Free-electron-laser-type interaction at 1 meter wavelength range

A free-electron-laser (FEL)-type interaction is observed between a low-energy electron beam (∼ 0.7 keV) and a radio wave in the VHF range (λ = 1.1 m). The experimental device consists of a transmission-line (TEM-mode) cavity, and a folded-foil planar wiggler (λw = 4 cm). Coherent oscillations are observed in the fundamental-cavity mode (0.28 GHz) and in higher-order modes. The FEL-type mechanism is demonstrated here in a new regime, λ ⪢ λw.