Active microphonics noise suppression based on DOB control in 166.6-MHz superconducting cavities for HEPS

Abstract

Superconducting 166.6-MHz cavities will be used to accelerate electron beams in high-energy photon source (HEPS). The radio-frequency (RF) fields inside these cavities have to be controlled better than 0.03% (rms error) for the amplitude and 0.03 $$^\circ $$ (rms error) for the phase. Adopting a quarter-wave geometry with $$\beta =1$$ , the 166.6-MHz cavity has two intrinsic mechanical modes at $$\sim $$ 100 Hz observed in both simulations and cryogenic tests. If coupled to external vibrations, these microphonics modes shall stress the existing proportional–integral (PI) feedback controller and inevitably deteriorate the field stabilities. Therefore, additional noise suppression may be required. A digital low-level RF system previously in-house developed was connected to a 166.6-MHz dressed cavity at room temperature in the laboratory. Piezo-tuners were used to “knock” on the cavity at various frequencies to excite cavity vibrations, and microphonics spectrum was subsequently measured. A disturbance observer (DOB)-based algorithm was adopted and integrated into the existing feedback controller. The performance of PI controller, DOB controller and a combination of PI and DOB controller was compared. The limitation of the DOB controller was also examined. The PI controller was proved to be insufficient in suppressing large cavity microphonics during the tests. By adding the DOB controller, the excellent field stabilities can be restored. Optimized loop parameters were obtained. The simple first-order filter was adequate thanks to the robustness of the DOB controller. This constitutes a first laboratory demonstration of the active microphonics noise suppression in the 166.6-MHz RF cavity for HEPS.