A high-precision electromagnetic undulator for a compact FIR FEL

A high-precision electromagnetic undulator has been developed for a compact FIR free-electron laser (FEL) driven by a magnetron-based microtron. Permanent magnets are placed between iron poles to compensate for the high-flux magnetic field generated by the main coils. The period of the undulator is 25 mm and the number of periods is 80. We can change the K-parameter from 1 to 1.6 (Bw=4.5–6.8 kG at a fixed gap of 5.6 mm) in order to cover the FEL wavelength range from 100 to 300 μm at an electron energy of 6.5–4 MeV. The undulator has a modular structure of iron poles and a novel correction scheme of field distribution to improve significantly the accuracy of the field distribution. The r.m.s. deviation of the field distribution was improved from 0.2% to 0.05% with the correction method. The undulator shows considerable advantages in performance, compactness, cost effectiveness, and simplicity in manufacturing and construction.