The undulator UE44 adopts the Apple-type scheme with 4 arrays of permanent magnets. Following the original idea of R. Car, its main design feature is fixed magnetic gap, with the polarization varied by movement of the opposite magnetic arrays, and photon energy of the two adjacent ones. A mathematical model is developed to remove coupling between the polarization and energy.
The fixed gap design (Figure 1) improves the undulator structure rigidity without compromising on the radiation parameters. The undulator has a period of 44 mm and length of 3.5 m. The magnetic gap is reduced to 11 mm, which delivers smooth transition between the undulator harmonics in all linear polarizations (Figure 2). The UE44 is the worldwide first fixed-gap undulator, realized in SLS by the group of T. Schmidt.
