Observation of a free-electron laser interaction in the UHF regime

A free-electron laser (FEL) oscillator experiment operating with a low-energy (<4 keV), low-current (<0.5 A) pulsed electron beam is presented in this paper. In the table-top FEL oscillator constructed in our laboratory the electrons travel along a planar undulator and an axial magnetic field, and interact with TEM-mode electromagnetic waves. In this experiment, microwave output is observed at /spl sim/40 cm wavelength. This radiation is much longer than the undulator period (2 cm). To the best of our knowledge, this is the first demonstration of an FEL operation in the UHF regime. For electron beam energy of /spl sim/3 keV, we observe also microwave output at /spl sim/5 GHz. This emission is related to the cyclotron resonance due to the axial magnetic field. These experiments may lead to the development of new high-power UHF and microwave radiation sources.