Double Focusing with Wedge-Shaped Magnetic Fields

Abstract

Geometric focusing in two directions for charged particles of a single energy is produced with a wedge‐shaped magnetic field. The uniform field in the gap focuses in the plane of the pole faces, while the fringe fields focus perpendicular to the pole faces. With a single wedge the energy dispersion is too large for the weak electron intensities available with our source; i.e., for are asonable size slit at the collector only a narrow energy width and, thus, a small number of electrons is accepted. With the use of two wedge‐shaped fields the energy dispersion can take a wide range of values, even zero in the first order. General formulas for either one or two magnet systems are derived for the focusing conditions, the energy dispersion, the second‐order beam spread, and the effect of source width. These formulas are then applied to two special cases from which practically all design requirements can be obtained. Finally, a design is recommended for which the following requirements are fulfilled : five percent energy spread in an inch collector slit, large angular collection, and the same size source and collector slits.