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24 January 2025

The High Luminosity Large Hadron Collider, edited by Oliver Brüning and Lucio Rossi, World Scientific

The High-Luminosity LHC test stand
Testing, one, two, three The High-Luminosity LHC test stand in November 2024. Credit: CERN

The High Luminosity Large Hadron Collider, edited by Oliver Brüning and Lucio Rossi, is a comprehensive review of an upgrade project designed to boost the total event statistics of CERN’s Large Hadron Collider (LHC) by nearly an order of magnitude. The LHC is the world’s largest and, in many respects, most performant particle accelerator. It may well represent the most complex infrastructure ever built for scientific research. The increase in event rate is achieved by higher beam intensities and smaller beam sizes at the collision points.

Brüning and Rossi’s book offers a comprehensive overview of this work across 31 chapters authored by more than 150 contributors. Due to the mentioned complexity of the HL-LHC, it is advisable to read the excellent introductory chapter first to obtain an overview on the various physics aspects, different components and project structure. After coverage of the physics case and the upgrades to the LHC experiments, the operational experiences with the LHC and its performance development are described.

The LHC’s upgrade is a significant project, as evidenced by the involvement of nine collaborating countries including China and the US, a materials budget that exceeds one billion Swiss Francs, more than 2200 years of integrated work, and the complexity of the physics and engineering. The safe operation of the enormous beam intensity represented a major challenge for the original LHC, and will be even more challenging with the upgraded beam parameters. For example, the instantaneous power carried by the circulating beam will be 7.6 TW, while the total beam energy is then 680 MJ – enough energy to boil two tonnes of water. Such numbers should be compared with the extremely low power density of 30 mW/cm3, which is sufficient to quench a superconducting magnet coil and interrupt the operation of the entire facility.

The book continues with descriptions of the two subsystems of greatest importance for the luminosity increase: the superconducting magnets and the RF systems including the crab cavities.

The High Luminosity Large Hadron Collider

Besides the increase in intensity, the primary factor for instantaneous luminosity gain is obtained by a reduction in beam size at the interaction points (IPs), partly through a smaller emittance but mainly through improved beam optics. This change results in a larger beam in the superconducting quadrupoles beside the IP. To accommodate the upgraded beam and to shield the magnet coils from radiation, the aperture of these magnets is increased by more than a factor of two to 150 mm. New quadrupoles have been developed, utilising the superconductor material Nb3Sn, allowing higher fields at the location of the coils. Further measures include the cancellation of the beam crossing angle during collision by dynamic tilting of the bunch orientation using the superconducting crab cavities that were designed for this special application in the LHC. The authors make fascinating observations, for example regarding the enhanced sensitivity to errors due to the extreme beam demagnification at the IPs: a typical relative error of 10–4 in the strength of the IP quadrupoles results in a significant distortion in beam optics, a so-called beta-beat of 7%.

Chapter eight describes the upgrade to the beam-collimation system, which is of particular importance for the safe operation of high-intensity beams. For ion collimation, halo particles are extracted most efficiently using collimators made from bent crystals.

The book continues with a description of the magnet-powering circuits. For the new superconducting magnets CERN is using “superconducting links” for the first time: cable sets made of a high-temperature superconductor that can carry enormous currents on many circuits in parallel in a small cross section; it suffices to cool them to temperatures of around 20 to 30K with gaseous helium by evaporating some of the liquid helium that is used for cooling the superconducting magnets in the accelerator.

Magnetic efforts

The next chapters cover machine protection, the interface with the detectors and the cryogenic system. Chapter 15 is dedicated to the effects of beam-induced stray radiation, in particular on electronics – an effect that has become quite important at high intensities in recent years. Another chapter covers the development of an 11 Tesla dipole magnet that was intended to replace a regular superconducting magnet, thereby gaining space for additional collimators in the arc of the ring. Despite considerable effort, this programme was eventually dropped from the project because the new magnet technology could not be mastered with the required reliability for routine operation; and, most importantly, alternative collimation solutions were identified.

Other chapters describe virtually all the remaining technical subsystems and beam-dynamics aspects of the collider, as well as the extensive test infrastructure required before installation in the LHC. A whole chapter is dedicated to high-field-magnet R&D – a field of utmost importance to the development of a next-generation hadron collider beyond the LHC.

Brüning and Rossi’s book will interest accelerator physicists in that it describes many outstanding beam-physics aspects of the HL-LHC. Engineers and readers with an interest in technology will also find many technical details on its subsystems.

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