LHC Machine Research Articles

Investigation and perspectives of using Graph Neural Networks to model complex systems: the simulation of the helium II bayonet heat exchanger in the LHC

Large cryogenic systems, like those installed at CERN, are complex systems relying on many diverse physical processes and phenomena that are difficult to simulate and monitor in detail. With only a limited number of properties measured and made available for monitoring and control purposes, several processes contributing to the dynamics of the systems are ignored. This lack of information can reduce the accuracy and the capability of a model to track, predict, and anticipate the behavior of the system. Accurate analytical or numerical computer modeling can be developed to simulate the non-linear dynamics of the processes but they are complex, computationally intensive, and cumbersome to test, validate, and implement with different configurations and limited measurements of the hidden properties. In this work, we present our investigation of using Graph Neural Networks (GNN) to build a model of the helium II bayonet heat exchanger operating in the LHC at CERN. We are proposing to use a hybrid machine learning approach, where the parameters of the GNN model are estimated by a combination of supervised learning algorithms trained on experimental data and bounding physics equations and parameters. The GNN model was initially trained on data from the experiments performed on the LHC prototype magnet strings and validated on data extracted during the operation of the LHC machine. We demonstrate the model’s accuracy, repeatability, and robustness in various configurations. The model is also well inspectable and explainable, providing the time evolution of all variables. We report on the results and expected applications, which include predictive control, diagnostic, and operator training.

RPC based tracking system at CERN GIF++ facility

With the HL-LHC upgrade of the LHC machine, an increase of the instantaneous luminosity by a factor of five is expected and the current detection systems need to be validated for such working conditions to ensure stable data taking. At the CERN Gamma Irradiation Facility (GIF++) many muon detectors undergo such studies, but the high gamma background can pose a challenge to the muon trigger system which is exposed to many fake hits from the gamma background. A tracking system using RPCs is implemented to clean the fake hits, taking profit of the high muon efficiency of these chambers. This work will present the tracking system configuration, used detector analysis algorithm and results.

The CMS pixel detector for the High Luminosity LHC

The LHC machine will be upgraded to increase its peak luminosity to 5–7.5 × 1034 cm−2 s−1 and to possibly reach an integrated luminosity of 3000–4000 fb−1, with an average number of pileup events of 140–200. The CMS experiment needs to be upgraded to keep up with the new challenges: the unprecedented radiation environment translates to the detector requirement of high resilience, while the increased number of events per bunch crossing requires higher detector granularity. Thus a completely new Inner Tracker will be installed. Design choices for the Inner Tracker Phase-2 upgrade, highlighting R&D activities and technological approaches, will be presented.

The CMS Tracker for the High Luminosity LHC

The LHC machine is planning an upgrade program which will smoothly bring the instantaneous luminosity to about 5–7.5 ×1034cm−2s−1, to reach an integrated luminosity of 3000−4000fb−1 by the end of 2039. This High Luminosity LHC scenario, HL-LHC, will require an upgrade program of the LHC detectors known as Phase-2 upgrade. The current CMS Tracker, already running beyond design specifications, and CMS Pixel Detector will not be able to survive HL-LHC radiation conditions and CMS will need completely new devices, in order to fully exploit the HL-LHC data which will be recorded under highly demanding background conditions.The Phase-2 Tracker will be made of two sections, an Inner Tracker and an Outer Tracker. Both detectors will feature increased radiation hardness, higher granularity and capability to handle higher data rate and longer trigger latency in order to ensure at least the same performances of the current detector, in terms of tracking and vertex reconstruction capabilities, at the high pileup (140–200 collisions per bunch crossing) expected at HL-LHC. Moreover the Phase-2 Outer Tracker will have also trigger capabilities since tracking information will be used at L1 trigger stage.This report is focusing on the replacement of the CMS Tracker system, describing new layout and technological choices together with some highlights on research and development activities.

Improved experimental layout for dipole moment measurements at the LHC

The electric and magnetic dipole moment of charm and bottom baryons can be measured for the first time by using bent crystal technology at the LHC. The experimental method, proposed in recent years, suffers from limited statistics, which dominates the uncertainty of the measurement. In this work, we present an alternative experimental layout, based on the use of crystal lenses, that improves the trapping efficiency by about a factor 15 (35) for a 2-cm (5-mm) target with respect to the nominal layout, with plain crystal faces. The efficiencies are evaluated taking into account the constraints from the LHC machine, and the technical challenges to realize this novel experimental method are discussed.

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.

Study of Beam-Gas Interactions at the LHC for the Physics Beyond Colliders Fixed-Target Study

Among several working groups formed in the framework of the Physics Beyond Colliders study, launched at CERN in September 2016, there is one investigating specific fixed- target experiment proposals. Of particular interest is the study of high-density unpolarized or polarized gas targets to be installed in the LHC, upstream of the LHCb detector, using storage cells to enhance the target density. This work studies the impact of the interactions of 7 TeV proton beams with such gas targets on the LHC machine in terms of particle losses.

The development programme of cathodes and electron guns for the Hollow Electron Lenses of the High Luminosity LHC project1

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, has to 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 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.

The CMS tracker upgrade for the high luminosity LHC

Abstract The LHC machine is planning an upgrade program which will smoothly bring the luminosity to about 5 × 1 0 34 cm −2s−1 in 2028, to possibly reach an integrated luminosity of 4000 fb−1 by the end of 2037. This High-Luminosity LHC scenario, HL-LHC, will require a preparation program of the LHC detectors known as Phase-2 upgrades. The current CMS Outer Tracker (OT), already running beyond design specifications, and CMS Phase1 Pixel Detector will not be able to survive the HL-LHC radiation conditions. Thus, CMS will need completely new devices, in order to fully exploit the delivered luminosity. The new OT should also have trigger capabilities. To achieve such demanding goals, there is ongoing R&D activity to explore the options for the Outer Tracker and for the pixel Inner Tracker. The developed solutions would allow including tracking information at the Level-1 trigger. The design choices for the Tracker upgrades have been documented in a dedicated Technical Design Report and will be reviewed along with some current highlights of the R&D activities.

A multi-physics approach to simulate the RF-heating 3D power map induced by the proton beam in a beam intercepting device

The project High Luminosity Large Hadron Collider (HL-LHC) calls for a streaking beam intensity and brightness in the LHC machine. In such a scenario, beam-environment electromagnetic interactions are a crucial topic: they could lead to uneven power deposition in machine equipment. The resulting irregular temperature distribution would generate local thermal gradients, this would create mechanical stresses which could lead to cracks and premature failure of accelerator devices. This work presents a method to study this phenomenon by means of coupled electro-thermo-mechanical simulations. Further, an example of application on a real HL-LHC device is also discussed.

Machine Learning Applied at the LHC for Beam Loss Pattern Classification

Beam losses at the LHC are constantly monitored because they can heavily impact the performance of the machine. One of the highest risks is to quench the LHC superconducting magnets in the presence of losses leading to a long machine downtime in order to recover cryogenic conditions. Smaller losses are more likely to occur and have an impact on the machine performance, reducing the luminosity production or reducing the lifetime of accelerator systems due to radiation effects, such as magnets. Understanding the characteristics of the beam loss, such as the beam and the plane, is crucial in order to correct them. Regularly during the year, dedicated loss map measurements are performed in order to validate the beam halo cleaning of the collimation system. These loss maps have the particular advantage that they are performed in well controlled conditions and can therefore be used by a machine learning algorithm to classify the type of losses during the LHC machine cycle. This study shows the result of the beam loss classification and its retrospective application to beam loss data from the 2017 run.

LHC and HL-LHC: Present and Future Radiation Environment in the High-Luminosity Collision Points and RHA Implications

The high-luminosity large hadron collider (HL-LHC) is a novel machine configuration which will rely on a number of key innovative technologies to enhance the performance of the present LHC machine as of 2025. The upgrade will also involve increased radiation levels that need to be predicted by combining scaled measurements and calculations in order to define the qualification requirements for electronic systems. In this paper, we describe such levels, first of all, by introducing the monitoring and calculation approaches used for the present LHC machine, and second, by applying scaling factors and dedicated simulations for the future HL-LHC accelerators. We present the levels according to the different areas relevant for the operation of electronics-based equipment, and discuss the associated radiation hardness assurance implications.

Outsourcing strategy and tendering methodology for the operation and maintenance of CERN’s cryogenic facilities

CERN operates and maintains the world largest cryogenic infrastructure ranging from ageing but well maintained installations feeding detectors, test facilities and general services, to the state-of-the-art cryogenic system serving the flagship LHC machine complex. A study was conducted and a methodology proposed to outsource to industry the operation and maintenance of the whole cryogenic infrastructure. The cryogenic installations coupled to non LHC-detectors, test facilities and general services infrastructure have been fully outsourced for operation and maintenance on the basis of performance obligations. The contractor is responsible for the operational performance of the installations based on a yearly operation schedule provided by CERN. The maintenance of the cryogenic system serving the LHC machine and its detectors has been outsourced on the basis of tasks oriented obligations, monitored by key performance indicators. CERN operation team, with the support of the contractor operation team, remains responsible for the operational strategy and performances. We report the analysis, strategy, definition of the requirements and technical specifications as well as the achieved technical and economic performances after one year of operation.

Beam screen cryogenic control improvements for the LHC run 2

This paper presents the improvements made on the cryogenic control system for the LHC beam screens. The regulation objective is to maintain an acceptable temperature range around 20 K which simultaneously ensures a good LHC beam vacuum and limits cryogenic heat loads. In total, through the 27 km of the LHC machine, there are 485 regulation loops affected by beam disturbances. Due to the increase of the LHC performance during Run 2, standard PID controllers cannot keeps the temperature transients of the beam screens within desired limits. Several alternative control techniques have been studied and validated using dynamic simulation and then deployed on the LHC cryogenic control system in 2015. The main contribution is the addition of a feed-forward control in order to compensate the beam effects on the beam screen temperature based on the main beam parameters of the machine in real time.

Impedance simulations and measurements on the LHC collimators with embedded beam position monitors

The LHC collimation system is a critical element for the safe operation of the LHC machine. The necessity of fast accurate positioning of the collimator's jaws, recently introduced the need to have button beam position monitors directly embedded in the jaws extremities of the LHC tertiary collimators and some secondary collimators. This addition led to a new design of these collimators including ferrites to damp higher order modes instead of rf fingers. In this work we will present the impedance bench measurements and simulations on a TCT (Transverse Tertiary Collimator) prototype including estimations for beam stability for the LHC.

Upgrade of the LHCb VELO detector

The LHCb experiment is a single-arm forward spectrometer optimised for performing heavy-flavour physics analyses, using proton-proton collisions provided by the LHC machine. A major upgrade of the LHCb experiment will take place prior to the start of Run 3 operations in 2021. The upgraded Vertex Locator (VELO) is an essential component of this upgrade. Its main role is to enable high precision track and vertex reconstruction, with data-driven readout to the software trigger at 40 MHz, in the higher-luminosity environment of Run 3. To achieve this goal, significant improvements are planned with respect to the current detector, including a switch from microstrips to pixels, upgraded electronics, and a new cooling system. I will briefly motiviate the need for an upgrade, describe the main aspects of the VELO upgrade design, and show highlights of recent sensor characterisation studies using the CERN SPS test beam.

A Time-to-Digital Converter Based on a Digitally Controlled Oscillator

Time measurements play a crucial role in trigger and data acquisition systems of high-energy physics experiments, where synchronization between system elements, signal calibration, and phase-measurement accuracy is often required. In this paper, we present a fully digital time-to-digital converter (TDC) architecture and its application, based on a synthesizable digitally controlled oscillator (DCO), where the TDC is used to measure the phase relationship between a timing signal and a 40-MHz reference clock. The DCO design is technology independent; it is described by means of a hardware description language and it can be placed and routed with automatic tools. The TDC will be used in the new readout chip that is under development for the muon detector electronic upgrade in Large Hadron Collider beauty experiment at CERN. The TDC presented in this paper has the fundamental task of measuring, with a resolution of about 1.5 ns, the phase difference between the 40-MHz LHC machine clock and a digital signal coming from the muon detector.

CMS Pixel Detector design for HL-LHC

The LHC machine is planning an upgrade program which will smoothly bring the luminosity to about 7.5×1034cm−2s−1 in 2028, to possibly reach an integrated luminosity of 3000 fb−1 by the end of 2037. This High Luminosity scenario, HL-LHC, will present new challenges in higher data rates and increased radiation. In order to maintain its physics reach the CMS collaboration has undertaken a preparation program of the detector known as Phase-2 upgrade. The CMS Phase-2 Pixel upgrade will require a high bandwidth readout system and high radiation tolerance for sensors and on-detector ASICs. Several technologies for the upgrade sensors are being studied. Serial powering schemes are under consideration to accommodate significant constraints on the system. These prospective designs, as well as new layout geometries that include very forward pixel discs, will be presented together with performance estimation.

Upgrades of the CMS Outer Tracker for HL-LHC

The LHC machine is planning an upgrade program which will smoothly bring the luminosity to about 5×1034cm−2s−1 around 2028, to possibly reach an integrated luminosity of 3000fb−1 in the following decade. This High Luminosity LHC scenario, HL-LHC, will require a preparation program of the LHC detectors known as Phase-2 upgrade. The current CMS Outer Tracker, already running close to its design limits, will not be able to survive HL-LHC radiation conditions and CMS will need a completely new device, in order to fully exploit the highly demanding operating conditions and the delivered luminosity. The new Tracker should have also L1 trigger capabilities. To achieve such goals, R&D activities are ongoing to explore options and develop solutions that would allow including tracking information at Level-1. The design choices for the CMS Outer Tracker upgrades are discussed along with some highlights of the R&D activities.

Analysis of the quench propagation along Nb3Sn Rutherford cables with the THELMA code. Part I: Geometric and thermal models

The paper describes the new lumped thermal model recently implemented in THELMA code for the coupled electromagnetic–thermal analysis of superconducting cables. A new geometrical model is also presented, which describes the Rutherford cables used for the accelerator magnets. A first validation of these models has been given by the analysis of the quench longitudinal propagation velocity in the Nb3Sn prototype coil SMC3, built and tested in the frame of the EUCARD project for the development of high field magnets for LHC machine. This paper shows in detail the models, while their application to the quench propagation analysis is presented in a companion paper.