Efficient expulsion of magnetic flux in superconducting RF cavities for high $Q_0$ applications

TL;DR

This study investigates how thermal gradients and heat treatments influence magnetic flux expulsion in superconducting niobium cavities, demonstrating that proper processing can significantly improve their quality factor for RF applications.

Contribution

It identifies key parameters affecting flux trapping during cooldown and shows that furnace treatments can convert poor expulsion to strong expulsion, enhancing cavity performance.

Findings

Thermal gradient and heat treatment are critical for flux expulsion.

Surface treatment has minimal effect on flux trapping.

Furnace treatment can significantly improve cavity quality factor.

Abstract

Even when cooled through its transition temperature in the presence of an external magnetic field, a superconductor can expel nearly all external magnetic flux. This Letter presents an experimental study to identify the parameters that most strongly influence flux trapping in high purity niobium during cooldown. This is critical to the operation of superconducting radiofrequency cavities, in which trapped flux degrades the quality factor and therefore cryogenic efficiency. Flux expulsion was measured on a large survey of 1.3 GHz cavities prepared in various ways. It is shown that both spatial thermal gradient and high temperature treatment are critical to expelling external magnetic fields, while surface treatment has minimal effect. For the first time, it is shown that a cavity can be converted from poor expulsion behavior to strong expulsion behavior after furnace treatment, resulting in a substantial improvement in quality factor. Future plans are described to build on this result in order to optimize treatment for future cavities.