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

Precise measurements of the cavity forward ( V f ) and reflected signals ( V r ) are essential for characterizing other key parameters such as the cavity detuning and forward power. In practice, it is challenging to measure V f and V r precisely because of cross talk between the forward and reflected channels (e.g., coupling between the cavity reflected and forward signals in a directional coupler with limited directivity). For DESY, a method based on the cavity differential equation was proposed to precisely calibrate the actual V f and V r . In this study, we verified the validity and practicability of this approach for the Chinese ADS front-end demo superconducting linac (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 V f signal using this method. The result demonstrated that the directivity of directional couplers might seriously affect the accuracy of V f measurement. At the cERL facility, we calibrated the Lorentz force detuning (LFD) using the actual V f . Our study confirmed that the precise calibration of V f significantly improves the accuracy of the cavity LFD measurement.