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,
l_{o} = 1.25
cm). The electron motion accordingly has a substantial transverse component (
\beta_{\perp} \approx 0.4
), and motion at the cyclotron (
\omega_{c} = eB_{o}/\gamma mc
) and undulator (
k_{o} V_{z}
) frequencies occurs. Theory predicts, in appropriate geometry, a convectively unstable electromagnetic wave having frequency
\omega_{s} \approx 2\gamma_{z}^{2} (\omega_{c} + k_{o}V_{z})
, where
\gamma_{z} = (1 - \beta_{z}^{2})^{-1/2}
, of the FEL type, where
k_{o} = 2\pi/l_{o}
. 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.