The results of experiments on the vertical instability observed in both positron and electron multibunch accumulations at the Photon Factory storage ring are presented. The betatron sidebands, which indicate a vertical instability, appear at different frequency regions depending on the polarities of the beam. The cause of these instabilities cannot be attributed to the well-known transverse wakefields induced by beam-wall interactions. We propose that these vertical instabilities are induced by electrons in a positron bunched beam, and by trapped ions in an electron bunched beam.

• Received 3 March 1995

DOI:https://doi.org/10.1103/PhysRevLett.74.5044