The European XFEL, which officially opened on 1 September , is the world’s largest x-ray laser facility. Eleven countries have collaborated on the project, which runs for 3.4km underground between Hamburg and Schleswig Holstein in Germany.
The machine can generate lasers that are a billion times brighter than conventional x-ray sources, and will open up a range of new opportunities for science.
“The special thing about the European XFEL will be the new science that it enables,” says Neil Pratt from the UK’s Science and Technology Facilities Council (STFC), which helped build and design a cutting-edge X-ray camera that can capture images of ultrafast processes such as chemical reactions within the XFEL.
From today, research groups from all over the world will be able to map the atomic details of viruses, dive into the molecular composition of cells, and create three-dimensional images of tiny nanoparticles.
Pratt highlights some of the facility’s unique abilities, such as superconductivity. This allows the creation of a high quality electron beam, enabling the XFEL to generate many more light flashes per second than existing facilities – up to 27,000.
Building the XFEL
Building such a powerful tool meant pushing the boundaries of engineering. “All of the lasers we have had up to date have generally run in the optical, ultraviolet or infra-red part of the spectrum,” explains John Collier, the director of STFC’s Central Laser Facility at the Rutherford Appleton Laboratory in Oxfordshire. “Getting something to work in the x-ray region has needed a completely different approach.”
Instead of exciting a ‘gain medium’ as in a traditional laser, the XFEL works more like a particle accelerator like the famous one at CERN. “You take an electron beam and accelerate it to high energy,” says Collier. “It takes a few hundred metres to do that. Then you put it through a series of devices called undulators – north-south magnets alternating along a very long stretch of pipe.” The undulators cause the electron beam to oscillate, giving off x-rays, which then act on the beam and turn it – after hundreds more meters of tunnel – into a laser-like beam.
“The engineering challenges are significant because this electron beam goes in a straight line,” says Collier. “You've got to maintain the alignment of all these large components over an extended distance. That's quite a common challenge for accelerators anyway, but it's in a free electron laser that is a significant challenge.”
To build the 800 accelerator resonant cavities, for example, the project had to enlist the help of two different industrial companies, which were extensively trained under guidance from experts. Another challenge was the detector or ‘camera’ at the end of the tunnel which captures the data after the x-rays have passed through the object of interest.
In order to take advantage of the XFEL’s 27,000 pulses per second, a detector had to be built which could record a high-quality image and then clear it very quickly ready for the next one. A number of European laboratories were involved, and the technology that has been developed could eventually trickle down to consumer electronic devices such as cameras.
Engineering applications
It has been a huge undertaking, with hundreds of engineers and scientists and a billion pounds of investment involved. It’s not just for academics either – although they will take the bulk of the time at first. Further down the line, it could play a key role in the pharmaceutical industry, and there has been interest from engineers too.
“Britain’s leading edge, high value-added industries rely on leading science to enable industrial innovation and competitiveness,” wrote David Dye of Imperial College and David Rugg of Rolls-Royce in a document making the case for building the XFEL. They highlighted the dynamics of fuel combustion and nuclear power, and the next generation of engineering tools, which work in timescales measured in femtoseconds. “Understanding the dynamics of these processing techniques on relevant timescales will enable improved understanding and optimisation, leading to enhanced material, component and therefore design performance,” they wrote.
It could be particularly useful in understanding an industrial process known as ‘laser shock peening,’ says Collier. This involves hardening a metal surface such as a turbine blade against cracks by converting tensile stresses into compressive stresses.
In the past, it was done by hitting the parts with a hammer or shooting them with ball-bearings. Now it’s done with lasers, which drive shockwaves through the surface. "That's not a very well understood surface in terms of fundamental physics,” says Collier. “With an x-ray laser, because you've got a combination of a laser hitting a surface and an x-ray probe that can pick up the structure on a very fast timescale, you've got a really interesting tool for being able to understand and better optimise that process and see how you could apply it to other materials other than metals.”