High current electron source

摘要

A high current electron source with a narrow energy band of the type employed in an electron beam printer has a beam generating system consisting of a cathode and focusing electrodes and an anode with at least one electrode functioning to astigmatically focus at least one electron crossover in the low velocity range. For varying the position and shape of the crossover in a simple manner for adaptation to particular use conditions of the high current electron source, the Wehnelt electrode of the electron source consists of one or more cylinder lenses or of an electrostatic multipole element, or an electromagnetic multipole element is disposed in the plane of a conventional non-ferromagnetic Wehnelt electrode with a circular aperture for the electron beam. By pole reversal of the Wehnelt voltage or of the coil current and by changing the size of the current, the shape and azimuth position of the first crossover in the cathode space can be varied to any configuration. By so doing, an energy spread of the electrons is prevented and an optimum illumination of surfaces to be imaged is achieved.