Summary

This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Physics. Please check back later for the full article.

Free electron lasers (FELs) are coherent radiation sources based on radiation from “free” relativistic electrons rather than electrons bound in atomic and molecular systems. FELs can, in principle, operate at any arbitrary wavelength, limited only by the energy and quality of the electron beam that is produced by accelerators. Therefore, FELs can be used to fill gaps in regions of the electromagnetic spectrum where no other coherent sources exist and can provide radiation of very high power and extreme brightness. More than 50 FELs have been built around the world, serving a diverse array of scientific fields and applications.

FELs are based on the resonant interaction of a high-quality electron beam with the radiation in a periodic magnetic device called an “undulator” and can have several operating modes. FEL oscillators use optical cavities to trap the radiation, so that the field is built up over many amplification passes through the undulator. FELs can also act as linear amplifiers that will magnify external radiation whose central frequency is close to the undulator resonance condition. Without any external signal, self-amplified spontaneous emission (SASE) can be used to generate intense coherent radiation starting from electron shot noise and is the most common approach for X-ray FELs. SASE will have limited temporal coherence and pulse stability due to its noisy start-up but is very flexible to generate ultrashort X-ray pulses down to attosecond durations. Various advanced schemes aiming at achieving fully coherent, stable X-ray pulses are proposed and are actively being investigated and developed.