Stability analysis of a coaxial-waveguide gyrotron traveling-wave amplifier

Abstract

The gyrotron traveling-wave tube (gyro-TWT) amplifier is known to be highly susceptible to spurious oscillations. This study develops a simulation approach to analyze the stability of a coaxial-waveguide gyro-TWT with distributed wall losses. The interplay among the absolute instabilities, the gyrotron backward-wave oscillations, and the circuit parameters is analyzed. Simulation results reveal that the distributed wall losses effectively stabilize spurious oscillations in the coaxial gyro-TWT. Furthermore, the wall resistivity of the center conductor is shown to be an additional effective mechanism for suppressing oscillations. Under stable operation conditions, the coaxial gyro-TWT with distributed losses is predicted to generate 435kW in the Ka band with 31% efficiency, a saturated gain of 45dB, and a bandwidth of 1.86GHz (≈5.8%) for a 70kV, 20A electron beam with an α(=ν⊥∕νz)=1.0 and an axial velocity spread of Δνz∕νz=5%.