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.

Date of Award	1977
Original language	English
Awarding Institution	Aston University