Facility for Nb 9-Cell Cavity (3) cavity, cathode, linac, controls 470
  • Y.I. Ida, V. Chouhan, K.N. Nii
    MGH, Hyogo-ken, Japan
  • T. Akabori, G. Mitoya, K. Miyano
    HKK, Morioka, Japan
  • Y. Anetai, F. Takahashi
    WING. Co.Ltd, Iwate-ken, Japan
  • H. Hayano, S. Kato, H. Monjushiro, T. Saeki, M. Sawabe
    KEK, Ibaraki, Japan
  The 1st report was delivered in May, 2018 at the IPAC 18 in Vancouver, Canada. The 2nd report was delivered in September, 2018 at the LINAC 18 in Beijing, China. We will make our 3rd report in July, 2019 at the SRF-19 in Dresden, Germany. There will be two main points this time. The first is that by using our improved Ninja Electrode Premium, we can out-perform our number one and number two competitors in terms of uniform electropolishing of the interior of the 9-cell cavity. The second point is that we can remove hydrogen gas, reacted during electropolishing, from the cavity chambers in a manner that has not been successfully achieved by 1st report, May 2018 and 2nd report, September 2018. We will report our 9-cell vertical polishing revolver-type unit that solves the above two problems.  
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DOI • reference for this paper ※  
About • paper received ※ 24 June 2019       paper accepted ※ 29 June 2019       issue date ※ 14 August 2019  
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TUP045 Ab Initio Calculations on Impurity Doped Niobium and Niobium Surfaces niobium, electron, experiment, lattice 523
  • N. Sitaraman, T. Arias
    Cornell University, Ithaca, New York, USA
  • R.G. Farber, S.J. Sibener, R.D. Veit
    The University of Chicago, Chicago, Illinois, USA
  • M. Liepe, J.T. Maniscalco
    Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
  Funding: This work was funded by the Center for Bright Beams
We develop and apply new tools to understand Nb surface chemistry and fundamental electronic processes using theoretical ab initio methods. We study the thermodynamics of impurities and hydrides in the near-surface region as well as their effect on the surface band gap. This makes it possible for experimentalists to relate changes in STM dI/dV measurements resulting from different preparations to changes in subsurface structure. We also calculate matrix elements for electron-impurity scattering in Nb for common impurities O, N, C, and H. By transforming these matrix elements into a Wannier function basis, we calculate lifetimes for a dense set of states on the Fermi surface and determine the mean free path as a function of impurity density. This technique can be generalized to calculate other scattering amplitudes and timescales relevant to SRF theory.
DOI • reference for this paper ※  
About • paper received ※ 02 July 2019       paper accepted ※ 03 July 2019       issue date ※ 14 August 2019  
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