Experimental study of phase deviation and pushing in a frequency doubling, second harmonic gyro-amplifier

The dependence of output signal phase on the electron beam accelerating voltage and guiding magnetic field has been studied in a frequency-doubling inverted gyrotwystron (phigtron). The phigtron is capable of producing 720 kW of peak power, centered at 33.75 GHz with a bandwidth of 0.7%, saturated gain of 30 dB and an electronic efficiency of 35%. This paper is motivated by the need for compact millimeter wave final amplifiers for systems such as high-resolution coherent radar and high-gradient linear accelerators, in other words, applications requiring very stable radio frequency (RF) phase. Previous studies of the phigtron concentrated on the operating parameters gain efficiency and bandwidth. Here, we investigate the phase characteristics of the frequency-doubling gain mechanism and the sensitivity of the phase to fluctuations in the external operating parameters. RF drive power is injected into a TE₀₂ gyro-TWT section to modulate a 51.2 keV, 20 A electron beam. In the output cavity, the prebunched beam excites the TE₀₃ mode at twice the input frequency near the second harmonic of the cyclotron frequency. Fluctuations in the accelerating voltage, guiding magnetic field, and RF phase were measured and analyzed for correlation. The results show that phase deviation is well correlated with fluctuations in pulse voltage and that straightforward improvements in the stability of power supplies would significantly reduce the measured phase deviation. Furthermore, the frequency-doubling mechanism employed in the phigtron is demonstrated to produce good gain near the second harmonic of the cyclotron frequency with low phase deviation