Vertical instability with transient characteristics in KEK-Photon Factory electron storage ring

A vertical instability of the KEK-Photon Factory electron storage ring was observed in a multibunch mode with empty buckets (bunch gap) using a bunch- by-bunch beam diagnostic system that consisted of a high-speed light shutte r and an optical betatron oscillation detector. It was clearly seen that ve rtical betatron frequencies of individual bunches varied along a bunch trai n. The dependence of the betatron frequencies on the bunch positions in the train was explained by modulation of trapped-ion density caused by passage of the beam. Agreement between experimental results and the theoretical pr ediction was quite good.