Free-electron lasers

A review of the rapidly changing field of free-electron lasers (FELs) is presented. Recent advances in increasing the power, improving the efficiency, and operating at shorter wavelengths have pushed the FEL to the forefront in the quest for ultrahigh power lasers. The tunability of FELs, long recognized as a major advantage, is beginning to be exploited in a new wave of devices that will be used for medical applications and other basic research programs. In this article, the important properties of the FEL will be discussed, and a history of the experiments leading up to the state-of-the-art devices will be presented. Some important technical issues, which must be dealt with in the design of any FEL, will be described. Finally, a brief look at the future directions of FEL research will be presented.