TE/sub 21/ second-harmonic gyro-TWT amplifier with an axis-encircling beam

Summary form only given. A second-harmonic TE/sub 21/ gyro-TWT is being constructed at UCD that is predicted to produce 50 kW in Ka-band with 20% efficiency, 30 dB saturated gain and 3% bandwidth. The device utilizes a novel Northrop Grumman cusp gun that provides a high quality axis-encircling electron beam. The axis-encircling beam from the cusp gun provides a significantly more efficient gyrotron interaction for harmonic operation than a non-axis-encircling beam from a magnetron injection gun (MIG) because the electrons are not constrained to a fixed azimuth so they rotate through both the peaks and nulls of the linearly polarized mode. An important advantage of harmonic gyro-TWTs is that they are capable of higher power than at the fundamental cyclotron harmonic because they are more stable to the absolute instability at cutoff of the operating mode. Due to weaker strength of the harmonic interactions, the start-oscillation current is significantly higher for higher harmonic operation. This allows higher harmonic gyro-TWTs to operate stably with appreciably higher beam current.