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After years of research
and design, the construction of the world's first Angstrom-wavelength
lasers will soon begin. Their X-rays will be produced by the collective
motion of free electrons from a particle accelerator, rather than
by electrons bound to atoms. These free-electron lasers (FELs) have
similarities to more familiar sources of electromagnetic radiation,
such as klystrons and magnetrons; however the electron beam of an
X-ray FEL must have much higher energy, in the multigigaelectron
volt (GeV) range.
The leading contender for becoming the world's first "hard"
X-ray laser is the Linac Coherent Light Source (LCLS), to be constructed
at Stanford Linear Accelerator Center in Menlo Park, California.
It will make use of the last kilometer of the SLAC accelerator to
produce the precisely controlled 14 GeV, 3,000 ampere electron beam
that will be manipulated to emit 8-16 gigawatt pulses of coherent
X-rays in the wavelength range 1.5-15 Angstroms. These pulses will
have full transverse coherence, coherence length of about 1,000
Angstroms, and overall duration of 200 femtoseconds (2 x 10-13 seconds)
or less. SLAC is proposing to produce first X-ray beams with the
LCLS by the end of 2008. The construction of the LCLS will be carried
out by SLAC in collaboration with Argonne National Laboratory and
Lawrence Livermore National Laboratory. Theoretical and computational
support is provided by the Particle Beam Physics Laboratory at UCLA.
The intensity and short pulse duration of the LCLS will, for the
first time, give researchers the ability to measure the evolution
of chemical processes, changes in materials, or plasma dynamics
on the natural time scales of these phenomena.
Angstrom-wavelength X-rays have wavelengths comparable to interatomic
distances in molecules and materials. For this reason X-ray sources
are valuable for determining the spacing and orientation of atoms
in molecules, crystals or other forms of matter. Radiation from
the LCLS is suited to this use.
The revolutionary feature of the LCLS is the short duration and
extreme brightness of the X-ray pulse. This makes it possible to
observe the changes in these properties over times of the order
of 100s of femtoseconds and less. It is anticipated that chemical
reactions will be initiated in a sample under study by a conventional
laser. By varying the time interval between the conventional laser
pulse and the LCLS pulse, X-ray scattering images that will provide
a picture of the departure of a single atom or radical can be directly
observed.
The creation and observation of high-density plasmas, the production
and observation of atomic excitations previously impossible to observe
in the laboratory, the determination of the form and structure of
clusters of only a few atoms, perhaps even the determination of
the structure of single molecules, these are some of the unprecedented
scientific opportunities that the Linac Coherent Light Source will
create.
Free-electron lasers are specialized examples of synchrotron light
sources, accelerator-based facilities designed to produce extraordinarily
intense beams of electromagnetic radiation ranging in wavelength
from infrared through X-rays. Synchrotron light sources have become
indispensable research tools to researchers in materials science,
chemistry, structural and molecular biology, and environmental science.
The variety of experimental techniques made possible by synchrotron
sources includes crystallography, microscopy, holography, tomography
and other techniques for imaging. Unlike the X-ray machines found
in hospitals or university research labs, synchrotron sources offer
selectable wavelength and beams up to 1010 times higher in brightness.
Brightness is a summary measure of collimation and spectral purity
of a light source, commonly measured in units of photons/ (second
x milliradian2 millimeter2 x 0.1% bandwidth). A brighter source
can deliver more photons of a desired wavelength and incident angle
to a very small sample.
Like radio waves, synchrotron light is produced by accelerating
charges. A familiar example of radiation from accelerating charges
is a radio transmitter, for which the accelerating charges are electrons
running up and down the antenna. If one could place such an antenna
on a rocket moving close to the speed of light, the radio waves
emitted in the direction of the rocket's flight would be Doppler
shifted to shorter wavelength like the sound of a train whistle.
Luckily it is not necessary to accelerate a radio transmitter on
a rocket ship to get this Doppler shift - one can accelerate just
the electrons themselves in a device like the Two-Mile Linac at
Stanford Linear Accelerator Center. Synchrotron radiation is produced
when electrons flying through a vacuum are deflected by a magnet.
A wiggler magnet is designed to make an electron follow a sinusoidally
wiggling path; the resultant radiation is a tone burst of light
with an equivalent number of +/- oscillations of its electric field.
The extraordinary intensity of synchrotron light sources is the
result of accelerating electrons to multi-GeV energies and compressing
them into current pulses of 100 - 1,000 amperes. Such pulses can
be trapped in a storage ring, retracing their path through specially
arranged steering and focusing magnets about a million times per
second. There are ten such storage-ring light sources in the US
serving many thousands of experimenters, and over fifty such sources
worldwide.
The temporal properties of the light from storage ring light sources
are those of a light bulb - the wiggling electrons and hence the
photons they produce have random phase correlation. This is quite
different than the electrons in a radio antenna, which are bunched
in the antenna wire and forced to move in unison. The bunching and
correlated motion of electrons (such as current modulation within
klystrons or magnetrons) is also responsible for the high power
output of radio transmitters and free electron lasers. What makes
an X-ray free-electron laser possible is the spontaneous bunching
of electrons bathed in their own synchrotron radiation as they travel
down a wiggler magnet. This bunching process, called self-amplified
spontaneous emission (SASE) can take place on Angstrom wavelength
scales. However, SASE requires an electron beam with sufficiently
high current, very low spread in kinetic energy, and a high degree
of collimation. The energy of the electron beam is determined by
both the desired output radiation wavelength and by the current
density requirements. While the energy spread and collimation or
emittance of electron beams in a storage ring are suitable for making
FELs down to around 2000 Angstroms in wavelength, only the electron
beam from a linear accelerator like the one at SLAC can have the
properties necessary for a hard X-ray laser. One must add, however,
that the 50 GeV SLAC linac is now the only one in the world capable
of producing electron beams for an Angstrom FEL.
While the LCLS is planned to be the world's first "hard"
X-ray laser, it will not be the last. X-ray laser facilities are
in varying stages of design in Germany, the United Kingdom, Italy,
and Japan. The TESLA Test Facility (TTF) at DESY in Hamburg, Germany
is a test bed for the technology to be used in the European X-ray
FEL Facility. It is also a scientific user facility designed to
provide 60 Angstrom radiation to a growing community of experimenters.
Since the discovery of X-rays in 1895, each major improvement in
the brightness of X-ray sources has created new opportunities for
groundbreaking science. The billionfold increase in brightness provided
by the LCLS will doubtless continue this tradition.
†The work described herein was performed for the U. S. Department
of Energy under contract number DE-AC03-76SF00515 and contract number
DE-AC02-76SF00515 by Stanford Linear Accelerator Center, Stanford
University; contract Number W-7405-ENG-48 by Lawrence Livermore
National Laboratory, University of California; and under contract
number W-31-109-ENG-38 by Argonne National Laboratory, University
of Chicago.
References
A fascinating graphical demonstration of how accelerated
electrons produce synchrotron radiation can be downloaded from:
http://www-xfel.spring*.or.jp/
John Galayda can be reached at the Stanford Linear Accelerator Center,
MS69, 2575 Sand Hill Road, Menlo Park, CA 94025; Phone: +1 650 926-2371;
Fax: +1 650 926-2371; E-mail: galayda@slac.stanford.edu.
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