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

Abstract

Precise measurements of the cavity forward ($V_\mathrm{f}$) and reflected signals ($V_\mathrm{r}$) are essential for characterizing other key parameters such as the cavity detuning and forward power. In practice, it is challenging to measure $V_\mathrm{f}$ and $V_\mathrm{r}$ 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 $V_\mathrm{f}$ and $V_\mathrm{r}$. 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 $V_\mathrm{f}$ signal using this method. The result demonstrated that the directivity of directional couplers might seriously affect the accuracy of $V_\mathrm{f}$ measurement. At the cERL facility, we calibrated the Lorentz force detuning (LFD) using the actual $V_\mathrm{f}$. Our study confirmed that the precise calibration of $V_\mathrm{f}$ significantly improves the accuracy of the cavity LFD measurement.