A Free-Electron Laser
A free-electron laser (FEL) generates intense, highly monochromatic (single-color) light. Electrons are accelerated to nearly the speed of light, then sent through a structure of alternating magnets called an undulator. The electrons wiggle back and forth as they pass over the magnets, emitting light at a frequency that depends on the undulator period, the magnets’ strength, and the electron’s energy. The frequency increases when the electron energy increases.
A pulse of coherent x-rays can be produced if a pulse of highly energetic electrons is sent through a very long undulator (80–100 meters or longer). Remarkably, the light from each pulse is a million to a billion times brighter than a pulse from a conventional x-ray storage ring. The reason is a nonlinear process called self-amplified spontaneous emission (SASE). With reference to the figure: (1) the electrons (red dots) spontaneously emit x-rays, (2) interactions with those x-rays cause the electrons to start bunching, increasing the pulse brightness, (3) electrons bunch at a spacing equal to the x-ray wavelength, so all x-rays are emitted in lock-step with each other, and (4) the bunching gets so tight that an instability develops, and the electrons begin to debunch. A single pulse emits enough photons to image a microscopic volume inside a dense solid.
“SASE was first demonstrated in the infrared at Los Alamos in 1997,” says Los Alamos XFEL designer Dinh Nguyen. “Today’s XFELs use the same SASE principle.” Nguyen and colleagues are developing a preliminary design for the first XFEL that can produce very high energy, 50–100-keV x-ray pulses needed for penetrating dense, thick samples. With x-ray pulses coming less than a nanosecond apart, one can create a movie of fast, dynamic processes.
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