View of the High-Luminosity LHC test stand during the installation of the separation and recombination dipole magnet. The metal line is the cryogenic line that will supply the superfluid helium needed to cool the superconducting magnets, and above it is the superconducting electrical transmission line. (Image: CERN)
The magnets that direct and focus the beams just before the collision point are one of the jewels of the High-Luminosity LHC (HL-LHC), the major upgrade of the LHC. The first magnets will be tested on a unique test stand at CERN, known as the IT (Inner Triplet) String. This test facility, located in a building on the surface, is an exact replica of the machine segments that will be installed on both sides of the ATLAS and CMS experiments.
"The aim of the test stand is to check how these magnets behave collectively when integrated with their powering systems and their innovative cryogenic cooling, protection and alignment systems and to test the installation procedures,", explains Marta Bajko, head of the IT String project.
After the impressive installation of the power supply system in September, two magnet assemblies have recently been installed in what was a complex and delicate operation (watch the video below). The first of these assemblies consists of a superconducting quadrupole magnet made at CERN and a correction magnet. The quadrupole magnet is one of the inner triplets that will squeeze the particles together more tightly to increase the luminosity of the accelerator. It's one of the new generation of magnets that are made of the superconductor niobium-tin instead of the niobium-titanium used for the LHC's current magnets. These new magnets generate more intense magnetic fields of 11.3 tesla, compared with 8.6 tesla, which allows them to better focus the beams. It took many years to develop them, with their winding and the use of the superconductor posing particular challenges.
"Developing magnets that generate very high magnetic fields is one of the major challenges for the accelerators of the future. Using quadrupole magnets with niobium-tin coils in the HL-LHC for the first time marks an important milestone", explains Susana Izquierdo Bermúdez, who is in charge of the construction of the HL-LHC superconducting magnets.
The cryostat of this quadrupole magnet also contains a correction magnet that was manufactured as part of a collaboration between CERN and CIEMAT and CDTI in Spain. "This correction magnet has a novel mechanical structure. It corrects the trajectory of the particle beams by generating a magnetic field of up to 4.1 tesla," explains Juan Carlos Perez, the CERN engineer in charge of the project.
The second cryostat installed on the test stand contains a dipole magnet known as a separation and recombination magnet. This magnet directs the beams on each side of the experiments to make them collide and then separate. Made of niobium-titanium like the dipole magnets in the LHC, it generates a field of 5.6 tesla. It was manufactured and tested at KEK in Japan.
In early 2025, an assembly of two quadrupole magnets that have been made in the United States is due to arrive at the test stand, which will be fully assembled by the summer The next step will be quality tests and cryogenic cooling to 1.9 K by the end of the year. 2026 will be a key year as the entire system will be tested in conditions equivalent to those of the tunnel.
