Abstract :
Two new methods of focusing a strip electron beam by means of periodic electrostatic fields are described. Focusing is obtained with all conductors of the periodic structures at the same direct voltage¿a condition which simplifies construction and operation. Wave crests in an undulating beam trajectory are made to coincide with regions of maximum axial r.f. field, giving good coupling between the two. Preliminary focusing experiments are described, together with backward-wave-oscillator structures incorporating the two focusing methods. Photographs show the beam trajectories and illustrate `ion drain¿ from the beam to the focusing plates¿a feature which may be significant in reducing the noise level of a travelling-wave tube.
