The European XFEL will feature the world’s longest undulator systems – magnetic structures which make electron bunches emit extremely bright X-ray flashes. After delivery of these components by industry partners, the magnetic tuning will begin. “XFELlow” Joachim Pflüger’s group is getting ready for this task.
As if one of the undulator segments wasn’t impressive enough: eight tons of steel, five metres long, 2,40 metres high, made with micrometre precision and comprising hundreds of magnets made of neodymium iron boron, alternating in a row. However, the European XFEL will host not just one of these devices, but 91. This adds up to a total undulator length of almost half a kilometre and is a world record: No other facility will be equipped with so many undulator metres. In comparison, the undulators of DESY’s current flagship light source PETRA III sum up to only about a tenth of that.
The reason for this is not megalomania, it is required by the laws of physics and the objective to construct a user facility for extremely bright X-ray flashes that have the coherent properties of laser light. To produce these X-ray flashes, an electron beam of outstanding quality has to be sent through extremely long undulator systems. (See: Inside an undulator)
The undulator segments themselves are delivered by industry partners under the supervision of the European XFEL GmbH, but many more partners are involved in the construction of the undulator systems. Among various other components, these also comprise monitors, phase shifters, vacuum chambers, and magnetic quadrupoles for controlling and adjusting the electron bunches. Like elsewhere in the European XFEL project, international collaborations were founded to achieve the technical goals.
Here, Joachim Pflüger plays a crucial and integrating role. The 58-year-old physicist has been participating in the development of undulators for free-electron lasers since the late 1990s – shortly before former DESY director Bjørn H. Wiik’s dream of a unique X-ray laser facility took concrete form for the first time in 2001.
Until April 2009, Joachim Pflüger was head of the undulator group at DESY. In that position, he was not only responsible for the European XFEL undulators, but also involved in the PETRA III project, currently the world’s brightest conventional X-ray source. His objective was to utilize synergetic effects that could arise from the two projects. “From the beginning, we planned to use the same undulator technology for PETRA III and the European XFEL. Apart from minor details, the undulators are identical. That is a win-win situation for both parties, true synergy”, Pflüger says.
Pflüger is one of the European XFEL staff members who quit their job at DESY to get hired at the European XFEL GmbH. That’s because DESY and the European XFEL GmbH are separate organizations – although with a share of over 50 percent, DESY is the biggest shareholder of the European XFEL company.
This autonomy is also the reason why there were some legal aspects to solve. The undulators utilize intellectual property of DESY. Regulations had to be found that ensure the European XFEL GmbH is allowed to use them. There is a general agreement between the two organisations for that. “But it hasn’t been specified that this is a paperless process”, Pflüger remarks with a smirk on his face.
Currently his team comprises 11 members and a guest scientist, and it is steadily growing. One of its main tasks is to supervise the production of the components that will be manufactured by industry partners. After delivery, a magnetic fine-tuning must be performed. “We have developed sophisticated means of producing perfect magnetic fields. My people have to become experts on these processes”, Pflüger says.
Until recently, Pflüger’s group had been busy designing the so-called pre-series prototype: “The position of every screw hole, of every bolt had to be specified. When you have to build 91 undulators, you think twice before you optimize anything. Implementations of changes that are not tested properly and turn out wrong can become very expensive.“
However, this does not mean there are no ideas for optimizations. The XFEL theory group at DESY has been producing a whole bouquet of upgrade possibilities – partly small modifications, partly big extensions. But from now on, these ideas will be used for future plans of an upgrade only. “The systems have to fulfil high precision and reliability requirements. Not only do they have to work, but they have to work reliably for 15 years. You have to steer a big ship over many years and hold direction.” Captain Pflüger makes the convincing impression to be the right person for this task.
Author: Dirk Rathje
The magnetic field of an undulator makes electrons follow a slalom path. Electric charges that are accelerated in such a way emit electromagnetic radiation. That’s in analogy to a radio antenna in which radio waves – a special kind of electromagnetic radiation – are produced. Because electrons move through an undulator with almost the velocity of light, Einstein’s theory of relativity has to be taken into account. As a result the wavelengths are shortened and the emitted electromagnetic radiation is strongly focussed in the direction of the electrons’ motion. Experts speak of synchrotron radiation.
In a free-electron laser such as the European XFEL, things are still more elaborate – and complex. The light emitted by the electrons interacts with the bunch of electrons itself. It slows down some of the particles and accelerates others. As a result the electron bunch gets organized into smaller microbunches that have exactly the distance of the wavelength of the undulator radiation. Such a microbunched beam emits high-intensity synchrotron radiation which is of many orders of magnitude more intensive than PETRA-III-type sources. The underlying process is called Self-Amplified Stimulated-Emission (SASE).