A free-electron laser (FEL) is a (fourth generation) synchrotron light source producing extremely brilliantand short pulses of synchrotron radiation. An FEL functions and behaves in many ways like a laser, but instead of using stimulated emission from atomic or molecular excitations, it employs relativistic electrons as a gain medium.[1][2] Synchrotron radiation is generated as a bunch of electrons passes through a magnetic structure (called undulator or wiggler). In an FEL, this radiation is further amplified as the synchrotron radiation re-interacts with the electron bunch such that the electrons start to emit coherently, thus allowing an exponential increase in overall radiation intensity.
As electron kinetic energy and undulator parameters can be adapted as desired, free-electron lasers are tunable and can be built for a wider frequency range than any type of laser,[3] currently ranging in wavelength from microwaves, through terahertz radiation and infrared, to the visible spectrum, ultraviolet, and X-ray.[4]
The first free-electron laser was developed by John Madey in 1971 at Stanford University[5] utilizing technology developed by Hans Motz and his coworkers, who built an undulator at Stanford in 1953,[6][7] using the wiggler magnetic configuration. Madey used a 43 MeV electron beam[8] and 5 m long wiggler to amplify a signal.
https://en.wikipedia.org/wiki/Free-electron_laser
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