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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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TUP034 |
Atomic-Scale Characterization of the Subsurface Region of Niobium for SRF Cavities Using Ultraviolet Laser-assisted Atom-probe Tomography | |
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Funding: This research was funded by USDOE (DE-AC02-07CH11359) and LEAP measurements were supported by NSF-MRI (DMR 0420532) and ONR-DURIP (N00014-0400798, N00014-0610539, N00014-0910781) programs. Niobium is the metal of choice for SRF cavities for a linear particle accelerator because it has the highest critical temperature of any element in the periodic table and can be deformed plastically into complex geometries. Differences in the sub-surface chemistry from bulk niobium are believed to determine the high-field Q-drop. In this study, the subsurface chemistry of niobium was characterized utilizing ultraviolet laser-assisted local-electrode atom-probe (LEAP) tomography employing picosecond laser pulsing. The superior spatial resolution and analytical sensitivity of a LEAP tomograph permits us to determine the subsurface composition on an atom-by-atom and atomic {hkl} plane-by-plane basis. The 3-D reconstructions from the LEAP tomographic analyses demonstrate different behaviors for Nb-oxides and Nb-hydrides in pure niobium as well as interactions with structural imperfections, dislocations and grain boundaries in SRF-grade Nb coupon material. Additionally, the chemistry and crystallographic structure of subsurface interstitial atoms were analyzed based on energy shifts of electron energy-loss spectroscopy in conjunction with a scanning transmission electron microscopy. |
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