The magnetic device (undulator) used to produce X-rays in an FEL at DESY Hamburg

A new light source generating intense bursts of coherent radiation over a wide range of wavelengths promises to revolutionise our understanding of matter

A diffraction pattern illustrating the spatial coherence of the FEL at DESY’s TESLA test facility

Layout of the proposed GLS accelerator complex to be built in Cheshire

Brilliant 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).

Experiments using an FEL at the Jefferson Laboratory

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.

	Component of an electron gun to produce electrons for an FEL

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 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.

	Intense X-ray radiation from an FEL causes a biomolecule to 'explode', so an image must be recorded very quickly

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 of a second). Key areas that will benefit include unravelling the dynamics of industrially or environmentally important chemical reactions, as well as the subtle molecular changes in biological systems. Carefully timed light pulses could be used to follow, and even control, the movement of electrons in atoms, molecules and advanced materials developed for nanotechnology and electronics. The FELs on 4GLS will also offer new opportunities to develop dynamic imaging techniques for diagnosing conditions such as progressive degenerative diseases and cancer.

FELs have a huge potential, and some US laboratories are exploring their application in industrial processing such as the modification of plastic surfaces, and also in surgery. They could even be deployed as a defence against guided missiles.

The availability of intense X-ray laser pulses in particular will open a new window on physical processes that have never before been explored. Two laboratories, the Stanford Linear Accelerator Center in California and the DESY laboratory in Hamburg, are developing X-ray FELs to operate down to wavelengths as short as a 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. These will allow researchers to take snapshots of chemical bonds being made and broken, and to look at detailed physical processes such as planes of atoms sliding over one another. Ultimately, researchers hope that it will be possible to image a single biological molecule before the X-rays’ powerful energy destroys it.

DESY scientists have already built a facility to test the technology needed for an X-ray FEL, and it has now been transformed into the VUV FEL (working in the high-energy ultraviolet and soft X-ray region) for use in scientific research. It provides pulses lasting only 25 femtoseconds, and is as powerful as predicted. The XFEL will be ready by 2012, if funding allows, and UK researchers are already making major contributions.

	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

Both 4GLS and the XFEL are extraordinarily exciting and challenging projects that will result in complementary FEL capabilities, offering UK researchers unrivalled research tools to probe the nature and dynamics of matter.

Further information on FELs can be found at:
http://sbfel3.ucsb.edu/www/vl_fel.html
(The World Wide Web Virtual Library: Free Electron Laser research and applications)

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Thanks go to Elaine Seddon of the Daresbury Laboratory and to Jörg Rossbach of the University of Hamburg and DESY for help with this paper

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