A Gun for Starting Electrons Straight in a Magnetic Field 



By J. R. PIERCE 



In a simple electron gun consisting of a cathode and two apertured planes 

 held at different potentials the apertures act as electron lenses. When the gun 

 is immersed in a uniform axial magnetic field the aperture spacings and poten- 

 tials can be chosen so that the emerging electrons have no radial velocities. 



IN 1931 Davisson and Calbick showed^ that a circular aperture in a con- 

 ducting plate which separates regions with different electric gradients 

 normal to the surfaces acts as an electron lens of focal length F given by 



F = — (1) 



Here V is the potential of the plate with respect to the cathode which 

 suppUes the electrons and VJ and Vi are the electric gradients on the far 

 and near sides of the aperture respectively. 



When an electron beam is produced by means of a plane cathode and an 

 opposed plane positive apertured anode, the fields about the anode aper- 

 ture form a diverging lens and cause the emerging beam to spread. Sometimes 

 this is very undesirable. A strong uniform magnetic field parallel to the 

 direction of electron flow may be used to reduce such spreading of the beam, 

 as well as the spreading caused by space charge and by thermal velocities. 



The magnetic field does not completely overcome the widening of the 

 beam caused by the lens action of the anode aperture, for the radial veloc- 

 ities which the electrons have on emerging from the aperture cause them to 

 spiral in the magnetic field, and the beam produced is alternately narrow 

 and broad along its length. 



This paper describes an electron gun consisting of a cathode and two 

 apertured plates together with a uniform axial magnetic field. The gun is 

 designed so that the net lens action is zero and the electrons emerge traveling 

 parallel to the magnetic field. 



The electrode system is shown in Fig. 1. The electrons travel from the 

 plane cathode to the aperture in plane electrode Ai in parallel lines. At Ai 

 they receive a radial velocity approximately vn, given by 



Vri = — fr n (2) 



^ C. J. Davisson and C. J. Calbick, "Electron Lenses," Phys. Rev., vol. 38, p. 585, Aug. 

 1931; vol. 42, p. 580, Nov. 1932. 



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