Magnetic field analysis of hybrid helical wiggler with multiple poles per period

It is useful to use a high-energy electron beam and a long-period wiggler to accomplish far-infrared (FIR), free-electron laser (FEL) operation with power as high as possible. In this study, the magnetic field of a wiggler has been analyzed by computational simulation and applied to the case of an FIR-FEL driven by an electron beam of the Institute of FEL (iFEL), Osaka University, with maximum energy of 165 MeV. The wiggler is a hybrid helical consisting of permanent magnets and ferromagnetic bodies and has some poles per one period used to obtain a uniform sinusoidal magnetic field. In most simulations, the period and gap length of the wiggler were assumed to be 14 and 2 cm, respectively. A uniform field higher than 1 T was calculated for the wiggler when the number of poles per period was greater than 8. It is expected by using this kind of wiggler and an electron beam energy of 165 MeV that the power of FIR-FEL will be >100 times as high as that for a usual FEL.