Figure 1. Alignment tests being performed on the TCLPX pre-series collimator for HiLumi LHC. Photo: Florence Thompson / CERN
By Emil Kaijanen, Francois-Xavier Nuiry, Stefano Redaelli and Florence Thompson (CERN)
Teams at CERN are working hard to perform essential tests on three pre-series collimators to validate their new designs and give the final green light for the full series production for the High-Luminosity LHC (HiLumi LHC). The pre-series includes a more-traditional single-beam collimator and two new X-series collimators which feature a double-beam design, new for the HiLumi LHC.
This comes after design, prototyping and validation efforts over the past 10 years, and represents a strong collaborative effort between HiLumi Work Package 5 (which deals with beam collimation for the project), several CERN groups and various companies in Europe.
Following manufacture of a large share of the components and the assembly of the collimator bodies by an Italian contractor, and delivery to CERN at the end of 2025, teams at CERN have equipped and installed the collimator bodies onto their supports and performed numerous Site Acceptance Tests (SAT). Once these SAT activities have been completed, the final go-ahead can be given for the remaining 33 series collimators, making this a crucial step for the HiLumi LHC and the first key milestone of production for the collimators.
What are collimators and what is special about these collimators for HiLumi LHC?
Collimators are devices that clean the beam of unwanted stray particles like the beam halo (a cloud of particles that stray from the beam) which needs to be safely disposed of in order to avoid losses that, amongst other negative effects, could otherwise quench superconducting magnets. To do this, two jaws sit symmetrically around the circulating beam and are moved closer or further away from the beam axis to intercept the unwanted particles. During operation, to remove the heat coming from the particles, the jaws are actively cooled by water circulating within the jaw.
The pre-series includes a single-beam collimator, TCSPM (low-impedence secondary collimator) for the IR7 main collimation section in the accelerator. It is based on a traditional design of collimators used in the LHC, where one beam passes through the collimator to be cleaned and the second beam passes through a standalone drift tube separate to the collimator.
The new X-series collimators – TCLPX and TCTPXH (physics-debris and tertiary collimators respectively) – are new designs for the HiLumi LHC which feature a common vacuum tank for both opposing beams; the collimated beam and the passthrough, non-collimated, second one. These double-beam collimators will be situated close to the collision points at ATLAS and CMS, where the two beams are very close to each other. The collimators here will be crucial to protect the new inner triplets and the experiments from incoming-beam losses (the role of tertiary collimators), and to efficiently catch the luminosity-debris losses emerging from the collision point (the role of the physics-debris collimators).
Depending on the collimator’s function, jaws are made of different materials; copper-coated graphite for the TCSPM single-beam collimator and heavy tungsten alloy for the TCTPXV and TCLPX double-beam collimators.
Figure 2. Aligment tests of TCLPX double-beam collimator (top), electrical cabling work of TCTPXV double-beam collimator (bottom left), and TCSPM single-beam collimator in the clean room (bottom right). Photos: Florence Thompson / CERN
What does the testing campaign involve, and what’s next?
The SAT campaign is fully underway, with plenty of activities already completed, including visual inspection of welds, hydroformed bellows and edge welded bellows, beam position monitor (BPM) tests, jaw mechanical stop measurements, jaw motor torque tests, impedance tests, coolant hose connection and leak tests, assembly on supports, installation of vacuum connection tools, electrical wiring and continuity tests, and survey and adjustment of the positioning of the collimator. Many teams from across the CERN ATS Sector have been involved in these activities.
The next step for the pre-series collimators will be an ultra high vacuum test to ensure the conformity of these devices against the challenging vacuum conditions required by the beams, and final checks of the full electrical system – including a jaw motor torque test, a BPM test and an impedence test.
They will then be stored ready for installation during Long Shutdown 3 (when the accelerator will be shut down for installation of the new machine) and the team will be ready for delivery and SAT activities of the series production collimators, due to start being delivered later this year.
Figure 3. Some people from the many teams working on the HiLumi LHC collimators in the clean room with the TCTPXV double-beam collimator. Photo: Florence Thompson / CERN