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Thermionic emission is the emission of electrons from a heated metal (cathode). This principle was first used in the Coolidge tube and then later in the modern day x-ray tubes. Before the discovery of the principle, gas tubes were used for x-ray production.
The cathode has its filament circuit that supplies it with necessary filament current to heat it up. As the temperature increases, the surface electrons gain energy. The energy acquired by the surface electrons allows them to move a short distance off the surface thus resulting in emission.
The electrons emitted from the surface are limited by the space charge effect.
Various factors affect the number of electrons emitted from thermionic emission. Higher temperature produces a higher number of electrons. Metals with smaller work functions require less energy to heat up and release the electrons such as tungsten, metallic oxides of barium and strontium, and thoriated tungsten. A pure tungsten filament must be heated to a temperature of 2300°C to emit a useful number of electrons. If tungsten is coated with oxide, only 750°C is required for thermionic emission. Other factors include the surface area and nature of the metal 2.