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.