Electron optical properties of three-electrode electron lenses

Abstract

The focal properties and chromatic and spherical aberrations of 3-electrode lenses represented by Glaser''s bell-shaped potential distribution and also by double bell-shaped potential distributions are extended. The results can be expressed simply in terms of the half-width of the potential distribution and the ratio of the potential at the centre of the lens to that of the accelerating potential. These parameters and their dependence on electrode geometry are derived approximately from Regenstreif''s potential theory modified by the aid of experimental measurements, leading to a lens model which may be considered to be the superposition of 4 or 5 aperture lenses. Focallengths deduced from the theory agree sufficiently closely with experimental values to be immediately applicable for design purposes.