Study of beam instability caused by APPLE-II undulator in vertical polarization mode in Aichi-SR

In the Aichi SR storage ring, an APPLE-II type undulator is mainly operated in the horizontal polarization mode. However, when the undulator is operated in the vertical polarization mode, transverse coupled bunch instability is excited and the oscillation amplitude grows till the electron beam is lost. We experimentally studied the instability and found that the main source of the instability is the higher order mode of the accelerating RF cavity. We also studied the effect of the magnetic field of the undulator on the electron beam by experiments and simulations and found that the multipole magnetic field of the undulator has some effect on the beam instability. (author)