Electron Guns and Focusing for High-Density Electron Beams

Abstract

Publisher Summary The incorporation of a high-density electron beam in an electron tube poses two problems: the design of an electron gun capable of providing a sufficient number of electrons, and the selection of an efficient method of holding the resulting electron beam together in the face of space-charge forces and other effects leading to divergence. This chapter describes the principal methods by which these two problems can be solved and provides a critical review of the various suggestions that have been made to improve these methods or to replace them by new techniques. It describes the Pierce method and evaluates several methods of arriving at a Pierce gun by alternate design procedures, which are thought to be more systematic than that proposed originally. Other interesting configurations that differ from Pierce's are evaluated in the chapter. To describe the methods of counteracting beam spreading three schemes are presented in the chapter: (1).the magnetic field surrounds the entire tube, so that the cathode is immersed in the field; (2). the magnetic field is excluded from the cathode by a ferromagnetic shield; and (3). the magnetic focusing field is periodic in nature, being produced by a series of magnetic lenses of alternating polarities.