Linear analysis of a coaxial-waveguide gyrotron traveling-wave tube

Linear theory provides an efficient analysis model for the preliminary design of a gyrotron traveling-wave tube (gyro-TWT). This study presents a linear theory, which is applicable to amplifications or self-excited oscillations induced by absolute instabilities in a coaxial waveguide of finite length. The effects of wall losses are incorporated in the theoretical formalism. The validity of the linear theory is verified by comparison with calculation results obtained using an existing self-consistent nonlinear theory. The linear theory is applied to analyze a TE01 mode coaxial gyro-TWT at the fundamental cyclotron harmonic. Numerical analysis of coupling between the beam cyclotron mode and the waveguide mode provides physical insight into the wave-growing mechanisms of various oscillations. The critical parameters for the onset of threatening oscillation modes are analyzed to determine the stable operating conditions. Finally, the dependencies of small-signal amplifications on system parameters are studied in great detail