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Dissertation/Thesis Abstract

Exploring and Understanding the Limitations of Nb SRF Cavity Performance
by Bafia, Daniel, Ph.D., Illinois Institute of Technology, 2020, 174; 28158052
Abstract (Summary)

Niobium superconducting radio-frequency (SRF) cavities are highly efficient resonators used to accelerate particles. Due to their high quality factors (Q0), these cavities may also serve as sensitive detectors and can be used in 3-D quantum computational architectures. To drive down cryogenic and production costs and improve performance in the low energy regime, it is necessary to increase the Q0 and quench field of these cavities. This dissertation presents work geared towards the understanding of limiting mechanisms in niobium SRF cavities and improving their performance.

Mechanisms responsible for performance limitations in in-situ baked cavities are discussed. The role of oxygen in the mitigation of high field Q slope (HFQS) is made apparent. Through material and cavity studies, HFQS onset is found to vary linearly with the depth to which oxygen diffuses. In addition, the 75/120 °C modified low temperature bake is presented, producing record quench fields of 50 MV/m. Furthermore, a bifurcation phenomenon is described, likely due to the suppression and growth of room temperature niobium nano-hydrides.

The mechanisms responsible for the high Q0 and anti-Q slope phenomenon in nitrogen doped cavities are investigated. Optimized variants of N-doping are studied. The mechanism responsible for quench in these cavities is found to be of magnetic origin. Furthermore, N-doped cavities with higher (lower) quench fields tend to quench closer to the equator (iris).

New phenomena of niobium SRF cavities are also presented: features in the resonant frequency before the transition temperature that vary with surface treatment and correlate with performance. One feature, a dip, is found to be a signature of dilute concentrations of impurities (N, Ti) in the RF layer. The effect of nitrogen concentration and frequency on this dip are investigated. The observed relationships set forth an N-doping level categorization scheme for cavities. One possible model to explain the origin of these features is presented. A phenomenological model shows that this dip is associated with larger average superconducting gaps and lower inelastic scattering within the RF surface. These results suggest that non-equilibrium effects in SRF cavities occur when the level of inelastic pair-breaking mechanisms, which would otherwise dominate the BCS resistance, is diminished.

Indexing (document details)
Advisor: Zasadzinski, John F.
School: Illinois Institute of Technology
Department: Physics
School Location: United States -- Illinois
Source: DAI-B 82/7(E), Dissertation Abstracts International
Subjects: Physics, Condensed matter physics
Keywords: Low temperature baking, Nitrogen doping, RF, SRF, SRF cavities, Superconducting
Publication Number: 28158052
ISBN: 9798557049511
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