Efficiency Enhancement of Multi-Stage Depressed Collector Using an External Magnet in TWTs

The dependence of overall traveling-wave tube (TWT) efficiency on the collector efficiency is of paramount importance. The objective of achieving high overall efficiency drives us to design a multi-stage depressed collector (MDC) with the theoretically highest possible efficiency with appropriate depressed potentials. Various design parameters, namely electrode geometry, depressed potentials, material parameters, etc., can be optimized to attain the desired theoretical maximum collector efficiency. But in a practical situation, the measured MDC efficiency is relatively less than the designed efficiency, which may be attributed to approximations involved in the nonlinear interaction of the electron beam and RF wave, fabrication inaccuracies, process limitations, simulation approximations, etc. This inexplicable practical decrease in MDC efficiency is typically accompanied by an increase in the backstreaming of electrons into the interaction structure. To compensate for this decrease, a permanent magnet has been suitably positioned on the base plate of the MDC, which results in an increase in collector efficiency. Simulation studies have been carried out to optimize the parameters of the magnet in terms of the geometrical dimensions, position, and magnetic field value, and has been validated experimentally. This technique of collector efficiency enhancement using an external magnet has proven to be very effective, contributing to a significant collector efficiency enhancement of > 5% from ~78% to ~83%, which in turn corresponds to an overall tube efficiency improvement of ~5.8% from 50.5% to 56.3%.