36 GHz, 10% efficiency, high-order mode CARM oscillator

A new stage of experimental study of cyclotron autoresonance maser (CARM) oscillator is reported. As compared to the previous stage [1] an improved microwave system was used and special attention to investigation of the quality of electron beam was paid In the experiments a thin tubular initially rectilinear electron beam (400ke V, 600A, 25nsec) producing in a coaxial diode with a field emission cathode was employed Rotational velocity up to v⊥/c≈. 6 was imparted to the electrons by means of a one- or two-coil kicker. Large measured rotational velocity spread (≈20%) was caused, mainly, by the presence of comparatively great initial oscillations in the beam. Such quality of the beam allowed to obtain CARM efficiency 3 times smaller than the predicted by numerical simulations value for a perfect electron beam. The use of a 2° conical section and a very short 100% resonant reflector in the CARM cavity allowed to avoid parasitic gyrotron excitation of lower modes. As a result of the “hot” experiments the generation of the H5,1 mode at the operating frequency 36 GHz with maximum output radiation power of 25 MW, efficiency 10 % and pulse duration 10–15 nsec was obtained