The ENEA activity on compact far infrared FELs

The Free Electron Laser (FEL) is a very interesting device with many peculiar characteristics that makes it unique [1]. It is essentially based on the interaction between a free electron and a photon. It is well known that a single electron cannot emit or absorb a single photon in vacuum, due to the fact that energy and momentum conservation laws cannot be violated. But if we move our problem from vacuum to a medium having a refractive index n, the photon dispersion relation changes, allowing an emission process called Cerenkov effect. This is not the only emission mechanism permitted, the most exploited one is related to the Compton Scattering. This process can be fully understood in a quantum picture, within which a simultaneous process of emission and absorption of photons can be explained by means of a virtual state; an intense magnetic field, in a relativistic regime, can be regarded as a virtual photon and, from the energy-moment conservation laws, the absorbed and emitted photon are simply connected by the electron kinetic energy.