Free Electron Lasers (FELs) are sources of coherent electromagnetic radiation having similar optical properties as conventional gas or solid-state lasers.

The relevant difference is the lasing medium being, for FELs, an electron beam which moves freely through a magnetic structure, the undulator which causes transverse electron oscillations (see figure).

 

While traveling in the undulator, the coupling of electron transverse velocity with an electromagnetic radiation field results in energy transfer. This causes micro-bunching of the electron beam in dependence of the radiation wavelength and leads to coherent emission.

In Linac based FELs, the micro-bunching instability can be created in a single pass through sufficiently long undulators, where an exponentially growing power radiation is produced. 

This spontaneous amplifying mechanism is called Self Amplified Spontaneous Emission (SASE) which ensures only the transverse coherence 

radiation, the longitudinal distribution showing large numbers of spikes generated by the noise in the electron bunch density.

“Seeding” the electron beam with an external radiation source allows to drive the instability and to control the longitudinal radiation properties.

The generated output radiation is extremely interesting in terms of brilliance, coherence, pulse duration and intensity; these unique features offer new opportunities in many fields of research: material science, chemistry, biology, medical science, industrial applications.