Investigation of nonlinear beam dynamics with APPLE II-type undulators at BESSY II

APPLE II-type undulators; are common sources of light with variable polarization. At BESSY II the UE56 insertion devices (IDs) consist of two parts. The closing of the upstream parts leads to an intolerable lifetime reduction of the beam. The paper describes the experimental investigation of this effect. Lifetime and beam loss measurements as a function of the working point clearly revealed the excitation of many resonances and especially a skew octupole driven resonance as the cause for the reduced lifetime. The nonlinearity stems from remaining field errors. In addition to existing bench measurements the integrated fields of the ID were determined with beam-based techniques. Skew and normal multipoles could be extracted from measurements of the vertical beam kick and the tune shift with horizontally displaced orbits inside the ID. Right now the impact of the ID on the lifetime can only be reduced by staying away from excited resonances. In the future for APPLE-type IDs a better control of the magnet block inhomogeneities is required.