Efficiency improvement of traveling-wave tubes by step velocity and voltage tapering

The objective of the work reported here is to substantially advance the efficiency capability of traveling-wave tubes. This has been pursued with the combination of two concepts. One is velocity tapering, designed to resynchronize the phase velocity of the circuit with the beam velocity at large signal levels. The other is a voltage step by which electron beam bunching is enhanced. This is explained by a stronger than usual coupling of the fast space-charge wave to the circuit in the driver section. This made it possible to achieve electronic efficiencies in excess of 40% and efficiencies approaching 60% could be achieved with single-stage collector depression. A coupled-cavity circuit at X-band without a sever was used in a demountable apparatus to facilitate variations of the test conditions. The collector included a secondary electron trap and provided an accurate analysis of the velocity distribution of the spent beam. The optimum design parameters, such as positioning, magnitude, and length of the tapers were determined and the results compared to those obtained by a large signal computer analysis.