InternalInternational resonance
The industrial production of the European XFEL's 800 accelerator cavities is a complex undertaking with many participants. Waldemar Singer and Detlef Reschke make sure that all adds up well.
How do you order complex structures made of superconducting niobium that no one—other than yourself—has ever produced before? This was what European XFEL shareholder Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany, and Istituto Nazionale di Fisica Nucleare (INFN) in Milan, Italy, had to do last year.
After 20 years of worldwide development and steady refinement, the production of the European XFEL accelerator cavities was finally ready to be tendered. Over the years, DESY and its collaborators around the globe had gained a tremendous amount of knowledge about the production of these sophisticated structures. What is more, they had proven with the Free-Electron Laser in Hamburg (FLASH), a smaller version of the European XFEL based on the same technology, that the cavities could be manufactured with the necessary properties. But FLASH comprises only 56 accelerator cavities. The European XFEL needs 800. This latter number is a world first, on an industrial scale that cannot be mastered by one research institute alone.
“What we do is technology transfer. We help our industry partners deliver what we need,” says Waldemar Singer about what had to be done next. Singer, a 61-year-old physicist, is the leader of the “Superconducting Cavities” work package. His doctoral thesis and many years of research were dedicated to material science and metallurgy. Now, he is in charge of the production of 800 accelerator cavities in which high-frequency electromagnetic fields oscillate to boost electrons to high energies. “When you need 800 of these cavities to provide a superb electron beam, you have to find industry partners who are willing to learn your recipes,” says Singer.
Close cooperation and steady supervision is crucial for production. “We ordered the cavities without any performance guarantee,” says Singer. “If we had not done so, we wouldn't have gotten them in time and at an affordable price. But, since we know how to build such cavities, we share our recipes with the manufactures and are in close contact to ensure that they do what we have already proven will work. With experts like Paolo Michelato, Axel Matheisen, and Jens Iversen, our team is very positive that all will run well.”
In this way, the private companies make money and gain knowledge, and the publicly funded European XFEL gets its accelerator elements. That is technology transfer at its best.
In contrast to other X-ray laser facilities, the European XFEL features an electron accelerator based on superconducting technology. Superconductivity is a property of materials like niobium that allow electric currents to flow without loss (or almost without loss for oscillating fields).
That saves energy. While it’s always good to keep your energy bill and carbon footprint small, one other big advantage of superconducting technology is the high number of electron bunches that can be accelerated per second. While conventional technology allows about 100 bunches per second before the elements get too hot, the European XFEL will deliver 300 times as many. Even a continuous beam will be possible for future extensions. More electron bunches result in more X-ray flashes which can reduce the duration of certain types of experiments enormously and increase the number of users that can benefit from the facility concurrently.
The downside of superconductivity is that you have to cool the accelerating elements to just a few degrees above absolute zero, and that superconducting materials like high purity niobium are a bit more difficult to get than, say, copper. The European XFEL needs about 20 tonnes; this is the largest local accumulation of high grade niobium in the world.
Producing the cavities is a long process: “We order the niobium from four suppliers, in Austria, China, Germany, and Japan,” Singer says. “The material for the cavity cells is, for example, delivered in square sheets with a side length of 265 mm and a thickness of 2.8 mm. We check the purity and surface conditions of the sheets and send them to our two cavity producers in Germany and Italy.”
Then the cavities are mechanically fabricated by deep drawing and electron beam welding of the parts. After that, steps like electropolishing of the internal surface, rinsing with ultrapure water, two heat treatments, tuning to the resonant frequency, integration into a helium tank, and assembly of the antennas have to be performed.
The finished cavities are then delivered back to DESY, where teams from the collaborator in Poland check whether the elements fulfil the specifications. From there, the cavities are transported to Commissariat à l’Énergie Atomique et aux Énergies Alternatives (CEA) in Saclay, France, where they are assembled into accelerator modules containing eight cavities each. These modules are finally brought back to DESY and tested again before being installed in the accelerator tunnel of the European XFEL.
The overall process sounds rather complicated. But, before you call for a consulting agency to streamline the process, keep in mind that the European XFEL is a truly international project with many partners around the globe. Sixteen institutes are involved in constructing the accelerator complex, contributing personnel and knowledge, thereby also fostering their national expertise.
Of course, someone has to keep track of things. That is the job of Detlef Reschke, the “cavity owner” in project management speak. The 49-year-old physicist is an expert in the field of superconducting cavities. After his doctoral thesis on the subject, he came to DESY in 1995, where he specialized in surface treatment and cavity testing.
“My job is to ensure that the process runs smoothly and to react if it doesn't. We have been setting up protocols and databases for that,” says Reschke. The production process will generate a big number of measured properties. Reschke has to make sure that these values are understood and the faults are spotted. “It is a real challenge—the numbers of cavities, the number of project participants. But we have lots of experience in the field, and it will work well.”
In the end, there will be 800 accelerating cavities in which electromagnetic fields oscillate resonantly to boost electron bunches—cavity by cavity.
Author: Dirk Rathje
