The Formation and Maintenance of Electron and Ion Beams

The radial potential distribution in an ionic beam of circular cross section is calculated and the maximum beam current which can be obtained in a beam of given radius and boundary conditions is computed. In the case of ions the maximum beam currents may be quite small. The ions in such beams have a considerable velocity distribution which in turn leads to a greater beam divergence than former calculations indicate. In Section 3 a method is worked out for holding the beam together during the initial accelerations. A disadvantage of cylindrical lenses is pointed out in this connection. The use of electrostatic lenses for holding together a beam composed of high energy particles is discussed in some detail. Calculations show the special arrangements of the lenses necessary for producing convergence of the beam. A few other arrangements to prevent divergence which were tried experimentally are described.