I. Introduction
By utilizing fast-wave interaction structures, gyrotron amplifiers have far greater power-handling capability at high frequencies than conventional slow-wave devices [1]. Gyrotron traveling-wave-tube (gyro-TWT) amplifiers offer a broader bandwidth than gyroklystrons, owing to the use of nonresonant traveling-wave circuits. In a gyro-TWT amplifier, electrons gyrate along an axial magnetic field at the electron cyclotron frequency and continuously amplify a copropagating electromagnetic wave. Interactions occur when the resonance condition is satisfied, where is the wave frequency, is the propagation constant, is the electron axial velocity, is the cyclotron harmonic number, and is the relativistic electron cyclotron frequency.