ABSTRACT
One might almost say that aperture lenses are the ‘traditional’ particle lens. A great variety of detailed shapes and proportions have been used which makes it difficult to prepare tables of focal data for general application. In the context of low energy beams there is no need for smoothly rounded electrodes: simple thin discs with circular holes can be used and, provided that there is some agreement about the spacing of the discs, it should be possible to make systems of two or three apertures which behave in a predictable fashion. Figure 3.10 shows the important dimensions of such systems. For the purposes of scaling, it is usual to express all the dimensions in terms of the diameter,D1, of the apertures themselves. The significance of the outer diameter of the discs, D3, was discussed earlier in the context of establishing a simple boundary condition to use in the relaxation calculation, but it has a further, practical, significance in that the lens system does not exist in a vacuum, but rather in a vacuum system which has walls which will either be of metal and therefore have some definite potential, or of an insulating material and have a rather indefinite potential. In both cases it is important that this should not influence the potential distribution within the lens and it is as well to ensure that (D3 −D1)/A > 3.
