Characteristics of a stable 200 kW second-harmonic gyro-TWT amplifier

Abstract

Summary form only given. The results of a second-harmonic TE/sub 21/ gyro-TWT amplifier experiment that generated an unprecedented 207 kW peak output power with 13% efficiency in Ku-band will be presented. This is a significant advance over the highest gyro-TWT power level previously reported which was 128 kW and achieved by Varian in C-band at the first harmonic. The amplifier had a saturated gain of 16 dB with a bandwidth of 2.1% and was completely zero-drive stable. Harmonic gyro-TWT's have the potential to stably generate significantly higher levels of power because the threshold beam current level for oscillation is raised dramatically due to the relatively weaker harmonic interaction. The success of this experiment validates the marginal stability design (MSD) procedure and also indicates the power level can be further increased by operating with higher harmonics. To suppress the potentially unstable TE/sub 11/ and TE/sub 31/ gyro-BWO modes, the interaction tube was cut with four slices separated in azimuth be 90/spl deg/. Cold test results show an attenuation of the TE/sub 11/ mode by at least 24 dB and less than 1 dB loss in the TE/sub 21/ mode. The length of the interaction circuit is shorter than the start-oscillation length of the dominant remaining gyro-BWO mode (fourth harmonic in the TE/sub 41/ mode), which was determined by linear theory. High return losses from the well matched rf couplers and loads were crucial to the stability of this high power amplifier. An 80 kV, 20 A MIG beam with a velocity ratio /spl alpha/= v/sub /spl perp///v/sub z/=1 was utilized in this second-harmonic experiment.