A major milestone of CERN’s high luminosity upgrade of its Large Hadron Collider, a project known as HL-LHC, is the construction of new over-ground test set-up known as the Inner Triplet String (HL-LHC IT String).

A lot of progress has been made recently on this test stand, which is approximately 95 metres long, and it is expected to be ready for testing under nominal operating conditions by late summer this year.

In this Q&A, Nicolás Heredia García, a project engineer at CERN working on the IT String, outlines the development of the system, why it is vital for HL-LHC, and the timeframe for its readiness.

What is the HL-LHC IT String, and why is it important?

The IT String is a test stand that represents a key intermediate milestone of the HL-LHC project. It is an above-ground replica of the new inner triplet (IT) region.  Four of these regions will be installed in the HL-LHC; one on each side of both the ATLAS and CMS experiments.

It provides a unique opportunity to test the combined performance of critical components, and serves as a tangible objective for all contributing teams to produce their systems and components, and also train for installation and operation of the underground versions of the region in the tunnel. While each element undergoes rigorous quality checks during manufacturing and individual testing, only a test stand like the IT String allows for the validation of their collective behaviour under conditions very similar to those of the future HL-LHC.

Has something similar been done before?

Yes. This test stand is inspired by previous IT String test stands, String 1 and String 2, conducted in 1994 and 2001 at CERN for the LHC. These tests confirmed the key technical choices for the LHC magnets and demonstrated their reliable operation under accelerator-like conditions. They also helped validate LHC operating procedures and better prepared teams for subsequent operations. The valuable feedback obtained from these test stands strongly influenced the approach taken by the project about the construction of a new IT String.

What are the key objectives of the IT String this time?

Many of the technologies for the HL-LHC have never been used in an accelerator before, like when the LHC was first developed, essentially creating a new machine. Assembling and testing them together in the IT String will provide insights into their collective behavior and how to manage them in the future HL-LHC.

Another key objective is to clarify the scope and responsibilities of different teams, particularly around equipment interfaces, which are common sources of issues. With 10 different work packages (WPs) – i.e. different teams working on their specific inputs for the HL-LHC – contributing to the IT String, managing these numerous interfaces effectively is essential.

Additionally, the IT String plays an important role in refining installation and commissioning procedures before their implementation in the HL-LHC, reducing risks and improving efficiency. The HL-LHC will require the installation and commissioning of four IT zones — essentially four IT Strings — so efficiency is crucial to ensuring the project is completed on time and successfully.

Finally, training the teams is a top priority. The IT String provides the most realistic mock-up available before teams move into the tunnel for the actual HL-LHC installation. The location of the IT String in a surface building allows teams to gain hands-on experience in a controlled, yet authentic, environment.

What are the main components of the HL-LHC IT String?

The magnet chain of the IT String comprises six quadrupole magnets made of niobium–tin (Nb₃Sn) and orbit correctors made of niobium–titanium (NbTi), grouped into four cryogenic assemblies (Q1, Q2a, Q2b, and Q3). It also includes the corrector package (CP) with additional corrector magnets and a long orbit corrector to fine-tune beam trajectories. And finally, the separation and recombination dipole magnet (D1) for transitioning beam particles from one to two beam pipes and vice versa. These magnets are operated in liquid helium at 1.9 K (-271.25°C) to maintain superconductivity.

The cold powering system is composed of a 75-metre-long superconducting power link made of magnesium diboride (MgB₂), a cryogenic distribution box (DFHX) with high-temperature superconducting leads to efficiently transfer electrical current from room to cryogenic temperatures, and a cryogenic distribution box (DFX) to connect this system with the magnet chain.

The warm powering system includes twenty power converters (from 120 A to 18 kA DC), water cooled busbars and cables to transmit high currents and circuit disconnector boxes that physically isolate the power converters from the rest of the circuit, for maintenance purposes.

Magnet protection elements include energy extraction systems, Coupling-Loss-Induced-Quench (CLIQ) devices, Quench Heater Discharge Power Supplies (DQHDS), and the quench detection system, specially tuned for the new technology of Nb₃Sn magnets.

A cryogenic system keeps the magnets and superconducting link cold using liquid helium. It includes a cryogenic distribution line (SQXL) which supplies helium to the magnets and a proximity cryogenic distribution system (PCDS) which connects the SQXL to the building’s cooling infrastructure.

Additionally, the IT String integrates a novel remote alignment system (FRAS), which ensures precise positioning of components without requiring human intervention close to the magnets.

Now that we’ve covered the basics and reasons of the IT String, what major milestones have been achieved?

Since the last update in Accelerating News in May 2024, significant progress has been made on the IT String. The installation of the cold powering system was a major milestone, requiring impressive maneuvers to lift the DFHX and the Superconducting Link (SC link) onto the platform. Following this, further work on the cold powering system took place, including the assembly of the DFX cold feedbox and the insertion of the SC Link into it. A video of this installation process is available on the HL-LHC YouTube channel.

With the cold powering system in place, the installation of IT String magnets followed. The first magnet installed was Q2a, followed by D1, then Q2b, and finally the CP. The transport and installation of the D1 magnet was nicely followed up in this video.

After the installation of the cold powering system, Electrical Quality Assurance (ElQA) and vacuum tests were conducted, confirming that everything was functioning correctly. Once the magnets were installed, they were also subjected to specific ElQA testing to ensure their performance.

Which challenges have been encountered?

The installation of the cold powering system was one of the most complex challenges, requiring a careful procedure, multiple trials, and specialised auxiliary equipment. The operation involved the coordinated use of two overhead cranes to lift the system onto the platform, followed by the precise unrolling of the SC Link into a wave shape. The final step required bending the last section vertically to align it for connection with the rest of the system.

Another challenge lay in the interconnections between the magnets and the already-commissioned cryogenic line. This line connects to several magnets to supply the liquid helium coolant via the so-called ‘jumpers’. During this process, several issues arose, in terms of misalignments, out-of-tolerance components, and other unforeseen non-conformities.

Finally, safety management continues to be a top priority; multiple tasks happening simultaneously requires strict procedures, careful coordination through meetings and access control, and continuous presence of the coordination team onsite.

What is in the pipeline right now?

Upon each magnet arrival, the alignment system is being installed, as well as anchors that fix the magnets firmly to the floor. Furthermore, individual electrical tests are being conducted on each magnet. Electronic systems for magnet protection are being positioned, connected and tested in the racks area.

The Q3 cryoassembly is expected to be in place by April, followed by Q1 in May. Once Q3 is installed, an extensive interconnection campaign will begin, linking all magnets together with the so-called ‘N lines’, and connecting the magnet line to the cold powering system to close the electrical circuits. Additionally, the helium circuits from the cryogenic line to the magnets and cold powering system will also be completed. During the interconnection phase, rigorous quality checks will be conducted, including leak tests and electrical tests, to verify the integrity of the connections.

Meanwhile, the development of control systems and software packages is in an advanced stage, benefiting from extensive collaboration across CERN, involving three departments and nine groups. The coordination team is launching tests in short-circuit mode and verifying system performance to provide feedback to developers. The test programme has been fully integrated into a sequencer, with 90% of the testing procedures finalised and approved.

When will the operation of the test stand begin?

The test programme is already underway. Individual system tests and short-circuit tests have been successfully performed, and transfer function measurements of some magnets are ongoing. Electrical and leak-tightness tests have also already been completed for some equipment.

The IT String is expected to be ready for testing under nominal operating conditions by late summer 2025. The first powering test of the full circuits is scheduled for the end of the year.

Hardware commissioning will continue into next year, followed by a thermal cycle, and a second round of powering tests. During this second phase, additional string-specific tests will be carried out before the final warmup in 2027.

This is an exciting time for the HL-LHC Project as we look forward to the Long Shutdown 3 in 2026, with the completion and operation of this unique test stand forming a major part of this preparation.