Axial modes of the gyrotron backward-wave oscillator

Summary form only given, as follows. 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. End reflections in principle play no role in forming the gyro-BWO field. The field pattern of the oscillating mode must then depend entirely on the beam-wave interaction. Field forming processes as well as the resultant axial field profile in the gyro-BWO are thus expected to be fundamentally different from those of the gyromonotron. Indeed, a recent study has shown that the field in the gyro-BWO contracts nonlinearly as the beam current builds up. Here, we examine the mechanisms for the formation of axial modes in the gyro-BWO in the linear regime.