A cold cathode refers to a type of electron source used in vacuum tubes and some types of display technologies (like cold cathode fluorescent lamps, or CCFLs) where electrons are emitted from a cathode without the need for significant heating. This is in contrast to hot cathodes, where the cathode is heated to facilitate electron emission through thermionic emission. In a cold cathode, the electron emission is typically achieved through processes such as field emission or Schottky emission.
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A cold cathode supplies electrons without relying on deliberate heating for thermionic emission. The term describes that operating distinction rather than one universal microscopic emission mechanism.
A strong surface electric field can produce field emission; in a gas discharge, particle impact and secondary electron emission can sustain the supply. A cathode may still warm during operation, so cold does not guarantee room temperature. Comparing emitted current as a function of field, illumination or discharge conditions helps distinguish mechanisms. The source's energy spread, stability and emitting geometry matter when it is used in an electron gun.