Precise calibration of cavity forward and reflected signals using low-level radio-frequency system

Precise measurements of the cavity forward (Vf) and reflected signals (Vr) are essential for characterizingother key parameters such as the cavity detuning and forward power. In practice, it is challenging to measureVf and Vr precisely because of crosstalk between the forward and reflected channels (e.g., coupling between thecavity reflected and forward signals in a directional coupler with limited directivity). For DESY, a method basedon the cavity differential equation was proposed to precisely calibrate the actual Vf and Vr. In this study, weverified the validity and practicability of this approach for the Chinese ADS front-end demo superconductinglinac (CAFe) facility at the Institute of Modern Physics and a compact energy recovery linac (cERL) test machine at KEK. At the CAFe facility, we successfully calibrated the actual Vf signal using this method. The resultdemonstrated that the directivity of directional couplers might seriously affect the accuracy of Vf measurement.At the cERL facility, we calibrated the Lorentz force detuning (LFD) using the actual Vf. Our study confirmedthat the precise calibration of Vf significantly improves the accuracy of the cavity LFD measurement."