[[abstract]]In a resonator structure, the axial field profile of a cold mode results from constructive superposition of reflected waves at both ends. For the gyromonotron oscillator, end reflections also constitute the feedback loop during the field build-up. The hot mode has essentially the same field profile and oscillation frequency as those of the cold mode. The gyrotron backward-wave oscillator (gyro-BWO), on the other hand, employs a waveguide structure in which no cold resonant modes exist. In addition, the feedback loop consists of 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 the axial modes in the gyro-BWO in the linear regime.
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