A lightweight single reversal field focused very low-noise traveling-wave tube

This paper describes a very low noise X-band TWT amplifier whose electron beam is focused by a single reversal permanent magnet field. Test results demonstrate that noise figures thus far only obtained on low noise TWT´s focused in a uniform PM field can be achieved in a single reversal PM field. Typical maximum terminal noise figures taken between 7 and 11 Gc are less than 7.5 db. Minimum gain and saturation power in this frequency range are 27 db and 8 mw, respectively. Developmental models have yielded maximum terminal noise figures of 7.0 db across the band and terminal noise figures between 5.5 and 5.7 db at 8.5 Gc. Over the full octave from 6 to 12 Gc, terminal noise figures less than 8.5 db and gains greater than 27.5 db have been measured. The magnetic circuit, whose principal elements are two main magnets and two ring-shaped aiding magnets, is described, and the influence of the magnetic field pattern on the energy relations of the beam electrons and therefore on tube performance is discussed. The magnet circuit is shielded by a cylinder 4 inches in diameter by 11 inches long which allows a maximum fringing field of 45 oersteds on the outer surface. The low noise TWT has a metal-ceramic construction and includes a conventional low noise gun. The completely shielded amplifier weighs 8.2 pounds, which is less than half the weight of a comparable tube focused in a uniform PM field. A further weight reduction is possible.