High Luminosity LHC Project Research Articles

Impact of beam coupling impedance on crab cavity noise induced emittance growth

Crab cavities will be deployed as a part of the High Luminosity Large Hadron Collider (HL-LHC) upgrade to mitigate the luminosity reduction induced by the crossing angle at the main experiments (ATLAS and CMS). Two prototype crab cavities have been installed in the CERN Super Proton Synchrotron (SPS) in 2018 for studies with proton beams. An issue of concern is the transverse emittance growth induced by noise in the crab cavity radio frequency (rf) system, which is anticipated to limit the performance of the HL-LHC. In measurements conducted in the SPS in 2018, the crab cavity noise-induced emittance growth was measured to be a factor of 4 lower than predicted from the existing analytical models. In this paper, it is shown that the observed discrepancy is explained by damping effects from the beam coupling impedance, which were not included in the models up to now. Using the van Kampen mode approach, a new theory is developed, suggesting that the impedance can separate the coherent tune from the incoherent spectrum leading to an effective reduction of the crab cavity rf noise-induced emittance growth. This mechanism is validated in tracking simulations using the SPS impedance model as well as in dedicated experimental measurements conducted in the SPS in 2022. The implications for the HL-LHC project are discussed. Published by the American Physical Society 2024

Two new helium refrigerators for the LHC accelerator upgrade (HL-LHC) at CERN; From concept and tender to contract

The High Luminosity LHC (HL-LHC) project is aiming to upgrade the Large Hadron Collider (LHC) at CERN by increasing its peak luminosity by a factor of five with respect to its nominal value. This upgrade will include the replacement of the final focusing superconducting magnets and additional superconducting radiofrequency crab cavities in the long straight sections of the interaction points 1 and 5 of LHC. The cryogenic heat loads in points 1 and 5 of the LHC accelerator will significantly increase, mainly because of the higher luminosity. Therefore, two new Refrigerators will be required in points 1 and 5, with each an equivalent capacity of 14 kW@4.5 K, including 3.25 kW@1.9 K. This paper presents the functional requirements and conceptual design, the key choices and specific challenges including the civil engineering constraints and the major technical requirements detailed in the Technical Specification documents for the supply from the European industry of two new helium Refrigerators for HL-LHC. A procurement contract based on this specification, was placed in 2022.

The PreProcessor module for the ATLAS Tile calorimeter at the HL-LHC

The Large Hadron Collider (LHC) has played a crucial role in advancing our understanding of fundamental physics. With the discovery of the Higgs boson in 2012, the ATLAS and CMS experiments have made significant progess in studying its properties and searching for new physics beyond the Standard Model. To maintain and expand the LHC’s discovery potential, the High-Luminosity LHC (HL-LHC) project is planned as a major upgrade. As part of this upgrade, the ATLAS experiment has developed a comprehensive roadmap for upgrades, including the installation of new detector components and advancements in data acquisition and processing systems. A crucial component of these enhancements involves the complete replacement of the ATLAS central Tile hadronic calorimeter readout electronics. This upgrade aims to optimize the system’s ability to handle higher data rates and improve its resilience to radiation. Through these comprehensive improvements, the ATLAS experiment aims to meet the requirements and seize the opportunities presented by the HL-LHC era. This paper presents the design and architecture of the TilePPr (Tile PreProcessor) module, which serves as a crucial interface between the on-detector electronics and the central systems of the ATLAS experiment, including Data Acquisition, Detector Control, and Trigger systems. The TilePPr module is based on the Advanced Telecommunications Computing Architecture (ATCA) and incorporates high-speed optical links, communication interfaces, and data processing capabilities. Through a series of certification tests, the module has demonstrated its compliance with industry standards and functional requirements, confirming its suitability for seamless integration into the ATLAS experiment during the HL-LHC phase.

Study of discharges in the CMS GEM GE1/1 station with LHC beam

The Large Hadron Collider (LHC) and the experiments installed there are being upgraded for the High Luminosity LHC project that will provide a greatly enlarged data sample in the search for physics beyond the Standard Model. During Long Shutdown 2, from 2018 to the first months of 2022, some of these upgrades were completed. In particular, the muon spectrometer of the CMS experiment was upgraded with installation of the GE1/1 detector station, based on Gas Electron Multiplier (GEM) technology. This station is positioned in the endcap region of the CMS muon system and covers the pseudorapidity range 1.55 < |η| < 2.18. On the 5th of July, 2022, Run 3 of the LHC began with collision energy of 13.6 TeV. The newly installed GEM detectors experienced high-voltage discharges when the beams were colliding inside CMS. To monitor these events and to understand how to safely operate the detectors, a study of these phenomena became necessary. The intensity of discharge current and the discharge rate were recorded in different detectors as a function of luminosity and time. It was observed that some chambers have a significantly higher discharge rate than the modal value of rate over the entire chamber population. The reasons responsible for this behavior are currently under investigation. In this contribution, an analysis of discharges will be presented, illustrating the dependence of discharge rate on different LHC beam configurations and the high voltage working point of the chambers.

Resistive Micromegas high-rate and long-term ageing studies at the CERN Gamma Irradiation Facility

The LHC experiments are undergoing important upgrades for the High Luminosity LHC project. Experiments will be required to operate under three times higher instantaneous luminosity and higher levels of radiation when compared to LHC Run2. The integrated luminosity will increase by a factor ten. To cope with these conditions resistive Micromegas chambers were chosen by the ATLAS experiment as one of the technologies for the upgrade of the first forward muon stations. This paper describes the results of two prototype Micromegas detectors that were installed in the CERN Gamma Irradiation Facility with the aim to evaluate the detector performance under high-rate irradiation and to assess ageing effects over a 2.5 year long irradiation period and an accumulated charge in excess of 0.3 C/cm2.

Quench Protection of the HL-LHC Hollow Electron Lens Superconducting Solenoid Magnets

The High-Luminosity LHC (HL-LHC) project is an upgrade of the Large Hadron Collider (LHC) and comprises the installation of two Hollow Electron Lens (HEL) systems, each acting on one beam on both sides of LHC Point 4. The systems aim for a controlled depletion of hadron beam tails and an enhanced hadron beam halo collimation. Each HEL unit contains 22 circuits made up of solenoid, saddle coils and canted cosine theta corrector magnets. The largest magnet is a 1.6 m long solenoid, with a 180 mm bore diameter and central field of 5 T at 300 A and 4.5 K. This contribution focuses on the powering and protection strategy of the solenoid magnet circuits. The circuits are protected by active quench detection and a timely switch-off of the power converter. For the circuit with the largest stored energy, a study is performed on the efficiency of an energy extraction system. The quench simulations performed using the STEAM-LEDET code are combined with uncertainty quantification methods available in the DAKOTA software. Quench simulation input parameters related to the superconducting wire, magnet, circuit, and detection have been parametrized and randomly varied to calculate statistics of key output parameters. These are compared to the allowed limits, namely 500 V maximum peak voltage-to-ground and 120 K maximum coil temperature.

Assembly and Pre-Loading Specifications for the Series Production of the Nb3Sn MQXFA Quadrupole Magnets for the HL-LHC

The High Luminosity LHC (HL-LHC) Project is planning to install 16 cold-masses made with Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn quadrupole magnets in the LHC Interaction Regions to significantly increase its luminosity. Half of these cold masses are fabricated at BNL, FNAL, and LBNL under the US Accelerator Research Program (AUP). Each cold mass includes two identical Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn quadrupole magnets, called MQXFA with a magnetic length of 4.2 m. Currently, the AUP project has completed the fabrication and test of the first 5 pre-series magnets, and is working on the following 16 magnets for the series production. The brittleness and strain sensitivity of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconducting material requires a careful definition of the allowable maximum stress in the windings during magnet assembly and pre-load, and a tight control of their variation within the whole coil length. Therefore, a series of assembly and pre-load specifications have been defined with the goals of minimizing the risk of conductor degradation and providing the mechanical support required to reach the nominal current during powering. In this paper we present the specifications defined for the MQXFA magnets and applied during the different assembly phases and during the pre-load process of the first 5 pre-series magnets.

Final design of the cryostat for the high luminosity LHC magnets

The high luminosity LHC project (HL-LHC) aims at increasing proton collisions by a factor of ten whilst extending physics exploitation until 2035. Its performance will rely on new focusing quadrupoles, beam separation dipoles and corrector magnets with large apertures to be installed on both sides of the ATLAS and CMS experiments. A dedicated cryostat design of about 1 m in diameter was developed for operation of these magnets at 1.9 K, comprising the required cryogenic circuits, interconnects, supports, insulation, and instrumentation systems. Six cryostats with various lengths in the range of 8 to 11 m are required on each side of the interaction points to house the triplet magnets, correctors and the first separation dipole. These cryostats will be linked through flexible interconnects to form a continuous vacuum insulation and cryogenic system of about 60 m in length. The second rearmost separation dipole requires a stand-alone cryostat of 15 m in length but nevertheless features common design principles. We present the design of these cryostats, from a conceptual stage towards the final design. Assembly aspects are also addressed, including a modular approach affording flexibility and simplification of the manufacturing and assembly processes.

Thermohydraulic simulation of quenches and quench recovery for the HL-LHC IT String test bench at CERN

In the framework of the High Luminosity LHC (HL-LHC) project, a new test system - the Inner Triplet (IT) magnets String - for the characterization and validation of the collective behaviour of a 60 m long string of magnets is being built in the SM18 cryogenic test facility at CERN. Among the main objectives of the IT String is the assessment of the magnet performance during the resistive transition from the superconducting to the normal state (quench) when energies up to 39.1 MJ are released into the magnet cold mass and into the helium. In this paper we present the thermohydraulic study performed to design the quench recovery system of the IT String, which limits the pressure rise in the magnets during a quench and ensures the recovery of the operating conditions within the required timeframe. As a new design method, a 1-D thermohydraulic model of the quench relief system has been created using the EcosimPro software with the CRYOLIB library developed at CERN.

Design of high-performance guns for the HL-LHC HEL

The High Luminosity LHC project (HL-LHC) foresees the construction and installation of important new equipment to increase the performance of the LHC machine. The Hollow Electron Lens (HEL) is a promising system to control the beam halo. It improves the beam collimation system of the HL-LHC and mitigates possible equipment damage in case of failure scenarios from halo losses. The halo can store up to 30 MJ energy. The specifications for this new device are quite demanding. The source, an electron gun with an annular shaped cathode, must deliver a current up to 5 A. This is five times higher than the current in the existing electron lenses in Fermi and Brookhaven national laboratories. This note describes the programme carried out to design and test high-perveance guns equipped with two types of high-performance scandate cathodes. The size of the final gun for the HL-LHC lenses is now considerably smaller than the one of the first prototype, allowing a reduction of diameter and cost of the superconducting magnet system used to steer the electron beam. The tests carried out at FNAL, BVERI and BJUT demonstrated that the developed cathodes fulfil the specifications and can supply a 5 A fully Space Charge Limited (SCL) current.

Quench Protection Studies for the High Luminosity LHC Nb$_3$Sn Quadrupole Magnets

Achieving the targets of the High Luminosity LHC project requires the installation of new inner triplet magnet circuits for the final focusing of the particle beams on each side of the two main interaction points. Each of the four circuits will include six 150 mm aperture, 132.2 T/m gradient, Nb <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_3$</tex-math></inline-formula> Sn quadrupole magnets to be installed in the LHC tunnel. The recently updated circuit topology is such that the protection of each magnet can be studied from a single magnet point-of-view. To limit the hot-spot temperature and the peak voltage-to-ground, a protection system was designed that quickly and reliably transfers voluminous parts of the coil to the normal-conducting state, hence distributing more homogeneously the magnets stored energy in the windings. This system is based on two elements: quench heaters attached to the outer layers of the magnet coils and CLIQ (Coupling-Loss Induced Quench). The performance of the protection system is investigated by simulating the electro-magnetic and thermal transients occurring after a quench with the program STEAM-LEDET, and by conducting dedicated experiments at the CERN and FNAL magnet test facilities. The effectiveness of the quench protection system is assessed at all representative operating current levels. Furthermore, the coils hot-spot temperature and peak voltage to ground are analyzed for various failure cases, conductor parameters, and parameter distribution among the four coils. It is concluded that the proposed design assures an effective, reliable, and fully redundant quench protection system.

Evolution of ATLAS analysis workflows and tools for the HL-LHC era

The High Luminosity LHC project at CERN, which is expected to deliver a ten-fold increase in the luminosity of proton-proton collisions over LHC, will start operation towards the end of this decade and will deliver an unprecedented scientific data volume of multi-exabyte scale. This vast amount of data has to be processed and analysed, and the corresponding computing facilities must ensure fast and reliable data processing for physics analyses by scientific groups distributed all over the world. The present LHC computing model will not be able to provide the required infrastructure growth, even taking into account the expected evolution in hardware technology. To address this challenge, several novel methods of how end-users analysis will be conducted are under evaluation by the ATLAS Collaboration. State-of-the-art workflow management technologies and tools to handle these methods within the existing distributed computing system are now being evaluated and developed. In addition the evolution of computing facilities and how this impacts ATLAS analysis workflows is being closely followed.

A methodology to compute the critical current limit in Nb3Sn magnets

Numerous experiments have shown that the loads applied to Nb3Sn strands and cables can reduce their critical current. Experiments, performed on uniaxially loaded strands, allowed to define clear laws to describe the evolution of the critical surface as a function of the applied current, field, temperature and strain. It is, however, still unclear how these laws can be applied to superconducting magnets. The present paper proposes a methodology to estimate the critical current and temperature margin reduction on superconducting magnets due to stress on the superconducting material. The methodology is tested on the MQXF magnets, a quadrupole developed for the High Luminosity LHC project, and successfully validated by comparing computed strain with data from strain gauge measurements. Results suggested that, because of the stresses arising in winding during assembly, cool-down and powering, the current limit of the magnet is lower than the expected short sample limit, and that the most critical region does not coincide with the peak field location.

Scientific Data Lake for High Luminosity LHC project and other data-intensive particle and astro-particle physics experiments

The next phase of LHC Operations – High Luminosity LHC (HL-LHC), which is aimed at ten-fold increase in the luminosity of proton-proton collisions at the energy of 14 TeV, is expected to start operation in 2027-2028 and will deliver an unprecedented scientific data volume of multi-exabyte scale. This amount of data has to be stored and the corresponding storage system should ensure fast and reliable data delivery for processing by scientific groups distributed all over the world. The present LHC computing and data processing model will not be able to provide the required infrastructure growth even taking into account the expected hardware technology evolution. To address this challenge the new state-of-the-art computing infrastructure technologies are now being developed and are presented here. The possibilities of application of the HL-LHC distributed data handling technique for other particle and astro-particle physics experiments dealing with large-scale data volumes like DUNE, LSST, Belle-II, JUNO, SKAO etc. are also discussed.

Novel triple-GEM mechanical design for the CMS-ME0 detector and its preliminary performance

In the framework of the High-Luminosity Large Hadron Collider project (HL-LHC), the LHC experiments will require upgrades to their detectors to cope with the new accelerator performance. The upgrade of the CMS Muon Spectrometer foresees the installation of three new muon stations based on the Gas Electron Multiplier (GEM) technology, referred to as GE1/1, GE2/1 and ME0 detectors. While the installation and commissioning of the GE1/1 detectors is currently underway, the GE2/1 and ME0 detectors are expected to be installed between 2023 and 2027. The CMS Muon Collaboration has developed a novel construction design of large-area, trapezoidal-shaped GE1/1 triple-GEM detectors; in particular, a new self-stretching technique has been introduced to mechanically stretch the GEM-foils without using spacer grids or glue inside the gas volume in order to avoid dead regions (several percent) or possibly outgassing contaminants which could trigger premature aging processes. As it has been observed, the PCB boards, which define the gas enclosure of the detector, get deformed under the internal gas over-pressure, introducing irregularities in the planarity of the GE1/1 detector, which could potentially affect the uniformity of the detector performance. Therefore, the collaboration has established a set of tests and quality controls in order to quantify these irregularities and mitigate their impact on detector performance. Additionally, new solutions and design upgrades have been implemented to prevent such effects in future GE2/1 and ME0 upgrade projects. We will focus in particular on the novel design solutions based on the PCB distance holders (pillars), which the collaboration adopted for realization of the latter projects, and their impact on the performance of the detector, with a summary of the ongoing R&D activities.

Distribution Feedbox for the Superconducting Link (SCLink) and Magnets of HL-LHC

The High Luminosity LHC (HL-LHC) project aims at upgrading the LHC collider to increase its luminosity by about a factor of five. The electrical connection between the magnets in the LHC tunnel and the power converters in a new transverse tunnel will be supplied by a superconducting line (SCLink), consisting of ten MgB2 cables housed into a 140 metre long flexible cryostat. This paper presents the detailed design for one of two types of distribution feedbox, (DFX) located between the magnet and the distribution feedbox. The vacuum barrier required to separate the vacuums of the upper SCLink and lower DFX sections; is to be integrated in the middle of the vertical section of DFX. A detailed study was performed, given the complexity of installing a vacuum barrier with a large diameter within a restricted height. Eccentric loading on the barrier is created by the “L-shape” vessel, necessary to accommodate the transition of the cable from vertical-to-horizontal. The solution considers a vacuum barrier assembly consisting of a flexible corrugated membrane (bellows) and deploys a lightweight “supporting cage” around the barrier and restraining rods in the horizontal section to ensure the barrier bares no substantial load or torque, and suffers no lateral or column type instability during operating and accidental conditions. The current design satisfies the mechanical and thermal design criteria outlined in the DFX specification.

Resistivity Characterization of Molybdenum-Coated Graphite-Based Substrates for High-Luminosity LHC Collimators

The High-Luminosity Large Hadron Collider (HL-LHC) project aims at extending the operability of the LHC by another decade and increasing by more than a factor of ten the integrated luminosity that the LHC will have collected by the end of Run 3. This will require doubling the beam intensity and reducing the transverse beam size compared to those of the LHC design. The higher beam brightness poses new challenges for machine safety, due to the large energy of 700 MJ stored in the beams, and for beam stability, mainly due to the collimator contribution to the total LHC beam coupling impedance. A rich research program was therefore started to identify suitable materials and collimator designs, not only fulfilling impedance reduction requirements but also granting adequate beam-cleaning and robustness against failures. The use of thin molybdenum coatings on a molybdenum–graphite substrate has been identified as the most promising solution to meet both collimation and impedance requirements, and it is now the baseline choice of the HL-LHC project. In this work we present the main results of the coating characterization, in particular addressing the impact of coating microstructure on the electrical resistivity with different techniques, from Direct Current (DC) to GHz frequency range.

Design, Procurement, Installation and Commissioning of the Cryogenic Infrastructure for a New Superconducting RF Test Facility with Beam at CERN

The High-Luminosity LHC project (HL–LHC), aiming at peak luminosity above 5.0 × 1034 cm −2 s −1, considers replacing the matching sections on both sides of the ATLAS and CMS experiments. To complement new focusing quadrupoles, this upgrade considers using the so-called superconducting crab cavities, never operated before with protons and therefore requiring qualification with beam. To this aim, a new cryogenic infrastructure for a superconducting RF test facility was initiated and recently installed at CERN SPS accelerator in 2018. From the early studies of heat load and design principles to the successful tests passed during late 2018, this paper describes the main cryogenic requirements for such a test facility, its design challenges, procurement, installation, and commissioning up to stable operation of the crab cavities module in superfluid helium at 2 K.

Manufacturing Error Analysis of Field Quality for the HL-LHC CCT Corrector

The High Luminosity LHC (HL-LHC) project, as an upgrade of the Large Hadron Collider (LHC), is to be started in the next decade. A set of canted cosine theta (CCT) twin aperture orbit corrector magnets will be developed in China, with a field quality requirement of 10−3 in the two apertures. Cooperating with CERN, twin aperture correctors are being fabricated as prototypes before the series production. During the fabrication of the CCT coils, certain manufacturing errors including winding error, rotating error and concentricity error appeared which will affect the field quality. This study analyzed the effects on field harmonic components by these manufacturing errors, and the manufacturing process and the first test result of the prototype were introduced briefly. Suggestions from the analysis results are provided for further series production of the 2.2-m orbit correctors.

Mitigation of intensity limitation in the CERN SPS using a double RF system

The Super Proton Synchrotron (SPS) at CERN is the injector of the Large Hadron Collider (LHC). Multi-bunch instabilities limit the intensity of the beam that can be accelerated to 450 GeV in the SPS and transferred to the LHC. Without mitigation measures, the threshold of bunch intensity of the longitudinal instability is three times below the nominal bunch intensity of the LHC beam. The High Luminosity LHC project (HL-LHC), which requires a doubling of the nominal bunch intensity, relies on improvement of beam stability in the SPS. A fourth harmonic RF system allows, presently, to stabilize the beam up to nominal LHC intensity. It increases the synchrotron frequency spread inside the bunch, providing more efficient Landau damping of beam instability. However, nonlinearities of the synchrotron frequency distribution inside the bunch pose a limitation on bunch length. This paper explores possible intensity increase in the SPS by studying the effect on beam stability of the voltage ratio between two RF systems. The results are substantiated by beam measurements and particle-tracking simulations. An optimized voltage program of the second RF system during the cycle has been tested in operation and beam stability and the quality have been successfully improved.