Approach to calibrate actual cavity forward and reflected signals for continuous wave-operated cavities

Precise measurement of the cavity forward (Vf) and reflected signals (Vr) are crucial for calibrating other key parameters such as cavity detuning and forward power. In practice, the accuracy of these measurements is impacted by the crosstalk between the forward and reflected channels. For a pulsed radio-frequency (RF) system, DESY has proposed an algorithm to calibrate the actual Vf and Vr based on the cavity differential equation. This method requires the value of cavity voltage (Vc) changing over time to establish a complete differential equation. However, for a continuous wave (CW)-operated RF system, the signal Vc is almost constant over time thanks to the high gain feedback control of the low-level RF system. As a result, it is difficult to locate the optimal calibration factors using the DESY’s algorithm. Therefore, we offer an alternative algorithm to determine the calibration factors using the CW RF waveforms during a nominal RF shutdown event (RF power source is forced to shut down while the cavity remains working appropriately without any RF-related faults). The validity of the proposed algorithm was demonstrated on a Chinese ADS front-end demo superconducting linac (CAFe) facility that operated in CW mode. Using the calibrated Vf signal, we successfully estimated the beam current and beam phase during the CAFe beam commissioning, and we confirmed the accuracy of these estimations with the measurement result.