Design considerations of a planar undulator applied in a terahertz FEL oscillator

To fulfill the physical requirement of a 50 – 100 μ m Free Electron Laser (FEL) oscillator, design considerations of a planar undulator are described. The main undulator parameters are optimized for a trade off between the gain and the FELʹs natural extraction efficiency, and the technical aspects including the structure of the permanent magnet (PM), the choice of PM materials and field tolerances are discussed. The lattice of the designed undulator is studied and the beam matching at the entrance of the undulator corresponding to various working points of electron beam energy is performed.