The Magnetic Design of a Double-Period Undulator Based on Magnetic Force Compensation Technology

The FEL-II beamline of the Shanghai High Repetition Rate XFEL and Extreme Light Facility (SHINE) will cover a photon energy range of 0.4 to 3 keV by utilizing 14 advanced double-period undulators. Two undulators with different periodic lengths are configured on the same set of bases, allowing the switching of the undulator through lateral translation. A fixed longitudinal offset is implemented between the magnetic arrays of the two undulators with different periodic lengths, and magnetic force compensation is achieved by translating the upper and lower magnet structure antiparallel along the beam direction accordingly. This design can reduce the magnetic force on the girder by about 61.5%, making the overall mechanical structure more compact. This article provides a detailed introduction to the magnetic force compensation scheme and magnetic field design.