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

A theory of the cyclotron-undulator interaction is briefly reviewed, using an especially simple orbit that is considered appropriate for the experiment conducted. New experimental results with the system configured as a multipass laser oscillator are presented, as are the dependences of the emission on the undulator field strength and guide magnetic field. It is found that the fast positive energy cyclotron-undulator interaction can account for the emission at millimeter wavelengths. The long-wavelength emission in the X and 2-cm microwave bands is also investigated. An interesting correlation between the 2-cm band emission and the millimeter wavelength emission is found. With an undulator of pitch 12.5 mm, relatively narrow-band radiation is observed at a wavelength near 1.7 mm, corresponding to the double Doppler unshifted sum of the undulator and cyclotron frequencies. Measurements of the growth rate and the radiation threshold in the undulator field strength are seen as consistent with linear kinetic theory of the cyclotron-undulator interaction.