Effect of a backward wave on the stability of an ultrahigh gain gyrotron traveling-wave amplifier

A systematic stability analysis method using theoretical tools combining linear and self-consistent nonlinear theory is presented to analyze an ultrahigh gain gyrotron traveling-wave (gyro-TWT) amplifier operated in the fundamental $TE11$ mode in the Ka-band. It characterizes the role that the backward-wave component plays in the internal feedback physical processes of two major kinds of self-induced oscillations associated with $TE11(1)$ absolute instability and $TE21(2)$ gyrobackward-wave oscillation. For the first time, self-induced constriction in $TE11(1)$ absolute instability caused by a strong backward-wave component is revealed through simulation. Both the thickness and resistivity of the distributed wall loss loaded on the inside of the interaction waveguide have obvious effects on stabilizing both kinds of oscillations. Following the stability analysis, a multistage interaction circuit is proposed by nonlinear analysis which shortens the length of the entire structure and enables the ultrahigh gain gyro-TWT to operate with high stability and wide bandwidth.