Development of a bunch-by-bunch longitudinal feedback system with a wide dynamic range for the HIGS facility

摘要

Electron beam coupled-bunch instabilities can limit and degrade the performance of storage ring based light sources. A longitudinal feedback system has been developed for the Duke storage ring to suppress multi-bunch beam instabilities which prevent stable,high-current operation of the storage ring based free-electron lasers (FELs) and an FEL driven Compton gamma source,the high intensity gamma-ray source (HIGS) at Duke University. In this work,we report the development of a state-of-the-art second generation longitudinal feedback system which employs a field programmable gate array (FPGA) based processor,and a broadband,high shunt-impedance kicker cavity. With two inputs and two outputs,the kicker cavity was designed with a resonant frequency of 937 MHz,a bandwidth of 97 MHz,and a shunt impedance of 1530Ω. We also developed an S-matrix based technique to fully characterize the performance of the kicker cavity in the cold test. This longitudinal feedback system has been commissioned and optimized to stabilize high-current electron beams with a wide range of electron beam energies (250 MeV to 1.15 GeV) and a number of electron beam bunch modes,including the single-bunch mode and all possible symmetric bunch modes. This feedback system has become a critical instrument to ensure stable,high-flux operation of HIGS to produce nearly monochromatic,highly polarized Compton gamma-ray beams.