Speaker
Description
We describe a method for determining the loaded (QL), unloaded (Q0), and external quality factors of superconducting radio-frequency (SRF) resonators based on S11 measurements using a vector network analyzer (VNA). Although S11 characterization is a straightforward measurement technique, it is not sufficiently mature to directly yield accurate QL and Q0 values. One limitation is that microphonics introduce significant noise into the S11 response. Another is that spurious reflections in the transmission line between the VNA and the resonator contaminate the reflected signal and distort the measured S11. To address these issues, we first apply averaging of the S11 response to mitigate noise induced by microphonics. We then introduce a mathematical model that isolates and accounts for spurious reflections in the transmission line, enabling accurate reconstruction of the resonator response and extraction of the true Q-factors. The method has been implemented for the characterization of overcoupled Nb₃Sn cavities at Jefferson Lab at 2 K.
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