Status of longitudinal feedback system for the PLS storage ring

Abstract

Originally, the Pohang Light Source (PLS) storage ring was designed to store the beam current up to 400 mA. But owing to the interactions between the HOMs of RF cavities and bunched beams which make the coupled bunch mode instabilities (CBMIs) such as dipole, quadrupole, and sextupole modes, the beam current can be stored up to 200 mA. So, to cure those CBMIs, a longitudinal feedback system (LFS) using parallel digital signal processors is necessary. After having considered the developing cost and the period, it was decided to install the LFS with electronics fabricated by SLAC and a pickup and a kicker designed by the PLS. At present, one aluminum kicker with 4 input/output ports and a nose cone is fabricated for the PLS longitudinal feedback system. With this LFS, it is possible to find the best operating condition for the temperatures of RF cavities and various fill patterns. The programmable LFS is useful for various beam diagnostics as well as for the cure of the CBMIs. Thus, it is also possible to measure the growth and damping rates of the instabilities, the HOM frequencies of RF cavities which generate the CBMIs, bunch-by-bunch current, bunch-by-bunch synchronous phase, and the longitudinal aliased impedances seen by the beam at revolution harmonics.