Browsing by Author "Tuggle, James Robert Jr."

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    Advanced Characterization of Materials for Superconducting Radiofrequency Accelerator Cavities
    Tuggle, James Robert Jr. (Virginia Tech, 2019-06-24)
    Particle accelerators are a leading tool for frontier science. Pushing that frontier further demands more machines with higher performance, and more of a very expensive technology: superconducting radio-frequency (SRF) acceleration. From a materials perspective this means reducing residual surface resistance or raising the operating temperature (currently ~2 K) of SRF cavities. Both are pursued by materials modification: nitrogen doping/infusion in the first instance and coating with Nb3Sn in the second. Materials characterization is key to achieving understanding and directing RandD. However, very little has been done. This present work aims to fill the knowledge gap and to provide needed, validated tools to the accelerator science community. In this connection, SIMS, XPS and EBSD have proven especially valuable and represent the majority of discussion in this dissertation.
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    Initial growth of tin on niobium for vapor diffusion coating of Nb3Sn
    Pudasaini, Uttar; Eremeev, Grigory, V.; Reece, Charles E.; Tuggle, James Robert Jr.; Kelley, Michael J. (2019-04)
    Nb3Sn offers significant potential to exceed the performance of niobium for superconducting radio frequency accelerator cavities. The most promising path toward deployment is by tin vapor diffusion coating of Nb cavity interiors via a two step nucleation-then-growth sequence. Reported here is a materials science study of the nucleation process. We manipulated the accessible range of process variables and determined the effect on composition and microstructure using an array of materials characterization tools. Broadly, nucleation deposits tin as a thin surface phase and, under some conditions, as near-micron sized particles as well, resembling Stranski-Krastanov growth. Conditions that impair nucleation promote the formation of defects, such as patches, in subsequent coating growth. Otherwise no significant effect on the subsequently grown coating was found for structures produced during nucleation.