A thermionic converter consists of a hot electrode which thermionically emits electrons over a potential energy barrier to a cooler electrode, producing a useful electric power output. Caesium vapor is used to optimize the electrode work functions and provide an ion supply (by surface ionization or electron impact ionization in a plasma) to neutralize the electron space charge. Definition From a physical electronic viewpoint, thermionic energy conversion is the direct production of electric power from heat by thermionic electron emission. From a thermodynamic viewpoint, it is the use of electron vapor as the working fluid in a power-producing cycle. A thermionic converter consists of a hot emitter electrode from which electrons are vaporized by thermionic emission and a colder collector electrode into which they are condensed after conduction through the inter-electrode plasma. The resulting current, typically several amperes per square centimeter of emitter surface, delivers electrical power to a load at a typical potential difference of 0.5–1 volt and thermal efficiency of 5–20%, depending on the emitter temperature (1500–2000 K) and mode of operation. History After the first demonstration of the practical arc-mode caesium vapor thermionic converter by V. Wilson in 1957, several applications of it were demonstrated in the following decade, including its use with solar, combustion, radioisotope, and nuclear reactor heat sources. The application most seriously pursued, however, was the integration of thermionic nuclear fuel elements directly into the core of nuclear reactors for production of electrical power in space. The exceptionally high operating temperature of thermionic converters, which makes their practical use difficult in other applications, gives the thermionic converter decisive advantages over competing energy conversion technologies in the space power application where radiant heat rejection is required. Substantial thermionic space reactor development programs were conducted in the U.S., France, and Germany in the period 1963–1973, and the US resumed a significant thermionic nuclear fuel element development program in the period 1983–1993. Thermionic power systems were used in combination with various nuclear reactors (BES-5, TOPAZ) as electrical power supply on a number of Soviet military surveillance satellites between 1967 and 1988. See Kosmos 954 for more details. Although the priority for thermionic reactor use diminished as the US and Russian space programs were curtailed, research and technology development in thermionic energy conversion have continued. In recent years technology development programs for solar-heated thermionic space power systems were conducted. Prototype combustion-heated thermionic systems for domestic heat and electric