1 2 M ay 1 99 9 Emission Optics of the Steigerwald Type Electron

The emission optics of a Steigerwald type electron gun is re-examined. The virtual and real points of divergence, divergence angles and beam-widths of the electron beams at different telefocusing strength are measured in detail for first time . Two different Wehnelt cylinders are used to establish a contrasting viewpoint. The original ‘focusing’ curves measured by Braucks are reconstructed and will be explained only through a ‘new’ interpretation which is different from the conventional views. While the image of the emitting surface in front of the filament is indeed telefocused beyond the anode, the envelope of the beam does not ‘focus’ as expected. A new model for the emission mechanism is established based on our results.