Improved RF Measurements of SRF Cavity Quality Factors

TL;DR

This paper introduces a calibration method that enhances RF measurement accuracy of SRF cavity quality factors across various coupling levels by compensating for systematic errors caused by non-ideal RF components.

Contribution

The paper presents a novel calibration technique using complex base-band signals and a trombone to accurately measure cavity quality factors over a wider coupling range.

Findings

Accurate measurements achieved at coupling factors near 7

Systematic errors can be characterized and suppressed

Calibration extends measurement accuracy beyond critical coupling

Abstract

SRF cavity quality factors can be accurately measured using RF-power based techniques only when the cavity is very close to critically coupled. This limitation is from systematic errors driven by non-ideal RF components. When the cavity is not close to critically coupled, these systematic effects limit the accuracy of the measurements. The combination of the complex base-band envelopes of the cavity RF signals in combination with a trombone in the circuit allow the relative calibration of the RF signals to be extracted from the data and systematic effects to be characterized and suppressed. The improved calibration allows accurate measurements to be made over a much wider range of couplings. Demonstration of these techniques during testing of a single-spoke resonator with a coupling factor of near 7 will be presented, along with recommendations for application of these techniques.