Focusing of an Electron Stream with Radio-frequency Fields†

Abstract

Abstract A stream of electrons has been focused solely by the fields of a slow (0p⁢c) electromagnetic wave An analysis was obtained by transforming to the wave frame in which the equations of motion of electrostatic periodic focusing are valid. Using the well-known results for the static case, and retransforming to the laboratory frame, focusing behaviour and design information (circuit size, frequency, power for a given stream) wore obtained. A second but more appropriate analysis was made by transforming to the average electron frame in order to predict both fast and slow-wave results; for the axially symmetric modes considered, the slow-wave focusing requires orders of magnitude less power than for fast waves and has a more favourable field shape Several experiments were performed using a helix as the slow waveguide. The stream was launched into the helix and was collected both on the helix and by a collector at the opposite end. Typically 10 to 50% of the current would be transmitted with no wave, ris...