Linear and Nonlinear Characteristics of a Low-Voltage Gyrotron Backward-Wave Oscillator in the Subterahertz Range

Gyrotrons operating at low voltage are highly attractive for subterahertz applications because such an operation can greatly reduce the size and cost of gyrotron systems. Among the oscillator versions of gyrotrons, the gyrotron backward-wave oscillator (gyro-BWO) features continuous frequency tuning in a broadband. This study investigates the characteristics of the beam-wave interaction in a low-voltage gyro-BWO in the subterahertz wave range. Operation at lower voltages has the advantage of a broader frequency tuning range and the suppression of negative $k_{z}$ competing modes. The phenomenon of a sharp increase in the start-oscillation current near the lower limit of the operating voltage is investigated. In addition to the linear characteristics, the dependence of the efficiency and tuning bandwidth on voltage is analyzed. The potential performance of a low-voltage gyro-BWO is predicted using the existing beam parameters: beam voltage of 2 kV, beam current of 0.2 A with a velocity ratio of 1.0, and axial velocity spread of 20%. Although the gyro-BWO operating at 2 kV exhibits the problem of low efficiency (an efficiency of 5.6%), it has a broad frequency tuning range of 3.0 GHz. The efficiency can be enhanced to 15% (a peak power of 60 W) by tapering the waveguide radius. Possible competing modes in low-voltage operation are considered and a stability analysis reveals stable operation.