The world’s first high-temperature superconducting (HTS) canted-cosing-theta (CCT) sextupole now exists and has been successfully tested at cryogenic temperatures at the Paul Scherrer Institute (PSI) in Switzerland.
The magnet has been developed by the FCCee-HTS4 project, a CERN–PSI collaboration funded through the CHART consortium, with EPFL, ETH Zurich and the University of Geneva as partners. It was presented publicly for the first time at the 17th International Particle Accelerator Conference (IPAC’26) in Deauville, France, this May.
It has been designed with the purpose of being used in the proposed Future Circular Collider (FCC), CERN’s proposal for its next flagship accelerator. The FCC proposal contains two stages, the first is an electron-positron collider (FCC-ee), and the second would be a hadron-hadron collider (FCC-hh).
As current plans stand, the FCC-ee baseline uses normal-conducting arc magnets, and the sextupoles among them account for a substantial share of the machine’s power consumption through ohmic losses. HTS4 proposes to replace them with HTS magnets, which dissipate no resistive power and, operating well above the few Kelvin required by low-temperature superconductors, carry a far smaller refrigeration penalty. Because they shape their fields without iron, the magnets can also be nested, improving the packing factor and reducing the RF voltage the collider needs.
“As a key demonstrator for FCC-ee, this magnet represents an important step forward in HTS magnet technology," said Francesco Bardi, an engineer at CERN involved in the HTS4 project.
"This achievement reflects the vision and perseverance of Michael Koratzinos, who led the project with determination and turned an ambitious idea into reality. It has been a privilege contributing to this milestone, which marked my first year and a half at CERN," he added.
What was built?
The demonstrator is a 240 mm-long, two-layer CCT sextupole with a 90 mm aperture, wound with insulated HTS ReBCO tape. Because ReBCO tape cannot tolerate hard-way bending, the conductor path, computed with the RAT software suite along a Frenet–Serret trajectory, keeps every bend within the tape’s limits as it winds around an aluminium former. Two commercial tapes, from Faraday Factory Japan and Shanghai Superconductor Technology, were qualified for the application.
It is designed to deliver a field gradient of 1000 T/m², with a peak field of 1.5 T on the conductor, at a modest operating current of 260 A and a temperature of 40 K. At these conditions the magnet’s critical current fraction is only 23 %, and has a temperature margin of roughly 27 K, with an inductance of 12.6 mH.
After winding, the coil was impregnated with paraffin wax to provide mechanical support and thermal contact, and the ten tapes were joined in nine soldered splices.
The cold test at PSI
The cold test was carried out at PSI on a cryogen-free stand using a 4 K cryocooler that needs no liquid helium. The magnet was held at a stable 46.5 K for the measurements. Although designed for 260 A, it was ramped to 300 A and held there for ten minutes, demonstrating stable operation beyond its nominal conditions in both transport current and temperature.
An array of six Hall probes was used to verify the field, which agreed with the simulations, validating the design and modelling approach.
“Powering-up the first CCT sextupole was an important milestone for the project. The cryogen-free stand lets us characterise the magnet across its full temperature range," said Michal Duda, responsible for magnet testing at PSI.
Next, the team will quantify the temperature margin and carry out detailed field-quality measurements. The full results are reported in the IPAC’26 proceedings contribution.
Work is already under way on the next, more demanding HTS magnets: a final-focus quadrupole and a crabbing sextupole at CERN, and a nested quadrupole-sextupole arrangement with innovative partial insulated technology at PSI.