Lawrence Berkeley National Laboratory

Elliptically Polarizing Undulators [1] that provide full photon polarization control also have fast, intrinsic transverse roll-off of the magnetic field. The roll-off is especially fast for vertical polarization settings, and can have big detrimental effects on the nonlinear single particle dynamics. Particularly low and medium energy light sources and long period EPUs are prone to those effects. The three existing 50 mm period EPUs at the ALS have been retrofitted with shims to correct for these dynamic multipole effects and a new 90 mm period device which otherwise would have caused a huge reduction in dynamic aperture has been shimmed before installation. Beam dynamics measurements on all devices show that the shimming works very well and user operation with the long period EPU has begun.