FCC - these three letters stand for the vision of a new ring accelerator that could be built at the European particle physics laboratory CERN in Geneva. With this long-term goal in mind, Swiss physicists founded the CHART research initiative five years ago. Now the demonstrator of a powerful magnet is available. If the tests are successful, a very first step towards decisive progress in infrastructure for basic research, but also in applications such as, e.g., innovative instrumentation for medical therapies will be achieved.
When you hear the word 'Tesla', the first thing you might think of today is a US manufacturer of electric cars. Elon Musk captured the name of the electrical engineer and inventor Nicola Tesla (1856 - 1943) for his commercial enterprise. But the fascination of the name goes back much further. Since 1960 Tesla has been the official unit of measurement for magnetic flux density. The earth's magnetic field is only a small fraction of a Tesla (ca. 50 micro Tesla), a horseshoe magnet is about a tenth of a Tesla strong. In medicine, tomographs with a field strength of one Tesla or so are used. The magnets that keep the protons on their orbits in the LHC circular accelerator at CERN generate up to 8.4 Tesla, which corresponds to a powerful magnetic field.
Future of CERN
To build magnets of this strength requires top technical performance. If magnets are to become even more powerful in the future, this can only be achieved through a combined effort of science and engineering; a collaborative initiative of the best minds is necessary. It is probably no exaggeration to say that the development of even more powerful magnets will decide the future of CERN. A huge circular accelerator with a circumference of 100 km is currently being planned there under the name 'Future Circular Collider' (FCC). From 2040, this is to replace the current circular accelerator LHC with its 27 km circumference and secure the future of the world's leading particle physics facility. "The name CERN is a brand like Coca Cola. If the research centre, which is so immensely important for Switzerland, is to continue its existence at the present site, the construction of a new accelerator is vital. One possible technical solution is the construction of a large ring accelerator. For such an accelerator to work, we have to develop a new generation of extremely powerful superconducting magnets", says Hans Rudolf Ott, ETH professor emeritus and expert on superconductivity. Ott is Chairman of the CHART Council, which is strategically leading the research initiative of the same name.
CHART stands for 'Swiss Accelerator Research & Technology'. Since 2015, the initiative has been bringing together the leading forces in Swiss accelerator research and development. Under the auspices of the State Secretariat for Education, Research and Innovation (SERI) and the Board of the ETH Domain (ETH Rat), the second funding period runs from 2019 to 2023. The CHART Collaboration includes researchers from the two Federal Institutes of Technology in Zurich and Lausanne, the University of Geneva and, of course, CERN. The total financial commitment by all these partners amounts to CHF 40 million. The home institute is the Paul Scherrer Institute (PSI), the largest research institute in Switzerland belonging to the ETH Domain. The central goal of CHART is the research and development of accelerators based on powerful magnets for basic research in Physics, Biology, Chemistry and for interdisciplinary issues as well as for the application in crystallography and medicine.
Hope rests on the compound Nb3Sn
The material plays a central role in the construction of powerful magnets. The current-carrying conductors of the 1200 magnets, each 15 m long, which keep the protons on their circular path in the current CERN particle accelerator LHC, are made of niobium-titanium (NbTi) alloy. The magnets are working at 1.9 Kelvin. At this temperature - just above absolute zero - electric current flows practically without resistance (superconductivity), allowing for large currents and consequently generating a strong magnetic field. The cooling medium is the liquid helium isotope 4He, which is in a superfluid state at this temperature and in this way guarantees a higher stability of the current-carrying conductor.
Scientists are discussing a different, even more powerful metal compound for the super magnets of the future. "For future magnets, we are relying on the niobium-based compound Nb3Sn; this compound is, at present, regarded as the key material for the construction of particularly strong but also very compact magnets," says Leonid Rivkin, PSI Deputy Director and main person responsible for the operational implementation of the CHART initiative. The crux: Nb3Sn is a very brittle material that is extremely demanding to process for the envisaged application.
CCT design minimizes risks
Strong forces on the current-carrying conductor prevail in a strong magnet. To tame these forces, the scientists of the CHART initiative have developed the Canted Cosine Theta (CCT) design. The main idea of this design is to capture the superconducting cable into grooves of a cylinder, reducing the forces exerted on the windings, thus minimizing the risks of local loss of superconductivity in the conductor (so-called quenches).
On paper, this design enables the construction of a magnet with 16 Tesla field strength. In order to find out whether this is also successful in practice, scientists at PSI built a 1 m long demonstrator as part of the CHART initiative, which works at temperatures between 2 and 4 Kelvin. The construction of the demonstrator was completed at the end of October 2019, then it was moved to California in the beginning of this year. In Berkeley it is currently being tested for its performance at the Lawrence Berkeley National Laboratory (LBNL).
Prototype until 2023
The CCT design is one of four construction types (see illustration 02) that are currently being discussed in the scientific community for magnets of the highest quality and investigated at various institutes worldwide. Which one will ultimately prove to be the most appropriate is currently still open. For the superconducting magnet in CCT design, the year 2023 is the next milestone: Until then, the prototype of a dipole magnet several meters long is to be built within the framework of CHART. Then it should become apparent which of the currently pursued concepts is best suited for the construction of a 16 Tesla magnet, as it is to be used in the FCC in the long term.
According to current planning, the 16 Tesla magnets will only be needed in the second operating phase of the FCC, which could start in 2061. While in the first operating phase of FCC, electrons and positrons will be the particles to be accelerated, in this second operating phase, the much heavier protons are to be kept on track on their circular orbit for which the strong 16 Tesla magnets are required. Although the use of these magnets is likely to start in approximately 40 years from now, it is essential that their development is under way already today. "Research, development and the subsequent industrialization of magnets is a lengthy process that will take around 20 years in total. Their subsequent installation into the prepared accelerator tunnel will again be a time-consuming process and therefore we must tackle this task today so that the magnets eventually are ready for the actual research periods," emphasizes Leonid Rivkin, Chairman of the CHART Executive Board.
Broad field of application
Research on magnets and their superconducting components is central to CERN but, as mentioned above, is also important for basic research in physics, chemistry, biology and material science as well as medical and industrial applications. For example, strong magnets are needed for generating light from electron synchrotrons (e.g. at the Swiss Light Source/SLS) or so-called free electron lasers (e.g. the SwissFEL), which are of great importance in studies and the characterization of a variety of materials in different forms and pharmaceutical research. Such magnets are also required for the provision of neutrons (e.g. at the spallation-neutron source SINQ the PSI), which again are used for the characterization of a broad variety of different materials, for example. Magnets also form the basis for modern methods for the diagnosis and therapy of cancer tumors.
If the size of required magnets can be reduced, this creates the prerequisites for the construction of compact medical devices that can be used directly in hospitals. Hopes here rest on high-temperature superconductors, which are also being studied within the framework of CHART. This refers to materials in which the state of superconductivity sets in at relatively high temperatures of 100 Kelvin and more. "CHART research opens up a promising field of technology that fosters scientific and technical excellence in Switzerland," says Hans Rudolf Ott. "With our project, we are particularly addressing the generation of younger academics and professionals. CHART opens up a wide variety of professional careers in research and industry for them and thus makes an essential contribution to a scientifically and technically educated society in Switzerland".
Author: Benedikt Vogel