Introduction to the Physics of Free Electron Laser and Comparison with Conventional Laser Sources

This report is a primer introduction on the sources of coherent syncrotron radiation from FELs compared to conventional lasers. Even though the underlying mechanisms are different, FELs can be considered a laser source to all effects. In fact FEL is based on the stimulated emission of virtual photons by a relativistic electron beam, passing through an undulator/wiggler, much like conventional lasers exploit the stimulated emission by an atomic/molecular system on which a population inversion has been realized. FEL operation is illustrated by means of the usual set of parameters (gain, saturation intensity...) used to describe the functioning of conventional lasers.