Cure of Coupled Bunch Instabilities in PLS Storage Ring

Abstract

The Pohang Light Source (PLS) storage ring whose design emittance is 12-nm uses four 500 MHz nosecone-structure rf cavities to store beam current up to 400-mA at 2-GeV. The stored beam current was limited to 180-mA at 2-GeV because of the coupled bunch instabilities (CBI) excited by higher order modes (HOMs) of rf cavity. In order to cure the CBIs three measures are incorporated: HOM frequency tuning by cavity temperature adjustment; a longitudinal feedback system (LFS); a transverse feedback system (TFS). Growth rate of longitudinal and transverse HOMs of all rf cavities as a function of cavity temperature was estimated with the low-power measurement data of frequency shift, and confirmed with the BPM amplitude of CBMs. The LFS which uses programmable digital signal processors supplied by SLAC was successfully commissioned at the end of 1999, and a very stable and low emittance electron beam could be stored up to 230-mA over which transverse CBIs grow severely and drive to beam loss. After completion of TFS at the beginning of 2000 we will be able to cure all CBIs by LFS and TFS, and store beam current higher than 300-mA.