Although most of the HL-LHC beam instrumentation work package is well into production and even integration, some of the more innovative designs are also taking advantage of the final beams in Run 3 to test and optimise their performance. In particular, an electro-optical beam position monitor (e-o BPM) and a Beam Gas Curtain (BGC) profile monitor have been extensively tested during 2024 operations and will continue in 2025 and 2026.
The e-o BPM gives a super-fast measurement of the beam position changing within a single bunch by “writing” the electrical signal from a beam pickup crystal onto a laser beam. This kind of fast diagnostic is essential for seeing the crabbing produced by the new HL crab cavities (CC). The instrument was developed by Royal Holloway, University of London (RHUL) as part of the successful HL-UK collaboration. It was installed in the SPS along with the CC prototypes and tested in 2024 (figure 1). Measurements demonstrated several key performance requirements including single shot sensitivity to bunch position, bandwidth and acquisition rate.
Figure 1 (left). Electro-optical BPM prototype installed in the SPS ring. CERN. Figure 2 (right). Beam Gas Curtain installed in the LHC ring. Maximilien Brice / CERN.
This is still very novel technology, and a prototype instrument will remain in the SPS for the remaining 2025 and 2026 runs, with upgrades and tests with beam whenever possible. A design review for this ‘Broadband Pickup’ requirement was held in January 2025, which acknowledged the remarkable long-term potential of this new technology. As key aspects will require additional time and validation tests to overcome the present shortcomings, the panel recommended to adopt conventional stripline BPMs as the baseline instrument for the HL era.
The other novel instrument under test for the HL-LHC is the BGC, also developed and manufactured as part of HL-UK by the Cockcroft Institute, Liverpool University and GSI. This is a ‘non-invasive’ beam profile monitor which works by projecting a thin curtain of gas across the beam and then directly viewing the light produced by the beam-gas interactions with a camera. Particular strengths of this instrument are its ability to monitor the beam non-destructively throughout the full LHC acceleration cycle and to give absolute beam size measurements for both proton and ion beams. A prototype was installed in the LHC in the year-end technical stop (YETS) 2022-2023 (figure 2) and has been taking data since then.
During the 2024 LHC run, the Beam Gas Curtain (BGC) instrument was extensively operated to record systematic measurements. The fluorescent light emitted by the beam-gas interaction was successfully exploited to measure the average transverse beam profile and its evolution throughout the machine cycle.
Notably, the BGC proved particularly effective during the ion run, enabling emittance measurements of high-intensity beams at injection energy, a regime where existing synchrotron radiation-based diagnostics are not available. These results are promising for the future use of the instrument to provide reliable average emittance measurements for both proton and ion beams, complementing the limitations of current transverse diagnostics systems.
Upgrades have been made in the YETS 2024/5 to improve the measurement in the vertical plane as well as to explore the capabilities of the BGC for measuring beam halo – one of the most challenging parts of the instrumentation requirements for the HL-LHC. 2025 and 2026 should see some exciting new results, validating further the importance of this instrument for the HL era and perhaps even giving completely new data for beam-gas cross-sections never before measured at these beam energies. To complement the system, a second BGC may be added for Beam 2 in long straight section 4 (LSS4) of the LHC. Further details on design, results and plans can be found in the BGC Collaboration Meeting of December 2024.
Finally, we are sad to mark the unexpected passing of Stefano Mazzoni last month. An instrumentation physicist in the peak of his career. He contributed so much to the HL-LHC and we will greatly miss him and his expertise.