Frequency Dependence of Superconducting Cavity Q and Magnetic Breakdown Field

TL;DR

This paper presents a theoretical model explaining why lower frequency superconducting cavities have reduced quality factors and magnetic breakdown fields, attributing these effects to flux trapping and fluxoid resistivity.

Contribution

It introduces a frequency-dependent model linking fluxoid resistivity to cavity performance, aligning theoretical predictions with experimental data.

Findings

Lower frequency cavities exhibit lower Q and breakdown fields.

Flux trapping in cavity walls affects performance.

Fluxoid resistivity varies with frequency, impacting cavity behavior.

Abstract

A theoretical explanation is given to account for the unexpected observation that L- and S-band Nb superconducting cavities were found to have lower Q and lower magnetic breakdown field than those of the higher X-band frequencies. Both effects can be related to the trapping of magnetic flux in the cavity walls. The frequency dependence arises from the frequency dependence of the resistivity of oscillating fluxoids. Calculations based on this model are in agreement with experimental observations.