ELECTRON LENSES 



335 



Such a lens is shown in Fig. VIII-13 as a small aperture in a thin metal 

 plate separating two fields of different electric intensities. The electric 

 field E 2 on the right side of the hole is of greater intensity than E\. 

 Note the narrow region around the edge of the hole in which there is a 

 rapid change in electrical intensity. Since the lines of force are continu- 

 ous, a field parallel to the axis of the lens can change in intensity only 



2R e;_ 



1 - 



£,>0 



Fig. VIII-13. Lines of 

 force terminating on an aper- 

 tured plate with field E<i > E\. 

 Equivalent optical lens sys- 

 tem below. (By courtesy 

 of the Bell System Technical 

 Journal and F. Gray [1939].) 



#2 V 



(a) 



G>) 



Fig. VTII-14. Electron trajectory 

 and principal points for cylindrical 

 tube lenses. Lines of force are the 

 broken lines; Hz second principal 

 plane; F% second principal focus. 

 (a) and (b) equivalent optical trains 

 for potential V<i > Vi. (By courtesy 

 of the Bell System Technical Journal 

 and F. Gray [1939].) 



by lines of force crowding into it or leaving it in a radial direction. These 

 radial components of the fields deviate the electrons in a radial direction. 

 Since the radial components of E 2 are greater than those of E\ in the 

 region around the hole, a parallel electron beam approaching the hole 

 from the left would converge on the right side of the aperture. In 

 this type of lens the non-uniform field at the aperture covers a distance, 

 along the axis, about equal to its diameter. 



Another electrical potential electron lens much favored in the con- 

 struction of " electron guns " is the coaxial tube type. 



These lenses are diagrammatically represented in Fig. VIII-14a 

 and b. The path of the electron is indicated as an unbroken line; the 

 lines of force, as broken lines. The divergent field has the same intensity 

 as the convergent field, but an electron is traveling faster in the diver- 



