Stability and Tunability of the Gyrotron Backward-Wave Oscillator

The gyrotron backward-wave oscillator (gyro-BWO) operates at a point in the ω versus k_z diagram where the group velocity is opposite to the beam velocity. This allows oscillations to build up in an internal feedback loop comprised of the forward moving electron beam and the backward propagating wave. By employing a nonresonant circuit, the gyro-BWO features continuous frequency tunability by either the voltage or magnetic field adjustment. However, frequency jumping and erratic power output were commonly observed during the frequency tuning. In this report, we begin with a study of the mode formation process in a tapered gyro-BWO. Since no cold resonant mode exists in the interaction structure, the identities of oscillating modes are shown to be determined by the beam-wave interaction. Stability analysis predicts broader stable tuning range with shorter interaction length. A Ka-band gyro-BWO experiment was performed employing a short interaction length. Continuous frequency tunability and stable output power spectra were demonstrated in both magnetic-field and beam-voltage tuning. Tunable bandwidth of 8.5 % was achieved at a peak power of 115 kW corresponding to an efficiency of 24 %.