Experimental study of a high-frequency megawatt gyrotron oscillator

A detailed experimental study of the efficiency and output power of a pulsed gyrotron operating in the TE16,2,1 mode at 148 GHz has been conducted. A peak efficiency of 30% was achieved at 80 kV and 20 A for an output power of 480 kW. The highest output power of 925 kW, corresponding to an efficiency of 19%, was measured at 120 kV and 40 A. Two cavities with different interaction lengths (6.0λ and 4.2λ) were investigated. In both cases, agreement was found between the theoretical and experimental efficiency for beam currents up to 15–20 A. At higher currents, the experimental efficiency saturated between 20% and 25%, well below the 35%– 40% predicted by theory. No increase was obtained for modest positive or negative linear tapering of the cavity magnetic field. Measurements indicate that the beam velocity ratio decreases as beam current increases, partially explaining the reduced efficiency at higher currents. Operation in different azimuthal rotations of the cavity modes was also observed. The measured rotation was found to be consistent with the theoretical coupling between the beam and rf field.