Measurement of the longitudinal wake potential in the photon factory electron storage ring

We tried to determine the longitudinal wake potential in the Photon Factory electron storage ring by measuring the longitudinal bunch structure with a photon counting method. A train of test bunches whose currents were small enough to neglect the interaction among them were injected behind a bunch with a large current. Deformation and the shift of the test bunches in the longitudinal direction due to the wake field generated by the large current bunch were measured, and the wake potential was estimated. We found the wake potential consists of two components: one is the short range wake which vanishes outside the bunch, and the other with the medium range of successive several bunches. The photon counting method is not always suitable for the multi-bunched beam. Development of a fast light shutter with a Pockels cell which enables us to measure the structure of a bunch in the multi-bunch mode is under way.