Abstract
By the use of oxide-cored cathode, which gives a very small electron source (J. Phys. Soc. Jap. 14, 180 (1959)), as well as the usual tungsten hair-pin filament, basic properties of electron beams from electron guns with various Wehnelt bias voltages were studied. It is proved that the image of a tip of cathode is formed on a plane at a certain bias voltage. Near this voltage, the brightness becomes maximum, agreeing with the ideal value. The decrease of the brightness at other valtages is attributed to an instrumental error due to the finite size of apertures used in the measurement, but is not due to the chromatic aberration as concluded by Haine and Einstein. Space charge limitation also makes the brightness lower near the cut-off voltage. Hollow beam does not appear for oxide-cored cathode. This is caused by electrons from the side surface of a filament.
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