Thermionic Cathode


One common way to form an electron beam is by a thermionic cathode (i.e. the process of thermionic emission), which works by heating a metal so that electrons with randomized energies tend to overcome the work function of the metal and boil off.

The emission current as a function of cathode temperature is provided by the Richardson-Dushman equation, though at higher temperatures space-charge effects dominate and limit the current to a threshold defined by Child’s law (i.e. space-charge rather than temperature limited case). In Schottky emission, a field in front of the cathode lowers the potential barrier, thereby increasing emission; it is described by the Schottky equation, which reduces to the Richardson-Dushman equation at low fields. At higher fields, extended Schottky emission occurs, involving tunneling effects (like in Field Emission). [1]. In cold Field Emission, unlike thermionic emission, electrons are tunneled from the surface with a very high field and no elevated temperature (Fowler-Nordheim effect).

There is a distribution of energies, normal and transverse, of electrode emitted from the cathode, baesd on the tail of the Fermi-Dirac distribution exceeding the work function, which for thermionic emission is roughly centered at E = k T, for Boltzmann constant k and cathode temperature T. [1] [2]. (See also chromatic aberration.)


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See Also

[1](1, 2) Orloff1997. Jon Orloff. Handbook of charged particle optics. G:_rYbAyf-iAkC. 1997. CRC Press. ISBN:978-0-8493-2513-7. pp. 81, 93.
[2]ReimerKohl2008. Ludwig Reimer. Helmut Kohl. Transmission electron microscopy: physics of image formation. G:o-OnI8hxEpMC. 2008. Springer. ISBN:978-0-387-40093-8. p. 81.