CERN Accelerator Chain Prepares for High-Luminosity Operations

Simulated image of the protons in an LHC bunch (left) and an HL-LHC bunch (right). (Image: CERN)

CERN’s network of particle accelerators — the engines that feed the mighty Large Hadron Collider — has reached a new milestone on its journey toward unprecedented luminosity. After years of upgrades and precision testing, the accelerator chain is now capable of delivering the most intense particle beams ever produced at CERN, marking a major step toward the High-Luminosity LHC era that promises deeper insights into the building blocks of the universe.

Record-Setting Beams Light the Way

Last week, the LHC achieved a remarkable feat: particle bunches containing over 230 billion protons (2.3 × 10¹¹) — around 40% more than standard bunches. When these high-intensity beams collided inside the detector chambers, they generated an average of 150 particle collisions per crossing, more than double the usual 65.

The multi-hour test runs simulated conditions similar to those expected once the HL-LHC begins operations, offering invaluable data on how long these dense beams can remain stable in collision mode.

Preparing for the High-Luminosity Era

Set to begin operation in 2030, the HL-LHC will dramatically increase the number of collisions within the accelerator, opening new frontiers for particle physics. Achieving this goal requires not only new focusing magnets — to tightly concentrate beams before they cross inside the ATLAS and CMS experiments — but also a complete upgrade of CERN’s accelerator complex.

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The Injector Chain: Revving Up the Protons

Before protons ever enter the LHC’s 27-kilometre ring, they journey through a succession of four accelerators known collectively as the injector chain. Over the past decade, this chain underwent an extensive modernization under the LHC Injectors Upgrade (LIU) project.

“Our objective was to double the number of protons in each bunch while keeping them tightly focused,” explained Giovanni Rumolo, accelerator physicist and former deputy leader of the LIU project.

This sweeping upgrade — completed in 2021 after ten years of effort — included the commissioning of Linac 4, a new linear accelerator that forms the first link in the chain, as well as major renovations to all subsequent injectors and their supporting infrastructure.

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Fine-Tuning the Beams

Once construction was complete, experts dedicated four more years to fine-tuning beam performance. In May 2025, the injector chain successfully produced a beam with the target specifications: 230 billion protons per bunch and a cross-sectional width (emittance) of 1.95 micrometres.

Last week’s milestone saw this powerful beam injected into the LHC, accelerated to 6.8 TeV, and brought into collision — a crucial demonstration of readiness for the next phase.

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From Testing to Transformation

During the recent tests, only 600 bunches circulated in the LHC — well below the roughly 2,500 used in standard operations — to ensure safe testing conditions. The HL-LHC, in contrast, will eventually handle over 2,700 bunches, with new, high-performance focusing magnets compressing the beams to achieve even greater luminosity.

“The next step is to confirm that these beams can be produced consistently and stably,” said Rumolo. “Beam tests will continue until summer 2026, leading into the long shutdown when new high-luminosity equipment will be installed.”

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As intensity rises and precision grows, CERN’s accelerator chain is proving ready to deliver — powering the discoveries that will define the next decade of particle physics.