A 5m-long combined function undulator used for the Peking University Infrared SASE FEL facility (PKU-FEL) is currently under construction at IHEP, and a 10 period prototype has already been developed. This paper describes the design study of the undulator, which is a hybrid planar magnet structure with extra focusing. The results of magnetic measurements performed on the prototype are also reported and discussed, and it demonstrates that the proposed combined function magnet structure is very promising for the use in the SASE FEL at Peking University.

An IR range SASE FEL test facility will be built at Peking University. The project is designed to get the SASE FEL at 7 micron driven by a superconducting accelerator. A hybrid planar Nd-Fe-B undulator is employed and the optimization of the external focus system for the undulator is studied. In the PKU-FEL facility, the electron energy is about 30-40 MeV. The combined function undulator with FODO lattice imposes quite stringent tolerances on the strength of the quads. To solve this problem, the weak natural focusing of the undulator in the vertical plane together with horizontally focusing quads, is proposed to supply the focusing in the facility. The combined function undulator of FOFO lattice and FF lattice in the horizontal plane are studied. Compared with the FODO lattice, the FOFO and FF lattice make the saturation a bit longer and the requirements of the field accuracy for the focusing system are much reduced.