Electron gun

An electron gun (also called electron emitter) is an electrical component in some vacuum tubes that produces a narrow, collimated electron beam that has a precise kinetic energy. The largest use is in cathode-ray tubes (CRTs), used in older television sets, computer displays and oscilloscopes, before the advent of flat-panel displays. Electron guns are also used in field-emission displays (FEDs), which are essentially flat-panel displays made out of rows of extremely small cathode-ray tubes. They are also used in microwave linear beam vacuum tubes such as klystrons, inductive output tubes, travelling wave tubes, and gyrotrons, as well as in scientific instruments such as electron microscopes and particle accelerators. Electron guns may be classified by the type of electric field generation (DC or RF), by emission mechanism (thermionic, photocathode, cold emission, plasmas source), by focusing (pure electrostatic or with magnetic fields), or by the number of electrodes. 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 to focus the electron beam (such as a Wehnelt cylinder); and one or more anode electrodes which accelerate and further focus the beam. A large voltage difference between the cathode and anode accelerates the electrons away from the cathode. A repulsive ring placed between the electrodes focuses the electrons onto a small spot on the anode, at the expense of a lower extraction field strength on the cathode surface. There is often a hole through the anode at this small spot, through which the electrons pass to form a collimated beam before reaching a second anode, called the collector. This arrangement is similar to an Einzel lens. The most common use of electron guns is in cathode-ray tubes, which were widely used in computer and television monitors before the advent of flat screen displays.