Free-electron lasers: Vacuum electronic generators of coherent radiation

Vacuum electronic sources have shown marked improvement since the invention of the magnetron before World War II, and dramatic increases in both avera ge powers and frequencies have been achieved. Of course, many of these gain s have been achieved by the development of different devices. The typical d evelopment pattern for a given device exhibits an initial period of rapid i mprovement followed by, a plateau determined by technological ol physical l imitations on the concept. Slow wave devices such as magnetrons and/or klys trons operate efficiently at frequencies up through X-band or somewhat high er. Helix or coupled cavity traveling-wave tubes are used for various appli cations at frequencies ranging up through W-band. At still higher frequenci es, fast wave devices such as gyrotrons and free-electron lasers are requir ed for high-power operation. The free-electron laser concept is unique in t hat the mechanism is applicable across the entire electromagnetic spectrum; free-electron lasers have been built at wavelengths from microwaves throug h the ultraviolet, and plans al-e under development for X-ray systems. Our put-pose in this paper is to describe the principal directions of free-elec tron laser research at the present time. To this end, we fir-st give a brie f tutorial of the physics underlying the concept and then describe the prin cipal development paths under way.