Polarization in free electron lasers

Polarized electromagnetic radiation is required often in numerous scientific and industrial applications, but polarization methods elaborated in optics can fail in far-infrared, vacuum-ultraviolet and X-ray regions. Polarization of untreated FEL radiation obtained with various types of undulators, with and without axial magnetic field, is studied. Some examples of how elliptical, circular or linear polarizations can be obtained or changed quickly are given. A method of obtaining a series of pulses with given type or direction of polarization in each is proposed.