An 85 GHz Quasioptical Gyroklystron Experiment

Abstract : The design and test of a new type of millimeter-wave tube called a quasioptical gyroklystron is reported. The device comprises a pair of quasioptical resonators separated by a drift region, which is powered by an annular electron beam with typical parameters 75 kV beam voltage, 6 A current, and an electron pitch angle of 1.9. The gyroklystron is operated as a free- running oscillator, a mode, primed oscillator, an amplifier, and a phase-locked oscillator. The output resonator is tilted by 2 deg relative to the plane perpendicular to the electron beam axis, which is predicted to increase the efficiency and operating region of stable, single-mode operation. Experimental measurements indicate that the output is single-moded over nearly all the gyroklystron operating parameter space. A new regime of operation has been studied called alpha priming, where the electron beam parameters are ramped to obtain higher power and efficiency. Peak electronic efficiencies of 22% are measured in a single mode with peak output powers up to 150 kW at 85.5 GHz in 13 microsecond pulses. A depressed collector is added to the experiment to increase the overall efficiency of the tube to greater than 30%. Capacitive probes in the drift tube are used to measure the average pitch angle of the beam electrons, and the results are compared to electron trajectory simulations. Experimental measurements of the output radiation are compared to time-dependent, multimode computer simulations, which demonstrate the advantages of mode priming by prebunching the electron beam and alpha priming. The experimental efficiencies are the highest obtained using a quasioptical gyrotron, and the present experiment also represents the state-of-art in high power phase-locked sources at frequencies near 100 GHz.