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 average powers and frequencies have been achieved. Of course, many of these gains have been achieved by the development of different devices. The typical development pattern for a given device exhibits and initial period of rapid improvement followed by a plateau determined by technological or physical limitations on the concept. Slow wave devices such as magnetrons and/or klystrons operate efficiently at frequencies up through X-band or somewhat higher. Helix or coupled cavity traveling-wave tubes are used for various applications at frequencies ranging up through W-band. At still higher frequencies, fast wave devices such as gyrotrons and free-electron lasers are required for high-power operation. The free-electron laser concept is unique in that the mechanism is applicable across the entire electromagnetic spectrum; free-electron lasers have been built at wavelengths from microwaves through the ultraviolet, and plans are under development for X-ray systems. Our purpose in this paper is to describe the principal directions of free-electron laser research at the present time. To this end, we first give a brief tutorial of the physics underlying the concept and then describe the principal development paths under way.