Figure 1: Photo of some of the many people involved in the IT String, to celebrate the successful cooldown of the system. Florence Thompson / CERN
By Aleksandra Onufrena and Antonio Perin on behalf of Cryogenic Commissioning team (CERN)
The High-Luminosity LHC (HiLumi LHC) IT String’s cryogenic system has been successfully validated during the IT String’s cooldown and cryogenic commissioning campaign. This confirmed the system’s performance and stability for the next IT String operating stages, including powering which has recently begun. Moreover, numerous aspects of the magnet cooling were validated during the test campaign, strengthening confidence in future cryogenic operation of the new niobium-tin (Nb₃Sn) magnets and the magnesium diboride (MgB2) cold powering system in the HiLumi LHC machine.
The HiLumi LHC IT String serves as a full-scale prototype of the final focusing region of the HiLumi LHC, bringing together magnets, power converters, cold and warm powering lines, protection systems, energy extraction elements, vacuum system, cryogenic distribution lines and their associated control systems in a configuration as close as possible to the HiLumi LHC. The test stand includes the first new quadrupoles (Q1, Q2a, Q2b and Q3) built with the niobium-tin (Nb3Sn) technology, the corrector package (CP), and the first dipole (D1). It also incorporates the new cold powering system with a magnesium diboride (MgB₂) superconducting link (SC-Link), as well as the quench protection system including the innovative CLIQ system for the first time. By testing the full system as an integrated assembly, any issues related to design can be identified early and the system performance can be validated prior to the HiLumi LHC installation underground.
A key milestone in the preparation of the IT String operations is its cooldown and cryogenic commissioning. During this phase, the system was first brought from room temperature to its operational conditions, both in the magnets and cold powering system. Once these conditions were reached, the cryogenic parameters were optimised, the system was characterised, and its various operation modes were validated.
Following a thorough preparation by the cryogenics team, cooldown of the magnet cold masses started on 23rd February 2026 and lasted approximately three weeks, with an average cooling rate of 1 kelvin (K) per hour, in line with expectations (see figure 2). Cooldown of the SC-Link (Figure 3) then began on 10th March 2026 and lasted several days. All specified operational constraints were respected, including limits on longitudinal temperature gradients in the Nb₃Sn magnets and MgB2 SC-Link, helium flow velocities, and maximum pressures.
Figure 2: Evolution of temperatures in the HiLumi LHC IT String magnets during the cooldown. CERN
Figure 3: Evolution of temperatures in the HiLumi LHC IT String cold powering system during its cooldown. CERN
After cooldown, nominal operating conditions were achieved for the first time: the magnets were successfully cooled to 1.9 K in superfluid helium (Helium II) while the SC-Link reached a nominal temperature of 4.5 K at its inlet, both meeting the “cryo-start” conditions required before magnet powering could begin.
The next commissioning objective was to validate the cryogenic performance during the magnet powering. During ramping of the current of the magnets, the cryogenic system must maintain stable temperatures in the cold masses while absorbing the magnetisation heat loads which are estimated to be at around 200 watts for the Nb3Sn magnets. This dynamic behaviour was successfully validated during dedicated tests with a controlled heat deposition. Results showed that the cryogenic system can maintain the Helium II conditions during all powering modes. Moreover, stable operation of the new bayonet heat exchangers, with a deposited heat load of several hundred watts, demonstrated the suitability of the cooling method foreseen for HiLumi LHC.
The IT String cryogenic distribution line (SQXL) is connected to the local cryogenic infrastructure of SM18, which, similarly to the HiLumi LHC, includes a cold compressor. Operation both with and without the cold compressor was tested with mass flows of up to 18 g/s, providing valuable insight into overall system performance.
Transient and fault scenarios were also successfully tested, with safe recovery demonstrated in the cases of pumping and cold box failures. A quench-like pressurisation event with helium discharge to the warm buffers was also tested. Static heat loads on the magnets and SC-Link were measured and estimated to be within the expected design values. Instrumentation was validated throughout the commissioning; temperature sensor calibration in Helium II within a millikelvin was confirmed, and the fast data acquisition system was tested.
Overall, the cryogenic commissioning demonstrated that the system is flexible, robust and capable of covering the full range of operating conditions required for the IT String validation program. The test program confirmed the system’s performance for the magnet powering phases and gave confidence in the future cryogenic operation of the magnets in the HiLumi LHC.
Figure 4: IT String control room after cooldown of the system. Florence Thompson / CERN
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