Characteristics
A direct current, electrostatic thermionic electron gun is formed from several parts: a hot cathode, which is heated to create a stream of electrons via thermionic emission, electrodes generating an electric field which focus the beam (such as a Wehnelt cylinder), and one or more anode electrodes which accelerate and further focus the electrons. A large voltage between the cathode and anode accelerates the electrons. A repulsive ring placed between them focuses the electrons onto a small spot on the anode at the expense of a lower extraction field strength on the cathode surface. Often at this spot is a hole so that the electrons that pass through the anode form a collimated beam and finally reach a second anode called a collector. This arrangement is similar to an Einzel lens.
An ion gun consists of a cylinder where gas enters from one end face, undergoes electron bombardment from the side walls, and is subjected to an extraction voltage from the other end face. The entire cage has the role of the cathode; the extractor acts as the anode, and an unnamed ring takes the role of the Wehnelt cylinder.
Most color cathode ray tubes — such as those used in color televisions — incorporate three electron guns, each one producing a different stream of electrons. Each stream travels through a shadow mask where the electrons will impinge upon either a red, green or blue phosphor to light up a color pixel on the screen. The resultant color that is seen by the viewer will be a combination of these three primary colors.
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