Electron Beam Focusing in Three-Anode Guns for Travelling Wave Tubes†

Abstract

ABSTRACT This paper analyses the focusing of three-anode immersed guns used for launching a large diameter parallel-flow electron beam into the helix region of travelling wave tubes. Such guns may be designed as parallel-flow guns for producing a beam of constant diameter, compensated guns operating under varying beam voltage or current, or compensated low noise guns incorporating a ‘ voltage jump ’ for reducing fluctuations in the beam. On the basis of small perturbations, the electron trajectory in the accelerating region is solved and the beam rippling in the drift region discussed. Electron guns for producing uniform beams are investigated and Pierce's theory of compensated guns is developed in the form of formulae and curves. Beam profiles and their perturbations at the point of launching are computed. The significant result of these analyses and experiments is that the use of compensated guns results in a reduction in the focusing magnetic field necessary to obtain parallel-flow beams.