2 | Photon Science
Beamlines and experimental stations in the FLASH hall ª
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 wave-lengths 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 great 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 freeelectron laser were already confirmed (see pages 1–23). Consequently there are many prospective users inter-ested 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 Xray flashes in the hard Xray 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 Xray flashes, the optical components, experimental setups and detector systems.
Operating FLASH is also helping them to gain valuable expe-rience 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 unprecedent-ed 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, for example 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 0, later between
and 0 nanometres. Of special importance is also the ex-tremely 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 strobo-scope to actually watch fast processes such as the formation of chemical bonds or the processes involved in magnetic data storage as they actually unfold. The high energy of the radia-tion makes it possible to produce in the laboratory energy densities in matter that can otherwise only be found in the far
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The FLASH free-electron laser at DESY is an absolute novelty: For the first time, experiments can now be conducted with highly intense, pulsed laser radiation at short wavelengths in the soft X-ray region. In other words, the researchers involved have ventured into entirely unexplored territory for which there was no prior data that they could rely on.
Most of the groups of scientists arriving at FLASH for the first measuring period (2005–2006) came equipped with entirely new instruments designed specifically for the special properties of this laser radiation. Despite the complexity of the equipment and initial teething troubles of a new radia-tion source that hadn’t been completely run in yet, the researchers were highly satisfied and returned home happily with new data by the diskful.
The early experiments ranged from the generation and measurement of plasmas, investigations of gases and clusters to initial studies of experi-mental methods for complex biomolecules – methods of the type intended to be used at the European X-ray laser XFEL, among others. As expected, the X-ray pulses of FLASH were shorter than 50 femtoseconds. Some groups were already able to use them for the first time to actually watch processes as they unfolded with extremely high temporal resolution – much as in a slow-motion film. The investigation of such time-resolved processes using short-wavelength radiation ranks among the most impor-tant new applications this type of X-ray laser will offer in the future (see pages 16–23).
Taking stock:
successes in the first measuring period
In the HIXSS experiment, the intense laser light from FLASH is used to explore catalytic processes on solid-state surfaces.
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SASE – the principle
So how does a free-electron laser work? During their slalom run through a periodic array of magnets (the undulator) the electron bunches emit radia-tion (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 homoge-neous charge density distribution becomes periodic – a chain of tiny indi-vidual 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.
reaches of the cosmos. FLASH thus also opens a new door to the exploration of open questions in plasma physics. The wavelength region around 13.5 nanometres is of particular interest as well, because radiation of this wavelength is required in the semiconductor industry for EUV (extreme ultra-violet) lithography, which will be used to manufacture the next generation of microprocessors.
Fundamentally important for the life sciences is the wave-length region between 2.3 and . 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 por-tion of the FLASH laser radiapor-tion, the socalled third and fifth harmonics (i.e. radiation with wavelengths of the correspond-ing multiple of the fundamental laser frequency), which pres-ently attain wavelengths of . and 2.8 nanometres, respect-ively. 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 selfamplified 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 supercon-ducting 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 200 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 superconduct-ing TESLA accelerator technology: the European Xray laser XFEL, whose linear accelerator is roughly 1.5 kilometres long, and the future major project of particle physics, the Interna-tional 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 consid-erable 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. ª
Taking stock:
successes in the first measuring period
The undulator section in the FLASH tunnel ª
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