A Gun for Starting Electrons Straight in a Magnetic Field

N 1931 Davisson and Calbick showed 1 that a circular aperture in a conducting plate which separates regions with different electric gradients normal to the surfaces acts as an electron lens of focal length F given by F = 4 7 i v - vy ' (1) K ' Here V is the potential of the plate with respect to the cathode which supplies the electrons and V-{ and V 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 aperture 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 velocities 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 A in parallel lines.