Progress in experimental studies of frequency-doubling gyro-amplifiers

Summary form only given, as follows. We report on the status of two ongoing, frequency-doubling gyro-amplifier experiments: a relativistic, 4 cavity, gyroklystron and a 3 stage, wideband, hybrid gyro-amplifier. Initial operation of the gyroklystron has been achieved using 460 keV, 510 A, 1 microsecond, electron beam pulses. Frequency doubling and amplification of the 8.6 GHz input signal was observed but the parameter range for stable operation was very limited. We are now investigating methods for suppressing the instabilities as a step toward realizing the design values of 57 dB gain and 35% efficiency. This amplifier is being developed to test the accelerating gradient limits in an experimental linac. Lower power but wider bandwidth are of interest for millimeter-wave radar applications. The 3-stage, wideband, hybrid gyro-amplifier with output frequency in Ka band aims to improve on results previously obtained with a 2-stage, frequency-doubling gyro-TWT by adding a 4-unit, TE mode cluster-cavity between the traveling wave input stage and the traveling wave output stage. We expect the addition of the cluster-cavity stage will improve electron phase punching and increase output efficiency without degradation of bandwidth. The three-stage amplifier has been fabricated and is currently undergoing vacuum conditioning. Its successful operation will correspond to the first use of a cluster-cavity in a gyrotron.