the free-electron laser works
David F. Salisbury
Oct. 9, 2001
free-electron laser (FEL) is an ideal instrument for charting the
interactions of light and matter in many of the still unexplored
regions of the electromagnetic spectrum.
As a laser ,
it produces light in a single wavelength. Ordinary white light contains
particles of light, or photons, with a broad range of different
colors. So, when white light strikes an object, it causes a multiplicity
of responses. By contrast laser light provokes a far more limited
set of reactions. This allows scientists to use it to measure the
physical properties of materials with great precision.
lasers, however, operate at a fixed frequency. That is, they produce
light in only one color. This has limited their usefulness. A number
of different types of lasers have been created that produce light
at a number of different wavelengths ranging through much of the
electromagnetic spectrum. Also, researchers have found ways to alter
their output frequencies by using lenses made of special optical
materials. Nevertheless, there are a number of regions of the spectrum
where few, if any lasers operate.
FEL is ideal for exploring the unknown regions in the spectrum because
it is tunable over a broad range of the spectrum. That enables researchers
to study how different materials respond as the wavelength of light
impinging on them changes. In addition, the FEL is capable of producing
very high power levels. The power level is important in applications
like surgery where the beam needs enough energy to vaporize soft
tissue and bone.
the FEL's tunability and power are the result of its unusual design.
In most other lasers, the lasing process occurs within a liquid,
solid or gas. So the wavelengths are limited by those permitted
by the electrical structure of the material. Similarly, the power
of the beam is limited by the amount of energy that the material
can withstand before breaking down.
FEL, however, is not subject to this limitation because it produces
laser light by sending bunches of electrons through a series of
magnets in a vacuum. These electrons are first accelerated to nearly
the speed of light
and then they are sent through a device called a "wiggler"
wiggler consists of a series of magnets with alternating north and
south poles. As a bunch of electrons travels through this alternating
field, it causes them to wiggle back and forth in a fashion that
causes them to emit some photons of a specific color. These photons
are directed onto a mirror that allows 15 percent of them through
and reflects 85 percent back along the beam line. At the end of
the beam line is another mirror that reflects the photons back up
the beam line. The distance between these two mirrors is set with
extreme precision so that each bundle of photons meets a new bunch
of electrons starting through the wiggler. These photons stimulate
the electrons to produce even more photons. After thousands of iterations
the power of the laser beam builds up until it reaches a steady
color of the laser beam can be varied in two ways: putting more
power into the electron beam and changing the spacing between the
magnets in the wiggler. The Vanderbilt FEL is designed to operate
at infrared frequencies and can be tuned from two to nine microns.
the production of laser light occurs in a vacuum, an FEL can be
designed to operate at extremely high power levels. The Vanderbilt
FEL is designed to produce a beam with a peak power of more than
10 Megawatts and an average power of 10 Watts.
come in two basic types: continuous and pulsed. One produces a continuous
beam of light and other produces light in pulses. The Vanderbilt
FEL is a pulsed laser. Its beam consists of a series of extremely
short pulses, each lasting less than a billionth of a second.
lasers and laser applications