Electron optics and space charge in strip-cathode emission systems

Abstract

A description of strip cathode systems is given and their merits are compared with those ot circular emission systems. It is shown how electron-optical laws can be applied to simple strip systems as long as the emission is sufficiently small. Results obtained under space-charge conditions reveal that the beam spread, due to mutual repulsion of the electrons, completely changes the emission distribution. These results, however, can be interpreted qualitatively by a simple space-charge theory. Special systems, proposed by Pierce, in which the electron-optical orbits are not upset by space charge, are investigated by measuring the current that can be transmitted through a tunnel of given dimensions. Large discrepancies are found between experimental results and predictions of the simple space-charge theory. With a view to finding the reasons for these discrepancies, ray-tracing results are discussed and the potential distribution in beams of relatively large space charge is investigated. Tracing results reveal lack of homocentricity, which is an essential supposition in the simple theory of beam spread. The lack of homocentricity, however, is found to be caused by the potential distribution in the beam. This potential distribution is probed in a large beam of high space charge by means of a fine beam of low intensity.