Improving the RF stabilities of BEPCII by a disturbance observer based controller

A digital low level radio frequency (DLLRF) system has been developed to replace the old analog LLRF and operated stably at BEPCII east RF station in the past two years. The RF stabilities required for BEPCII are ± 1% in magnitude and ± 1 degree in phase. These are satisfied both by the new DLLRF and the old analog LLRF. Yet, the DLLRF did not improve the RF stabilities so much as expected, especially when the beam current was high in collision mode. The purpose is to improve the RF stabilities further and meet the requirements by the upgraded BEPCII project (BEPC3). A disturbance observer based (DOB) controller to suppress the noise was developed and tested with beam at BEPCII RF system. The DOB controller works and the RF stabilities with beam may be improved from ± 1% to less than ± 0.5% in magnitude and from ± 1 degree to less than ± 0.5 degrees in phase.