Electron-Optical Properties of Single-Pole Magnetic Electron Lenses

Abstract

The thesis is concerned with the study of Single-polepiece lens characteristics. These show, among other things, that the correction of spiral distortion in the electron microscope is feasible with a projector system using single-polepiece lenses for the intermediate and final projector lenses. It is shown that the optimum design for such a system is two single-pole lenses facing each other, in which the intermediate lens, with large bore, works as a correcting lens. Such an arrangement has the advantage of increasing the field of view and considerably reducing the length of the viewing chamber compared with that of current electron microscopes, especially high voltage electron microscopes. A critical appraisal of Scherzer's equation for the spiral distortion coefficient shows why it is difficult to design a correcting system for spiral distortion consisting entirely of conventional electron lenses. Calculations for iron-free coils, which are relevant to superconducting windings have also been made. These show that there is a real optimum shape for such a lens, when used as an objective, in order to have minimum spherical aberration. However no such optimum could be found for coil lenses when used as projectors. It is shown that the focal properties and aberration of practical single-polepiece lenses, can be deduced from the focal properties of the little known mathematical model of the exponential field distribution of Glaser. Finally some experimental results are described showing the feasibility of an improved high voltage projector lens and the possibility of correcting spiral distortion in magnetic projector lenses.