Free-electron laser FLASH

Since 2005, researchers at DESY have had access to a unique new light source: FLASH, the world’s first and, until 2009, only free-electron laser in the soft X-ray range. Among current light sources FLASH is an absolutely pioneering facility with a performance that surpasses not only the best synchrotron radiation sources but also the very latest laser systems in the X-ray range.


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 ultra-short flashes, it provides the researchers using the new DESY facility with experimental capabilities not available from any other radiation source on the globe.

Builders of radiation sources on several continents have been competing for years to develop the first high-performance laser for the X-ray range. In this race, the international FLASH team is clearly out in front: The 260-metre-long free-electron laser at DESY is presently the world’s only laser facility that delivers fast pulsed, powerful and ultra-short light flashes in the soft X-ray region. Even during the first measuring period from 2005 into 2006, FLASH set a new record of 32 nanometres (billionths of a metre) – the shortest wavelength ever produced with a free-electron laser. In 2006 the FLASH team bettered that record with a wavelength of only 13.1 nanometres – and, on top of that, they generated laser power that exceeded anything even the world’s largest plasma X-ray laser facilities can produce.

In the summer of 2007, FLASH was expanded further with the aim of reducing the wavelength of the generated radiation to the planned design value of six nanometres. This will enable DESY to retain its worldwide leadership until 2009, when the LCLS (Linac Coherent Light Source) free-electron laser in Stanford (USA) goes into operation with even shorter wavelengths in the hard X-ray region.

In demand

The FLASH facility at DESY is being used for research with short-wavelength ultraviolet radiation and soft X-rays. User time at the initial four of 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 LCLS in Stanford and 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 XFEL as well as 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. Furthermore, FLASH is presently the world’s only radiation source where researchers can explore new experimental methods for the 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 a special, less intense portion of the FLASH laser radiation, the so-called third and fifth harmonics (i.e. radiation with wavelengths of the corresponding multiple of the fundamental laser frequency), which presently attain wavelengths of 4.4 and 2.8 nanometres, respectively. 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.

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 European X-ray laser 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 XFEL and other linear accelerators around the globe.

Examples from research

HIGH LIGHTS (5.1 MB) The easy to understand HIGHLIGHTS brochure gives a choice of FLASH research examples