336 



COMPOUND AND ELECTRON MICROSCOPES 



gent field so that it receives a corresponding smaller radial deflection in 

 that field, thus making the total lens system convergent. 



Electron Gun 



The use of the apertured lens plate as an electron lens unit is illustrated 

 in Law's [1937] design of an electron gun, Fig. VIII-15. It is composed 



re 6 mm »i 



Cathode 



c 



v< 



Volts 100 

 Type of element + + 



200 2000 10,000 

 + + - 



Fig. VIII-15. Details of a high-density electron gun. Indirectly heated cath- 

 ode. Coated area 0.13-cm diameter. Electron beam, broken line. Apertures 

 3.86-, 2.24-, 1.70-, 1.32-, and 0.10-mm diameter. (By courtesy of R.R. Law [1937].) 



of an oxide-coated cathode (C) with aperture Vq acting as its " grid." 

 This is followed by a train of aperture anodes. The potentials of the 

 grid and cathode are V , and the others rise in the order indicated. This 

 lens gives a high-density electron stream with a cross-over at the hole 

 in F 4 - 



A coaxial type of electrostatic electron lens train as designed by 

 Zworykin [1933] is shown in Fig. VIII-16. The electrons emitted by 

 the cathode are accelerated by the positive potential of the first anode 

 and converged by the higher positive potential of the second anode. 

 Note that a cross-over of the beam occurs near the aperture of the first 

 anode. 



An electron gun, featuring both the circular diaphragm and the 

 coaxial cylinder as parts of an electron lens system to obtain either a 

 divergent or a convergent beam, is illustrated diagrammatically in 

 Fig. VIII-17. If the electrons are traveling with low velocity near the 

 cathodes as in (6) the field must be perfectly uniform. If the first aper- 

 ture lens draws off the electrons with high velocity as in (a), the first 

 cylindrical electron lens acts as a condenser lens ; otherwise an accelerat- 

 ing cylinder lens must precede it, as in (b), to impart the necessary 

 velocity before convergence takes place. In this way a condenser lens 



