New high-power coherent radiation sources

The status of the development of sources of high-power electromagnetic radiation (EMR) driven by relativistic electron beams is assessed, primarily in terms of free electron lasers (FEL) and cyclotron-resonance masers (CRM). Current efforts are focusing on devices which operate at shorter wavelengths, have high efficiencies, and function over a broad range of frequencies. The physical components and operation of the FEL are reviewed, along with the operating regimes thus far attained or predicted. Techniques of enhancing the wave-particle interaction efficiency are discussed, e.g., by recoving beam energy. Current FEL research programs are described and the directions future research will follow are indicated. The physical mechanism of the CRM is also summarized and attention is given to methods of improving the efficiency of these devices and developing suitable power sources using, e.g., superconductors or magnetron injection guns. Other high power EMR sources are briefly surveyed, including the nonisochronic electron system and coherent Cherenkov radiation. Finally, applications of these mm-wave devices to spectroscopy, accelerators, plasma heating and radars are described.