| Paper | Title | Page |
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| WE105 | An Investigation of the influence of grain boundaries on flux penetration in high purity large grain niobium for particle accelerators |
| | - Z. H. Sung, P. J. Lee, A. Gurevich, A. A. Polyanskii, D. C. Larbalestier
NHMFL, FSU - C. Antoine
Saclay - C. Boffo, H. T. Edwards
Fermilab
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| | Grain boundaries (GBs) in niobium cavities may be one of the important causes of extra power dissipation by reducing the field of first vortex penetration because the superconducting gap and the local depinning current density Jb on the GB are reduced. It is therefore important to measure the critical current density Jb and investigate the microstructure at grain boundaries to better understand whether or how grain boundary weakness can affect SRF cavity performance. Our experiments are currently correlating the global (by magnetometer) and local magnetization (by magneto-optical imaging), transport critical current density and atomic scale structure of Nb samples so that a DC analog of the RF surface currents can be developed for real Nb surfaces prepared using cavity optimization treatments. To measure Jb we apply transport current as a function of perpendicular magnetic field on BCP-treated bi-crystals of as-received, high-purity, large-grain niobium sheet. After measurement, we thin the very same grain boundary so that we image the microstructure of the external surface adjoining each GB by scanning transmission electron microscopy (STEM) in conjunction with EELS (Electron Energy Loss Spectroscopy). EELS has shown the presence of stoichiometric niobium oxide on the topmost layers, well within the typical superconducting niobium penetration depth (~ 50nm). 1. now at SACLAY | |
 | Slides(PDF) | |