One of the keys to pushing the energy limits of accelerators is being able to reach higher magnetic fields. CERN and several other laboratories around the world have launched R&D programmes aimed at improving existing magnet technology. In February, a demonstrator magnet using superconducting niobium-tin, cooled to 1.9 kelvins, achieved a peak magnetic field of 16.5 tesla on the conductor, exceeding the previous record of 16.2 tesla in 2015.
The demonstrator, known as an enhanced Racetrack Model Coil (eRMC) magnet, consists of two superimposed flat coils in the shape of a racetrack, hence its name. The coils are produced using a cable composed of multifilament composite wire made of niobium-tin, a superconductor that can reach higher magnetic fields than the niobium-titanium superconductor currently used for the magnets of the Large Hadron Collider (LHC). The dipole magnets in the LHC operate at a nominal field of 8.3 tesla.
Niobium-tin is the material being used for some of the new magnets in the High-Luminosity LHC, the successor to the LHC, which will make use of dipole and quadrupole magnets generating a magnetic field of around 12 tesla. This increase is already significant in comparison with what can be achieved with niobium-titanium, but niobium-tin will allow even higher magnetic fields to be produced. This potential is now being explored further, notably as part of the Future Circular Collider (FCC) study. To reach a collision energy of 100 TeV using a ring with a circumference of 100 km, dipole magnets generating magnetic fields of 16 tesla are needed.
Even though the eRMC demonstrator isn’t an accelerator magnet, its configuration allows the performance of niobium-tin conductors to be tested. During the tests, the eRMC magnet, cooled to 1.9 kelvins (the LHC’s operating temperature), reached a peak magnetic field on the conductor of 16.5 tesla. At 4.5 kelvins, this field peaked at 16.3 tesla, which corresponds to 98% of the maximum estimated performance of the superconducting cable.
“These results and recent advances with niobium-tin magnets demonstrate the potential of this technology for a next-generation hadron collider,” emphasises Luca Bottura, leader of the Magnets, Superconductors and Cryostats (TE-MSC) group at CERN. This record is just one of many promising advances at several laboratories. Another magnet, FRESCA2, which has a 100 mm aperture, reached a magnetic field of 14.6 tesla in 2018 at CERN. FRESCA2 was developed for integration into a test station for superconducting cables. Last year, Fermilab in the United States tested an accelerator-type short model dipole magnet, with a 60 mm aperture, which reached a field of 14.1 tesla at 4.5 kelvins.
The CERN teams will continue their work to develop an accelerator magnet configuration. The eRMC demonstrator will therefore be dismantled and reassembled with a third coil on the median plane to create a 50 mm cavity.