Abstract
A generalized linear theory that allows for mismatched boundary conditions was used in determining the starting oscillation conditions for gyrotron backward wave oscillators (gyro-BWO). The results reveal that the gyro-BWO interaction is normally caused by the interference between a constant-amplitude backward wave and a decaying backward wave. The latter becomes a forward-growing wave if the operating frequency is close to the waveguide cutoff frequency. The starting oscillation length for a reflection-type gyro-BWO is shorter than that of a matched-type gyro-BWO. The mismatch has more of an effect upon the onset of a gyro-BWO, which is operated close to the cutoff frequency. © 1994 American Institute of Physics.