CERN’s High-Luminosity Large Hadron Collider (HL-LHC) project, which aims to increase the performance of the LHC to raise the potential for discoveries after 2030, has reached some major milestones in 2024.

The project aims to increase the integrated luminosity in the LHC by a factor of 10 beyond the original design, meaning that there will be a drastic increase in the number of particle collisions, providing scientists with more data and more chances of understanding the properties of particles present at the beginning of the universe.

In this interview, HL-LHC project leader Oliver Brüning looks at some of the major technological progress achieved in 2024 and ahead to the coming steps and challenges expected in 2025.

Looking back on the past year, what are some of the major achievements for the project?

Series production well underway

“We really are well into series production and that’s fantastic! For many components we are already past the midpoint for their series production. For example, our American collaboration has produced over half of their magnets, and the Japanese collaboration has produced three series magnets out of their five total, and they've produced all coils. We [CERN] have also produced over half of our quadrupole magnets, and the fifth cryoassembly (with eight required for installation in the LHC) has been validated and tested at CERN.

“And for each required type of HL magnet, at least one series item is already validated, which is a really important step. So, we really can see the end of the production process which is very encouraging and very positive!”

Cold Powering System

The Cold Powering System, consisting of the superconducting link (SC-link) made of novel superconducting materials (magnesium diboride) and complex feed boxes DF and DFH, has also received a lot of attention this year. This novel superconducting system has undergone several milestones in its development and delivery in 2024.

“We have not only validated the cable and the overall principle – as we've done in previous steps – but now also the complete system with feed boxes. It's an absolutely brilliant milestone, and the prototype system has now already been moved to the IT string (figure 2) which will take centre-stage next year.”

IT String

The IT String test stand will serve as a full system validation of a complete side of one of the four HL-LHC insertions on each side of the experimental points ATLAS and CMS.

It will consist of the first 130-150 metres from the interaction point, so all the new focusing quadrupole magnets (the inner triplet consisting of the Q1, Q2a and Q2b and Q3 magnets), the D1 separation dipole from Japan, the corrector packages, the connection cryostat including the diode box, the feed boxes, superconducting link, quench protection system, the power converters, full remote alignment system; everything in one system installed in the back of the SM18 test facility.

“This year has seen significant progress with the IT String (figure 1), with power converters, CLIQ racks, water-cooled busbars, and major cabling infrastructure all installed in the first half of the year, as well as the full commissioning of the cryogenic distribution system for the IT string just before summer. In the second half of the year, we saw the installation of the prototype cold powering system into the IT String, followed by the installation of the first two magnets to make up the String, Q2a produced at CERN and D1 produced by the Japanese Collaboration (figure 3).”

What would you say are the main challenges still to face?

“With such a large and complex project, that is using innovative technologies, many of which have never been used in an accelerator, there are naturally challenges that the experts in each aspect of the project must overcome.”

Crab cavities

“The main challenges we are currently facing are with the crab cavities; I think we are just learning that these are extremely complex radiofrequency devices which are not easy to pass over to industrial production. We currently see many what we call ‘nonconformities’, meaning that the pieces and final products produced in industry deviate from what we wished for. We're well into this year's production, but there is still a long list of nonconformities coming up. For part of the production, we also find that we run out of schedule, as several important components have suffered from additional delays.

“The challenge for us right now and into next year is to identify which nonconformities are critical and which ones we can accept, and at what pace we can then continue series production. We are also coming up with mitigation plans we can put in place to tackle any scheduling issues, and how we can make sure that we still have a complete and operable system for the start of the High-Luminosity era of the machine.”

Schedule

“The other major challenge we are facing now, and will continue to tackle into the new year, is staying on schedule. During the Long Shutdown 3 (LS3) Readiness Review in September we assessed to what extent we would be ready for LS3, which will last between July 2026 and June 2030 . It was clearly observed that the project would be ready for a LS3 starting in November 2025, but the time for the HL-LHC works within the shutdown period currently don't fit in the allotted time for the work. The current length of time given is 47 months, and the current HL-LHC schedule requires 51.5 months, so about four and a half months too long. We are working hard on reducing this.

“The main conclusion of the Cost & Schedule Review in November was, if I can summarise it in one statement, that the project really has moved from budget and technical risks to just schedule risk.

“This is positive for the budget and technical parts, and I think also natural for a project in this phase of its life. We've done the technical demonstrations, we've finished the research and development programme, and as we approach the actual installation dates, the schedule becomes more and more the focal point. And the closer you get to the actual installation work, the less movement you have timewise until you reach the actual date, when you just have to be ready!”

Looking forward to 2025, what can we expect for the next year with the Project, and what are you most looking forward to coming to fruition?

Series production

“Now that we are in full swing with series production, next year I would like to see a number of different items really come to the second half of their production. This is particularly true for the magnets from our American collaboration. For their contribution, they combine two magnets to form one cryoassembly. They have produced over half of the magnets, but currently we have just one full cryoassembly delivered to CERN (figure 3). So, I am really looking forward to getting the second one from them, so that we can get a better feeling and more confidence about how they handle the magnet system.”

Dynamic and evolving collaborations

The is a hugely collaborative project that requires and relies on successful collaborations and the dedication of all parties for its success.

“The project is a continuous effort to look for potential contributions from collaborators until the very end of the project.

“We are still discussing options with Japan on how they can contribute; they just added major contributions last year with the powering system for the crab cavities, which is a big step for us and was a very successful negotiation. They still want to contribute also to personnel, so we are discussing that now. And then we have the discussions with

Brazil, Greece and Poland, mainly for manpower, and with Pakistan looking at whether their laboratories in their own countries could contribute with manpower during the whole commissioning and installation phase.

“So different collaborative avenues are still being pursued as the project evolves, and collaborations are not yet done and dusted. It’s a very dynamic process.”

IT String

“We already have the D1 and Q2a installed into the IT String and coming at the beginning of 2025 will be the second CERN prototype magnet, the Q2b. It is already being made ready for installation, so that will come next. The Q3 magnet (figure 4) will be the first of the American magnet assemblies to be installed, and it is also already at CERN and being prepared for the installation. And then that just leaves the second American magnet, the Q1, the last of the triplet magnets to be installed into the String, and this we expect to come to CERN in early January next year.

“The corrector package is about to finish testing at CERN, and once that test is completed that will also move over to the IT string next to the D1. And then we have essentially everything; we will just need to finish the DFX feed box for the IT string for the connection of the superconducting link with the magnet system.

“All of this should be done by the second quarter of next year, and the cooldown should start hopefully in the second half of next year ready for operating the IT String. It will really allow us to study the behaviour of the new components we build for HiLumi as a system, and just in general to check all the interfaces on the surface before we go down into the tunnels. It’s also a good training ground for our engineers for the installation work in the tunnel, because they see how the pieces fit together, what kind of works are challenging, what to watch out for, and how to optimise some of the procedures.

“With the IT string we will validate and learn a lot for the commissioning of the actual machine, so it’s a very valuable exercise. I think that will be phenomenal.”