Magnetic measurements and tuning of undulators for the APS FEL project

Two insertion device magnetic structures have been prepared for the Advanced Photon Source (APS) FEL project. The magnetic structures are standard APS undulators, 2.4 m long with 3.3-cm period. Measurements and tuning of the undulators have been completed at a magnetic gap of about 9.3 mm, where K is 3.1. Special measurement and tuning techniques were used to satisfy the tight trajectory straightness requirement that the second field integral be less than 3.3 kG-cm². The magnetic field strengths of the undulators must be well matched; this leads to the requirement that the magnetic gap must be controlled to better than 10 microns. Proper phasing between the undulators is ensured by adjusting the length of the drift space between the undulators. The drift space length that is needed is strongly affected by the end fields of the magnetic structures. The results of measurements of the magnetic field and calculations of the drift length are provided