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TUP012 |
Understanding the Role of Strain Induced Defects in the Degradation of Surfacesuperconductivity for SRF Quality Niobium | |
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Funding: This work was supported by the US DOE under awards DE-FG02-05ER41392, DE-SC0009960 and FNAL PO 570362, and the State of Florida. Some years ago Casalbuoni showed that the r32 of (Hc3/Hc2) of SRF-processed Nb could deviate markedly from the GL values of 1.695 due to nanostructure difference between surface and bulk. Here, we address the impact of increasing levels of cold work introduced by wire drawing on the localized surface superconducting properties of SRF Nb. We used AC susceptibility measurements to explore the surface and bulk superconductivity of the wires after applying different levels of EP and post baking. Then, we quantified the changes in microstructure by EBSD to map the crystallographic texture and micro-scale grain misorientation. These combined characterizations showed that the r32 of heavily deformed Nb surfaces, though initially very enhanced, can revert to or become even lower than 1.695 after long EP and high T baking. However, the marked difference in surface superconductivity compared to the bulk appears after a mild bake (120°C/48h). This distinct surface property may be associated with light element diffusion through the highly deformed GBs or dislocations during low baking. AC susceptibility made on single and bi-crystal from large grain sheet strongly supports this hypothesis. |
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