Even when cooled through its transition temperature in the presence of anexternal magnetic field, a superconductor can expel nearly all externalmagnetic flux. This Letter presents an experimental study to identify theparameters that most strongly influence flux trapping in high purity niobiumduring cooldown. This is critical to the operation of superconductingradiofrequency cavities, in which trapped flux degrades the quality factor andtherefore cryogenic efficiency. Flux expulsion was measured on a large surveyof 1.3 GHz cavities prepared in various ways. It is shown that both spatialthermal gradient and high temperature treatment are critical to expellingexternal magnetic fields, while surface treatment has minimal effect. For thefirst time, it is shown that a cavity can be converted from poor expulsionbehavior to strong expulsion behavior after furnace treatment, resulting in asubstantial improvement in quality factor. Future plans are described to buildon this result in order to optimize treatment for future cavities.
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