Two-stage depressed collector with enhanced efficiency for Ka-band TWTs

High efficiency is a key requirement for power amplifiers in transmitters for terrestrial and non-terrestrial wireless links. Traveling wave tubes (TWTs) have intrinsically higher efficiency than solid state power amplifiers (SSPAs) thanks to their working mechanism where a substantial portion of the spent beam energy is recovered by the collector. In back-off condition, the total efficiency and the energy spread of electrons reduce, posing challenges to the collector design. This article presents a realistic design of a two-stage single-ceramic depressed collector based on a novel simulation approach where the depressed collector is modeled as a separate component, without any assumptions or dimensionality reduction in the data provided as input to the collector design. The electrons backstreaming from the collector electrodes towards the slow wave structure (SWS) have been thoroughly analyzed. The role that the different categories of backstreaming electrons play on the efficiency of the collector has been investigated and presented in this work. As a reference, the study is based on data from a Ka-band TWT operating in the 27.5-29.5 GHz frequency range. A two-stage collector with an efficiency better than 86% over the considered frequency band is discussed. Thermal analysis has also been carried out to determine the temperature distribution at different operating regimes of the TWT, an important parameter for housing and deployment. The achieved high efficiency of the two- stage collector in back-off permits producing TWTs with high total efficiency and low complexity power supply suitable for enabling long range high capacity wireless links.