[Seminars] Accelerator Science Seminar Today! - May 2
holly2 at uchicago.edu
Mon May 2 09:56:29 CDT 2016
ACCELERATOR SCIENCE SEMINAR TODAY
Monday April 18, 2016
Speaker: Steven Sibener (UChicago)
Title: Surface Chemistry of SRF Materials for use in Particle Accelerators
Abstract: Superconducting radio frequency (SRF) technology is the key enabler for high-energy and high- beam-power accelerators. The performance of such SRF cavities is characterized by their quality factor, Q0, a measure of their efficiency of operation, and the maximum accelerating field, Eacc, that they can sustain before their quality factor degrades. Chemical and structural defects at the surface and in the near-surface region of niobium SRF cavities can negatively affect cavity performance, and, in some instances, render cavities inoperable especially when placed under high field gradients. Visualization studies across multiple length- scales demonstrate that despite extensive chemical polishing on a polycrystalline cavity substrate that a large number of surface defects and weld pits persist on the surface. This presentation will examine the surface structure, chemistry, and oxidative states of single-grain niobium crystals and polycrystalline SRF cavity samples using a number of surface science techniques including STM, AFM, SEM, AES, and XPS. These tools allow us to study processes such as the adsorption and dissolution of common dopants such as oxygen and nitrogen onto and through the niobium surface and into the bulk, and how changes in local surface structure (such as crystal face, step edges, and domain boundaries) affect these processes. It will be shown that the crystalline orientation of the surface significantly influences surface oxide formation; we observe that the (111) face does not form long-range ordered oxides, while the (100) surface presents remarkable highly- organized and spontaneously forming (nx1) ladder structures on the atomic scale. The stability and interconversion of the (nx1) oxides arising from thermal cycling with or without the addition of gas-phase oxygen will be discussed, with particular focus on oxygen dissolution kinetics induced by relatively low- temperature annealing of the substrate. If time permits, comments will be given on our initial efforts to examine the interactions of N and H with regard to hydride formation, as well as our planned studies on next-generation SRF materials such as Nb3Sn and MgB2.
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