HOW A FREE-ELECTRON LASER WORKS
In a free-electron laser, as the name suggests, bunches of high-energy
electrons move through a vacuum while magnets cause them to wiggle and produce
light. The wavelength of the emitted light is controlled by the speed of
the electrons and by the magnets.
A free-electron laser produces a high-intensity beam that can be tuned
across the light spectrum. Most other lasers can produce light only in a
few discrete colors, or wavelengths. Also, the FEL's beam is delivered in
pulses rather than a steady stream. These bursts of light can be timed,
with pulse sequences as short as a trillionth of a second.
In theory, the FEL is an extremely adaptable source of light. Because the
electrons are not attached to the atoms of a lasing medium, such as gas
or dye, they can shift freely from one energy level to another. That means
the wavelength of the radiation given off in a free-electron laser - the
Vanderbilt FEL is a mid-infrared version - can be varied from the far infrared
to the ultraviolet. Also, where conventional lasers convert only a few percent
of their input energy, free-electron lasers convert as high as 65 percent.
This allows researchers to explore a whole range of effects specific to
different wavelengths, and FELs have been used in experiments ranging from
solid-state physics to molecular biology.
Vanderbilt's Mark-III free-electron laser is tunable from 2 to 10 microns
(a micron is 1/1000th of a millimeter). The peak power of Vanderbilt's FEL
is more than 10 megawatts; its average power exceeds 10 watts. It is the
most powerful FEL in the world. The FEL accelerates electrons to an energy
of 40 million volts, injects them into a wiggler and the emitted laser beam
is transported into the center's laboratories and operating rooms for use
in experiments. It can be retuned to the specifications and needs of different
researchers in as little as five minutes.
In practice, the FEL has been largely confined to the physics laboratory.
Only recently have free-electron lasers begun to come into their own as
tools for multidisciplinary research. Electron accelerators are being designed
for their specific FEL needs, and facilities similar to the Keck FEL Center
at Vanderbilt are being set up so that researchers at other institutions
can take advantage of this new source of intense light.
At this time Vanderbilt's FEL is classified as a research laser, but current
hardware and software upgrades will soon meet the performance criteria of
a medical laser. By showing which wavelengths are useful and which are not,
FELs are helping surgeons bring new laser applications to the clinic.
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This document created November 18, 1996