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TUP030 |
Elimination of post annealing chemistry: a route to high Q cavities and processing simplification | |
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Funding: Fermilab is operated by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the United States Department of Energy. We investigate the effect of high temperature treatments followed by only high-pressure water rinse (HPR) of superconducting radio frequency (SRF) niobium cavities. The objective is to provide a cost effective alternative to the typical cavity processing sequence, by eliminating the material removal step post furnace treatment while preserving or improving the RF performance. The studies have been conducted in the temperature range 800-1000C for different conditions of the starting substrate: large grain and fine grain, electro-polished (EP) and centrifugal barrel polished (CBP) to mirror finish. An interesting effect of the grain size on the performances is found. Cavity results and samples characterization show that furnace contaminants cause poor cavity performance, and a practical solution is found to prevent surface contamination. Extraordinary values of residual resistances ~ 1 nOhm and below are then consistently achieved for the contamination-free cavities. We explore the addition of a small partial pressure of gas during the anneal to further increase the cavity quality factor by reducing the BCS resistance. |
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TUP031 |
Muon Spin Rotation Studies of Bulk Electropolished Cavity Cutouts and Thin Films of Alternative Materials | |
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Funding: Fermilab is operated by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the United States Department of Energy. In the previous studies [*] magnetic flux penetration into fine and large grain BCP cavity cutouts was investigated using the muon spin rotation (muSR) technique. The technique is based on implanting muons, which serve as sensitive magnetic probes inside the material. Here we report muSR studies on fine grain EP cavity cutouts, both before and after 120C baking, and on the films of new materials. [*] A. Grassellino et al, Phys. Rev. ST Accel. Beams 16, 062002 (2013) |
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| TUP043 | Nanostructural TEM/STEM Studies of Hot and Cold Spots in SRF Cavities | 504 |
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Direct TEM/STEM imaging and spectroscopic chemical characterization by EELS/EDS of the surface of the SRF cavity cutouts before and after the treatments (e.g. in situ mild vacuum bake and rinsing with hydrofluoric acid) down to subnanometer scale is implemented to correspond the changes in niobium surface to the SRF performance of the cavities. We also report current results of the direct search, using cryogenic TEM stage, for suggested phase transformations in the niobium-hydrogen system* on “hot” and “cold” spot cavity cutouts, which may help clarifying the mechanism of the high field Q slope and its empirical cure.
*A. Romanenko, F. Barkov, L. D. Cooley, A. Grassellino, Supercond. Sci. Technol. 26 (2013) 035003. |
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| TUP065 | Chemical Structure of Niobium Samples Vacuum Treated in Nitrogen in Parallel With Very High Q0 Cavities | 583 |
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| XPS in combination with subsequent material removal via Ar sputtering as well as XRD are used for the surface analysis and bulk phase characterization of nitrogen treated samples processed parallel with SRF cavities. We investigated the surface chemistry of the samples treated with nitrogen in order to understand this treatment effect on SRF cavity performance for several baking temperatures and durations in order to find cost efficient post-furnace chemistry free procedures to enable high Q-values. | ||