ACCURATE CALCULATIONS OF DOUBLE-APERTURE ELECTROSTATIC IMMERSION LENSES

A new and simple modification of the method of expansion in terms of Bessel functions has been developed for the accurate calculation of the potential distributions of cylindrically symmetric electrostatic lenses. The method has been used to give the potential distributions of some double-aperture immersion lenses, and the positions of the cardinal points and the spherical aberration coefficients of these lenses have been calculated by numerical integration of the ray equations. The calculated values are accurate to better than 1%, and differ significantly from previous experimental values.