Magnetic Flux Dynamics in Horizontally Cooled Superconducting Cavities

TL;DR

This study investigates how the orientation of superconducting cavities relative to helium cooling flow affects magnetic flux trapping and surface resistance, revealing new flux concentration phenomena impacting cavity performance.

Contribution

It provides new insights into flux dynamics in horizontally cooled SRF cavities, including the effects of magnetic field orientation and a novel flux concentration phenomenon.

Findings

Flux trapping varies with cavity orientation and magnetic field direction.

Flux line concentration at the cavity top causes temperature rise.

Different field components influence surface resistance differently.

Abstract

Previous studies on magnetic flux expulsion as a function of cooling details have been performed for superconducting niobium cavities with the cavity beam axis placed parallel respect to the helium cooling flow, and findings showed that for sufficient cooling thermogradients all magnetic flux could be expelled and very low residual resistance could be achieved. In this paper we investigate the flux trapping and its impact on radio frequency surface resistance when the resonators are positioned perpendicularly to the helium cooling flow, which is representative of how superconducting radio-frequency (SRF) cavities are cooled in an accelerator. We also extend the studies to different directions of applied magnetic field surrounding the resonator. Results show that in the cavity horizontal configuration there is a different impact of the various field components on the final surface resistance, and that several parameters have to be considered to understand flux dynamics. A newly discovered phenomenon of concentration of flux lines at the cavity top leading to cavity equator temperature rise is presented.