Stable 0.3-THz Gyrotron Backward-Wave Oscillator With a Tapered Coaxial Interaction Waveguide

Mode competition is a severe problem in the development of a high-power gyrotron backward-wave oscillator (gyro-BWO) in terahertz (THz) region. To improve the stability of this device, this paper investigates the possibility of using a tapered coaxial waveguide as the interaction structure of a 0.3-THz gyro-BWO. The inner cylinder rarefies the mode density near the operating mode. Moreover, tapering the wall of the outer cylinder suppresses almost all the competing modes and shifts the optimum magnetic tuning range of the operating mode under stable operating conditions. The gyro-BWO is predicted to stably generate a peak power of 17.2 kW with 11.5% efficiency at 0.307 THz, and a continuous tuning bandwidth of 3.4 GHz (≈1.1%) for a 30-kV, 5-A electron beam with α = 1.0 and an axial velocity spread of 5%. Compared with the uniform case, the tapered coaxial interaction waveguide enhances both the stability and the frequency tunability of the THz gyro-BWO.