The HL-LHC IT String installation is entering its final phase in the SM18 building at CERN. All major components are now in place, following a series of installation and validation activities executed by the end of 2024 and beginning of 2025. These tasks have been carried out on schedule, within scope and following extensive preparations carried out beforehand. This is an important achievement given the number of teams working simultaneously on site, providing valuable lessons for the HL-LHC machine installations. The collaboration between teams has been crucial to safely deal with multiple co-activities onsite (figure 1).
Figure 1. Multiple intervening teams in the IT String. Nicolás Heredia García / CERN.
Installation of components
All the superconducting magnets of the Inner Triplet continuous cryostat– namely the Q1, Q2a, Q2b, Q3, CP and D1 – have been successfully installed in their final positions. Videos showcasing these installations can be found here. The installation process involved a wide variety of tools, including a ROCLA vehicle, an overhead crane, a tow tractor, and two transfer tables. In addition to the coordination team and the equipment owner, special acknowledgment must be given to the EN-HE transport team, who executed each manoeuvre in a very precise manner with magnets weighing from 10 to 18 tons.
The cold powering system composed of the distribution feed boxes (DFHX & DFX), as well as the Superconducting Link (SC-link) is in place. The last component to be installed was the DFX, which was assembled on-site in a carefully defined sequence. It began with the installation of the DFX support structure, followed by the vertical section, and concluded with the horizontal part. Particular care was taken to safely insert the SC-link into the DFX. Leak detection was an important part of the process to ensure the vacuum tightness of the complete system. The process also included the installation of the cryogenic line to supply helium to the DFX from the cryogenic distribution line (SQXL). On the other side of the SC-link, next to the DFHX, final arrangements were made, including the installation of dispatching boxes for the instrumentation signal and the installation of the helium Gas Management System (GMS) which recovers and contributes to the control of helium gas flow from the current leads. The electrical connection of the current leads to the warm powering side of the circuits using ultra flexible cables was also installed.
Figure 2 (left). Installation of Q2a magnet in the HL-LHC IT String. Marta Bajko / CERN. Figure 3 (right). Figure 3: View of the N-lines in an IT String interconnection. Melanie Arnold / CERN
Pulling of the bus bar bundles called N1 and N2 lines, was carefully performed along the full length of the magnet chain. Their function is to provide connectivity for the individual magnets throughout the various cryogenic assemblies, from D1 to Q1. Operators were stationed at each interconnection to monitor the progress as the lines were guided from one end of the chain to the other. To perform this operation, a motor was placed at one end of the magnet chain to pull the guiding wire with a force equivalent to approximately 400 kg.
The installation of the D1-to-DFX Connection Module (DCM), which ensures the electrical connections between the D1 and the DFX, was also successfully completed. It houses the lambda plate, a component that serves as barrier-separating regions of superfluid helium and normal liquid helium. Additionally, the DCM contains the bypass diode stack, important for the global circuit protection. The DCM was the last major component to be installed since it had to be placed after the N lines had been successfully pulled.
Once each magnet was in place, the installation of its dedicated Fully Remote Alignment System (FRAS) started. This included the installation of the wire positioning system, the hydraulic levelling system and other components serving to accurately measure the vertical, radial and longitudinal position of each magnet. The FRAS acquisition system has been already operational for a while to gather data. Vertical and radial actuators were also installed on the supporting jacks to allow fine adjustments of the magnets, as needed, based on measurements.
Other activities included the installation of quench heater power supplies (DQHDS) in the IT String racks area, and the installation of the water cooling plates for the busbars operating at room temperature and transmitting the current to the RQX.SF and RD1.SF circuits.
Figure 4 (left). DCM installed into the HL-LHC IT String. Samer Yammine / CERN. Figure 5 (right). Components of the FRAS system installed in the HL-LHC IT String. Nicolás Heredia García / CERN
Interconnection activities
The electrical and mechanical interconnection activities between magnets, the DCM and the DFX were performed for the first time in the IT String after type tests conducted in the workshops and on dedicated mock-ups. This effectively makes it a full-scale exercise for the machine.
First, the preparation of the jumpers took place. These connect three magnets (D1, CP and Q2b) to the cryogenic distribution line (SQXL). This required careful alignment, in the order of a fraction of a millimetre, to allow for proper connection at a later stage.
This was followed by splice preparation, soldering, and electrical insulation of each superconducting cable, ensuring reliable electrical connections. A splice is the process of joining two electrical cables together to create a continuous electrical path. In total, there were 7 interconnections between cryomagnets to be performed, and more than 65 splices were made.
With these steps completed, the closure of the lines began. More than 80 welds between the lines have been performed to date. Once the lines are fully closed, leak and pressure tests will be carried out to confirm the integrity and tightness of the cryogenic circuits.
Figure 6 (left). Splices at the interconnection DCM – D1. Nicolás Heredia García. Figure 7 (right). Lines welded at the interconnection Q2a – Q2b. Florence Thompson / CERN
Testing activities
Electrical Quality Assurance (ElQA) test campaigns have accompanied the installation and electrical interconnection between components.
For instance, these checks have been conducted on each of the magnets following their final installation in the IT String, and were also performed on the N lines after their pulling along the magnets to assess electrical integrity before starting the interconnection works.
After that, ElQA verifications were carried out at regular intervals to verify the electrical continuity, polarity and dielectric insulation during the electrical interconnection activities. The tests executed during this period required close collaboration between the teams. Before being electrically connected to the magnets, the SC-link also underwent an ElQA testing campaign to validate the electrical integrity in a warm gaseous helium environment, applying test voltages in the order of 1 kV.
Leak testing activities were carried out on both the SC-link and DFX to ensure vacuum tightness of the components. These operations involved several weeks of pumping the components and precise leak detection using specific ancillaries. Leak tests will also be performed on the interconnection welds to verify their integrity. After checking the welds and closing the outer envelope of the interconnections, the global leak tests can begin. This entailed checking the tightness of the entire system – from the insulation vacuum to ambient air, and from the helium lines to the insulation vacuum. The mandatory pressure test will also be performed.
Individual System Tests (ISTs) were conducted on the Coupling Loss Induced Quench (CLIQ) protection system, which is designed to act rapidly in the event of a quench, ensuring the safety of the magnets. ISTs were also performed on the DQHDS units using dedicated inductances to replicate the performance when connected to the magnet quench heater strip circuits.
Figure 8. ElQA test set up for Q1 magnet in the IT String. Antoine Kosmicki / CERN
Other important activities
While much of the attention has been focused on on-site activities, it is worth highlighting the significant work accomplished in control systems and software interfaces to ensure they are ready for IT String operation. These efforts required close collaboration across three departments and nine groups, from development teams to equipment owners and the IT String operation team.
Two out of three dry run tests have already been conducted to validate the functionalities of the new device types within the control and software applications. The IT String facility offers a unique opportunity to validate all control and software systems and the tools required for the HL-LHC hardware commissioning (HWC) and operation phases.
Another key milestone has been the completion of the commissioning procedures for components and electrical circuits, including the 200 A Corrector Circuits (RQSX3), 120 A High-Order Corrector Circuits, Interlock Tests for the HL-LHC IT String Circuits, the Separation Dipole Circuit (RD1), 2 kA Corrector Circuits (RCBX) and the Inner Triplet Circuit (RQX). 23 of these procedures have been drafted and are approved or currently undergoing approval.
Final comments
All the work and efforts done have paved the way for launching the String Validation Program (SVP) in early autumn 2025. The goal is to cool down the entire IT String at nominal cryogenic conditions and power all superconducting circuits at low current already by the end of the year.
The diverse set of activities carried out on the IT String clearly illustrates the complexity of the construction site, where multiple installation and commissioning tasks are executed in parallel. This coactivity, while executed safely, is essential to maintain progress in line with the project schedule. All steps undergone so far during the installation and commissioning phases have served as valuable hands-on training for the teams that will later work on the HL-LHC installation during Long Shutdown 3.