Thermionic cathode

In vacuum tubes, a hot cathode or thermionic cathode is a cathode electrode which is heated to make it emit electrons due to thermionic emission. This is in contrast to a cold cathode which does not have a heating element. The heating element is usually an electrical filament, heated by a separate electric current passing through it. Hot cathodes typically achieve much higher power density than cold cathodes, emitting significantly more electrons from the same surface area. Cold cathodes rely on field electron emission or secondary electron emission from positive ion bombardment and do not require heating. There are two types of hot cathode. In a directly heated cathode, the filament is the cathode and emits the electrons. In an indirectly heated cathode, the filament or heater heats a separate metal cathode electrode which emits the electrons.

From the 1920s to the 1960s, virtually every electronic device used hot cathode vacuum tubes. Today, hot cathodes are used as the source of electrons in fluorescent lamps, vacuum tubes, and electron guns in cathode ray tubes and laboratory equipment such as electron microscopes.

A cathode electrode in a vacuum tube or other vacuum system is a metal surface which emits electrons into the evacuated space of the tube. Since the negatively charged electrons are attracted to the positive nuclei of the metal atoms, they normally stay inside the metal, and require energy to leave it. This energy is called the work function of the metal. In a hot cathode, the cathode surface is induced to emit electrons by heating it with a filament, a thin wire of refractory metal like tungsten with current flowing through it. The cathode heats to a temperature that causes electrons to be 'boiled out' of its surface into the evacuated space in the tube, a process called thermionic emission.

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