Design and operation of high-harmonic gyrotron oscillators and gyro-klystron amplifiers

The design and performance of a millimeter-wave harmonic gyrotron is described in which the interaction is between axis-encircling electrons and cylindrical cavity TEnllmodes. Gyrotron cavities operating on this principle are well matched to low current, moderate energy, rf-accelerated electron beams (∼ 100 mA, ∼ 250 keV). Efficiencies up to 15% have been measured for moderate harmonic interactions, multi-kW power levels have been attained at the eleventh harmonic of the cyclotron frequency, and 65 GHz waves have been produced. The concept allows the magnetic field of the gyrotron to be reduced by an order of magnitude, thereby making a submillimeter-wave gyrotron feasible. However, mode competition problems must be considered for high-harmonic, submillimeter wave operation. We have been successful in suppressing mode competition by destroying unwanted modes. The beam current required for oscillation is highly dependent on the electron perpendicular energy. The energy requirement can be reduced by dielectric loading of the gyrotron cavity. We have also begun testing a 30 dB gain, twelfth-- harmonic gyro-klystron amplifier.