ABSTRACT
When the free-electron laser (FEL) appeared in the scenario of lasers, it was immediately clear that a new family of coherent sources of radiation was born, having unique and peculiar characteristics [1]. It must be pointed out that the story of the free-electronbased sources is older than that of conventional lasers [2] and starts from the technological development of electronic tubes (ET), that goes back to the ‘30s of the last century [3]. The FEL is capable to exploit all the positive aspects of the ET, such as the possibility of continuously tuning the output frequency, the wide range of output power operation, and a temporal structure of the emitted radiation that can be chosen during the design phase.
