| Paper | Title | Page |
|---|---|---|
TUP012 |
Understanding the Role of Strain Induced Defects in the Degradation of Surfacesuperconductivity for SRF Quality Niobium | |
|
||
|
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. |
||
| TUP019 | Probing Hot Spot and Cold Spot of SRF Cavities with Tunneling and Raman Spectroscopies | 466 |
|
||
| Point contact tunneling and Raman spectroscopies are presented on high purity Nb samples, including pieces from hot and col spot regions of tested SRF cavities and Nb coupons subject to similar treatment. High quality tunneling spectra were observed on cold spots, revealing the bulk Nb gap, indicating minimal surface contamination. Hot spots exhibit high smearing suggestive of pair breaking along with generally lower superconducting gap. In addition, pronounced zero bias conductance peaks were frequently observed indicative of spin-flip tunneling and thus magnetic impurities in the oxide layer. Optical microscopy reveals higher density of surface blemishes on hot spots. Raman spectra inside those blemishes show clear difference from surrounding areas, exhibiting enhanced intensity peaks identified as either amorphous carbon, hydrocarbons or the ordered NbC phase. The presence of surface NbC is consistent with TEM studies, and these inclusions exhibit enhanced second order phonon response. Such regions with high concentrations of impurities are expected to suppress the local superconductivity and may explain the formation of hot spots. | ||
TUP034 |
Atomic-Scale Characterization of the Subsurface Region of Niobium for SRF Cavities Using Ultraviolet Laser-assisted Atom-probe Tomography | |
|
||
|
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. |
||
TUP042 |
In-Situ Study of Nb Oxide and Hydride for SRF Cavity Applications Using Aberration-Corrected STEM and Electron Energy Loss Spectroscopy | |
|
||
|
Funding: supported by the University Research Associate (URA) Visiting Scholars Program at the FNAL. UIC JEOL JEM-ARM200CF is supported by an MRI-R2 grant from the U.S. National Science Foundation We present an atomic-resolution study of the effects that 48hr bake at 120 °C in vacuum has on the high-field properties of Nb-based SRF cavities. This bake results a significant increase in high-field Q, reversely, 800 °C bake for 2hr reduces the Hc3/Hc2-ratio. Several mechanisms have been proposed, including an increased NbOx surface layer thickness and the precipitation of NbHy. Using combination of atomic-resolution Z-contrast imaging and electron energy-loss spectroscopy with in-situ heating and cooling experiments, we examine the atomic and electronic structures of Nb and related oxides/hydrides near the cavity surface. We quantify the oxygen diffusion on surface during bake by measuring the local Nb valence using EELS. We demonstrate that hydrogen atoms incorporated into the Nb crystal, forming β-NbH precipitates, can be directly visualized by annular bright field imaging in our aberration-corrected JEOL ARM-200CF. The effects of baking on the local hydrogen and other impurity will be examined by imaging, EEL spectra and strain analysis. Our results will be combined with atom-probe tomography to develop a 3-D impurity and phase profile of Nb near the SRF cavity surface. "R. Tao, R. F. Klie et al, Journal of Applied Physics, 110, Issue 12, (2011)" "Y.J. Kim, R. Tao, R.F. Klie and D. Seidman, ACS nano 7 (1), pp 732–739, (2013)" |
||