Analysis of Oscillations in Confocal Gyrotron Travelling-Wave Amplifiers

In this article, key parameters that characterize field distribution and beam-wave coupling in confocal circuits are studied. The kinetic theory of confocal gyro-TWT considering launching loss is deduced based on it, which is capable of predicting the linear gain rate of confocal circuits with arbitrary mirror widths. Numerical results manifest that the output power/gain versus interaction position agrees well with the nonlinear results in the small-signal regime. In addition, main oscillations in confocal circuits including the near-cutoff oscillation and backward-wave oscillation (BWO) have been examined. Analytical expressions that can predict the near-cutoff oscillation threshold of uniform circuits without considering loss are derived and kz-f plots are introduced for lossy circuits. Furthermore, an algorithm that can automatically search the BWO start-oscillation thresholds of confocal circuits with arbitrary mirror widths has been developed.