An important step in the high-luminosity upgrade of CERN’s Large Hadron Collider (LHC) has been achieved this year with the successful modernisation of the chain of injector accelerators. In May, a proton beam was extracted from the Super Proton Synchrotron (SPS) – the final injector before the LHC – with the target intensity and emittance for the upgrade.
The revamped injector complex means that beam intensities can be provided at the required emittance (beam size) for sustaining the luminosities of the High-Luminosity LHC for a sufficiently long period of time.
A major factor in ensuring this beam lifetime is the number of protons that can be provided to the LHC, as beam lifetime is directly proportional to the number of protons provided. The upgraded injector complex tackles this and will provide enough protons at an acceptable beam size to allow the increased luminosity, which is steered mainly by the innovative quadrupoles and crab cavity powering in the HL-LHC upgrade.
The project to upgrade the series of injectors, which starts at a linear accelerator and includes the Proton Synchrotron (PS) and the SPS, is part of a series of improvements to the LHC known as the High-Luminosity LHC (HL-LHC) project.
The HL-LHC project aims to crank up the current collider’s performance by increasing the integrated luminosity by a factor of 10 beyond the original design. Alongside the modernisation of the injectors, there is also work being undertaken to upgrade the magnets, install new crab cavities and introduce a new novel superconducting system, among other updates.
Modernising the injectors
The project to modernise the injectors to CERN’s LHC, called the LHC Injectors Upgrade (LIU), has been several years in the making.
The goal has been to create much more intense and higher quality proton beams. In other words, to increase the number of protons in every package (or “bunch”), while reducing their dispersion, making the beam more precise. This allows for a considerable increase in the number of useful collisions, which in turn means more data and more chances of new physics discoveries.
The LIU is also important for the LHC ion programme, when heavy ions are collided instead of proton beams, as upgrades to the accelerators Linac3 and LEIR aim to increase luminosity of lead ions.
The main objective of the LIU consisted of extracting a beam from the SPS composed of four trains or 72 bunches spaced 25 ns apart, each one containing 2.3x1011 protons per bunch (p/b), with a normalised transverse emittance (measure of the size of the beam) of 2.1 μm, explained Giovanni Rumolo, joint leader of the LIU project.
The work to upgrade the injector chain essentially started during the previous long shutdown of the LHC at CERN that lasted between 2019 - 2022. CERN periodically carries out these shutdowns to allow maintenance and upgrades of the LHC. During this time, the new Linac4 was installed. This linear accelerator serves as the source of proton beams that are eventually injected into the LHC.
Following that, work got underway along the rest of the injector chain. In 2023, Malika Meddahi, head of the LIU project, announced that the LIU beam was successfully working up until extraction from the PS – the accelerator ring that comes before the SPS in the chain.
Then, in spring 2024, the LIU beam was extracted from the SPS for the first time, although the emittance was still slightly higher than targeted. Work continued and in May 2025, the target was achieved: the beam produced at SPS extraction had the expected intensity and emittance of 2.3x1011 p/b and 1.95 μm.
The next milestone is the LIU Reliability Run, planned for 2026. During many months, the injectors will function daily with the LIU beam for 2 hours, in order to prove that the beam can be produced in a stable and repeatable manner, ready to fill the LHC during the HL-LHC era.