Electrostatic Electron Lenses

Abstract

AN article by K. Spangenberg and L. M. Field (Elec. Comm., 21, No. 3; 1943) describes and discusses the measured characteristics of a number of electrostatic lenses, giving the characteristic curves of nine different lenses belonging to three basic types. The forms tested were cylinder lenses of various spacings and diameter-ratios, aperture lenses (parallel plates with circular apertures on the beam axis) of various spacings, and, for comparisons, a lens formed by a cylinder and an aperture in a plate. By interpolating between the sets of curves given, approximate predictions of the properties of lenses of slightly different spacings or diameter-ratio may be made. The test method employed makes use of a conventional electron-gun with parallel wire grids which, for measurement purposes, are placed before and after the lens to be tested. All the lens characteristics are deducible from measurements made on shadows cast by these measuring grids upon a fluorescent screen placed in the electron beam at a suitable distance from the gun.