High Luminosity Upgrade Of The Large Hadron Collider Research Articles

Beam test measurements of Low Gain Avalanche Detector single pads and arrays for the ATLAS High Granularity Timing Detector

For the high luminosity upgrade of the LHC at CERN, ATLAS is considering the addition of a High Granularity Timing Detector (HGTD) in front of the end cap and forward calorimeters at |z|= 3.5 m and covering the region 2.4 <|η|< 4 to help reducing the effect of pile-up. The chosen sensors are arrays of 50 μm thin Low Gain Avalanche Detectors (LGAD). This paper presents results on single LGAD sensors with a surface area of 1.3×1.3 mm2 and arrays with 2×2 pads with a surface area of 2×2 mm2 or 3×3 mm2 each and different implant doses of the p+ multiplication layer. They are obtained from data collected during a beam test campaign in autumn 2016 with a pion beam of 120 GeV energy at the CERN SPS. In addition to several quantities measured inclusively for each pad, the gain, efficiency and time resolution have been estimated as a function of the position of the incident particle inside the pad by using a beam telescope with a position resolution of few μm. Different methods to measure the time resolution are compared, yielding consistent results. The sensors with a surface area of 1.3×1.3 mm2 have a time resolution of about 40 ps for a gain of 20 and of about 27 ps for a gain of 50 and fulfil the HGTD requirements. Larger sensors have, as expected, a degraded time resolution. All sensors show very good efficiency and time resolution uniformity.

Cryogenic Considerations for Cooling Superconducting Links and Application to the High Luminosity Upgrade of the Large Hadron Collider (LHC)

Powering superconducting (SC) magnets in particle accelerators remotely through SC links allow to install the power supplies, the current leads, and the ancillary equipment away from the limited space, high radiation areas next to the magnets. Accelerator facilities with SC magnets at 4.5 K always need a significant cryogenic infrastructure; integrating the SC links with the cryogenic system requires a variety of considerations besides the thermal performance of the link's cryostat. In this paper, we describe the main factors and possible choices for cooling SC links for some selected configurations of the cryogenic infrastructure, focussing on cooling schemes applicable to the high luminosity upgrade of the LHC. The study covers SC links using NbTi or higher temperature superconductors like MgB2, interfaces for supply and recovery of the cooling fluid, hardware constraints, and flow requirements for the current leads. We compare in a parametric study the different factors in terms of equivalent refrigeration power at 4.5 K, and also present the economic implications at the refrigerator level in terms of capital and operational costs.

Performance of the CMS precision electromagnetic calorimeter at LHC Run II and prospects for High-Luminosity LHC

Many physics analyses using the Compact Muon Solenoid (CMS) detector at the LHC require accurate, high-resolution electron and photon energy measurements. Following the excellent performance achieved during LHC Run I at center-of-mass energies of 7 and 8 TeV, the CMS electromagnetic calorimeter (ECAL) is operating at the LHC with proton-proton collisions at 13 TeV center-of-mass energy. The instantaneous luminosity delivered by the LHC during Run II has achieved unprecedented levels. The average number of concurrent proton-proton collisions per bunch-crossing (pileup) has reached up to 40 interactions in 2016 and may increase further in 2017. These high pileup levels necessitate a retuning of the ECAL readout and trigger thresholds and reconstruction algorithms. In addition, the energy response of the detector must be precisely calibrated and monitored. We present new reconstruction algorithms and calibration strategies that were implemented to maintain the excellent performance of the CMS ECAL throughout Run II. We will show performance results from the 2015-2016 data taking periods and provide an outlook on the expected Run II performance in the years to come. Beyond the LHC, challenging running conditions for CMS are expected after the High-Luminosity upgrade of the LHC (HL-LHC) . We review the design and R&D studies for the CMS ECAL and present first test beam studies. Particular challenges at HL-LHC are the harsh radiation environment, the increasing data rates, and the extreme level of pile-up events, with up to 200 simultaneous proton-proton collisions. We present test beam results of hadron irradiated PbWO crystals up to fluences expected at the HL-LHC . We also report on the R&D for the new readout and trigger electronics, which must be upgraded due to the increased trigger and latency requirements at the HL-LHC.

Quench Behavior of the HL-LHC Twin Aperture Orbit Correctors

A study was performed to understand the quench behavior and ensure adequate quench protection of the canted cosine theta (CCT) twin aperture orbit corrector magnet, a superconducting magnet under development as part of the high-luminosity upgrade of the Large Hadron Collider (HL-LHC). The cosine theta geometry features canted superconducting coils, which together produce a magnetic dipole field. The NbTi/Cu strands are placed in slots inside formers that maintain the shape of the coils. The presence of these formers affects the quench behavior of the magnet by preventing direct thermal contact between adjacent groups of strands. At the same time, a discharge of the stored energy over an external resistor results in significant eddy current heating inside the formers, which quickly brings the entire superconducting magnet to a normal state. A calculation model was developed that describes the electrical and thermal behavior of this type of magnet, and the results of this model are compared to experimental observations on a 0.5 m CCT model coil. It is found that the calculation results and experimental observations are generally consistent, although the simplified manner in which the eddy current heating is described in the model leads to a modest overestimation of the hotspot temperature. The calculation results indicate that a proposed quench protection configuration, featuring a discharge over a 0.7 $\Omega$ energy extractor and a 0.05 $\Omega$ crowbar, is sufficient to protect both the 0.5 m CCT model magnet and the 2.2 m CCT prototype magnet, resulting in hotspot temperatures of 63 and 193 K, and peak voltages to ground of 300 and 310 V, respectively.

Comparison of two hardware-based hit filtering methods for trackers in high-pileup environments

As experiments in high energy physics aim to measure increasingly rare processes, the experiments continually strive to increase the expected signal yields. In the case of the High Luminosity upgrade of the LHC, the luminosity is raised by increasing the number of simultaneous proton-proton interactions, so-called pile-up. This increases the expected yields of signal and background processes alike. The signal is embedded in a large background of processes that mimic that of signal events. It is therefore imperative for the experiments to develop new triggering methods to effectively distinguish the interesting events from the background. We present a comparison of two methods for filtering detector hits to be used for triggering on particle tracks: one based on a pattern matching technique using Associative Memory (AM) chips and the other based on the Hough transform. Their efficiency and hit rejection are evaluated for proton-proton collisions with varying amounts of pile-up using a simulation of a generic silicon tracking detector. It is found that, while both methods are feasible options for a track trigger with single muon efficiencies around 98–99%, the AM based pattern matching produces a lower number of hit combinations with respect to the Hough transform whilst keeping more of the true signal hits. We also present the effect on the two methods of increasing the amount of support material in the detector and of introducing inefficiencies by deactivating detector modules. The increased support material has negligable effects on the efficiency for both methods, while dropping 5% (10%) of the available modules decreases the efficiency to about 95% (87%) for both methods, irrespective of the amount of pile-up.

Training Performance With Increased Coil Prestress of the 2 m Model Magnet of Beam Separation Dipole for the HL-LHC Upgrade

A large aperture beam separation dipole (D1 magnet) is required to generate a field integral of 35 Tm at 12 kA and 1.9 K for the high luminosity upgrade of the large hadron collider (HL-LHC). High-energy accelerator Research Organization (KEK) fabricated the first 2 m model (MBXFS1) and a magnet test was performed in 2015–2016. Although the quench current reached the nominal operating current, training performance was not satisfactory. The measured azimuthal coil pre-stress was completely released below the nominal current, and this was suggested to be the main reason for the insufficient training performance. After the magnet test, MBXFS1 was disassembled and reassembled as MBXFS1b after increasing the coil pre-stress. In this paper, we report the reassembly of the magnet and test results focusing on the training performance.

Field Quality of MBH 11-T Dipoles for HL-LHC and Impact on Beam Dynamic Aperture

For the high-luminosity upgrade of the Large Hadron Collider (HL-LHC), the development of the 11-T Nb $_3$ Sn dipole (MBH) is progressing. At present, one double-aperture and five single-aperture short-model magnets have been built and tested. Magnetic measurements have been performed both at ambient and cryogenic temperatures. In addition, a double-aperture short model and the first full-length collared-coil assemblies have been produced and measured at ambient temperature. In this paper, the measurement results collected so far are presented and discussed. The geometrical field multipoles, the iron saturation effects, as well as the effects of persistent currents are summarized. Experimental data are compared with the magnetic calculations using the CERN field computation program ROXIE and discussed in view of the construction of the final magnets. In addition, based on the measured field errors, beam dynamics simulations have been carried out by taking into consideration different scenarios with and without MBH magnets installed in the accelerator.

Windability Tests of Nb3Sn Rutherford Cables for HL-LHC and FCC

In the framework of the development of high field magnets made of Nb3Sn superconductors for projects like the high luminosity upgrade of the large hadron collider (HL-LHC) and FCC studies, it is necessary to refine the understanding of the winding process and its impact of the overall mechanical integrity of the conductor. The mechanical behavior during winding of the unreacted Rutherford-type Nb3Sn cables has been studied experimentally. In order to quantify the windability of the various cable designs, the measurement method and a dedicated specimen-scanning device has been developed. The validation of the device and achieved repeatability are presented in this paper. The results from the first winding test campaign with cables 11 T dipole RRP, 11 T dipole dummy Cu, MQXF, FRESCA 2, and NbTi are shown.

Magnetic Measurements on the First CERN-Built Models of the Insertion Quadrupole MQXF for HL-LHC

The high-luminosity upgrade of the large hadron collider (HL-LHC) requires new high-field and large-aperture quadrupole magnets for the low-beta inner triplets (MQXF). CERN and LARP are currently collaborating to develop a 150-mm-aperture quadrupole based on Nb $_3$ Sn superconducting cables for the coils, and an aluminum shell with the bladder-key technology for the support structure. This paper presents the test setup for magnetic measurements, both at ambient and cryogenic temperatures, and the instrumentation being used for the first two short-models of MQXF built and tested at CERN. Finally, the measurement results, in terms of field quality, effects of persistent currents, and iron saturation are reported and discussed.

STEAM: A Hierarchical Cosimulation Framework for Superconducting Accelerator Magnet Circuits

Simulating the transient effects occurring in superconducting accelerator magnet circuits requires including the mutual electro-thermo-dynamic interaction among the circuit elements, such as power converters, magnets, and protection systems. Nevertheless, the numerical analysis is traditionally done separately for each element in the circuit, leading to possible non-consistent results. We present STEAM, a hierarchical co-simulation framework featuring the waveform relaxation method. The framework simulates a complex system as a composition of simpler, independent models that exchange information. The convergence of the coupling algorithm ensures the consistency of the solution. The modularity of the framework allows integrating models developed with both proprietary and in-house tools. The framework implements a user-customizable hierarchical algorithm to schedule how models participate to the co-simulation, for the purpose of using computational resources efficiently. As a case study, a quench scenario is co-simulated for the inner triplet circuit for the High Luminosity upgrade of the LHC at CERN.

The CMS ECAL Upgrade for Precision Crystal Calorimetry at the HL-LHC

The electromagnetic calorimeter (ECAL) of the Compact Muon Solenoid Experiment (CMS) is operating at the Large Hadron Collider (LHC) with proton-proton collisions at 13 TeV center-of-mass energy and at a bunch spacing of 25 ns. Upgrades are necessary for the High-Luminosity upgrade of the LHC (HL-LHC). We review the design and R&amp;D studies for the CMS ECAL crystal calorimeter upgrade. We present test beam results of hadron irradiated PbWO4 crystals up to fluences expected at the HL-LHC. We also report on the R&amp;D for the new readout and trigger electronics, which must be upgraded due to the increased trigger and latency requirements at the HL-LHC.

Social Cost-Benefit Analysis of HL-LHC

We present a Social Cost–Benefit Analysis (CBA) of the High Luminosity upgrade of the Large Hadron Collider (HL-LHC), assessing its economic costs and benefits up to 2038. The Net Present Value (NPV) of the HL-LHC project is positive at the end of the observation period. The ratio between incremental benefits and incremental costs of the HL-LHC with respect to continue operating the LHC under normal consolidation (i.e. without high-luminosity upgrade) is slightly over 1.7, meaning that each Swiss Franc invested in the HL-LHC upgrade project pays back approximately 1.7 CHF in societal benefits. Simulations based on 50000 Monte Carlo rounds show that there is a 94% chance to observe a positive NPV (i.e. a quantifiable economic benefit for the society). The attractiveness of CERN for Early Stage Researchers (ESR) is key for a positive CBA result. Given that benefits to ESRs are the single most important societal benefit, CERN should invest more in activities facilitating the transition to the international job market. Technological spillovers are another very important ingredient in the CBA evaluation of CERN research infrastructures. Relations with firms in the supply chain and development of Information and Communication Technologies (ICT) are strategic levers CERN should use to boost these social benefits. Cultural effects, especially those related to onsite visitors, have a great development potential for generating societal benefits. This part needs to be quantified in greater detail to understand, how this benefit potential can be further increased efficiently in cooperation with the host states.

Parametric study on the thermal performance of beam screen samples of the High-Luminosity LHC upgrade

The High-Luminosity upgrade of the Large Hadron Collider (HL-LHC) will increase the accelerator’s luminosity by a factor 10 beyond its original design value, giving rise to more collisions and generating an intense flow of debris. A new beam screen has been designed for the inner triplets that incorporates tungsten alloy blocks to shield the superconducting magnets and the 1.9 K superfluid helium bath from incoming radiation. These screens will operate between 60 K and 80 K and are designed to sustain a nominal head load of 15 Wm−1, over 10 times the nominal heat load for the original LHC design. Their overall new and more complex design requires them and their constituent parts to be characterised from a thermal performance standpoint. In this paper we describe the experimental parametric study carried out on two principal thermal components: a representative sample of the beam screen with a tungsten-based alloy block and thermal link and the supporting structure composed of an assembly of ceramic spheres and titanium springs. Results from both studies are shown and discussed regarding their impact on the baseline considerations for the thermal design of the beam screens.

The design of the new LHC connection cryostats

In the frame of the High Luminosity upgrade of the LHC, improved collimation schemes are needed to cope with the superconducting magnet quench limitations due to the increasing beam intensities and particle debris produced in the collision points. Two new TCLD collimators have to be installed on either side of the ALICE experiment to intercept heavy-ion particle debris. Beam optics solutions were found to place these collimators in the continuous cryostat of the machine, in the locations where connection cryostats, bridging a gap of about 13 m between adjacent magnets, are already present. It is therefore planned to replace these connection cryostats with two new shorter ones separated by a bypass cryostat allowing the collimators to be placed close to the beam pipes. The connection cryostats, of a new design when compared to the existing ones, will still have to ensure the continuity of the technical systems of the machine cryostat (i.e. beam lines, cryogenic and electrical circuits, insulation vacuum). This paper describes the functionalities and the design solutions implemented, as well as the plans for their construction.

Cryogenic test facility instrumentation with fiber optic and fiber optic sensors for testing superconducting accelerator magnets

The magnets for the next steps in accelerator physics, such as the High Luminosity upgrade of the LHC (HL- LHC) and the Future Circular Collider (FCC), require the development of new technologies for manufacturing and monitoring. To meet the HL-LHC new requirements, a large upgrade of the CERN SM18 cryogenic test facilities is ongoing with the implementation of new cryostats and cryogenic instrumentation. The paper deals with the advances in the development and the calibration of fiber optic sensors in the range 300 - 4 K using a dedicated closed-cycle refrigerator system composed of a pulse tube and a cryogen-free cryostat. The calibrated fiber optic sensors (FOS) have been installed in three vertical cryostats used for testing superconducting magnets down to 1.9 K or 4.2 K and in the variable temperature test bench (100 - 4.2 K). Some examples of FOS measurements of cryostat temperature evolution are presented as well as measurements of strain performed on a subscale of High Temperature Superconducting magnet during its powering tests.

Precision Timing with Silicon Sensors for Use in Calorimetry

The high luminosity upgrade of the Large Hadron Collider (HL-LHC) at CERN is expected to provide instantaneous luminosities of 5 × 1034cm−2s−1. The high luminosities expected at the HL-LHC will be accompanied by a factor of 5 to 10 more pileup compared with LHC conditions in 2015, causing general confusion for particle identification and event reconstruction. Precision timing allows to extend calorimetric measurements into such a high density environment by subtracting the energy deposits from pileup interactions. Calorimeters employing silicon as the active component have recently become a popular choice for the HL- LHC and future collider experiments which face very high radiation environments. We present studies of basic calorimetric and precision timing measurements using a prototype composed of tungsten absorber and silicon sensor as the active medium. We show that for the bulk of electromagnetic showers induced by electrons in the range of 20 GeV to 30 GeV, we can achieve time resolutions better than 25 ps per single pad sensor.

The CMS ECAL Upgrade for Precision Crystal Calorimetry at the HL-LHC

The Compact Muon Solenoid (CMS) experiment is operating at the Large Hadron Collider (LHC) with proton–proton collisions at 13-TeV center-of-mass energy and at a bunch spacing of 25 ns. Challenging running conditions for CMS are expected after the High-Luminosity upgrade of the LHC (HL-LHC). We review the CMS ECAL crystal calorimeter upgrade and present results from the first test beam studies. Particular challenges at HL-LHC are the harsh radiation environment, the increasing data rates, and the extreme level of pile-up events, with up to 200 simultaneous proton–proton collisions. Precision timing can be exploited to reduce the effect of the pileup. We report on the timing resolution studies performed with test beams. We discuss the new readout and trigger electronics, which must be upgraded due to the increased trigger and latency requirements at the HL-LHC.

Investigations into the impact of locally modified sensor architectures on the detection efficiency of silicon micro-strip sensors

The High Luminosity Upgrade of the LHC will require the replacement of the Inner Detector of ATLAS with the Inner Tracker (ITk) in order to cope with higher radiation levels and higher track densities. Prototype silicon strip detector modules are currently developed and their performance is studied in both particle test beams and X-ray beams. In previous test beam measurements of prototype modules, the response of silicon sensors has been studied in detailed scans across individual sensor strips. These scans found instances of sensor strips collecting charge across areas on the sensor deviating from the geometrical width of a sensor strip. The variations have been linked to local features of the sensor architecture. This paper presents results of detailed sensor measurements in both X-ray and particle beams investigating the impact of sensor features (metal pads and p-stops) on the sensor strip response.

Transient Thermoelectrical Behavior of MgB2 Cables in the Superconducting Links for the High Luminosity Upgrade of the LHC

High-current superconducting links (SC-Link) consisting of multiple MgB2 cables are being developed for the cold powering of the high luminosity upgrade of the LHC (HL-LHC). The SC-Links are designed to work over a temperature gradient between 4.2 and 20 K cooled by flowing subcritical helium gas to provide up to 20 kA per cable to the new inner-triplets to gain a higher luminosity. The SC-Links have a compact configuration of six 20-kA cables for three magnet circuits and several lower current cables. Upon the quench of one magnet, the neighboring circuits are exposed to a transient field up to 0.3 T, which may induce significant magnetization and coupling current losses with the potential of quenching the cables in these circuits. In this paper, we present a thermoelectrical analysis of the transient electromagnetics of the cable system, the thermal stability of the cables in temperature gradient, and the effect of lateral cooling by the flowing helium gas on the quench behavior. Furthermore, the analysis was compared with a comprehensive experimental study of direct thermometric and electrical measurements of the transient magnetic losses at different temperatures (4–30 K) and field sweep rates (0.3–30 T/s) as well as quench measurements under different cooling configurations. Cables samples of different stabilization matrices and strand configurations were measured to provide insight for the overall optimization by compromising between loss reduction and cryogenic stabilization.

Fabrication and Test Results of the First 2 m Model Magnet of Beam Separation Dipole for the HL-LHC Upgrade

The large aperture superconducting beam separation dipole (D1) must be developed as part of the high luminosity upgrade of the Large Hadron Collider (HL-LHC) at CERN. The most important specifications of the D1 are a coil aperture of 150 mm and field integral of 35 T·m at 12.0 kA and 1.9 K. The technical challenges of this magnet include predicting the changes in the size of the coils during fabrication, achieving a high radiation resistance, and management of the iron saturation. KEK is in charge of developing the D1. After conducting a series of design studies, a design consisting of a single-layer coil based on Nb-Ti technology with a collared yoke structure was selected. The fabrication of the first 2 m model of the D1 started at KEK in 2015. Newly developed radiation resistant glass-fiber-reinforced-plastics were utilized as coil parts in the accelerator magnets for the first time. The 2 m model was subjected to two cycles of cold test in the vertical cryostat at KEK. In this paper, we report on the fabrication of the 2 m model. We also present and discuss the quench test results from the perspective of the coil prestress.