The High Luminosity Large Hadron Collider

The following sections are included:

• Foreword
• Contents
• List of Authors

The Large Hadron Collider (LHC) is one of largest scientific instruments ever built. It has been exploring the new energy frontier since 2010, gathering a global user community of 10,000 scientists. To extend its discovery potential, the LHC requires a major upgrade in the 2020s to increase its luminosity (rate of collisions) by a factor of five beyond its design value, and the integrated luminosity by a factor of ten. Being a highly complex and optimized machine, such an upgrade of the LHC must be carefully studied and requires about 10 years to implement. The novel machine configuration, called High Luminosity LHC (HL-LHC), relies on a number of key innovative technologies, each representing exceptional technological challenges, such as: cutting-edge 11-12 tesla superconducting magnets, very compact superconducting cavities for beam rotation with ultra-precise phase control, new technology for beam collimation and 100-metre-long high-power super-conducting links with negligible energy dissipation, very precise 2-Q high current power converter, new surface treatment for e-could suppression, and many others. All these constitute major breakthroughs in accelerator technology.

HL-LHC federates efforts, R&D, and construction of a large community in Europe, the USA, Japan, China and Canada, thereby consolidating CERN and LHC as the center of a world-wide collaboration for basic science and technology.

This contribution reviews the physics potential of the HL-LHC experimental programme. The first 10 years of the LHC has demonstrated the vast open range of opportunities for measurements and discoveries of new phenomena. Starting from the results of the first two runs of the LHC, extensive experimental and theoretical studies have now defined a broad set of goals for the future high-luminosity phase of the project, reviewed here. The precision measurement of the Higgs boson properties, which has greatly expanded our knowledge today, represents the primary guaranteed deliverable. This target is complemented by a vast array of additional measurements, ranging from the continued search for phenomena beyond the Standard Model, to the study of the Standard Model dynamics and parameters, flavour phenomena, and the study of matter at high density and temperature.

The HL-LHC upgrade plans of ALICE, ATLAS, CMS and LHCb are briefly outlined in this chapter.

By the end of Run 2 in December 2018, the LHC had seen seven full years of operation and a wealth of knowledge and experience has been built up. The key operational procedures and tools are well established. The understanding of beam dynamics is profound and utilized online by well-honed measurement and correction techniques. Key beam-related systems have been thoroughly optimised and functionality sufficiently enhanced to deal with most of the challenges encountered. Availability has been optimised significantly across all systems. This collected experience will form the initial operational basis for Run 3 and subsequent HL-LHC operation.

A brief review of Run 1 and Run 2 is given below, firstly to outline the progress made, and secondly to highlight the issues encountered and surmounted along the way. A synthesis of operational features of the machine and the lessons learnt is then presented. The chapter concludes with brief look at consolidation activities in view of the need to sustain high availability and safe operation given the considerable challenges of the HL-LHC operational regime and the time-frame over which it will operate.

The main beam and machine parameter choices and the underlying beam dynamics considerations are reviewed together with the main challenges and expected performance.

In this section we present the magnet technology for the High Luminosity LHC. After a short review of the project targets and constraints, we discuss the main guidelines used to determine the technology, the field/gradients, the operational margins, and the choice of the current density for each type of magnet. Then we discuss the peculiar aspects of the design of each class of magnet, with special emphasis on the triplet.

The HL-LHC beams are injected, accelerated to and stored at their nominal energy of 7 TeV by the existing 400 MHz superconducting RF system of the LHC. A new superconducting RF system consisting of eight cavities per beam for transverse deflection (aka crab cavities) of the bunches will be used to compensate the geometric loss in luminosity due to the non-zero crossing angle and the extreme focusing of the bunches in the HL-LHC.

High-performance collimation systems are essential for operating modern hadron accelerators with large beam intensities efficiently and safely. In particular, at the LHC the collimation system ensures a clean disposal of beam halos in the superconducting environment. The challenges of the HL-LHC study pose more demanding requests for beam collimation. In this chapter, the upgraded collimation system for HL-LHC is presented. Various collimation solutions were elaborated to address the HL-LHC requirements and challenges. These are reviewed in the following, identifying the main upgrade baseline and pointing out advanced collimation concepts under consideration for further enhancement of the collimation performance.

The HL-LHC upgrade will impose changes to the magnet circuits in Points 1, 5 and 7 with respect to the present LHC configuration. This chapter describes those changes and describes the powering characteristics and protection strategies applicable to the new magnet circuits. The electrical design criteria for magnets and other elements of the circuit are also presented.

This chapter describes the cold powering, i.e. the transfer of current from room temperature to the liquid helium environment, of the High-Luminosity LHC (HL-LHC) [1] magnets. Target R&D was done to develop novel systems relying on long superconducting transfer lines, with up to |120| kA current capability, called Superconducting Links, based on MgB₂ superconductor. The Superconducting Links are part of the so-called Cold Powering Systems that include complex terminations at the two extremities for interfacing with the magnets and the power converters. High Temperature Superconducting REBCO based current leads, housed in a cryostat of novel concept, make the electrical transition between room temperature and 17 K. Two types of system were conceived and designed for the powering of the HL-LHC magnets in the Triplets and the D2 separation dipole in the Matching Sections. Following the successful completion of a staged and focused R&D, which included qualification of prototype systems, series production has been launched. Aspects associated with the integration and operation of the systems in the final LHC configuration were also studied in-depth and optimized.

New power converters are needed for the powering of the HL-LHC circuits in the insertion regions of LHC points 1 and 5. In addition to the unprecedented precision, the HL-LHC power converters are also designed to assure very high availability in order to maximize operation time. Moreover, energy storage system will be designed to not only recuperate the magnet energy but to optimize the electrical infrastructure for accelerator applications as well. Furthermore, with an even more complex inner triplet circuit than in the LHC, a new nested circuit control needs to be developed for the HL-LHC. This chapter will present the main novelties and challenges in the development of the HL-LHC power converters.

In the high luminosity era of the LHC, the energy stored in the two beams will nearly double as compared with the design value of the current LHC and the beta functions will significantly increase in critical locations. In addition, novel equipment like Nb₃Sn based superconducting magnets, crab cavities, hollow e-lenses and others will be installed. These changes require the careful review of known fast failure cases and the study of newly emerging ones. Furthermore, a new generation of magnet and circuit protection systems will be applied in the HL-LHC, which will set a new standard in this field. Finally, a high machine availability will be one of the key factors to achieve the challenging integrated luminosity goals of the HL-LHC era. Therefore, the overall availability targets in terms of the allowed number of beam dumps and fault recovery time are defined.

We describe the main general changes as relevant for the experiments, and in particular the changes in the machine-detector interface (MDI) region extending from the merging of the LHC beampipes into a single experimental chamber before the inner triplet to the start of the experimental beampipe in the experimental cavern. The geometry of this region is optimized to provide a maximum decoupling of the activities in the tunnel and inside the experimental caverns. Massive absorbers are used to strongly reduce the flux of secondary particles produced in collisions in the interaction region into the tunnel, and at the same time help to protect the experiments from beam induced losses and backgrounds. This chapter explains the main challenges within that region and how they will be solved to cope with the new requirements coming from the luminosity upgrade.

The discovery of a Higgs boson at CERN in 2012 was the start of a major program working to measure this particle’s properties with the highest possible precision for testing the validity of the Standard Model and to search for further new physics at the energy frontier. The LHC is in a unique position to pursue this program. Europe’s top priority is the exploitation of the full potential of the LHC, including the high-luminosity upgrade of the machine and detectors with an objective to, by around 2030, collect ten times more data than in the initial design. To reach this objective, the LHC cryogenic system must be upgraded to withstand higher beam current and higher luminosity at top energy while keeping the same operation availability by improving the collimation system and the protection of electronics sensitive to radiation. This paper will present the conceptual design of the cryogenic system upgrade with recent updates in performance requirements, the corresponding layout and architecture of the system as well as the main technical challenges which have to be met in the coming years.

The radiation impact on the machine elements and the electronics equipment in the high luminosity insertions is discussed, distinguishing the different loss regions, and respective mitigation measures are highlighted.

This chapter describes the design of, and parameters for, the 11 T dipole [11] developed at FNAL and the European Organization for Nuclear Research (CERN) for the High Luminosity Large Hadron Collider (HL-LHC) project.

The HL-LHC project requires an important upgrade of the LHC vacuum system to ensure sufficient beam lifetime, limit the background to the high-luminosity experiments, protect the new final focusing system from ionizing radiation, increase the mechanical robustness, maximize beam aperture and mitigate electron multipacting. The technologies developed to tackle these challenges are exposed in this chapter.

The following sections are included:

• Introduction
• Beam Position Monitoring for the HL-LHC
• Beam Loss Monitoring for the HL-LHC
• Emittance Measurement for the HL-LHC
• Diagnostics for Crab Cavities
• Luminosity Measurement for HL-LHC
• Gas Curtain Diagnostics
• References

Some of the elements of the LHC injection and extraction systems will be upgraded or replaced to adapt to the increased beam brightness and intensity of the HL-LHC beams [1; 2]. The injection main protection absorber will be replaced with new hardware which will be able to absorb and withstand 288 HL-LHC bunches in case of an injection kicker failure. The compatibility with injection of 320 bunches (four batches of 80 HL-LHC bunches [3]) was also verified. One auxiliary injection protection collimator in Point 2 will be displaced closer to the interaction point (IP) to increase the acceptance of the ALICE Zero-Degree Calorimeter. The injection kickers, which suffered already from electron cloud, degraded vacuum and beam induced heating while operating with LHC beams, will be upgraded with several modifications to mitigate these effects. The compatibility of the LHC beam dump system with the increased beam intensities of the HL-LHC beams still needs to be fully assessed. However, the dump protection devices, as well as the dump absorber block and its entrance and exit windows needs an upgrade or replacement. The studies include the definition of the possible worst failure scenarios for the extraction and dilution kickers and the consequences on the different dump elements. Finally, the extraction and dilution system will be upgraded to improve its reliability by reducing the risk of erratics, monitoring the status of the system and reacting faster in case of failures.

HL-LHC will pose new challenges on the accelerator control system. Although the overall architecture will be preserved and most of the currently deployed equipment will continue its operation, three areas have been identified for renovation in response to the new requirements: logging system, new hardware platform in the distributed I/O tier and radiation-tolerant fieldbus.

The LHC Injectors Upgrade (LIU) project aims at increasing the intensity and brightness in the LHC injectors in order to match the challenging requirements of the High-Luminosity LHC (HL-LHC) project, while ensuring high availability and reliable operation of the injectors complex up to the end of the HL-LHC era. Fulfilling this goal requires extensive hardware modifications and new beam dynamics solutions across the entire LHC proton and ion injection chains: the new Linac4, the Proton Synchrotron Booster (PSB), the Proton Synchrotron (PS), the Super Proton Synchrotron (SPS), together with Linac3 and the Low Energy Ion Ring (LEIR) as ion PS injectors. The great majority of the LIU hardware modifications have been implemented during the 2019-2020 CERN accelerators shutdown. This chapter describes the various project phases, highlights the past and future challenges, and concludes on the expected beam parameter reach and ramp-up, together with the risks and mitigations.

Optimized space allocation for each equipment is instrumental to ensure that the various systems perform according to specification and to minimize installation and maintenance time. The general optimization effort goes under the name of “integration”, and it is strictly linked to the de-installation of the previous machine, to the management of the interfaces with existing infrastructures and to the installation of the new HL-LHC equipment. One of the machine setups, which is most deeply integrated with the others, is the alignment system that plays a prominent role in allowing achieving the HL-LHC performance goals and that is conceived to minimize human presence in the machine in order to maximize operational time, to provide new operational flexibility, and – of paramount importance – to reduce the radiation dose to personnel.

The upgrade in luminosity of the LHC requires large and new technical infrastructures like civil engineering, electrical distribution, cooling, ventilation, access system, alarm system, transport, and operational safety system. This chapter describes the new technical infrastructure including layout, performance requirements and architecture as well as the main technical challenges.

The HL-LHC IT String is the test stand to validate the collective behavior of the Inner Triplet (IT) magnets and circuits in conditions as near as possible to the operational ones.

Several test stands were assembled for testing integrated operation of individual components with and without beam. Amongst these, at CERN, the SPS SRF test stands allows to qualify crab cavity modules with beam, while the test facilities situated in the SM18 buildings underwent important upgrades for testing SRF single-crab-cavities, SRF modules, superconducting cryomagnets and cold powering systems. Other test stands are available off-site in the framework of collaborations. This chapter describes the test facilities at CERN and presents a summary overview of the other test facilities outside of CERN.

This chapter describes the test facilities upgraded in the framework of HL-LHC for testing magnets, cryo assemblies, SC Link systems, HTS leads and cold diodes at CERN, and at the collaborators’ premises.

The compensation of the long-range beam-beam interactions using DC wires is presently under study as an option for enhancing the Large Hadron Collider (LHC) performance in the framework of the High-Luminosity LHC (HL-LHC) Project. After the installation of four wire demonstrators in the LHC, a successful experimental campaign was performed, with various beam conditions and wire set-ups. In parallel, a simulation framework was established to support the experimental results in terms of the impact of the wires on dynamic aperture and thereby lifetime, for both LHC and HL-LHC.

Nonlinear optics errors in low-β* insertions pose a serious challenge to successful operation of the HL-LHC. LHC experience however has demonstrated that the previously assumed correction strategy, based upon ideal compensation of selected nonlinear resonances, as determined from magnetic measurements, suffers from several limitations. A beam-based correction approach yielded a positive operational impact in the LHC, and dedicated machine studies have helped establish new methods for nonlinear optics corrections in HL-LHC.

The beam response to an external excitation may result in a growth of the emittance, or more generally a modification of its particle distribution. The former reduces the luminosity, and the latter might lead to a loss of Landau damping of coherent instabilities. The corresponding beam dynamics model, experimental studies at the LHC as well as extrapolations to the HL-LHC are discussed in this chapter.

Crystal collimation is an advanced technique where a silicon crystal, only a few millimeters long and bent to a curvature of about 50 μrad, coherently deflects the beam halo onto a collimator absorber. This technique can improve beam collimation in the HL-LHC. Since 2015, a test stand has been operational in the betatron cleaning insertion of the LHC for beam tests at the unprecedented hadron beam energy of up to 6.5 Z TeV, where Z is the atomic number. For the first time, channeling was observed at this energy and the crystal collimation concept was validated, demonstrating that the cleaning of lead heavy-ion beams at 6.37 Z TeV can be improved by up to a factor 10. Crystal collimation has become part of the HL-LHC baseline in 2019 and will be the key upgrade for improving the cleaning efficiency for ion beam operation in Run 3.

The Hadron Collider (LHC) at CERN can be regarded as the ultimate collider built with Nb₃ Ti magnets. Its main dipoles have reached a field of approximately 8 T, which is very likely close to the highest practical field for this superconductor in accelerators. The next major step is the High Luminosity upgrade of the LHC at CERN, which among the many upgrades of the accelerator, calls for a few tens of Nb3Sn dipole and quadrupole magnets, operated at 1.9 K and at conductor peak fields up to about 12 T. HL-LHC magnets are in the production phase, marking an historical milestone in accelerator technology and the culmination of 20 years of worldwide R&D. Here we describe the rationale for high field accelerator magnet R&D beyond HL-LHC, consisting of two complementary axes: (i) development of an ultimate Nb₃Sn technology, increasing the field reach and achieving maturity and robustness level required for deployment on a large scale and (ii) demonstrating suitability of high-temperature superconductors for accelerator magnet applications. We start with a review of the state-of-the-art, review the main goals, and identify the drivers for an R&D program responding to the declared priorities of the European Strategy Upgrade. This chapter is intended as the starting point in the formation of a structured High Field Accelerator Magnet R&D Program.