Flux Expulsion in Coaxial Superconducting Radiofrequency Cavities




Gregory, RuthAnn Rose

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This thesis explores the effects of different cool-down speeds and applied magnetic fields on TRIUMF’s coaxial cavities using COMSOL® simulations and experimental results. Magnetic sensitivity describes how sensitive the surface resistance of a material is to an external magnetic field, and is an important characteristic of SRF accelerator design. Reducing magnetic sensitivity can improve cavity performance. Previous studies have shown that nitrogen doped elliptical cavities are very sensitive to external fields compared to conventionally treated cavities, resulting in stringent requirements for the residual field and cavity cool-down speed. Few such studies have been done on non-elliptical cavities such as half wave resonators (HWRs) and quarter wave resonators (QWRs). Factors affecting magnetic sensitivity include cavity treatment, rf field distribution inside the cavity, external magnetic field direction, cool-down speed, and thermal gradient during transition to the superconducting state. Reducing the magnetic sensitivity can improve cavity performance since in practice it is impossible to eliminate all residual magnetic fields from external sources such as Earth’s natural magnetic field during a cool-down. It was found that magnetic sensitivity is not an ideal parameter for characterizing TRIUMF’s HWR and QWR since these cavities exhibit non-uniform flux trapping. Therefore, the parameter normalized Rmag is introduced. Normalized Rmag, or RmagN is the additional surface resistance introduced by applying a dc magnetic field to the cavity, divided by the applied magnetic field. The HWR’s normalized Rmag is compared for different resonant frequencies after 400 and 120℃ bakes, with the 120℃ bake resulting in lower normalized Rmag. The normalized Rmag was found to generally increase with frequency for both the HWR and QWR. The study also seeks to maximize flux expulsion, which occurs when a cavity is cooled down through its superconducting temperature. Flux expulsion is affected by cool-down speed, temperature gradient, and cavity orientation relative to an applied magnetic field. The effects of cool-down speed and temperature gradient on flux expulsion were found to be insignificant for the QWR with a vertically applied magnetic field. However, a horizontal magnetic field can be nearly completely expelled by a fast, high temperature gradient cool-down.



TRIUMF, coaxial cavity, HWR, QWR, flux trapping, flux expulsion, superconductor, SRF, superconducting radio frequency