Suppression of longitudinal coupled-bunch instability in UVSOR

The UVSOR is a 750-MeV electron storage ring used solely for vacuum ultraviolet synchrotron radiation research in molecular science and related fields. A longitudinal coupled-bunch instability excited by higher-order-mode resonances of the RF cavity was observed when routine operation of the ring was started. Two methods of curing the instability were tried, i.e. a feedback method and a decoupling method. The former system consists of 16 independent feedback loops; each of them corrects the energy deviation of one of 16 bunches. Although the system can suppress the instability, critical adjustment is required, i.e. the feedback loops have to be finely adjusting according to parameters such as the electron energy, the beam current, and the RF acceleration voltage. The latter system spreads synchrotron frequencies of individual bunches by the modulation of the effective acceleration voltage to decouple these synchrotron oscillations. The instability is damped by this method when the beam current is not high (less than 100 mA). The limitation of this method is in the maximum modulation index available in the experiment. Therefore, the former system is being used in routine operation in spite of its drawback