Since 2005 researchers at DESY have had access to a unique new type of light source: FLASH, the world’s only free-electron laser to generate radiation in the vacuum ultraviolet and soft X-ray regions. This is a pioneering facility in a number of ways. As the world’s first-ever X-ray free-electron laser with a superconducting linear accelerator, FLASH is a source of indispensable knowledge for the development of future accelerators and X-ray lasers. At the same time, it provides researchers from virtually all the natural sciences with unprecedented experimental possibilities.
Record laser in the X-ray region
For many years there was fierce competition between constructors of radiation sources around the world to see who would be the first to develop a high-power laser in the X-ray region. During this period the international FLASH team set one record after another, progressively reducing the wavelength of the laser radiation until the target of 6.5 nanometres was finally achieved in 2007. This remained unsurpassed for two years, until the start-up of the Linac Coherent Light Source LCLS in California. This free-electron laser produces radiation of an even shorter wavelength – in the region of 0.15 nanometres, which is in the hard X-ray range. Nevertheless, FLASH remains unique as the world’s only free-electron laser to generate high-power ultra-short pulses of laser light in the vacuum ultraviolet and soft X-ray ranges, down to a wavelength of slightly over 4 nanometres.
As such, FLASH outperforms not only the world’s best synchrotron radiation sources but also the very latest conventional X-ray lasers. While it is true that synchrotron radiation sources also deliver tightly collimated radiation, FLASH generates light with real laser properties, i.e. which is perfectly collimated. In the X-ray range, conventional lasers can only deliver low-intensity beams. In contrast, the peak luminosity of the FLASH radiation is several orders of magnitude higher, even than that of the most advanced synchrotron radiation sources. In addition, since the laser radiation from FLASH is emitted in ultrashort flashes, it provides the researchers using the new DESY facility with unique experimental capabilities.
So how does a free-electron laser work? During their slalom run through a periodic array of magnets (the undulator) the electron bunches emit radiation (photons) of a distinct wavelength. The photon beam propagates in a straight line so that it overlaps with the electron bunch. It imprints its periodic structure on the electron bunch, so that the initially homogeneous charge density distribution becomes periodic – a chain of tiny individual charge “disks” regularly separated by a single wavelength. Now all the electron disks emit radiation in synchronism, and the light can amplify itself to form high-intensity laser radiation.
The FLASH facility at DESY is being used for research with short-wavelength ultraviolet radiation and soft X-rays. User time at the five experimental stations is in demand – just a year after the start of user operations, the facility was already threefold overbooked. Even during the first measuring period, the high hopes that the researchers had placed on the revolutionary new experimental capabilities of the free-electron laser were confirmed. Consequently there are many prospective users interested in other projects at FLASH, for instance in the fields of physics, chemistry and molecular biology.
However, FLASH is not only in demand as a new kind of research instrument. The facility is also playing an important pioneering role for the larger free-electron lasers to come, such as the European X-ray laser XFEL, which will generate X-ray flashes in the hard X-ray region. At FLASH, scientists, technicians and engineers are testing the superconducting accelerator technology which will be used in the European XFEL as well as the undulators – the special magnet arrangements for generating the X-ray flashes –, the optical components, experimental setups and detector systems. Operating FLASH is also helping them to gain valuable experience with the electronic processing of large data volumes. And, last but not least, FLASH offers researchers an opportunity to explore new experimental methods for use with future X-ray lasers.
Unique experimental capabilities
The extraordinary properties of the FLASH radiation provide researchers in virtually all natural sciences with unprecedented experimental capabilities. The peak luminosity of FLASH for instance exceeds that of the most advanced synchrotron radiation sources by a factor of ten million, and consequently opens the door to previously impossible studies of processes in astrophysics using extremely diluted samples. The radiation is laser-like, i.e. coherent, and the wavelength can currently be adjusted between 13 and 60 nanometres. Later it will be possible to achieve wavelengths down to 6 nanometres. Of special importance is also the extremely short duration of the radiation pulses, which last only 10 to 50 femtoseconds (quadrillionths of a second). Scientists will be able to use this radiation much like an ultra-fast stroboscope to actually watch fast processes such as the formation of chemical bonds or those involved in magnetic data storage as they actually unfold.
The high energy of the radiation makes it possible to produce in the laboratory energy densities in matter that can otherwise only be found at other locations in the universe and consequently opens a new door to the exploration of open questions in plasma physics. Of particular interest is, for example, the wavelength region around 13.5 nanometres, because radiation of this wavelength is required in the semiconductor industry for EUV (extreme ultraviolet) lithography, which will be used to manufacture the next generation of microprocessors.
Fundamentally important for the life sciences is the wavelength region between 2.3 and 4.4 nanometres, known as the “water window.” In the water window, carbon atoms in matter are highly opaque to the radiation, while the surrounding water is transparent and therefore remains invisible. This wavelength region is covered by the higher harmonics of the FLASH laser radiation, and since 2010 also by the fundamental wavelength. This enables biologists to perform previously impossible studies – such as generating holographic images of cellular systems with the aid of a single radiation pulse from the FLASH facility.
- Free-electron laser with superconducting linear accelerator in TESLA technology
- Total length: 260 metres
- Generates extremely brilliant laser radiation in the vacuum ultraviolet (VUV) and the soft X-ray range using the SASE principle (wavelengths tunable between 4 and 60 nanometres)
- In user operation since 2005
- Five experimental stations
Technology for tomorrow’s accelerators
With respect to technology too, FLASH is advancing far into new territory. The free-electron laser’s operation is based on the innovative SASE principle of self-amplified spontaneous emission. In this special amplification process, electrons from a particle accelerator fly through an undulator – a periodic array of magnets – which causes them to follow a high-speed slalom course, forcing them to emit flashes of radiation. These flashes reinforce each other in accordance with the SASE principle to form short-wavelength, high-intensity laser flashes.
A distinguishing feature of FLASH is the use of superconducting accelerator technology to propel the electrons to the required high energy.
The technology used to achieve this was developed and tested by the international team of the TESLA Collaboration between 1992 and 2004 at DESY. The accelerating elements, the resonators, which are cooled to minus 271 degrees Celsius, conduct electric current loss-free, so that practically all of the electric power they consume can be transferred to the particles – an extremely efficient acceleration method. What’s more, the superconducting resonators deliver a very thin and homogeneous electron beam of extremely high quality. A particle beam with such special properties is a prerequisite to operate a free-electron laser in the X-ray region.
Two other large projects are based on the superconducting TESLA accelerator technology: the X-ray laser European XFEL with its linear accelerator, which is roughly 1.5 kilometres long, and the future international particle physics project, the International Linear Collider ILC, which is currently being planned in a worldwide cooperation. Its two accelerating sections will be up to 20 kilometres long and will also be equipped with superconducting resonators. Scientists and engineers can therefore gather valuable information for both projects from the operation of the 120-metre-long linear accelerator of FLASH. Participation in the FLASH project is also of considerable interest to industrial companies that can leverage the acquired technical know-how to qualify for participation in the construction of the European XFEL and other linear accelerators around the globe.