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

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