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What Is a Free Electron Laser?

Free Electron Lasers (FELs) represent an increasingly important kind of light source with a brightness that can be up to one billion times higher than that of ordinary synchrotron light. FEL’s differ from conventional lasers in that they use the electron beam as the lasing medium rather than a gas or a solid.

FELs are usually based on the combination of a linear accelerator followed by a high-precision insertion device, which may also be placed in an optical cavity formed by mirrors. Under certain circumstances, the accelerated electrons in the insertion device bunch together more tightly than usual (microbunching). Over the length of the insertion device (or during multiple passes back and forth through the optical cavity), the electrons in the microbunches begin to oscillate in step (coherently), thereby giving rise to light with properties characteristic of conventional lasers. Because the microbunches are so tiny, the light generated comes in ultrashort pulses that can be used for strobe-like investigations of extremely rapid processes. Current FEL’s cover wavelengths from millimeter to visible and are nudging into the ultraviolet. New facilities designed specifically to produce x rays are under construction.


Free Electron Laser Graphic

Image 1. Electrons are released from the source at the lower left, and are accelerated in a linear accelerator (linac). After emerging from this linac, the electrons pass into a laser cavity which has a wiggler at its center. This wiggler causes the electrons to oscillate and emit light which is captured in the cavity, and used to induce new electrons to emit even more light. (Courtesy: Jefferson Lab)