Optics of electron guns

Abstract

Electron guns are important special cases of systems with large ray gradients. Most electron guns consist of a cathode, a Wehnelt electrode, and an anode. The latter electrode is at positive potential with respect to the cathode. The Wehnelt electrode is held at negative potential, which defines the spatial distribution of the zero-volt equipotential and hence the size of the emitting area of the cathode. Raising the potential of the Wehnelt enlarges the emission area, while a larger negative potential reduces it. The negative Wehnelt potential has a strong focusing effect on the emitted electrons and guarantees that they pass through the hole of the anode electrode. By varying the Wehnelt potential, we can alter the intensity of the emitted beam without changing the anode potential or the cathode temperature. The shape of the cathode surface largely affects the properties of the electron gun because the curvature of the emitting tip determines the strength of the electric field. The weak electric field of a flat cathode surface cannot immediately remove the thermally emitted electrons, resulting in the buildup of an electron cloud. The negative space charge reduces the emission current and broadens the energy width of the emitted electrons. This so-called Boersch effect results from stochastic Coulomb interactions between electrons at regions of high-current density within the beam. Systems for imaging surface layers with photoemission electrons (PEEM) or with low-energy emitted electrons also involve large ray gradients. Low-energy electron microscopes (LEEM) use either reflected or secondary electrons for image formation. Accordingly, we can treat the optics of these system like that of cathode lenses. © 2009 Springer-Verlag Berlin Heidelberg.