Browsing by Author "Reece, Charles E."

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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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    Enhanced Field Emission Studies on Nioboim Surfaces Relevant to High Field Superconducting Radio-Frequency Devices
    Wang, Tong (Virginia Tech, 2002-09-18)
    Enhanced field emission (EFE) presents the main impediment to higher acceleration gradients in superconducting niobium (Nb) radiofrequency cavities for particle accelerators. The strength, number and sources of EFE sites strongly depend on surface preparation and handling. The main objective of this thesis project is to systematically investigate the sources of EFE from Nb, to evaluate the best available surface preparation techniques with respect to resulting field emission, and to establish an optimized process to minimize or eliminate EFE. To achieve these goals, a scanning field emission microscope (SFEM) was designed and built as an extension to an existing commercial scanning electron microscope (SEM). In the SFEM chamber of ultra high vacuum, a sample is moved laterally in a raster pattern under a high voltage anode tip for EFE detection and localization. The sample is then transferred under vacuum to the SEM chamber equipped with an energy-dispersive x-ray spectrometer for individual emitting site characterization. Compared to other systems built for similar purposes, this apparatus has low cost and maintenance, high operational flexibility, considerably bigger scan area, as well as reliable performance. EFE sources from planar Nb have been studied after various surface preparation, including chemical etching and electropolishing, combined with ultrasonic or high-pressure water rinse. Emitters have been identified, analyzed and the preparation process has been examined and improved based on EFE results. As a result, field-emission-free or near field-emission-free surfaces at ~140 MV/m have been consistently achieved with the above techniques. Characterization on the remaining emitters leads to the conclusion that no evidence of intrinsic emitters, i.e., no fundamental electric field limit induced by EFE, has been observed up to ~140 MV/m. Chemically etched and electropolished Nb are compared and no significant difference is observed up to ~140 MV/m. To address concerns on the effect of natural air drying process on EFE, a comparative study was conducted on Nb and the results showed insignificant difference under the experimental conditions. Nb thin films deposited on Cu present a possible alternative to bulk Nb in superconducting cavities. The EFE performance of a preliminary energetically deposited Nb thin film sample are presented.
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    Finite Element Modeling of Electrochemical Polishing of Niobium in Hydrofluoric-Sulfuric Acid Electrolyte
    Wang, Kaiwen; Cai, Wenjun; Tian, Hui; Reece, Charles E. (Electrochemical Soc Inc, 2022-06)
    Niobium (Nb) used in superconducting radio-frequency cavities requires smooth surface to achieve optimal performance. In this work, a finite element model that coupled electrochemistry, heat transfer, and fluid dynamics was developed to investigate the electrochemical polishing mechanisms of Nb, using experimentally measured polarization results of coupon samples as validations. The current and potential distribution, oxide growth kinetics of Nb in a complex cavity geometry was investigated as a function of temperature and coolant flow. A low temperature coolant with intermediate flow rate was found to reduce surface current and ensure oxide uniformity. These results could shed light on the design of future particle accelerators. (C) 2022 The Electrochemical Society ("ECS"). Published on behalf of ECS by IOP Publishing Limited.
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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.
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    Trace Level Impurity Quantitation and the Reduction of Calibration Uncertainty for Secondary Ion Mass Spectrometry Analysis of Niobium Superconducting Radio Frequency Materials
    Angle, Jonathan Willis (Virginia Tech, 2022-04-08)
    Over the last decade, the interstitial alloying of niobium has proven to be essential for enabling superconducting radiofrequency (SRF) cavities to operate more efficiently at high accelerating gradients. The discovery of "nitrogen doping" was the first readily accessible avenue of interstitial alloying in which researchers saw an increase in cavity performance. However, the serendipitous nature of the discovery led to additional research to fundamentally understand the physics behind the increase in cavity performance. This knowledge gap is bridged by materials characterization. Secondary ion mass spectrometry (SIMS) is a characterization technique which has become a staple of SRF cavity characterization that details elemental concentration profiles as a function of depth into the niobium surface with submicron resolution. SIMS has been widely used by the semiconductor industry for decades but has found less application in other fields due to the difficulty to produce reproducible data for polycrystalline materials. Much effort has been given to reduce the uncertainty of SIMS results to as low as 1% - 2% for single crystals. However, less attention has been given to polycrystalline materials with uncertainty values reported between 40% - 50% The sources of uncertainty were found to be deterministic in nature and therefore could be mitigated to produce reliable results. This dissertation documents the efforts to reduce SIMS method uncertainty which has been further used to solve mysteries regarding the characterization of SRF cavities which include predictive modeling of oxygen diffusion as well as the identification of contaminants resulting from cavity furnace treatments.