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t e ch n o l o g i c a l i m p l i c a t i o n s .
AVAILABLE VISION PAPERS:
Components of
an infrared FEL
at the Jefferson
Laboratory in
the US
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Front cover:
The magnetic device
(undulator) used to
produce X-rays in an FEL
at DESY Hamburg
Inset:
A diffraction pattern
illustrating the spatial
coherence of the FEL at
DESY’s TESLA test facility
Below:
Artist's impression of the
building that will house
the 4GLS FEL complex
Free electron lasers
A n ew l i g h t s o u r c e
generating intense bursts of
c o h e r e n t r a d i a t i o n ove r a
w i d e r a n g e o f wave l e n g t h s
p r o m i s e s t o r evo l u t i o n i s e
o u r u n d e r s t a n d i n g o f m a tt e r
VISIONS 17
B R I E F I N G P A P E R S F O R
P O L I C Y M A K E R S
Jefferson Laboratory
Component of an
electron gun to
produce electrons
for an FEL
Experiments
using an FEL at
the Jefferson
Laboratory
B
rilliant sources of
electromagnetic radiation,
from microwaves, through
visible light to X-rays, provide
scientists with powerful tools for
studying and even transforming
all kinds of matter. During the
past 40 years, various sources
have been developed: the laser
is well known; another is a large
ring-shaped machine which
deflects a beam of electrons
moving close to the speed of
light in a magnetic field, causing
them to emit intense light. This
‘synchrotron radiation’ is a tool
much used by physicists,
chemists, materials scientists
and biologists alike. Now,
however, a radical new type of
light source is coming of age,
which combines the best of
lasers and synchrotron sources,
the free electron laser (FEL).
FELs offer pulses of light that
are a million times more intense
than those from synchrotron
facilities. They are tunable to
different wavelengths and the
light they emit is coherent (the
light waves are in synchrony).
FELs can also be made to work
at wavelengths not easily
accessible by conventional
lasers, and can operate
continuously at high power –
spitting out blips of brilliant light
in rapid succession. Researchers
believe that FELs will open up
uncharted territory in exploring
how matter behaves at the
microscopic scale.
How do FELs differ from
ordinary lasers?
The light from the laser in a CD
player or barcode scanner, for
example, is emitted from the
electrons bound in atoms (in a
semiconductor). In an FEL, the
electrons are not bound but
clustered into bunches
comprising a carefully controlled
beam, which is accelerated
close to the speed of light down
a linear accelerator (or linac). The
beam passes through an array of
magnets, with alternating
polarities called an undulator,
which causes the electron beam
to oscillate and emit synchrotron
radiation in the process. The
wavelength of the radiation can
be tuned by altering the beam
energy, or the strength of the
magnetic fields.
The ingenious thing about this
set-up is that the electrons and
light waves interact in a way that
generates the intense, coherent
radiation typifying laser
emission. The electrons within
the bunches are either speeded
up or slowed down so that they
gather into dense ‘microbunches’,
leading to a rapid build-up of
laser light as the bunches race
down the undulator. This is called
self-amplified spontaneous
emission (SASE).
For longer wavelengths, the
emission can be amplified by
placing mirrors beyond the ends
of the undulator; the mirrors
A new tool for research and indust
XUV-FEL injector
High average
current injector
Main linac
Beam
separator
Seed laser
XUV-FEL
Linac
Undulator / FEL
VUV-FEL
IR-FEL
High average
current loop
Dipole
Beam dump
Photoinjector / gun
Layout of the proposed
GLS accelerator complex
to be built in Cheshire
Laser
Optical mirror
THANKS
GO TO
ELAINE SEDDON
OF THE
DARESBURY LABORATORY
AND TO
JÖRG ROSSBACH
OF THE
U
ry
A UK facility
The UK is currently considering
building a suite of FELs as part
of a world-leading accelerator
facility called 4GLS at the
Daresbury Laboratory in
Cheshire. It will include three
FELs generating light in the far
infrared, the high-energy
ultraviolet and the soft X-ray
regions of the spectrum.
Operating the facility using novel
energy-recovery technology will
be a crucial element in
generating and maintaining
extremely high-quality electron
bunches. Using this approach,
light from a variety of sources
can be combined to give
researchers unprecedented
opportunities to use a range of
spectroscopic tools for imaging,
and to probe dynamical
processes in real time on
timescales down to tens of
femtoseconds (million-billionths
UNIVERSITY
Intense X-ray radiation
from an FEL causes a
biomolecule to 'explode',
so an image must be
recorded very quickly
DESY Hamburg
These will allow researchers to
of a second). Key areas that will
take snapshots of chemical
benefit include unravelling the
bonds being made and broken,
dynamics of industrially or
and to look at detailed physical
environmentally important
processes such as planes of
chemical reactions, as well as
atoms sliding over one another.
the subtle molecular changes in
Ultimately, researchers hope
biological systems. Carefully
that it will be possible to image a
timed light pulses could be used
single biological molecule before
to follow, and even control, the
the X-rays’ powerful energy
movement of electrons in
destroys it.
atoms, molecules and advanced
DESY scientists have already
materials developed for
built a facility to test the
nanotechnology and electronics.
technology needed for an X-ray
The FELs on 4GLS will also offer
FEL, and it has now been
new opportunities to develop
transformed into the VUV FEL
dynamic imaging techniques for
(working in the high-energy
diagnosing conditions such as
ultraviolet and soft X-ray region)
progressive degenerative
for use in scientific research.
diseases and cancer.
It provides pulses lasting only
FELs have a huge potential,
25 femtoseconds, and is as
and some US laboratories are
powerful as predicted. The XFEL
exploring their application in
will be ready by 2012, if funding
industrial processing such as the
allows, and UK researchers are
modification of plastic surfaces,
already making major
and also in surgery. They could
contributions.
even be deployed as a defence
Both 4GLS and the XFEL are
against guided missiles.
extraordinarily exciting and
The availability of intense
challenging projects that will
X-ray laser pulses in particular
result in complementary FEL
will open a new window on
capabilities, offering UK
physical processes that have
researchers unrivalled research
never before been explored. Two
tools to probe the nature and
laboratories, the Stanford Linear
dynamics of matter.
Accelerator Center in California
and the DESY laboratory in
Hamburg, are developing X-ray
FELs to operate down to
Electron source and accelerator
wavelengths as short as a
Magnetic structure
fraction of a nanometre. The
European X-ray FEL programme
(XFEL) at DESY aims to provide
coherent X-ray pulses one billion
times brighter than those from a
synchrotron source, the bursts
of light lasting only tens of
femtoseconds at wavelengths
down to 0.1 of a nanometre.
DESY Hamburg
bounce the light beam back and
forth to increase interaction with
the electron beam. But, for
shorter wavelengths, there are
no suitable mirrors and so to
achieve enough gain, an intense
electron beam is sent down a
much longer undulator with
several thousand alternating
magnets.
Although the principle behind
the FEL was first explored in the
1970s, it is only in the past few
years that people have started to
exploit its potential at shorter
wavelengths. There have been
enormous advances in
developing high-intensity
electron sources, as well as
superconducting accelerating
devices and magnet technology
– all necessary for the
development of FELs. Today,
there are a dozen or so FEL
facilities, operating in the infrared
and visible-to-ultraviolet range,
in the US, Europe and Japan.
How an FEL works
After an electron beam is
accelerated, it passes down a
special arrangement of magnets
called an undulator, which
causes the electrons to emit
laser-like bundles of radiation
Electron trap
Light beam
Experiment
OF
HAMBURG
AND
DESY
FOR HELP WITH THIS PAPER