Abstract

The free-electron laser (FEL) is a promising new source of high power, tunable, coherent radiation in the extreme ultraviolet (XUV, 10-100-nm) and soft x-ray (<10-nm) regions of the electromagnetic spectrum. The basic principles of an FEL have long been verified experimentally by the operation of about one dozen devices at wavelengths ranging from 240 nm to ~8.8 mm. To shorten the operating wavelength of a given FEL by increasing the energy of the incident electrons is possible according to the FEL resonance relation which shows that the laser wavelength is proportional to the undulator's wavelength and inversely proportional to the electrons' energy. However, the optical gain decreases with decreasing wavelength. Also, mirror reflectivities decrease sharply in the XUV, thus raising the threshold gain for an oscillator. Brighter electron beams and longer undulator magnets are required to exceed the threshold gain. Thus, operation of an XUV FEL requires specific improvements in each of the three major contributing technologies: relativistic electron beams, optics, and undulator magnets. We shall discuss in detail the required improvements for the operation of an XUV FEL oscillator and also requirements for mirrorless amplified spontaneous emission sources.

© 1989 Optical Society of America