Beam instability caused by APPLE-II undulator and its suppression at Aichi-SR

In Aichi SR storage ring, excitation of a transverse coupled bunch instability is observed when APPLE-II type undulator is operated in vertical or helical polarization mode. We studied on the instability and found that the main source of the instability is the higher order mode of the RF accelerating cavity. Observation of the betatron tune spread and simulations on the undulator magnetic field strongly suggest that the Landau damping which suppress the instability is degraded by the dynamical multipole field of the undulator. We performed an experiment to compensate the octupole field of the undulator using multi-wires method and succeeded in restoration of the tune spread and in widening the stable undulator operating region. However, complete suppression of the instability around the minimum undulator gap have not yet been realized. (author)