Design of high-efficiency wide-bandwidth coupled-cavity traveling-wave tube phase velocity tapers with simulated annealing algorithms

The output circuit section of a traveling-wave tube (TWT) routinely contains an RF phase velocity taper for the purpose of increasing RF output power and efficiency. By slowing the RF phase velocity in approximate synchronism with the decelerating electron beam bunches, the taper increases power transfer from the beam to the RF wave. Recently, the computational optimization technique of simulated annealing was shown to be very effective in the design of an RF phase velocity taper that significantly increased computed RF power and efficiency of a coupled-cavity TWT. In this paper, two new broadband simulated annealing algorithms are presented that optimize (1) minimum saturated efficiency over a frequency bandwidth and (2) simultaneous bandwidth and minimum efficiency over the frequency band with constant input power. The algorithms were incorporated into the NASA 2.5-dimensional (2.5-D) coupled-cavity TWT computer model and used to design optimal phase velocity tapers using a 59-64 GHz coupled-cavity TWT as a baseline model. Compared to the baseline taper design, the computational results of the first broadband algorithm showed an improvement of 73.9% in minimum saturated efficiency. The second broadband algorithm indicates an improvement of 272.7% in minimum RF efficiency with constant input power drive and an increase in simultaneous bandwidth of 0.5 GHz over that calculated for the baseline TWT.