Microwave plasma devices—Promise and progress

Over the last few years, considerable effort has been devoted to employing the plasma state in applications as widely differing as microwave and optical frequency devices; power generation by thermonuclear fusion; magnetohydrodynamic (MHD) and thermionic energy conversion, and the propulsion of spacecraft. In some of these areas intense activity still continues, and encouraging progress is being made. In others the results have been disappointing, and after an initial period of enthusiasm interest has waned. This paper is restricted to such an area, and reviews microwave plasma device research. A few years ago it seemed that a new and useful range of components, including phase shifters, beam-plasma amplifiers, and harmonic generators, might be developed. After a great deal of work, there is still a dearth of serious commercial plasma competitors to existing vacuum tube and solid-state devices. The paper discusses some of the obstacles to microwave plasma device development that have been encountered along the way, and some of the paths that have been taken to circumvent them. It is concluded that the present disappointing situation is likely to persist unless a major breakthrough is made in production of a simple quiescent plasma source, or there is a departure from the present device concepts.