Free-electron laser

The free-electron laser FELIX at Radboud University, Netherlands

A free-electron laser (FEL) is a fourth-generation light source producing extremely brilliant and short pulses of radiation. An FEL functions much as a laser but employs relativistic electrons as a gain medium instead of using stimulated emission from atomic or molecular excitations. In an FEL, a bunch of electrons passes through a magnetic structure called an undulator or wiggler to generate radiation, which re-interacts with the electrons to make them emit coherent light, exponentially increasing its intensity.

As electron kinetic energy and undulator parameters are adaptable, free-electron lasers can be tuned and can offer a wider frequency range than any other type of laser, with wavelengths ranging from microwaves through terahertz radiation and infrared, to the visible spectrum, ultraviolet, and X-ray.

Schematic representation of an undulator, at the core of a free-electron laser

The first free-electron laser was developed by John Madey in 1971 at Stanford University using technology developed by Hans Motz and his coworkers, who built an undulator at Stanford in 1953, using the wiggler magnetic configuration. Madey used a 43 MeV electron beam and 5-metre-long (16 ft) wiggler to amplify a signal.

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