Coupling Correction of a Circularly Polarizing Undulator at the Advanced Photon Source

The electromagnetic circularly polarizing undulator (CPU) installed at the Advanced Photon Source (APS) storage ring produces skew quadrupole field errors, which were initially corrected by a small skew quadrupole magnet at one end of the device. Because the storage ring is operated at 1% coupling or less, a correction not located at the source inside the CPU is insufficient, as we have confirmed in simulation. Adding a skew coil at the other end of the CPU allows us to make a complete correction of the coupling source in the undulator. Correction set points are determined by APS’s general optimizing software with the vertical beam size of an x-ray pinhole image as a readback.