Investigations of the focal properties and magnification of multielement electrostatic lenses

Abstract

We developed a technique for measuring the focal properties and the magnification behaviour oi electrostatic lenses with cylindrical geometry. We applied the method to the study of three-element lenses of various proportions, and we found excellent agreement between our experimental results and a simple theoretical model of lenses with centre elements longer than 1.25 internal lens diameters. We extended our study to lenses with centre element 1 and 0.5 lens diameters where the simple theoretical model is inappropriate but there are detailed calculations on the focal properties and the magnification of such lenses. Again we found very good agreement. Having established a sound measuring technique we studied the properties of lenses with controlled magnification. We constructed a five-element lens whose linear magnification can be varied over a range of 9:1 at a given overall voltage ratio. The same lens can also be operated to give a constant magnification over an extremely wide range (10000:1) of overall voltage ratios. Such a lens can prove a powerful tool in electron spectroscopy for uses in electron guns and as the output lens of monochromators. The potential distribution required for a specific purpose can be obtained simply from design curves. By imposing certain constraints on the focusing voltages of the five-element lens we can form a system with a magnification that depends only on the overall voltage ratio as (Vs/V1)-0.2S and with the property of forming an image at a fixed distance from an object. We have made an experimental study of such a lens and found that a prediction of a simple scaling law for the potentials was valid. We developed two independent methods for measuring the spherical aberration of some of the above lenses. We have measured the third order coefficients Cs, Cso, Cs4 for two and three-element lenses of various proportions and the coefficient Cs for a five-element afocal lens. We have observed fifth order effects for large launching angles. The object of this thesis was to develop and use a technique to investigate the focal properties, magnification and spherical aberration of electrostatic lenses.