S^6 



THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



Here r is the radial position of the electron, Fi is the focal length of the lens 

 at ^1, and Vi is the longitudinal velocity at ^i. 



CATHODE 



l«-Lr*'*-— -Lg — J 



Fig. 1 — The gun consists of a planar cathode and two apertured plane electrodes Ai 

 and Ai , with the spacings and the voltages with respect to cathode which are shown 

 above. 



^Ct^N ;.^ 



,/ \'-y \ 



APERTURE 



V 



Fig. 2 — Between electrodes Ai and i4a of Fig. 1, an electron path as seen looking paral- 

 lel to the axis is a sector of a circle of angular extent *. 



The magnetic field strength is so adjusted as to return the electrons to 

 the radius r at v4 2. Figure 2 shows the motion of an outer electron between 

 Ai and Aij seen looking along the axis. Since there is no radial electric field 



