Slow fluctuation of synchrotron radiation and bunch lengthening in SOR

Slow fluctuation of synchrotron radiation and bunch lengthening of the stored beam current in SOR, a 400 MeV electron storage ring for synchrotron radiation, were studied experimentally and theoretically and found to be induced by a longitudinal coupled-bunch instability. The lengthening which follows a 1/5.8 power dependence on the beam current is explained well as an equilibrium state with a balance of the growth and damping rates of the instability. The damping is brought about by the frequency spread of synchrotron oscillations. The radiation fluctuation is due to a weak disturbance of the bunch length about the equilibrium state and a simple relation was found between the fluctuation frequency and the growth rate of the instability. Horizontal bunch widening is clearly related to the bunch lengthening. A quadratic frequency dependence of coupling impedance is derived from the data on bunch lengthening. (orig.)