The study of novel electrostatic electron lenses

Abstract

This thesis is concerned with the investigation, both numerically and experimentally of novel electrostatic lenses. The properties of a five-element lens are described. This lens allows the variation of the magnification of an image of fixed position with fixed overall energy, and can be therefore considered a 'zoom' lens. This lens can also be constrained so that it is afocal and the separation between any pair of conjugate points is constant, and therefore independent of V5/V1, with the magnification related very simply to V5/V1. A numerical technique involving matrix multiplication is used to compute the properties of the five-element lens from the tabulated properties of two-element lenses. Manipulation of the calculated data revealed that it is possible to define two 'universal' curves to summarise its properties. The calculated lens properties are compared with those previously obtained by experiment, (Heddle and Papadovassilakis 1984). The aberration behaviour of a five-element lens was investigated. In particular, the dependence of the spherical aberration coefficient Cson V3/V1 where V5/V1 = 1, and V2/V1 = V4/V5. Cs was also investigated for a number of afocal lenses. Finally, Cs was investigated for the lens where V5/V1 = V3/V1 = 1, V2/V1 is the variable and V4/V3. This lens was found to have a minimum value for the product MAG x Cs, therefore, optimum values of V2/V1 and the magnification exist for this lens. The values for Cs obtained by experiment are compared with those calculated by my supervisor Professor Heddle using the Bessel Function Expansion Method, and the Fox-Goodwin Method. Finally, the properties of a three-element lens constructed from 31 discs electrically insulated from each other, and sandwiched between two ordinary cylindrical elements was investigated. Voltages were applied to this lens so that it simulated a three-element lens with a 'movable' centre element of variable length. The obtained experimental properties are also compared with those calculated by Professor Heddle.