A free electron laser oscillator based on a cyclotron-undulator interaction

A free electron laser (FEL) oscillator experiment is described, in which an intense electron beam (750 kV, 20 kA) is prepared in a region of increasing guide magnetic field before injection into an undulator (60 periods, cm). The electron motion accordingly has a substantial transverse component ( ), and motion at the cyclotron ( ) and undulator ( ) frequencies occurs. Theory predicts, in appropriate geometry, a convectively unstable electromagnetic wave having frequency , where , of the FEL type, where . Megawatt level radiation at 1.7 mm, together with millimeter-harmonics and other microwave emission, is observed in a simple Fabry-Perot resonator system driven above oscillation threshold by a sufficiently large undulator field amplitude. Comparison of spectroscopic data with features of the theory permits identification of the cyclotron-undulator interaction.