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High frequency, medium power gyrotrons (Gyrotron FU series) have been developed at the Fukui University as radiation sources convering a broad frequency band from millimeter to submillimeter wave region. We have performed cw operation of the Gyrotron FU IV. However, the output power was not stable (approximately several tens %) due to the fluctuation of the cathode potential (40 V). The output power was stabilized within several percent by controlling the anode potential. Such a technique was also used for amplitude modulation of the gyrotron output. Improved stabilization of output power and effective modulation will be achieved by stabilizing the cathode potential

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Special experiments devoted to studying hysteresis in gyrotron oscillators have been performed for the first time. Clear hysteresis-like effects with respect to variation of the cathode voltage have been observed in the mode competition scenario of the Forschungszentrum Karlsruhe coaxial gyrotron [B. Piosczyk et al., IEEE Trans. Plasma Sci. 30, 818 (2002)] and with respect to variation of the magnetic field and voltage in a single-mode operation of the Fukui IV gyrotron [T. Idehara et al., Int. J. Infrared Millim. Waves 19, 793 (1998)]. The observed phenomena are explained theoretically