ELECTROSTA TIC ELECTRON-OPTICS 



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Any one of these electric fields can be produced by shaping and 

 positioning electrodes to correspond with two of its equipotential 

 surfaces. These fields are, however, in general not well adapted to 

 production with practical electrode structures. The one exception is 

 the field defined by equation 79, and electrodes for producing it in a 

 practical form are shown in Fig. 13. They are suitable for giving an 



Figs. 13, 14 — Lenses with reduced aberration. 



electron stream its initial acceleration. The electric field constitutes 

 a divergent lens, as do practically all initial accelerating fields. 



As expressed by equation 79, this field is followed by a symmetrically 

 reversed field, and for some purposes it may be desirable to include 

 the reversed field. This is done by locating a low potential electrode 

 along its corresponding equipotential surface as shown in Fig. 14. 

 A small aperture may be cut in this electrode for the passage of 

 electrons. The aperture then acts as a lens to bring the beam to a 

 focus, but this lens has its own aberration, and the whole system is 

 then only partially free from first order aberration. 



Appendix I — Calculations for a Complete System 



The electrode arrangement of Fig. 15 is chosen for giving a simple 

 example of the calculations for a complete optical system. The final 

 focal distance is found by calculating the focal distances at the points 

 m, n, o, p in succession. Electrons leave the cathode and travel 

 parallel to the axis in the uniform field between the first and second 

 plates, so their focal distance is — oo when they arrive at the point m. 

 The electrons then pass through the aperture in the second plate, 



