In-Vacuum Undulator

Undulator is an array of permanent magnets, which generates vertical (or horizontal) transverse magnetic field of periodically changing its polarity. Passing electron beam inside undulator experience periodic transverse acceleration from the magnetic field, and generates radiation strongly focused in to beam axis. The radiation electric field pattern is exact copy of the undulator field, and the Doppler effect squeezes longitudinal pattern, thus the radiation wavelength becomes very short, such as, VUV, Soft-X-ray and X-ray according to the electron beam energy. The radiation becomes beautiful sin-wave whose spectrum becomes quasi-monochromatic.