STUDIES OF THE NONLINEAR DYNAMICS EFFECTS OF APPLE-II TYPE EPUs AT THE ALS ∗

Elliptically Polarizing Undulators (EPUs) have become more and more popular at synchrotron radiation sources, providingfull polarizationcontrol of the photon beam. The fields of the most commonly used APPLE-II type EPUs have a very fast, intrinsic field roll-off, creating significant non-linearitiesof the beam motion with in some cases large impact on the dynamic (momentum) aperture. In general, the nonlinear effects get stronger with longer periods and higher undulator magnetic fields. One of the planned future beamlines at the ALS (MERLIN) will use a quasiperiodic EPU with 9 cm period and maximum B fields of about 1.3 T. We will present simulation studies for the proposed shimming schemes for this future device to reduce the nonlinear effects to acceptable values, as well as experimental studies for the existing 5 cm period EPUs already installed in the ALS.