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The method by which free electrons are emitted from the surface of a metal on applying the external heat energy, is called thermionic emission. In general thermionic emission occurs, when large amount of external energy in the form of heat is supplied to the free electrons in the metals. It occurs in metals that are heated to a very high temperature.
·Thermionic emission is actually the emission of electrons from a heated metal (cathode). The electrons emitted from the surface are then limited by the space charge effect.
·The metal (cathode) saturation voltage is reached when increasing the Kvp no longer causes an increase in tube current to cause thermionic emission. Any slight increase in tube current (undesirable) is controlled by decreasing the filament heating. Current below the saturation voltage is limited by the space-charge effect & compensation circuitry

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