SPHERICAL AND CHROMATIC ABERRATION CORRECTION USING APERTURE LENS - COMPUTER-SIMULATION

The electron trajectories from a Pierce-type electron gun are calculated. The aperture lens which is formed by an anode hole acts as a concave lens and produces negative-sign spherical and chromatic aberrations. The negative-sign focal length which is calculated by the trajectories coincides with the value calculated by the formula derived by Davisson and Calbick 60 years ago. For an electrode of practical size, the negative-sign spherical and chromatic aberration coefficients are on the order of 10(0) to 10(2) m and of 10(-3) to 10(-1) m, respectively. It is shown that when the electron beam from this type of electron gun is focused by a magnetic lens, most of the spherical aberration is compensated. These aperture lenses are expected to improve the characteristics of the charged particle beam systems.