Second beamline for SwissFEL

If researchers want to follow the creation of new molecules in chemical reactions, to decipher the functional mechanisms of vitally important proteins or materials for electronic components, they quickly hit their limits. Here the processes run too fast to be captured in detail. X-ray free-electron lasers such as SwissFEL open up new possibilities to study them, with promising prospects for applications: These range from environmentally friendly processes for the chemical industry to custom-tailored drugs for targeted treatment of diseases and further to the development of new materials for the electronics of the future.

SwissFEL works like a film camera, where the rapid succession of individual photographic images produces a motion picture. With its intense X-ray pulses, it X-rays the sample to be analysed for a fraction of a trillionth of a second. These snapshots are recorded by detectors and can then be assembled into a "film". With SwissFEL, 100 such "photos" can be taken every second, which then can be joined together into a film.

An offer custom-tailored for researchers

This year the first pilot experiments are starting at the X-ray free-electron laser SwissFEL. Beginning in 2018, two experiment stations will be available for researchers from Switzerland and around the world. A third is currently in the planning stages. These stations are equipped with different experimental options designed to meet their users' expected demands. Each problem to be studied – whether biological, chemical, or physical – sets different requirements with regard to the experimental setup and the most suitable methods for the investigation.

But the type of X-ray light also determines what can best be studied with it: At the three experiment stations, the researchers can carry out experiments using so-called "hard" X-ray light. This X-ray light has an extremely short wavelength. It is generated by accelerating electrons to high energy and then using a special arrangement of magnets (the undulators) to force them onto a slalom track. The electrons then emit X-ray light with the characteristics of laser light.

This very high-energy X-ray light is optimally suited for following how and where atoms move during an ultrafast process. However, if the researchers want to understand more precisely what is happening with atoms or molecules while they are forming a new chemical bond, or how they react to external influences such as electromagnetic fields or light, they need "soft" X-ray light with a longer wavelength. The reason: Because of its relatively lower energy – in comparison to "hard" X-ray light – it can be used to target those processes that take place in the outer levels of the atomic shell.

These processes are of great interest to researchers because they provide information about how atoms or molecules interact with each other. They can reveal exactly what is happening during the formation of chemical bonds. Through their research the scientists can, for example, better understand how catalysts, which are used for a great many processes in the chemical industry, actually work. Thus they can contribute to making these processes more environmentally friendly and more efficient. Yet the researchers also gain deep insights into the mode of operation of fundamental vital functions by studying processes in the outer levels of the atomic shell. Furthermore, these play an important role in the investigation of new materials that are expected to improve the performance of electronic components – after all, they are responsible for essential properties such as a material's conductivity or its magnetic behaviour.

Second beamline as of 2020

When the SwissFEL facility was built, a second beamline was already included in the plan. This has now been under construction since the beginning of 2017, in parallel with preparing the first beamline with "hard" X-ray light for its first experiments. This second beamline will generate longer-wavelength, "soft" X-ray light and is expected to start operations in 2020.

Whether X-ray light will be "hard" or "soft" is determined by the choice of the energy the electrons reach in the linear accelerator, and through the selection of the undulators. With the first beamline, the undulators are technically configured to generate X-ray light at the shortest possible wavelength. With the second beamline, a different undulator technology is employed. Besides the wavelength, another priority is the direction in which the generated X-ray light oscillates – that is, its polarity. Depending on which direction of oscillation the light strikes the sample with, it causes different reactions. Thus the researchers can – in much the same way as with a polariser on a camera – examine different aspects of the samples. The researchers themselves can adjust the polarisation as desired. That way, they can resolve their research questions even more specifically.

The cost of the second beamline amounts to around 44 million Swiss francs and is borne in large part by the federal government. The canton of Aargau shares in the financing with 4 million francs from its Swiss lottery endowment fund. The cost of SwissFEL with the first beamline is around 275 million francs. Of this, the canton Aargau is contributing 30 million francs.

Text: Paul Scherrer Institute/Martina Gröschl