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14 July 2000 Suppression of parasitic self-excitation in Cherenkov amplifiers
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A means for removing the parasitic feedback in microwave amplifiers, the main obstacle to achieving a high gain, is described here. The method is based on utilizing the resonant interaction between fast cyclotron waves on an electron beam and the electromagnetic waves that are propagating in the opposite direction. This is an effective method to prevent detrimental self-excitation in amplifiers whose operation is based upon the stimulated Cherenkov radiation of a forward-propagating electron beam in a guiding magnetic field. Conditions for the resonant interaction are provided by proper choice of the guiding magnetic field. At such resonances the counter-propagating waves are in stop-bands and, therefore, cannot propagate. Results of theoretical and experimental investigations of cyclotron absorption of counter-propagating waves in amplifiers are given in the present work. It is shown that the resonant cyclotron interaction leads to a complete suppression of the feedback and that the threshold of self- excitation becomes unachievable even for large reflections. Only a minor decrease in the amplification results in comparison with an ideal amplifier without reflections. It follows from these results that a spatially varying magnetic field can be applied along the axis of the amplifier to expand the zone of the cyclotron absorption and thereby exclude a re-tuning of the self-excitation frequency.
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Michael Fuchs, Czeslaw Golkowski, Theodore Grabowski, Nikolay F. Kovalev, and Alexey V. Palitsin "Suppression of parasitic self-excitation in Cherenkov amplifiers", Proc. SPIE 4031, Intense Microwave Pulses VII, (14 July 2000);

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