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A method of computing the electrostatic potential and electron trajectories in electron guns the electron microscope type is described. Potentials are computed digitally using the successive over-relaxation technique, using two different mesh sizes. Electron trajectories are computed by a predictor-corrector method, using a bivariate interpolation formula for the electrostatic fields. Gun parameters of interest are readily obtained from the slope and displacement of trajectories as they leave the gun. Two electron guns with different grid shapes are investigated, and the grid shape is shown to influence the gun performance considerably. A re-entrant cone on the grid is shown to be deleterious in all respects, giving a source position of -12.1 mm compared with -5.1 mm, a source size of 20m compared with 8m, and a spherical aberration of 2.5 mm compared with 0.7 mm. [ABSTRACT FROM AUTHOR]

The focal properties and spherical aberration of a simple model of a field emission electron gun are investigated theoretically. Results are presented graphically. For a fixed source position, spherical aberration decreases as the grid aperture increases, but for a fixed image position, spherical aberration decreases as the grid aperture decreases, down to 01S, where S is the grid-anode distance. The results are almost independent of the anode aperture. [ABSTRACT FROM AUTHOR]