Feedforward correction of the pulsed circularly polarizing undulator at the Advanced Photon Source

A circularly polarizing undulator capable of switching the polarization very rapidly was installed at the Advanced Photon Source. The net magnetic field perturbation is characterized in both planes by a transient orbit motion, which lasts about 30 ms, and a DC orbit shift. In addition, multipole magnetic moment errors are present. The correction system consists of small dipole and multipole correction magnets at the ends of the undulator, a multichannel arbitrary function generator (AFG) to program the corrector magnet current triggered on the polarization change event, low-level software to load and interpolate the AFG waveforms, and high-level software running on a workstation to determine the optimum AFG waveforms for the dipole correctors. We rely on the existing real-time feedback system to acquire the orbit transient and to automatically generate a close approximation of the required corrector wave forms. A choice of deterministic correction or trial-and-error manual adjustments of the wave forms is available in the high-level software.