Active disturbance rejection control for suppressing Klystron ripple noises in BEPCII-U RF system

Abstract The Beijing Electron-Positron Collider II (BEPCII) has recently been upgraded to increase the optimal collision energy from 1.89 GeV to 2.35 GeV. The radio frequency (RF) system was reconfigured from a single-cavity to a dual-cavity scheme per ring, improsing field stability requirements of ± 0.5 % in amplitude and ± 0.5° in phase. Three of the four RF stations employ solid-state amplifiers and achieve amplitude and phase stabilities of ± 0.1 % and ± 0.2°, respectively, whereas the remaining station retains the existing klystron transmitter and is limited to ± 0.3 % and ± 1°, mainly due to the 1.2 kHz ripple from the klystron high-voltage power supply and its harmonics. Conventional proportional-integral (PI) control is ineffective because the disturbance frequency lies close to the cavity half-bandwidth, and increasing loop gain compromises system stability. Active noise control (ANC) is effective for single fixed-frequency distrurbance but becomes complex for multiple harmonics. An Active Disturbance Rejection Control (ADRC) scheme is therefore developed for the IQ vector control loop. A Luenberger extended state observer (LESO) is designed and implemented on a field-programmable gate array (FPGA). Offline bench tests show that ADRC outperforms PI control in suppressing both single-tone and harmonic disturbances. The algorithm was then applied to the BEPCII-U East-2 RF cavity, where it reduces the 1.2 kHz sideband by more than 30 dB and achieves amplitude and phase stabilities of ± 0.06 % and ± 0.1°, respectively, at a beam current of 100 mA. These results demonstrate that ADRC provides robust suppresses of power-supply-induced ripple and its harmonics with minimal parameter tuning, offering an effective solution for high-precision RF field control in particle accelerators.