In the present study, large‐signal analysis of a disc‐loaded gyro‐travelling wave tube amplifier has been presented. For this purpose, analytical theory for non‐linear analysis of an arbitrary‐shaped interaction structure has been extended for a disc‐loaded interaction structure. Power profile results of the present theory, taking waveguide disc‐hole radius equal to waveguide‐wall radius where the structure becomes a smooth wall circular waveguide, closely matches with the published results of a circular waveguide. Comparison of already published small‐signal theory results and the present large‐signal theory has been done and a fair agreement (within 4%) has been achieved. Spatial power and efficiency profile for a disc‐loaded gyro‐traveling wave tube with structure parameters r _D / r _W = 0.6, L / r _W = 0.4 and T / r _W = 0.1 operating in TE ₀₂ ‐mode at 91.4 GHz has been presented.

The present paper is devoted to the study of mode competition and control in a 170 GHz gyrotron cavity for potential application in electron cyclotron resonance heating (ECRH) of plasmas. An extensive analysis has been carried out for coupling coefficient, start oscillation current, voltage depression, and limiting current for the candidate modes in the gyrotron cavity resonator. The study reveals that the main competing modes for TE _{28,8}^{-} come from TE _{25,9}^{+} and TE _{27,8}^{-} . The optimum beam position for TE _{28,8}^{ - } mode is found to be at 8.28 mm. To avoid excitation of competing modes, it has been found that slight increase in electron beam radius from 8.28 mm to 8.40 mm avoids the excitation of all the competing modes. Based on the considerations of coupling coefficient, start oscillation current, voltage depression, and limiting current; the TE _{28,8}^{-} mode is selected as the operating mode.

In the present paper, a disc-loaded circular waveguide structure has been field analyzed for the power loss and attenuation characteristics. The structure has also been simulated with commercially available simulation tool'CST Microwave Studio'. The analytical results have been found in close agreement with the simulation results. Attenuation characteristics have been observed for a disc-loaded circular waveguide interaction structure made of materials of different electrical conductivity and has been observed that the material with higher conductivity results in lower attenuation. Further, the variation of attenuation characteristics has been observed for two different azimuthally symmetric modes typically TE _{01} and TE _{02} . Power loss at different surfaces of the disc-loaded circular waveguide excited in TE _{01} mode has also been observed and it has been found that the tip of the discs is the surface where maximum power loss per unit area occurs. Further, it has been found that with increasing the disc-hole radius the ohmic power loss as well as attenuation constant decreases and with increasing the structure periodicity, the ohmic power loss and attenuation increases. Hence, the structure dimensions should be optimally selected for higher bandwidths and gains.

A self-consistent nonlinear simulation model has been presented to analyze the nonlinear behaviour of a gyro-TWT amplifier. It employs a slow-time scale formulation such that the numerical calculation time and the number of numerical integration steps have been drastically reduced. Beam-wave interaction mechanism has been presented with the help of electron flight diagram. The effect of variation of the static magnetic field on the bunching of the electrons along axial direction has also been clearly presented. The electron flight diagram thus obtained clearly demonstrates the electron defocusing and its effect on the saturation behaviour of the device.