Balanced Acceleration and Deflection Electrostatic Focusing†

Abstract

have the following property: as the strength of the focusing field is increased from zero, the system is initially stable but subsequently passes through alternate bands of stability and instability, of which the former are normally the narrower. In consequence, operation of a given system must usually be restricted to the first band. A suitable combination of acceleration focusing and deflection focusing leads to a model for a periodic electrostatic focusing system that is free from instabilities. Convenient electrode structures are shown which will approximate the required field; the resulting system should have only narrow bands of instability. Such structures are similar to the Slalom configuration with the important difference being that the beam travels on one side only of the central electrodes. This scheme has the following characteristics that should make it suitable for incorporation in a tape-helix travelingwave tube: the beam velocity varies in such a way that electrons spend more time between the central electrodes, and the beam thickness varies in such a way that the beam is thinnest where it passes the central electrodes. (auth)