Cryogenics is a key enabling technology for present and future particle accelerators and detectors, providing the conditions required for the operation of superconducting magnets, superconducting RF cavities, vacuum systems, and particle detection devices. However, extracting heat at very low temperatures requires large amounts of energy, often representing a major share of the total energy demands of the facilities. This article presents the main factors driving energy consumption, the status of the technology for a large spectrum of temperatures, and possible developments for improving the efficiency of cryogenic systems. It discusses the impact of cryogenic cooling configurations and the potential of new superconducting materials towards improved sustainability of future accelerators and particle detectors.

The High-Luminosity upgrade of the Large Hadron Collider (HL-LHC) at CERN is a major upgrade project of the LHC accelerator, which will allow increasing the peak luminosity by at least a factor of five beyond LHC’s design value. This upgrade will include the replacement of the final focusing superconducting magnets and the implementation of superconducting radiofrequency crab cavities in the long straight sections of the interaction points 1 and 5 of LHC. The cryogenic part of this upgrade consists in the design, specification, procurement, installation, commissioning, and handover to operation of two new cryogenic plants and associated cryogenic distribution lines at the machine interaction points for the high luminosity insertions dedicated to CMS and ATLAS detectors. The two new cryogenic plants, with an equivalent capacity of 14 kW@4.5 K, including 3.25 kW@1.9 K, were defined based on the heat loads of the new superconducting magnets and radiofrequency crab cavities, of the cold powering systems, and of distribution heat loads. This paper presents the details of the static and dynamic heat loads applied to each cryogenic element added for the HL-LHC, the methodology for addressing the maturity of their design, the defined nominal operating modes and finally the required helium refrigerators cooling capacity for each temperature level, considering the effect of luminosity and beam energy.

The Large Hadron Collider (LHC) Long Shutdown 2 (2019-2021), following LHC Run 2, was primarily dedicated to the upgrade of the LHC Injectors but it included also a significant amount of activities aimed at consolidation of the LHC machine components, removal of known limitations and initial upgrades in view of the High -Luminosity LHC (HL-LHC) to favour the intensity ramp -up during Run 3 (2022-2025). An overview of the major modifications to the accelerator and its systems is followed by a summary of the results of the superconducting magnet training campaign to increase the LHC operation energy beyond the maximum value of 6.5 TeV reached during Run 2. The LHC configuration and the scenarios for proton and ion operation for Run 3 are presented considering the expected performance of the upgraded LHC Injectors and the proton beam intensity limitations resulting from the heat load on the cryogenic system due to beam -induced electron cloud and impedance.

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.

In the context of HL-LHC project, the duty of the existing LHC helium refrigerators will face an increase of beam induced heat loads in the arcs. The cryogenic sectors around the LHC Point 4 (P 4), are considered in priority since the respective refrigerators must cover in addition to the superconducting (sc) magnetic system the sc Radio Frequency (SRF) cavities. Initial studies demonstrated that the upgrade of one of the existing two cryoplants at P 4 would prevent adding a third new refrigerator as foreseen in the baseline scenario. Complementary studies demonstrated that only one helium refrigerator delivered in 1993 by Linde Kryotechnik® needed to be upgraded, representing a particular challenge as it had already been modified twice to match the LEP 200 and the LHC project requirements. This paper will present the upgrade of ex-LEP helium refrigerator, located at LHC P 4 with split cold boxes (surface and underground) emphasizing the challenges to obtain the required additional refrigeration capacity equivalent to 2 kW@4.5 K with respect to the existing plant capacity of 16.5 kW@4.5 K. The major action has been to integrate and replace all expansion turbines using new state-of-the-art turbines, while verifying existing equipment manufactured since the nineties. The constraints, singularities of each surface and underground cold boxes and project risks assessment led to different technical solutions for the integration of the new turbines will be described. The performance obtained for the upgraded ex-LEP refrigerator will be presented as well as 18 months of operation facts with cool-down of the corresponding LHC sector and preparation for beams.

In the framework of the future High Luminosity upgrade of the Large Hadron Collider (HL-LHC) at CERN, most superconducting magnets in the Long Straight Sections will be replaced. Among them, the new D2 recombination dipole will be a He II conduction-cooled magnet with a larger aperture than the LHC dipoles. To provide the required cooling (up to 70 W) to the D2 and to comply with its cryostat integration constraints, a compact heat exchanger was designed by the CEA Département des Systèmes Basses Températures (DSBT) based on a CERN preliminary analysis and a CEA review of the possible cooling schemes. This heat exchanger provides the required heat transfer between the He II pressurized bath and the He II saturated bath to cool the D2 magnet in different operating conditions at 1.8 K and 2 K. The detailed design of the heat exchanger was defined and one prototype was manufactured by industry under the CEA supervision. The heat exchanger prototype is composed of roughly one hundred oxygen-free high purity copper tubes, electron beam welded to the stainless steel He II bath enclosure. The present paper describes the successful cryogenic performance tests of the prototype of the D2 heat exchanger measured in the CEA 400W@1.8K test facility in Grenoble.

After the successful completion of the cryogenic performance tests of the He II heat exchanger prototype for the D2 recombination dipole of the future HL-LHC project at CERN, specific measurements were performed to determine the operation safety margin in case of abnormal operating conditions. This is particularly relevant in case of the failure of liquid helium supply in the He II cold source. For nominal operation, the liquid level is regulated at a constant value and it is not necessary to know its value very accurately. However, in case of a partial drying of the heat exchanger due to discontinuation of the helium liquid supply, it is essential to monitor the absolute value of the liquid level to anticipate any cooling malfunction. This paper describes the procedure for an accurate in-situ He II level measurement as well as for the heat loss and mass flow rate estimates in a He II phase separator. The operation safety margins of the He II heat exchanger prototype for the D2 magnet are then analyzed for the different operating conditions considered during the HL-LHC runs in case of non-nominal liquid level in the He II cold source.

The High Luminosity Large Hadron Collider (HL-LHC) upgrade is planned to take place during next long shut down of the LHC, starting in 2025. During this period the matching sections of both ATLAS and CMS experiments will be upgraded to allow for increasing collisions rate and more efficient luminosity production. One of the new element which will be introduced to the layout is the beam deflecting RF system so-called as crab cavities. Eight crab cavities modules, operating in superfluid helium, will be installed in this new machine layout. This paper will focus on the cryogenic design solutions adopted and integrated in the crab cavities cryogenic modules. The concept of the cryogenic modules was created in 2012 and evolved over the years with introduced optimizations of the cryogenic local cooling loops and related safety system. Design aspects of the chosen solutions for the first prototype and LHC compatible solution will be discussed. The thermal behavior results from operation on SPS proton beam from the first prototype module will be developed.

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.

The upgrade of the cryogenics for the HL-LHC will consist of the following: - The design and installation of two new cryogenic plants at P1 and P5 for high luminosity insertions. This upgrade will be based on a new sectorization scheme aimed at separating the cooling of the magnets in these insertion regions from the arc magnets and considering the newt feedboxes and superconducting links located in underground infrastructures. - The design and installation of a new cryogenic distribution lines (QXL) at P1 and P5 in the LHC tunnel and in a new underground service galleries. - The upgrade of the existing cryogenic plant (QSRA and QURA) cooling the LHC sector 3-4 located at P4. - The cryogenic design support for superconducting devices, such as magnets, crab cavities, superconducting links, and the hollow electron lenses. Some other options such as new cryogenic circuits at P7 for the HTS links and displaced current feedboxes or a new cryoplant in P4 have been discarded.

This document specifies the functional requirements for the MQXFA magnet readapted for the American contribution. If all the requirements specified in this document are met, then the U.S. HL-LHC AUP MQXFA deliverables will be accepted by CERN for the HL-LHC project. Another separate document will be issued by the American contribution for the MQXFA cold mass functional requirements. Please note that the definition of threshold as it is being used by the American contribution is not the same as objective, according to the HL-LHC quality policy.

The high luminosity upgrade of the Large Hadron Collider will require the replacement of the triplet of final focusing superconducting magnets at interaction points 1 and 5 with a new set of helium II cooled magnets. To validate the technologies and assembly procedures and to investigate the collective effects, a string of magnets representative of the final HL-LHC configuration, will be installed in the SM18 test facility at CERN in 2021. The local cryogenic distribution connected to the string of magnets will also be representative of the final configuration. This paper first describes the cryogenic configuration of the magnet string and the functional cryogenic requirements for the operation and test program. The cryogenic parameters for the different circuits and operation phases are presented. The paper details the cryogenic test program foreseen to investigate the specific challenges of cooling Nb3Sn magnets and their beam screens during high heat load (several hundred watts at 1.9 K), steady state, and transient operation. The paper presents the complete cryogenic system planned for the string, its integration into the SM18 facility, the required adaptation of the SM18 cryogenic infrastructure, and the conceptual design of the main components.

New D2 recombination dipoles with a larger aperture than in the LHC dipoles are required for the future High Luminosity LHC at CERN. These 13.5 m-long D2 magnets are proposed to be conduction cooled in a static bath of pressurized He II. Their cooling is provided via pressurized He II channels located in the D2 iron yoke and thermally connected to a saturated bath installed at one end of each D2 dipole. The heat transfer between the pressurized He II static bath and the bath pumped down to 16.4 mbar (1.8 K) is performed in a heat exchanger under study at CEA. Various design solutions were studied and evaluated to define the more suitable solution fulfilling on the one hand D2 cooling requirements (up to 70 W) and on the other hand D2 cryostat integration constraints. The paper will report on the D2 cooling needs and constraints, present the studied options and detail the main design features of the selected solution for a compact heat exchanger for D2 dipoles.

In the framework of HL-LHC, a new infrastructure was installed in 2018, to test SRF structures in the proton beams of the SPS. Scope of the test stand is to study the operational performance of crab-cavities for HL-LHC more generally, SRF cavities through a wide range of proton beam parameters up to high energy and current, under safe conditions for equipment and personnel. The SPS beam instrumentation is used to monitor orbit centering, RF phase scans, bunch rotation. To minimize impact on beam time, infrastructure and services allow for full remote control. Critical aperture restrictions are overcome by placing the test structure and its ancillaries on a motorized table for lateral translation inand out of beam. Two articulated Yshaped vacuum chambers connect the test cryomodule on a beam by-pass. A new cryogenic refrigerator is installed in a split scheme, with an underground cold box fed from a surface compressor. The two Inductive Output Tubes (IOT) power amplifiers deliver up to 50kW cw via coaxial transmission lines to the two cavities and charges and circulators, the latter installed on the translation table. Interlocks and safety equipment complete the test stand.

Recurrent beam dumps significantly perturbed the operation of the CERN LHC in the summer months of 2017, especially in August. These unexpected beam dumps were triggered by fast beam losses that built up in the cryogenic beam vacuum at the half-cell 16 left of LHC-IP2 and were detected either at that location, but mainly in the collimation insertions. This contribution details the experimental observables (beam losses, coherent instabilities, heat load to cryogenic system, vacuum signals), the extent of the understanding of the beam loss and instability mechanisms and the mitigation steps and new settings that allowed recovering the luminosity performance of the LHC for the rest of the Run.

During the second LHC physics operation period (Run2), between 2015 and 2018, the accelerator operation modes and beam parameters have been adapted thus allowing significantly improved integrated luminosity production. Increased energy, intensity and adapted beam operation schemes with 25 ns of inter-bunches spacing have an essential influence on the dynamic heat load generation with direct impact on the cryogenic cooling system. In order to cope with significantly higher than expected beam induced thermal load, the cryogenic system was tuned and optimized to adapt the required refrigeration capacity to the beam operational requirements. The most challenging part of tuning was focused on the dynamic heat load compensation on the beam screens circuits. The paper will provide the overview on the main differences between the theoretical heat load values considered for initial design and the on-line measurements performed on cryogenic LHC sectors. Finally, the paper will summarize the methodology and tools implemented in the cryogenic process control system allowing the highly efficient on-line adaptation of the refrigeration power with respect to the beam induced heat load distribution.

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.

The cryogenic system of the LHC will be upgraded by 2025 to comply with a considerable increase of beam induced heat loads deriving from higher beam currents and peak luminosity levels from the High Luminosity LHC. The current baseline foresees a modified sectorisation scheme with three additional cryogenic plants dedicated to cool the insertions at LHC's points 1, 4 and 5, reducing the refrigeration duty of the existent adjacent plants. This paper assesses the refrigeration duty of the eight existing plants considering the modified sectorisation and increased heat load deposition. The accelerator loads and distribution losses are quantified for each plant and compared to the existing refrigeration capacity. The heat load values were obtained from the extrapolation of previous LHC assessments as well as from new calculations. Specifically for the LHC point 4 cryogenic equipment, based on updated refrigeration requirements, the upgrade of an existing plant is proposed as an alternative to the baseline scenario.