Unique Characteristics of X-rays from a Free-Electron Laser

Very-high-energy electrons, traveling at nearly the speed of light, can produce x-rays with the useful properties of being very intense and highly collimated. Such high-energy electrons are exploited in modern synchrotron radiation light sources. The x-rays from a synchrotron source can be a billion times brighter than, for example, those from a medical x-ray machine in a doctor’s office. However, even the synchrotron source converts only a tiny fraction of the energy of the electrons into radiation. And though the synchrotron x-rays are highly collimated, they are not as well-directed as a laser beam, nor are they monochromatic like laser radiation.

An x-ray Free-Electron Laser (FEL) such as the Linac Coherent Light Source (LCLS) will exploit a process called Self-Amplified Spontaneous Emission (SASE) to create a qualitatively new type of x-ray source, as different from the synchrotron source as a laser pointer is from a flashlight. The technical differences between a synchrotron light source and a SASE FEL are subtle, involving a linear accelerator to create very short, intense electron pulses and a very long undulator magnet array in which the SASE x-ray emission takes place. But the differences between synchrotron radiation and FEL radiation are profound.

The FEL radiation is pulsed. Whereas synchrotron radiation consists of tens of millions of small pulses per second adding up to a quasi-DC source, FEL radiation consists of a relatively small rate (tens or hundreds per second) of short, intense pulses.
The FEL pulses are very short. This is one of their most exciting features. The FEL pulses are a thousand times shorter than typical synchrotron pulses, short enough to resolve atomic motions during chemical reactions.
The FEL radiation is monochromatic. Whereas the synchrotron produces a broad spectrum of radiation, the FEL radiation is concentrated into a very narrow bandwidth.
The FEL radiation is extremely bright. The SASE process extracts a much greater fraction of the electrons’ energy than does synchrotron radiation, and puts the energy into a more narrow beam and into a much more narrow bandwidth. FEL x-rays can be more than a billion times brighter than synchrotron x-rays.
The FEL radiation is spatially coherent. As with optical laser light, the high coherence allows the use of interference techniques such as holographic imaging.