Digital transverse bunch-by-bunch feedback system for HLS

HLS (Hefei Light Source) is an 800 MeV storage ring with the bunch rate of 204 MHz, the harmonics of 45, and the circumference of 66 meters. HLS injection works at 200 MeV, where the beam is easily affected by transverse instabilities and, previously, the octupole magnet was used to suppress the transverse coupled bunch instabilities during injection. However, the octupole magnet itself limits further current increase and we developed a digital transverse bunch-by-bunch feedback system to suppress the transverse instability during injection. For this, we employ the SPring-8 FPGA based feedback processor, which can operate 500 MS/s with six ADCs and two 20-tap FIR filters. Such performance is two times more than that for one-dimensional feedback for HLS: sampling rate of 204 MS/s with three ADCs and one FIR filter. Then the FPGA code of the processor was modified by NSRL/USTC to process horizontal and vertical oscillation signal, independently and simultaneously by one single processor. The design of the digital feedback system and primary experiment results are presented in this paper. Further improving of the feedback system and investigating of the characteristics of the feedback loop are the future work. (author)