Conventional Free Electron Lasers (FELs) suffer from some drawbacks related to the largesize of most electron beam (e-beam) accelerators that implies high costs, system complexity andshielding requirements. In the Far InfraRed (FIR) and Millimetre (MM) wave region the FEL canmeet the demand of compactness following several approaches involving the different sub-systemsof the device. For example the low e-beam energies required (< 10 MeV) allow the use of small sizeaccelerators like Radio-Frequency (RF) Linacs or Microtrons. The increased performance in termsof gain at wavelengths in the FIR and MM also requires a shorter length of the interaction region,thus allowing the use of short period undulators and small number of periods and resulting in abetter efficiency. Following these considerations a compact FEL source in the MM wave range hasbeen realised at ENEA Frascati.
"The ENE a compact millimetre wave FEL", Proc. SPIE 2104, 18th International Conference on Infrared and Millimeter Waves, 210408 (30 August 1993); doi: 10.1117/12.2298440; https://doi.org/10.1117/12.2298440