Accelerator Report: HiLumi LHC beam reliability runs pave the way to the future

After the major upgrades carried out during Long Shutdown 2 (LS2, 2019-2020), the LHC injector complex entered a new phase of operation. The LHC Injectors Upgrade (LIU) project consolidated and enhanced the accelerator chain to meet the demanding beam requirements of the High-Luminosity LHC (HiLumi LHC), scheduled to come into operation after Long Shutdown 3 (LS3, 2026-2030).

The LIU objective was clear: significantly increase the beam brightness and almost double the beam intensity delivered to the LHC. Each of the two LHC beams consists of more than 2000 tightly packed proton bunches that are spaced by just 25 nanoseconds, structured by the 40-MHz LHC radiofrequency system. After LS3, denser bunches will produce a substantially higher number of particle collisions in the LHC, opening the door to more precise measurements of the Higgs boson and rare processes and potentially revealing signs of new physics.

In the period between LS2 and LS3, efforts in the injector complex have focused on demonstrating that the upgraded machines could achieve the demanding LIU beam parameters. With this milestone now reached, attention has shifted towards ensuring reliable, stable and sustainable delivery of high-quality beams. This is a crucial step to guarantee that the HiLumi LHC can operate at peak performance from the very start of physics running, planned for 2030.

To this end, dedicated HiLumi LHC beam reliability runs have been introduced in the injector schedule. During selected weeks in 2026, short periods of beam time - typically around 30 minutes following each LHC fill - are reserved to simulate HiLumi LHC-type filling schemes with the new beam parameters. These runs are designed to test not only performance but also the robustness and reproducibility of operation and technical systems.

HiLumi LHC beam reliability runs already took place successfully in parallel operation last year in the machines of the Proton Synchrotron complex (Linac4, PSB and PS), and the first such reliability run in the Super Proton Synchrotron (SPS) was successfully carried out last week. Operating mainly during daytime on weekdays, and carefully scheduled around LHC operation and machine development periods, the SPS performed eight injection attempts, six of which reached flat-top energy. Typically, during these runs, 15 to 20 injections were accumulated, all meeting the beam quality criteria required for transfer to the LHC.

As expected for such high-intensity beams, stability posed some initial challenges. Special beam adjustments were required at the start of each run to maintain stable conditions. Continuous optimisation of the SPS cycle - including improved energy matching, orbit corrections, fine-tuning of local bumps and commissioning of the beam scraper - resulted in a clear improvement in performance over the course of the week, as illustrated inthe figure below. Throughout the run, vacuum conditions remained comfortably within operational limits. Thus, by the end of the run, reaching flat top had become significantly more routine.

By the conclusion of last week's run, the SPS was routinely delivering beams at full HiLumi LHC nominal parameters: bunch intensities of 2.3×10¹¹ protons, transverse emittances of 2.1 micrometres and bunch lengths of 1.65 nanoseconds.

Following a short interruption this week to give priority to HiRadMat operations, the reliability programme will resume with a three-week period of more intensive running. This next phase will extend operation into nights and weekends, further testing the endurance of the injector complex under realistic conditions. The goal is clear: to build on the strong performance achieved so far and establish the level of reliability required for the HiLumi LHC era.