Tunable non-relativistic FEL

A super broadband regime of operation of a long wavelength Free Electron Laser (FEL) has been studied. It has been shown that because of the physical nature of the underlying instability, a frequency region within which amplification and/or generation of the electromagnetic waves occurs, ranges from frequencies slightly below to many times above the resonant FEL frequency. The upper limit of the frequency band is imposed by the thermal spread in an electron beam. Therefore, in this regime, the FEL operating frequency is determined by the frequency characteristics of a FEL resonator, and can be tuned over a broad band without changing the electron beam energy or wiggler period. A theory of this regime has been developed and available experiments have been discussed. The most striking result of the study is that, although it cannot be understood (and, consequently, was not discovered) without using the relativistic equation of motion, the regime does not rely upon relativism of an electron beam. A non-relativistic implementation of this FEL regime in a submillimeter/THz device has been proposed.