2017 Volume 52 Issue 3 Pages 160-165
Lectures on “electron sources and electron guns” are presented in two consecutive articles. The present article comprises the first part and discusses fundamental physics and optics of the electron source, especially the theoretical brightness and the electron emission mechanisms. Both the transmission and scanning type electron microscopes have dramatically improved the performance by adopting high brightness electron sources. The theoretical brightness of electron sources is determined dominantly by the cathode current density. The improvement in the source brightness is done by developing cathodes that provide high density currents. The cathode current density is a function of the ‘work function,’ ‘cathode temperature,’ and ‘electric field intensity.,’ combination of which determines the electron emission mode. The dependence of the cathode current density on those parameters as well as the expected theoretical brightness of the thermionic, Schottky, and cold field emission cathodes are described. It is shown that the cathode current density can substantially be enhanced by the application of electric field.
