Studies of bunch lengthening at the BEPC

The Beijing Electron–Positron Collider (BEPC) has been operating for more than ten years. In addition to providing colliding beams for high energy physics and a single beam for synchrotron radiation study, we carried out many machine related beam experiments for understanding better the beam physics and for improving performance. Among them, measurements of bunch length and its lengthening, and the impedance implied thereby, play crucial roles in the future plans for luminosity upgrades. This paper reviews the measurement of the bunch length in the BEPC storage ring, concluding with the scaling law for the bunch lengthening above the microwave instability threshold. The low frequency impedance of the storage ring is also obtained from the bunch lengthening due to the potential well distortion in low bunch current.