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

A free-electron laser (FEL) oscillator experiments operating with a low-energy (< 4 keV), low-current (< 0.5 A) pulsed electron beam are 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 fields, and interact with TEM-mode electromagnetic waves. In this experiment, the microwave output is observed at approx. 40 cm wavelength. This radiation is much longer than the undulator period (2 cm). To the best of our knowledge, this is a first demonstration of an FEL operation in the UHF regime. For electron beam energy of approx. 3 keV, we observe also microwave output at approx. 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.