The Large Hadron Collider (LHC), the world's largest and most powerful particle accelerator, is also the largest single machine operating in the world today that uses superconductivity. The proton beams inside the LHC are bent and focused around the accelerator ring using superconducting electromagnets. These electromagnets are built from coils, made of niobium-titanium (Nb-Ti) cables, that have to operate at a temperature colder than that of outer space in order to be superconducting. This allows the current to flow without any resistance or loss of energy. The High-Luminosity LHC (HL-LHC), an upgrade of the LHC, will for the first time feature innovative electrical transfer lines known as the "Superconducting Links".
Recently, CERN's SM18 magnet test facility witnessed the successful integration of the first series of magnesium diboride superconducting cables into a novel, flexible cryostat. Together with high-temperature superconducting (HTS) magnesium diboride (MgB2) cables, they will form a unique superconducting transfer line to power the HL-LHC triplets. The triplets are the focusing magnets that focus the beam, right before collisions, to a diameter as narrow as 5 micrometres.
Colloquially known as the "python", the flexible, double-wall, corrugated cryostat comprises 19 MgB2 superconducting cables in a single assembly, twisted together to form a compact bundle. Each MgB2 cable is about 140 metres long, with the diameter of the bundle measuring about 90 mm. Together, these 19 superconducting cables can transfer a DC current of about 120 kA at 25 K (-248 °C) - a temperature higher than that at which conventional superconductors operate. In the LHC, niobium-titanium (Nb-Ti) and niobium-tin (Nb3Sn) cables are operated in superfluid helium at 1.9 K (‑271.3 °C) - a temperature colder than the 2.7 K (-270.5 °C) of outer space. The MgB2 cables of the Superconducting Link are cooled by a forced flow of helium gas. "The R&D done in the initial phase of the LHC project has made the ongoing production reliable and repeatable," says HL-LHC project leader Oliver Brüning.
This new type of superconducting transmission line also has potential outside accelerator technology. These lines can transfer vast amounts of current within a small diameter and could therefore be used to deliver electricity in big cities or to connect renewable energy sources to populated areas. Recently, CERN and Airbus UpNext signed a collaboration agreement to assess the use of superconducting transmission for future low-emission aeroplanes.
But the novelty of this superconducting material is not the only secret component for a sustainable, superconducting transfer line.
"One of the beauties of this new system is that the cryogenic operation of the Superconducting Link is done at zero cost because it transfers the helium gas that in any case is needed to cool the current leads," says Amalia Ballarino, the deputy leader of CERN's Magnets, Superconductors and Cryostat group. "So, the Superconducting Links act as both helium and electrical transfer line."
CERN's SM18 facility will continue to host the assembly and testing of the Superconducting Links - ten, in total, for the HL-LHC - until they are installed in the LHC tunnel during Long Shutdown 3, scheduled to start in 2026. The first HL-LHC Superconducting Link will come into operation this year, when it will be connected to the cryostat with the REBCO (rare-earth barium copper oxide) HTS current leads on one side and to the Nb-Ti connections on the other. Integrating these key new technologies (novel superconducting cables made of MgB2, long and low static heat load flexible cryostats, and REBCO HTS current leads) marks the beginning of a sustainable approach to electrical transmission for the future of CERN's accelerators, starting with the HL-LHC.
