Abstract
The gyrotron backward-wave oscillator (gyro-BWO) employs a waveguide structure in which no cold resonant modes exist. In addition, the feedback loop comprises the forward moving beam and the backward propagating wave. Since the end reflections in principle play no role in forming the gyro-BWO field, the field pattern of the oscillating mode must depend entirely on the beam-wave interaction. This paper examines the mechanisms for the formation of axial modes in the gyro-BWO in the linear regime.