A Low-Voltage Gyrotron Backward-Wave Oscillator at 200 GHz

The gyrotrons are primarily developed for radars, plasma heating in controlled fusion reactors, and material processing, so they are usually operated at high voltages (about 100 kV) to generate high power coherent radiation in the millimeter to terahertz band. Recently, some high frequency applications require a relatively low-power gyrotron. In this paper, we analyze the stability and performance of a 200 GHz gyrotron backward-wave oscillator (gyro- BWO) operating at a beam voltage of 10 kV and a beam current of 0.5 A. The beam-wave interaction in a cylindrical waveguide is simulated by a single-mode, self-consistent nonlinear code. All the possible oscillating modes, including the operating mode and the competing modes, are considered. Simulation results show that the low-voltage gyro-BWO operating in the TE 42 and the fundamental cyclotron harmonic is stable in the magnetic range of 73.5−77.0 kG. Under this stable operating condition, the low-voltage gyro-BWO was predicted to generate a peak power of 466 W with 9.3% efficiency and a frequency tuning bandwidth of 2.5 GHz using a uniform interaction waveguide. If the interaction waveguide is tapered to enhance the efficiency, the low- voltage gyro-BWO was predicted to generate a peak power of 1.25 kW with 25% efficiency and a frequency tuning bandwidth of 0.93 GHz.