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 ( Q L ) and half bandwidth ( f 0.5 ) is essential for monitoring the performance of superconducting radio-frequency cavities. However, the conventional “field decay method" employed to calibrate these values requires the cavity to satisfy a “zero-input" condition. This can be challenging when the source impedance is mismatched and produce nonzero forward signals ( V f ) 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 f 0.5 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.