Thermocurrents and their role in high Q cavity performance

Abstract

Over the past years it became evident that the quality factor of a superconducting cavity is not only determined by its surface preparation procedure, but is also influenced by the way the cavity is cooled down. Moreover, different data sets exist, some of which indicate that a slow cooldown through the critical temperature is favorable while other data states the exact opposite. Even though there were speculations and some models about the role of thermocurrents and flux-pinning, the difference in behavior remained a mystery. In this paper we will present a consistent theoretical model which we confirmed by data that describes the role of thermocurrents, driven by temperature gradients and material transitions. We will clearly show how they impact the quality factor of a cavity, discuss our findings, relate it to findings at other labs and develop mitigation strategies which especially address the issue of achieving high quality factors of so-called nitrogen doped cavities in horizontal tests.