THERMIONIC POWER CONVERTER

also called thermionic generator, thermionic power generator, or thermoelectric engine, any of a class of devices that convert heat directly into electricity rather than first changing it to some other form of energy. also called thermionic generator, thermionic power generator, or thermoelectric engine any of a class of devices that convert heat directly into electricity rather than first changing it to some other form of energy. Thermionic devices consist of evacuated or plasma-filled cells in which electrons are boiled out of a hot cathode and collected at the cooler anode. In 1883 the American inventor Thomas A. Edison applied for a patent on a direct-conversion device that employed thermionic emission, although at the time he was not aware of the physical principles of the device. The Edison tube, which proved very inefficient, was essentially a carbon-filament light bulb into which an extra electrode had been inserted. No systematic effort was made to develop practical thermionic converters until the mid-1950s, at which time the impetus of space exploration awakened considerable interest in their potential. Thermionic converters seem particularly well suited as an electrical-power source for deep-space probes and various other space vehicles. Unlike conventional heat-power devices (e.g., Rankine and Stirling cycle power plants), they have no rotating or reciprocating components, thereby eliminating the need for frequent maintenance. Moreover, heat for thermionic devices may come from any convenient source; it may be solar, chemical, or nuclear in origin. Other advantages include low system weight and the ability to withstand acceleration higher than 30 times the force of gravity. In addition, their high efficiency makes them suitable energy sources for use in low-power radio transmitters and for other applications in remote and hostile terrestrial environments (e.g., undersea operations). There are two principal types of thermionic devicesthe vacuum converter and the gas-filled converter. The first variety has a vacuum gap of about 0.025 mm (0.001 inch) or less between the component electrodes. After the electron gas has been boiled out of the heated cathode (at temperatures from 1,000 to 2,500 K), it is passed through this gap to the comparatively cool anode, where it is condensed. The electron gas then passes through an external circuit and back to the cathode side to deliver electric power. The vacuum converter has only had limited practical applications. The second type of thermionic converter is filled with the vapour of such metals as cesium, rubidium, or potassium. Those containing cesium are the most efficient because cesium has a particularly low ionization potential (3.87 electron volts). As soon as electrons escape from the cathode, they are accelerated by the potential generated by cesium ions immediately outside of that electrode. A few of the electrons help to produce ions, but most of them flow through the almost neutral plasma to the anode, with negligible energy loss. Additional reading Stanley W. Angrist, Direct Energy Conversion, 4th ed. (1987), provides a historical introduction and overview. Texts on thermodynamics in general include Leighton E. Sissom and Donald R. Pitts, Elements of Transport Phenomena (1972); and Francis F. Huang, Engineering Thermodynamics: Fundamentals and Applications, 2nd ed. (1988). Discussions on thermionic converters in particular are G.N. Hatsopoulos and E.P. Gyftopoulos, Thermionic Energy Conversion, 2 vol. (197379); and F.G. Baksht et al., Thermionic Converters and Low-Temperature Plasma, trans. from Russian (1978). Leighton E. Sissom