Measurement of the Cavity-Loaded Quality Factor in Superconducting Radio-Frequency Systems with Mismatched Source Impedance

The accurate measurement of parameters such as the cavity-loaded quality factor (QL) and half bandwidth (f0.5) is essential for monitoring the performance of superconducting radio-frequency (SRF) cavities. However, the conventional “field decay method” employed to calibrate these values requires the cavity to satisfy a “zeroinput” condition. This can be challenging when the source impedance is mismatched and produce nonzero forward signals (Vf ) that significantly affect the measurement accuracy. To address this limitation, we developed a modified version of the “field decay method” based on the cavity differential equation. The proposed approach enables the precise calibration of f0.5 even under mismatch conditions. We tested the proposed approach on the SRF cavities of the Chinese Accelerator Driven System Front-End Demo Superconducting Linac and compared the results with those obtained from a network analyzer. The two sets of results were consistent, indicating the usefulness of the proposed approach.