Focussing of Electron Beam from Biased Electron Gun. A Study with Oxide-Cored Cathode as Well as Tungsten Hair-Pin Filament.

Abstract

Various characteristics of electron beam (beam diameter, total current, brightness, etc.) from electron guns such as used in electron microscopes and other equipments were studied systematically. The present paper describes the general results which are independent of detailed design of electron guns. It was found that the image of the tip of cathode is formed on a plane at a certain bias voltage. At this bias voltage, the brightness measured on the plane takes maximum value which agrees with the theoretical value. It was made clear that the decrease of brightness from the maximum value is mainly due to the finite size of apertures used for the measurement. Near the cut-off voltage, space charge limitation also makes the brightness lower. These results are against the conclusions of Haine & Einstein. Nature of the so-called hollow beam was studied. This is due to electrons from the side surface of cathode. Thus it does not appear for oxide-cored cathode.