Editing Thermionic converter (section)

== Description ==
The scientific aspects of thermionic energy conversion primarily concern the fields of [[surface physics]] and [[plasma physics]]. The electrode surface properties determine the magnitude of [[electron emission]] current and [[electric potential]] at the electrode surfaces, and the plasma properties determine the transport of electron current from the emitter to the collector. All practical thermionic converters to date employ caesium vapor between the electrodes, which determines both the surface and plasma properties. Caesium is employed because it is the most easily ionized of all stable elements.

A thermionic generator is like a cyclic [[heat engine]] and its maximum efficiency is limited by Carnot's law. It is a low-Voltage high current device where current densities of 25–50 (A/squarecm) have been achieved at voltage from 1–2V. The energy of high temperature gases can be partly converted into electricity if the riser tubes of the boiler are provided cathode and [[anode]] of a thermionic generator with the interspace filled with ionized caesium vapor.

The surface property of primary interest is the [[work function]], which is the barrier that limits electron emission current from the surface and essentially is the [[heat of vaporization]] of electrons from the surface. The work function is determined primarily by a layer of caesium atoms adsorbed on the electrode surfaces.<ref>{{cite journal |last=Rasor |first=Ned S.|author2=Charles Warner |title=Correlation of Emission Processes for Adsorbed Alkali Films on Metal Surfaces |journal=Journal of Applied Physics |volume=35 |issue=9 |page=2589 |publisher=The American Institute of Physics |date=September 1964 |url=http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JAPIAU000035000009002589000001&idtype=cvips&gifs=yes |issn=0021-8979 |doi=10.1063/1.1713806 |bibcode=1964JAP....35.2589R|url-access=subscription }}</ref> The properties of the interelectrode plasma are determined by the mode of operation of the thermionic converter.<ref>{{cite journal |last=Rasor |first=Ned S. |title=Thermionic Energy Conversion Plasmas |journal=IEEE Transactions on Plasma Science |volume=19 |issue=6 |pages=1191–1208 |date=December 1991 |doi=10.1109/27.125041|bibcode=1991ITPS...19.1191R}}</ref> In the ignited (or "arc") mode the plasma is maintained via ionization internally by hot plasma electrons (~ 3300&nbsp;K); in the unignited mode the plasma is maintained via injection of externally produced positive ions into a cold plasma; in the hybrid mode the plasma is maintained by ions from a hot-plasma interelectrode region transferred into a cold-plasma interelectrode region.