Designing Magnets for the World’s Largest Particle Collider

To produce a field of 16 teslas for CERN’s Future Circular Collider, scientists must invent a new class of magnets

Artistic impression of a collision event at the center of a future detector.
Illustration: CERN
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At the Large Hadron Collider (LHC) in Switzerland, man-made magnetic fields—roughly 200,000 times stronger than the Earth’s—accelerate particles into one another at nearly the speed of light. Experiments by this world-renowned particle accelerator have provided critical data supporting the Standard Model, a theory describing how particles in our universe behave and interact. But physicists are keen to know whether the model holds up for particles at even higher energies.

More experiments must also be done to extend the Standard Model, to answer questions such as why the universe has so much matter compared to antimatter, what exactly is dark matter, and why neutrinos have mass. CERN, the research organization behind the LHC, hopes to explore these mysteries with its proposed next-generation facility, the Future Circular Collider (FCC).

On 15 January, CERN released its first report outlining potential designs for the FCC, with input from more than 130 institutions. The facility would come with an estimated price tag between €9 billion (US $10.2 billion) and €21 billion, and part of the facility could open as soon as 2040.

If realized, the FCC will produce magnetic fields nearly twice as strong as the LHC, and accelerate particles to unprecedented energies of 100 tera-electron volts, compared to the LHC’s energies of 13 TeV. Whereas the magnetic system at the LHC can achieve strengths of 8.3 teslas, the FCC system would be able to achieve 16 T.

Now, for those working on the design, the question is: What technical feats must come next for scientists to be able to produce such a strong magnetic field?

More than 4,500 magnets will be required for the FCC, and those magnets must rely on new designs and materials. The magnets currently used in the LHC are made of niobium titanium (NbTi)—but for the FCC, scientists will switch to the more powerful superconducting material niobium tin (Nb3Sn).

“The only superconducting material suitable in producing accelerator magnets achieving 16 T is Nb3Sn,” says Arnaud Marsollier, head of media relations at CERN. However, this material is sensitive to the tiniest deformation. This is problematic considering the extreme changes the material will undergo, which could degrade its superconducting properties. Therefore, scientists need new designs to minimize stress on these systems.

One team working on making more powerful and durable magnets is Stefania Farinon and her colleagues at Italy’s National Institute of Nuclear Physics. They’ve been trying to improve a particular type of magnet called D2. A D2 magnet is the second-to-last one along the circular track of a particle collider, positioned right before the particles smash into each other. Its main function is to separate and recombine the particles of the two beams.

On 14 January, a day before CERN unveiled its plans for the FCC, Farinon and her colleagues published a study in IEEE Transactions on Applied Superconductivity describing how to build a D2 magnet for the FCC that is made of Nb3Sn and won’t degrade when exposed to its stronger magnetic fields.

The two magnetic fields of a D2 magnet have identical polarities and direction—which also means the magnetic fields can interfere with one another (a phenomenon described as “cross-talk”), degrading performance of the system. For lower magnetic fields, such as those at the LHC, this cross-talk can be resolved by including a component known as an iron yoke between the two apertures. But an iron yoke is not sufficient to stop cross-talk at higher magnetic field strengths.

To address this issue, Farinon’s group has developed a new type of D2 magnet that has a differently shaped iron yoke, and a system of asymmetric coils. A similar design will be used first in the LHC, as part of an upgrade project called High Luminosity LHC (HL-LHC) that will be completed in 2026.

The upgrade will allow for better steering of beams, increasing the total number of collisions that occur by a factor of 10. The magnetic fields produced by this new D2 magnet will be able to reach 4.3 T, while the most powerful magnets of the HL-LHC may reach 10 to 12 T. Recently, the D2 magnet was shipped to CERN and Farinon’s team will begin testing it there in February.

In their recent study, Farinon’s team proposes that the D2 magnet for the FCC, which will be made of Nb3Sn and reach 10 T, be manufactured using a technique recently developed by a team at Lawrence Berkeley National Laboratory, in California, whereby an aluminum shell and pressurized water bladders are applied to pre-stress the system sufficiently.

Developing these upgrades for the HL-LHC—and the FCC, if it is implemented—could open up new areas of particle physics and improve our understanding of the universe. While many physicists are excited about the potential of the FCC, some scientists have argued that the money it will cost to build the facility could be better spent on health or global warming initiatives. The decision on which components of the FCC proposal to fund—or whether to fund it at all—will be made by the CERN Council, shortly after the next European Strategy is finalized in the first half of 2020.

Yifang Wang, director of the Institute of High Energy Physics of the Chinese Academy of Sciences, notes how the electron-electron collisions that will be produced by the proposed FCC could take our understanding further, saying the facility “[could] measure Higgs properties with a factor of 10 improvement on precision, which is critical for our understanding of physics within and beyond the Standard Model.”

While Wang supports the development of the electron-electron component of the FCC design, he is less convinced that the part of the proposal for a proton-proton collider is worth the high price tag. He says, “Personally, I think Nb3Sn is not ideal—it’s too expensive. But if you want to have something now, this is the only choice.”

His team at the Institute of High Energy Physics is exploring the prospect of a Super proton-proton Collider (SppC)—but its magnetic field would be made from cables comprising an iron-based high-temperature superconductor. “Iron-based high Tc cable have a lot of promising features, but [the technology] is still too far away, since its current density is still a factor of 10 short,” he says.

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