Theory and performance of depressed trochoidal collectors for improving traveling-wave tube efficiency

This paper discusses the theory and performance of depressed trochoidal collectors which are used to improve the efficiency of S-band traveling-wave amplifiers. These crossed-field multistage collectors sort the electrons in the spent beam into velocity classes, and collect each class at an appropriately low potential. The sorting and collecting actions depend on the tendency of electrons to flow to the lower-potential anode in a split-anode cylindrical magnetron. The magnetic field required in the collector region to produce sorting action is evaluated, and the design parameters of the collector structure are determined from an analysis of electron ballistics inside the collector tunnel. In the trochoidal-collector tube, spent electrons are sorted into eight energy levels and collected by eight electrodes biased at different potentials. This tube provides information on the fraction of electrons collected at a voltage equal to, or greater than, the equivalent voltage lost by the electrons in the process of RF interaction, and on the collection voltages required for spent electrons at different levels of RF drive. Complete performance curves for the assembly are included for both an unmodulated beam and an RF driven beam. When the tube produces a power output of 16 watts and a saturated gain of 35 db at 2360 Mc, the collector efficiency is 42 per cent, the over-all efficiency 38 per cent, and the electronic efficiency 16 per cent. These results are achieved without adverse effects on the RF performance as a result of beam instabilities and regenerations caused by space-charge blocking of electron flow or by motion of electrons returning to the helix region.