Technology Archives – Page 4 of 112

CLEAR highlights and goals

Particle accelerators have revolutionised our understanding of nature at the smallest scales, and continue to do so with facilities such as the LHC at CERN. Surprisingly, however, the number of accelerators used for fundamental research represents a mere fraction of the 50,000 or so accelerators currently in operation worldwide. Around two thirds of these are employed in industry, for example in chip manufacturing, while the rest are used for medical purposes, in particular radiotherapy. While many of these devices are available “off-the-shelf”, accelerator R&D in particle physics remains the principal driver of innovative, next-generation accelerators for applications further afield.

The CERN Linear Electron Accelerator for Research (CLEAR) is a prominent example. Launched in August 2017 (CERN Courier November 2017 p8), CLEAR is a user facility developed from the former CTF3 project which existed to test technologies for the Compact Linear Collider (CLIC) – a proposed e⁺e^– collider at CERN that would follow the LHC. During the past five years, beams with a wide range of parameters have been provided to groups from more than 30 institutions across more than 10 nations.

CLEAR was proposed as a response to the low availability of test-beam facilities in Europe. In particular, there was very little time available to users on accelerators with electron beams with an energy of a few hundred MeV, as these tend to be used in dedicated X-ray light-source and other specialist facilities. CLEAR therefore serves as a unique facility to perform R&D towards a wide range of accelerator-based technologies in this energy range. Independent of CERN’s other accelerator installations, CLEAR has been able to provide beams for around 35 weeks per year since 2018, as well as during long shutdowns, and even managing successful operation during the COVID-19 pandemic.

Flexible physics

As a relatively small facility, CLEAR operates in a flexible fashion. Operators can vary the range of beams available with relative ease by tailoring many different parameters, such as the bunch charge, length and energy, for each user. There is regular weekly access to the machine and, thanks to the low levels of radioactivity, it is possible to gain access to the facility several times per day to adjust experimental setups if needed. Along with CLEAR’s location at the heart of CERN, the facility has attracted an eager stream of users from day one.

CLEAR has attracted an eager stream of users from day one

Among the first was a team from the European Space Agency working in collaboration with the Radiation to Electronics (R2E) group at CERN. The users irradiated electronic components for the JUICE (Jupiter Icy Moons Explorer) mission with 200 MeV electron beams. Their experiments demonstrated that high-energy electrons trapped in the strong magnetic fields around Jupiter could induce faults, so-called single event upsets, in the craft’s electronics, leading to the development and validation of components with the appropriate radiation-hardness. The initial experiment has been built upon by the R2E group to investigate the effect of electron beams on electronics.

**Access all areas** Inspecting beamline equipment to ensure that CLEAR continues to operate efficiently. Credit: CERN-PHOTO-201711-271-2

As the daughter of CTF3, CLEAR has continued to be used to test the key technological developments necessary for CLIC. There are two prototype CLIC accelerating structures in the facility’s beamline. Originally installed to test CLIC’s unique two-beam acceleration scheme, the structures have been used to study short-range “wakefield kicks” that can deflect the beam away from the planned path and reduce the luminosity of a linear collider. Additionally, prototypes of the high-resolution cavity beam position monitors, which are vital to measure and control the CLIC beam, have been tested, showing promising initial results.

One of the main activities at CLEAR concerns the development and testing of beam instrumentation. Here, the flexibility and the large beam-parameter range provided by the facility, together with easy access, especially in its dedicated in-air test station, have proven to be very effective. CLEAR covers all phases of the development of novel beam diagnostics devices, from the initial exploration of a concept or physical mechanism to the first prototyping and to the testing of the final instrument adapted for use in an operational accelerator. Examples are beam-loss monitors based on optical fibres, and beam-position and bunch-length monitors based on Cherenkov diffraction radiation under development by the beam instrumentation group at CERN.

Advanced accelerator R&D

There is a strong collaboration between CLEAR and the Advanced Wakefield Experiment (AWAKE), a facility at CERN used to investigate proton-driven plasma wakefield acceleration. In this scheme, which promises higher acceleration gradients than conventional radio-frequency accelerator technology and thus more compact accelerators, charged particles such as electrons are accelerated by forcing them to “surf” atop a longitudinal plasma wave that contains regions of positive and negative charges. Several beam diagnostics for the AWAKE beamline were first tested and optimised at CLEAR. A second phase of the AWAKE project, presently being commissioned for operation in 2026, requires a new source of electron beams to provide shorter, higher quality beams. Before its final installation in AWAKE, it is proposed to use this source to increase the range of beam parameters available at CLEAR.

Installation of novel microbeam position monitors — **In position** The installation of novel microbeam position monitors on the in-air test station prior to a beam test in 2020. Credit: CERN-PHOTO-202010-130-4

Further research into compact, plasma-based accelerators has been undertaken at CLEAR thanks to the installation of an active plasma lens on the beamline. Such lenses use gases ionised by very high electric currents to provide focusing for beams many orders of magnitude stronger than can be achieved with conventional magnets. Previous work on active plasma lenses had shown that the focusing force was nonlinear and reduced the beam quality. However, experiments performed at CLEAR showed, for the first time, that by simply swapping the commonly used helium gas for a heavier gas like argon, a linear magnetic field could be produced and focusing could be achieved without reducing the beam quality (CERN Courier December 2018 p8).

Plasma acceleration is not the only novel accelerator technology that has been studied at CLEAR over the past five years. The significant potential of using accelerators to produce intense beams of radiation in the THz frequency range has also been demonstrated. Such light, on the boundary between microwaves and infrared, is difficult to produce, but has a variety of different uses ranging from imaging and security scanning to the control of materials at the quantum level. Compact linear accelerator-based sources of THz light could potentially be advantageous to other sources as they tend to produce significantly higher photon fluxes. By using long trains of ultrashort, sub-ps bunches, it was shown at CLEAR that THz radiation can be generated through coherent transition radiation in thin metal foils, through coherent Cherenkov radiation, and through coherent “Smith–Purcell” radiation in periodic gratings. The peak power emitted in experiments at CLEAR was around 0.1 MW. However, simulations have shown that with relatively minor reductions in the length of the electron bunches it will be possible to generate a peak power of more than 100 MW.

FLASH forward

Advances in high-gradient accelerator technology for projects like CLIC (CERN Courier April 2018 p32) have led to a surge of interest in using electron beams with energies between 50–250 MeV to perform radiotherapy, which is one of the key tools used in the treatment of cancer. The use of so-called very-high energy electron (VHEE) beams could provide advantages over existing treatment types. Of particular interest is using VHEE beams to perform radiotherapy at ultra-high dose rates, which could potentially generate the so-called FLASH effect in patients. Here, tumour cells are killed while sparing the surrounding healthy tissues, with the potential to significantly improve treatment outcomes.

**In a flash** Advances in high-gradient accelerator technology for projects such as CLIC have opened up the possibility of using very high-energy electrons to perform FLASH radiotherapy. Credit: CERN-PHOTO-202008-108-13

So far, CLEAR has been the only facility in the world studying VHEE radiotherapy and FLASH with 200 MeV electron beams. As such, there has been a large increase in beam-time requests in this field. Initial tests performed by researchers from the University of Manchester demonstrated that, unlike other types of radiotherapy beams, VHEE beams are relatively insensitive to inhomogeneities in tissue that typically result in less targeted treatment. The team, along with another from the University of Strathclyde, also looked at how focused VHEE beams could be used to further target doses inside a patient by mimicking the Bragg peak seen in proton radiotherapy. Experiments with the University Hospital of Lausanne to try to demonstrate whether the FLASH effect can be induced with VHEE beams are ongoing (CERN Courier January/February 2023 p8).

Even if the FLASH effect can be produced in the lab, there are issues that need to be overcome to bring it to the clinic. Chief among them is the development of novel dosimetric methods. As CLEAR and other facilities have shown, conventional real-time dosimetric methods do not work at ultra-high dose rates. Ionisation chambers, the main pillar of conventional radiotherapy dosimetry, were shown to have very nonlinear behaviour at such dose rates, and recombination times that were too long. Due to this, CLEAR has been involved in the testing of modified ionisation chambers as well as other more innovative detector technologies from the world of particle physics for use in a future FLASH facility.

High impact

As well as being a test-bed for new technologies and experiments, CLEAR has provided an excellent training infrastructure for the next generation of physicists and engineers. Numerous masters and doctoral students have spent a large portion of their time performing experiments at CLEAR either as one-time users or long-term collaborators. Additionally, CLEAR is used for practical accelerator training for the Joint Universities Accelerator School.

Numerous masters and doctoral students have spent time performing experiments at CLEAR

As in all aspects of life, the COVID-19 pandemic placed significant strain on the facility. The planned beam schedule for 2020 and beyond had to be scrapped as beam operation was halted during the first lockdown and external users were barred from travelling. However, through the hard work of the team, CLEAR was able to recover and run at almost full capacity within weeks. Several internal CERN users, many of whom were unable to travel to external facilities, were able to use CLEAR during this period to continue their research. Furthermore, CLEAR was involved in CERN’s own response to the pandemic by undertaking sterilisation tests of personal protective equipment.

Test-beam facilities such as CLEAR are vital for developing future physics technology, and the impact that such a small facility has been able to produce in just a few years is impressive. A variety of different experiments from several different fields of research have been performed, with many more that are not mentioned in this article. Unfortunately for the world of high-energy physics, the aforementioned shortage of accelerator test facilities has not gone away. CLEAR will continue to play its role in helping provide test beams, with operations due to continue until at least 2025 and perhaps long after. There is an exciting physics programme lined up for the next few years, featuring many experiments similar to those that have already been performed but also many that are new, to ensure that accelerator technology continues to benefit both science and society.

LHCb looks forward to the 2030s

**Looking ahead** Schematic side-view of the LHCb Upgrade II detector, which will push the limits of technology to enable precision flavour physics in the daunting conditions of the HL-LHC. (Credit: LHCb)

The LHCb collaboration is never idle. While building and commissioning its brand new Upgrade I detector, which entered operation last year with the start of LHC Run 3, planning for Upgrade II was already under way. This proposed new detector, envisioned to be installed during Long Shutdown 4 in time for High-Luminosity LHC (HL-LHC) operations continuing in Run 5, scheduled to begin in 2034/2035, would operate at a peak luminosity of 1.5 × 10³⁴ cm^–2 s^–1. This is 7.5 times higher than at Run 3 and would generate data samples of heavy-flavoured hadron decays six times larger than those obtainable at the LHC, allowing the collaboration to explore a wide range of flavour-physics observables with extreme precision. Unprecedented tests of the CP-violation paradigm (see “On point” figure) and searches for new physics at double the mass scales possible during Run 3 are among the physics goals on offer.

Attaining the same excellent performance as the original detector has been a pivotal constraint in the design of LHCb Upgrade I. While achieving the same in the much harsher collision environments at the HL-LHC remains the guiding principle for Upgrade II, the LHCb collaboration is investigating the possibilities to go even further. And these challenges need to be met while keeping the existing footprint and arrangement of the detector (see “Looking forward” figure). Radiation-hard and fast 3D silicon pixels, a new generation of extremely fast and efficient photodetectors, and front-end electronics chips based on 28 nm semiconductor technology are just a few examples of the innovations foreseen for LHCb Upgrade II, and will also set the direction of R&D for future experiments.

**On point** LHCb constraints on the apex of the Cabibbo–Kobayashi–Maskawa (CKM) unitary triangle with (left) inputs as of 2018 and (right) anticipated improvements with a 300 fb^–1 HL-LHC dataset, assuming consistency with the Standard Model and expected improvements in lattice QCD. Credit: CKMfitter

Rethinking the data acquisition, trigger and data processing, along with intense use of hardware accelerators such as field-programmable gate arrays (FPGAs) and graphics processing units (GPUs), will be fundamental to manage the expected five-times higher average data rate than in Upgrade I. The Upgrade II “framework technical design report”, completed in 2022, is also the first to consider the experiment’s energy consumption and greenhouse-gas emissions, as part of a close collaboration with CERN to define an effective environmental protection strategy.

Extreme tracking

At the maximum expected luminosity of the HL-LHC, around 2000 charged particles will be produced per bunch crossing within the LHCb apparatus. Efficiently reconstructing these particles and their associated decay vertices in real time represents a significant challenge. It requires the existing detector components to be modified to increase the granularity, reduce the amount of material and benefit from the use of precision timing.

The future VELO will be a true 4D-tracking detector

The new Vertex Locator (VELO) will be based, as it was for Upgrade I (CERN Courier May/June 2022 p38), on high-granularity pixels operated in vacuum in close proximity to the LHC beams. For Upgrade II, the trigger and online reconstruction will rely on the selection of events, or parts of events, with displaced tracks at the early stage of the event. The VELO must therefore be capable of independently reconstructing primary vertices and identifying displaced tracks, while coping with a dramatic increase in event rate and radiation dose. Excellent spatial resolution will not be sufficient, given the large density of primary interactions along the beam axis expected under HL-LHC conditions. A new coordinate – time – must be introduced. The future VELO will be a true 4D-tracking detector that includes timing information with a precision of better than 50 ps per hit, leading to a track time-stamp resolution of about 20 ps (see “Precision timing” figure).

**Precision timing** Illustration of the primary interactions and corresponding track density (left) generated for each bunch crossing at Upgrade II luminosities. Right: the same event with a time window of 20 ps applied to the primary interactions. Credit: LHCb

The new VELO sensors, which include 28 nm technology application-specific integrated circuits (ASICs), will need to achieve this time resolution while being radiation-hard. The important goal of a 10 ps time resolution has recently been achieved with irradiated prototype 3D-trench silicon sensors. Depending on the rate-capability of the new detectors, the pitch may have to be reduced and the material budget significantly decreased to reach comparable spatial resolution to the current Run 3 detector. The VELO mechanics have to be redesigned, in particular to reduce the material of the radio-frequency foil that separates the secondary vacuum – where the sensors are located – from the machine vacuum. The detector must be built with micron-level precision to control systematic uncertainties.

The tracking system will take advantage of a detector located upstream of the dipole magnet, the Upstream Tracker (UT), and of a detector made of three tracking stations, the Mighty Tracker (MT), located downstream of the magnet. In conjunction with the VELO, the tracking system ensures the ability to reconstruct the trajectory of charged particles bending through the detector due to the magnetic field, and provides a high-precision momentum measurement for each particle. The track direction is a necessary input to the photon-ring searches in Ring Imaging Cherenkov (RICH) detectors, which identify the particle species. Efficient real-time charged-particle reconstruction in a very high particle-density environment requires not only good detector efficiency and granularity, but also the ability to quickly reject combinations of hits not produced by the same particle.

**Mighty pixels** A microscope image of the first LHCb-dedicated high-voltage CMOS sensor. Credit: Karlsruhe Institute of Technology

The UT and the inner region of the MT will be instrumented with high-granularity silicon pixels. The emerging radiation-hard monolithic active pixel sensor (MAPS) technology is a strong candidate for these detectors. LHCb Upgrade II would represent the first large-scale implementation of MAPS in a high-radiation environment, with the first prototypes currently being tested (see “Mighty pixels” figure). The outer region of the MT will be covered by scintillating fibres, as in Run 3, with significant developments foreseen to cope with the radiation damage. The availability of high-precision vertical-coordinate hit information in the tracking, provided for the first time in LHCb by pixels in the high-occupancy regions of the tracker, will be crucial to reject combinations of track segments or hits not produced by the same particle. To substantially extend the coverage of the tracking system to lower momenta, with consequent gains for physics measurements, the internal surfaces of the magnet side walls will be instrumented with scintillating bar detectors, the so-called magnet stations (MS).

Extreme particle identification

A key factor in the success of the LHCb experiment has been its excellent particle identification (PID) capabilities. PID is crucial to distinguish different decays with final-state topologies that are backgrounds to each other, and to tag the flavour of beauty mesons at production, which is a vital ingredient to many mixing and CP-violation measurements. For particle momenta from a few GeV/c up to 100 GeV/c, efficient hadron identification at LHCb is provided by two RICH detectors. Cherenkov light emitted by particles traversing the gaseous radiators of the RICHes is projected by mirrors onto a plane of photodetectors. To maintain Upgrade I performances, the maximum occupancy over the photodetector plane must be kept below 30%, the single-photon Cherenkov-angle resolution must be below 0.5 mrad, and the time resolution on single-photon hits should be well below 100 ps (see “RICH rewards” figure).

Photon hits on the RICH photodetector plane — **RICH rewards** A simulation of the distribution of photon hits on the RICH photodetector plane for a typical Upgrade I event (left), and the same event applying a time window of 100 ps (right), allowing a cleaner identification of Cherenkov rings to be achieved. Credit: LHCb

Next-generation silicon photomultipliers (SiPMs) with improved timing and a pixel size of 1 × 1 mm², together with re-optimised optics, are deemed capable of delivering these specifications. The high “dark” rates of SiPMs, especially after elevated radiation doses, would be controlled with cryogenic cooling and neutron shielding. Vacuum tubes based on micro-channel plates (MCPs) are a potential alternative due to their excellent time resolution (30 ps) for single-photon hits and lower dark rate, but suffer in high-rate environments. New eco-friendly gaseous radiators with a lower refractive index can improve the PID performance at higher momenta (above 80 GeV/c), but meta-materials such as photonic crystals are also being studied. In the momentum region below 10 GeV/c, PID will profit from TORCH – an innovative 30 m² time-of-flight detector consisting of quartz plates where charged particles produce Cherenkov light. The light propagates by internal reflection to arrays of high-granularity MCP–PMTs optimised to operate at high rates, with a prototype already showing performances close to the target of 70 ps per photon.

Excellent photon and π⁰ reconstruction and e–π separation are provided by LHCb’s electromagnetic calorimeter (ECAL). But the harsh occupancy conditions of the HL-LHC impose the development of 5D calorimetry, which complements precise position and energy measurements of electromagnetic clusters with a time resolution of about 20 ps. The most crowded inner regions will be equipped with so-called spaghetti calorimeter (SPACAL) technology, which consists of arrays of scintillating fibres either made of plastic or garnet crystals arranged along the beam direction, embedded in a lead or tungsten matrix. The less-crowded outer regions of the calorimeter will continue to be instrumented with the current “Shashlik” technology with refurbished modules and increased granularity. A timing layer, either based on MCPs or on alternated tungsten and silicon-sensor layers placed within the front and back ECAL sections, is also a possibility to achieve the ultimate time resolution. Several SPACAL prototypes have already demonstrated that time resolutions down to an impressive 15 ps are feasible (see “Spaghetti calorimetry” image).

**Spaghetti calorimetry** A SPACAL prototype being prepared for beam tests, with an LAPPD (by Incom) inserted between the front and back sections as a demonstrator for the MCP-based timing layer. Credit: LHCb

The final main LHCb subdetector is the muon system, based on four stations of multiwire proportional chambers (MWPCs) interleaved with iron absorbers. For Upgrade II, it is proposed that MWPCs in the inner regions, where the rate will be as high as a few MHz/cm², are replaced with new-generation micro-pattern gaseous detectors, the micro-RWELL, a prototype of which has proved able to reach a detection efficiency of approximately 97% and a rate-capability of around 10 MHz/cm². The outer regions, characterised by lower rates, will be instrumented either by reusing a large fraction (∼95%) of the current MWPCs or by implementing other solutions based on resistive plate chambers or scintillating-tile-based detectors. As with all Upgrade II subdetectors, dedicated ASICs in the front-end electronics, which integrate fast time-to-digital converters or high-frequency waveform samplers, will be necessary to measure time with the required precision.

Trigger and computing

The detectors for LHCb Upgrade II will produce data at a rate of up to 200 Tbit/s (see “On the up” figure), which for practical reasons needs to be reduced by four orders of magnitude before being written to permanent storage. The data acquisition therefore needs to be reliable, scalable and cost-efficient. It will consist of a single type of custom-made readout board combined with readily available data-centre hardware. The readout boards collect the data from the various sub-detectors using the radiation-hard, low-power GBit transceiver links developed at CERN and transfer the data to a farm of readout servers via next- generation “PCI Express” connections or Ethernet. For every collision, the information from the subdetectors is merged by passing through a local area network to the builder server farm.

With up to 40 proton–proton interactions, every bunch crossing at the HL-LHC will contain multiple heavy-flavour hadrons within the LHCb acceptance. For efficient event selection, hits not associated with the proton–proton collision of interest need to be discarded as early as possible in the data-processing chain. The real-time analysis system performs reconstruction and data reduction in two high-level-trigger (HLT) stages. HLT1 performs track reconstruction and partial PID to apply inclusive selections, after which the data is stored in a large disk buffer while alignment and calibration tasks run in semi real-time. The final data reduction occurs at the HLT2 level, with exclusive selections based on full offline-quality event reconstruction. Starting from Upgrade I, all HLT1 algorithms are running on a farm of GPUs, which enabled, for the first time at the LHC, track reconstruction to be performed at a rate of 30 MHz. The HLT2 sequence, on the other hand, is run on a farm of CPU servers – a model that would be prohibitively costly for Upgrade II. Given the current evolution of processor performance, the baseline approach for Upgrade II is to perform the reconstruction algorithms of both HLT1 and HLT2 on GPUs. A strong R&D activity is also foreseen to explore alternative co-processors such as FPGAs and new emerging architectures.

Real-time versus the start date of various high-energy physics experiments — **On the up** Bandwidth of data analysed in real-time versus the start date of various high-energy physics experiments. Credit: A Cerri/University of Sussex

The second computing challenge for LHCb Upgrade II derives from detector simulations. A naive extrapolation from the computing needs of the current detector implies that 2.5 million cores will be needed for simulation in Run 5, which is one order of magnitude above what is available with a flat budget assuming a 10% performance increase of processors per year. All experiments in high-energy physics face this challenge, motivating a vigorous R&D programme across the community to improve the processing time of simulation tools such as GEANT4, both by exploiting co-processors and by parametrising the detector response with machine-learning algorithms.

Intimately linked with digital technologies today are energy consumption and efficiency. Already in Run 3, the GPU-based HLT1 is up to 30% more energy-efficient than the originally planned CPU-based version. The data centre is designed for the highest energy-efficiency, resulting in a power usage that compares favourably with other large computing centres. Also for Upgrade II, special focus will be placed on designing efficient code and fully exploiting efficient technologies, as well as designing a compact data acquisition system and optimally using the data centre.

A flavour of the future

The LHC is a remarkable machine that has already made a paradigm-shifting discovery with the observation of the Higgs boson. Exploration of the flavour-physics domain, which is a complementary but equally powerful way to search for new particles in high-energy collisions, is essential to pursue the next major milestone. The proposed LHCb Upgrade II detector will be able to accomplish this by exploring energy scales well beyond those reachable by direct searches. The proposal has received strong support from the 2020 update of the European strategy for particle physics, and the framework technical design report was positively reviewed by the LHC experiments committee. The challenges of performing precision flavour physics in the very harsh conditions of the HL-LHC are daunting, triggering a vast R&D programme at the forefront of technology. The goal of the LHCb teams is to begin construction of all detector components in the next few years, ready to install the new detector at the time of Long Shutdown 4.

ALICE 3: a heavy-ion detector for the 2030s

The ALICE experiment at the LHC was conceived to study the properties of the quark–gluon plasma (QGP), the state of matter prevailing a few microseconds after the Big Bang. Collisions between large nuclei in the LHC produce matter at temperatures of about 3 × 10¹² K, sufficiently high to liberate quarks and gluons, and thus to study the deconfined QGP state in the laboratory. The heavy-ion programme at LHC Runs 1 and 2 has already enabled the ALICE collaboration to study the formation of the QGP, its collective expansion and its properties, using for example the interactions of heavy quarks and high-energy partons with the QGP. ALICE 3 builds on these discoveries to reach the next level of understanding.

One of the most striking discoveries at the LHC is that J/ψ mesons not only “melt” in the QGP but can also be regenerated from charm quarks produced in independent hard scatterings. The LHC programme has also shown that the energy loss of partons propagating through the plasma depends on their mass. Furthermore, collective behaviour and enhanced strange-baryon production have been observed in selected proton–proton collisions in which large numbers of particles are produced, signalling that high densities may be reached in such collisions.

During Long Shutdown 2, a major upgrade of the ALICE detector (ALICE 2) was completed on budget and in time for the start of Run 3 in 2022. Together with improvements in the LHC itself, the experiment will profit from a factor-50 higher Pb–Pb collision rate and also provide a better pointing resolution. This will bring qualitative improvements for the entire physics programme, in particular for the detection of heavy-flavour hadrons and thermal di-electron radiation. However, several important questions – for example concerning the mechanisms leading to thermal equilibrium and the formation of hadrons in the QGP – will remain open even after Runs 3 and 4. To address these, the collaboration is pursuing next-generation technologies to build a new detector with a significantly larger rapidity coverage and excellent pointing resolution and particle identification (see “Brand new” figure). A letter of intent for ALICE 3, to be installed in 2033/2034 (Long Shutdown 4) and operated during Runs 5 and 6 (starting in 2035), was submitted to the LHC experiments committee in 2021 and led to a positive evaluation by the extended review panel in March 2022.

Behind the curtain of hadronisation

In heavy-ion collisions at the LHC, a large amount of energy is deposited in a small volume, forming a QGP. The plasma immediately starts expanding and cooling down, eventually reaching a temperature at which hadrons are formed. Although hadrons formed at the boundary of this phase transition carry information about the expansion of the plasma, they do not inform us directly about the temperature and other properties of the hot plasma phase of the collision before hadronisation takes place. Photons and di-lepton pairs, which are produced as thermal radiation in electromagnetic processes and do not participate in the strong interaction, allow us to look behind the curtain of hadronisation. However, measurements of photon and dilepton emission are challenging due to the large background from electromagnetic decays of light hadrons and weak decays of heavy-flavour hadrons.

Distribution of electron–positron pairs in Pb–Pb collisions at the LHC — **Taking the temperature** The expected invariant-mass distribution of electron–positron pairs in Pb–Pb collisions at the LHC, with coloured bands indicating the projected systematic uncertainties for ALICE 3. The slope of the spectrum above the ρ peak gives access to the plasma temperature. In the absence of ρ and a₁ mixing for chiral-symmetry restoration, a depletion appears in the intermediate mass range. Source: arXiv:2211.02491

One of the goals of the current ALICE 2 upgrades is to enable the first measurements of the thermal emission of electron–positron pairs (from virtual photons), and thus to determine the average temperature of the system before the formation of hadrons, during Runs 3 and 4. To further understand the evolution of temperature with time, larger data samples and excellent background rejection are needed. The early-stage temperature is determined from the exponential slope of the mass distribution above the ρ resonance, i.e. pair masses larger than 1.2 GeV/c² (see “Taking the temperature” figure, upper panel). ALICE 3 would be able to explore the time dependence of the temperature before hadronisation using more differential measurements, e.g. of the azimuthal asymmetry of di-electron emission and of the slope of the mass spectrum as a function of transverse momentum.

The di-electron mass spectrum also carries unique information about the mechanism of chiral symmetry breaking – a fundamental quantum-chromodynamics (QCD) effect that generates most of the hadron mass. At the phase transition to the QGP, chiral symmetry is restored and quarks and gluons are deconfined. One of the predicted signals of this transition is mixing between the ρ and a₁vector-meson states, which gives the di-electron invariant mass spectrum a characteristic exponential shape in the mass range above the ρ meson peak (0.8–1.1 GeV/c²). Only the excellent electron identification and rejection of electrons from heavy-flavour decays possible with ALICE 3 can give physicists experimental access to this effect (see “Taking the temperature” figure, lower panel).

**Multi-charm production** Expected production yields of double and triple charmed hadrons normalised to the Λ_c yield as a function of the number of participation nucleons, from pp (N_part = 2) to central Pb–Pb (N_part = 416). Statistical thermal models predict an enhancement by several orders of magnitude for central Pb–Pb collisions. The data points indicate the measurements that could be performed with ALICE 3. Source: arXiv:2211.02491

Another important goal of the ALICE physics programme is to understand how energetic quarks and gluons interact with the QGP and eventually thermalise and form a plasma that behaves as a fluid with very low internal friction. The thermalisation process and the properties of the QGP are governed by low-momentum interactions between quarks and gluons, which cannot be calculated using perturbative techniques. Experimental input is therefore important to understand these phenomena and to link them to fundamental QCD.

Heavy quarks

The heavy charm and beauty quarks are of particular interest because their interactions with the plasma can be calculated using lattice-QCD techniques with good theoretical control. Heavy quarks and antiquarks are mostly produced as back-to-back pairs in hard scatterings in the early phase of the collision. Subsequent interactions between the quarks and the plasma change the angle between the quark and antiquark. In addition, the “drag” from the plasma leads to an asymmetry in the overall azimuthal distributions of heavy quarks (elliptic flow) with respect to the reaction plane. The size of these effects is a measure of the strength of the interactions with the plasma. Since quark flavour is conserved in interactions in the plasma, measurements of hadrons containing heavy quarks, such as the D meson and Λ_c baryon, are directly sensitive to the interactions between heavy quarks and the plasma. While the increase in statistics and the improved spatial resolution of ALICE 2 will already allow us to measure the production of charm baryons, measurements of azimuthal correlations of charm–hadron pairs are needed to directly address how they interact with the plasma. These will only become possible with the precision, statistics and acceptance of ALICE 3.

Heavier beauty quarks are expected to take longer to thermalise and therefore lose less information through their interactions with the QGP. Therefore, systematic measurements of transverse-momentum distributions and azimuthal asymmetries of beauty mesons and baryons in heavy-ion collisions are essential to map out the interactions of heavy-flavour quarks with the QGP and to understand the mechanisms that drive the system towards thermal equilibrium.

To understand how hadrons emerge from the QGP, those containing multiple heavy quarks are of particular interest because they can only be formed from quarks that were produced in separate hard-scattering processes. If full thermal equilibrium is reached in Pb–Pb collisions, the production rates of such states are expected to be enhanced by up to three orders of magnitude with respect to pp collisions. This implies enormous sensitivity to the probability for combining independently produced quarks during hadronisation and to the degree of thermalisation. At ALICE 3, the precision with which multi-charm baryon yields can be measured is enhanced (see “Multi-charm production” figure).

**Close encounters** Model of a novel design for a retractable tracker. The four segments can be rotated to bring the tracker sensors closer to the beam pipe. Credit: C Gargiulo

In addition to precision measurements of di-electrons and heavy-flavour hadrons, ALICE 3 will allow us to investigate many more aspects of the QGP. These include fluctuations of conserved quantum numbers, such as flavour and baryon number, which are sensitive to the nature of the deconfinement phase transition of QCD. ALICE 3 will also aim to answer questions in hadron physics, for example by searching for the existence of nuclei containing charm baryons (analogous to strange baryons in hypernuclei) and by studying the interaction potentials between unstable hadrons, which may elucidate the structure of exotic hadronic states that have recently been discovered in electron–positron collisions and in hadronic collisions at the LHC. In addition, ALICE 3 will use ultra-peripheral collisions to study the structure of resonances such as the ρ′ and to look for new fundamental particles, such as axion-like particles and dark photons. A dedicated detector system is foreseen to study very low-energy photon production, which can be used to test “soft theorems” that link the production of very soft photons in a collision to the hadronic final state.

Pushing the experimental limits

To pursue this ambitious physics programme, ALICE 3 is designed to be a compact, large-acceptance tracking and particle-identification detector with excellent pointing resolution as well as high readout rates. The main tracking information is provided by an all-silicon tracker in a magnetic field provided by a superconducting magnet system, complemented by a dedicated vertex detector that will have to be retractable to provide the required aperture for the LHC at injection energy. To achieve the ultimate pointing resolution, the first hits must be detected as close as possible to the interaction point (5 mm at the highest energy) and the amount of material in front of it be kept to a minimum. The inner tracking layers will also enable so-called strangeness tracking – the direct detection of strange baryons before they decay – to improve the pointing resolution and suppress combinatorial background, for example in the measurement of multi-charm baryon decays.

ALICE 3 is a compact, large-acceptance tracking and particle-identification detector with excellent pointing resolution as well as high readout rates

First feasibility studies of the mechanical design and the integration with the LHC for the vertex tracker have been conducted and engineering models have been produced to demonstrate the concept and explore production techniques for the components (see “Close encounters” image). The detection layers are to be constructed from bent, wafer-scale pixel sensors. The development of the next generation of CMOS pixel sensors in 65 nm technology with higher radiation tolerance and improved spatial resolution has already started in the context of the ITS 3 project in ALICE, which will be an important milestone on the way to ALICE 3 (see “Next-gen tracking” image). The outer tracker, which has to cover the cylindrical volume to a radius of 80 cm over a total length of ±4 m, will also use CMOS pixel sensors. These will be integrated into larger modules for an effective instrumentation of about 60 m² while minimising the material used for mechanical support and services. The foreseen material budget for the tracker is 1% of a radiation length per layer for the outer tracker, and only 0.05% per layer for the vertex tracker.

**Next-gen tracking** An engineering model of ITS 3, demonstrating the mechanical stability of three bent wafers supported by only carbon foam wedges. Credit: C Gargiulo

For particle identification, five different detector systems are foreseen: a silicon-based time-of-flight system and a ring-imaging Cherenkov (RICH) detector that provide hadron and electron identification over a broad momentum range, a muon identifier starting from a transverse momentum of about 1.5 GeV/c, an electromagnetic calorimeter for photon detection and identification, and a forward tracker to reconstruct photons at very low momentum from their conversions to electron–positron pairs. For the time-of-flight system, the main R&D line aims at the integration of a gain layer in monolithic CMOS sensors to achieve the required time resolution of at least 20 ps (alternatively, low-gain avalanche diodes with external readout circuitry can be used). The calorimeter is based on a combination of lead-sampling and lead-tungstate segments, both of which would be read out by commercially available silicon photomultipliers (SiPMs). For the detection layers of the muon identifier, both resistive plate chambers and scintillating bars are being considered. Finally, for the RICH design, the R&D goal is to integrate the digital readout circuitry in SiPMs to enable efficient detection of photons in the visible range.

ALICE 3 provides a roadmap for an exciting heavy-ion physics programme, along with the other three large LHC experiments, in Runs 5 and 6. An R&D programme for the coming years is being set up to establish the technologies and enable the preparation of technical design reports in 2026/2027. These developments not only constitute an important contribution to the full physics exploitation of the LHC, but are of strategic interest for future particle detectors and will benefit the particle and nuclear physics community at large.

Plasma acceleration under the microscope

A team led by DESY researchers has used a noninvasive technique to measure the energy evolution of an electron bunch inside a laser-plasma accelerator for the first time, opening new possibilities to understand the fundamental mechanisms behind this next-generation accelerator technology.

Laser-driven plasma-wakefield acceleration, which is under study at DESY, SLAC and several other labs worldwide, promises to significantly reduce the size of particle accelerators. The idea is to use a high-power laser to create a plasma in a gas, in which charge displacements generate electric fields of the order 100 GV/m. Such fields can accelerate electron bunches to highly relativistic energies over short distances, outperforming conventional radio-frequency technologies by orders of magnitude. The AWAKE experiment at CERN, meanwhile, is a unique facility for the investigation of proton-driven plasma acceleration, which could enable even higher energies to be reached. Turning the concept of wakefield acceleration into a practical device, on the other hand, is a major challenge.

Turning the concept of wakefield acceleration into a practical device is a major challenge

In order to understand and thus improve the process of laser-plasma acceleration, which lasts for a period of femtoseconds to picoseconds, it is essential to observe as precisely as possible how the properties of the accelerated particles change in the plasma. Publishing their results in December, a team led by DESY’s Simon Bohlen and Kristjan Põder tracked the evolution of the electron beam energy inside a laser-plasma accelerator with high spatial resolution. The feat was performed within a project called PLASMED X, which aims to develop a compact, narrowband and tunable X-ray source for medical imaging.

The team began by splitting the laser beam into two parts: one was used for electron acceleration, while the other was superimposed so that the light could be scattered by the electrons. Using an X-ray detector to measure the energy of Thomson-scattered photons at 20 points over a 400 μm section of the plasma, the team was able to reconstruct the energy evolution of the electrons over most of the accelerator length without disturbing either the electron beam or the acceleration process itself.

“We were able to show in our measurements that the acceleration gradient can change significantly over very short distances,” says Bohlen. “With the new measurement method, we now have direct insight into a plasma acceleration process and can thus investigate the direct influence of different laser parameters or geometries of plasma cells on the acceleration process.”

CERN and Airbus collaboration aims high

Superconducting rare-earth barium copper oxide — **Hot stuff** CERN’s superconducting rare-earth barium copper oxide (also referred to as REBCO) power-transmission cable, used to study the feasibility of high-temperature superconductivity for aircraft. Credit: CERN

On 1 December, CERN and Airbus UpNext, a wholly owned subsidiary of Airbus, launched a collaboration to explore the use of superconducting technologies in the electrical distribution systems of future hydrogen-powered aircraft. The partnership will bring together CERN’s expertise in superconducting technologies for particle accelerators and Airbus UpNext’s capabilities in aircraft design and manufacturing to develop a demonstrator known as SCALE (Super-Conductor for Aviation with Low Emissions).

Superconducting technologies could drastically reduce the weight of next-generation aircraft and increase their efficiency. If its expected performances and reliability objectives are achieved, the CERN–Airbus collaboration could reach the ambitious target of flying a fully integrated prototype within the next decade, says the firm. The joint initiative seeks to develop and test in laboratory conditions, an optimised generic superconductor cryogenic (~500 kW) powertrain by the end of 2025. SCALE will be designed, constructed and tested by CERN using Airbus UpNext specifications and CERN technology. It will consist of a DC link (cable and cryostat) with two current leads, and a cooling system based on gaseous helium.

“Partnering with a leading research institute like CERN, which has brought the world some of the most important findings in fundamental physics, will help to push the boundaries of research in clean aerospace as we work to make sustainable aviation a reality,” said Sandra Bour-Schaeffer, CEO of Airbus UpNext. “We are already developing a superconductivity demonstrator called ASCEND (Advanced Superconducting and Cryogenic Experimental powertrain Demonstrator) to study the feasibility of this technology for electrically powered and hybrid aircraft. Combining knowledge obtained from our demonstrator and CERN’s unique capabilities in the field of superconductors makes for a natural partnership.”

Italy ramps up superconductor R&D

Developing high-temperature and high-magnetic-field superconducting technologies both for societal applications and next-generation particle accelerators is the goal of a new project in Italy called IRIS, launched in November and led by the INFN. IRIS (Innovative Research Infrastructure on applied Superconductivity) has received a €60 million grant from the Piano Nazionale di Ripresa e Resilienza to create a distributed R&D infrastructure throughout the country. It will focus on cables for low-loss electricity transport, and on the construction of superconducting magnets with high-temperature superconductors (HTS) in synergy with R&D for the proposed Future Circular Collider (FCC) at CERN. The project is estimated to last for 30 months, with more than 50% of the funds going to laboratories in the South of Italy.

One of the main objectives will be the construction in Salerno of a large infrastructure that will host not only a superconducting connection line, but also a centre of excellence for testing future industrial products for high-power connections, with the aim of making high-temperature superconductors less difficult and less expensive to work with.

“With the IRIS project, Italy assumes a leading position in applied superconductivity, creating a real synergy between research institutions and universities, which will offer an important collaboration opportunity for particle physicists and those involved in the fields of superconductivity and magnetism,” explains IRIS technical coordinator Lucio Rossi of the University of Milan. “An aspect not to be overlooked is also the high educational value of the project, which will guarantee numerous doctoral and high-level training opportunities for about a 100 students, young researchers and technicians.”

The activities of IRIS will be coordinated by the Laboratory of Accelerators and Applied Superconductivity (LASA) in Milan, with many partners including the universities of Genova, Milano, Naples, Salento and Salerno, and the CNR Institute for Superconductors, Innovative Materials and Devices (SPIN).

“IRIS is a virtuous example of how basic research, and in this case particle and accelerator physics, can provide an important application in other science areas, such as the development of new materials for energy saving that is essential for the creation of high-power cables without dissipation and suitable for the needs of future electricity networks serving new energy sources,” says Pierluigi Campana of INFN Frascati, IRIS scientific coordinator.

Radiotherapy debut for proton linac

Hadron therapy, to which particle and accelerator physicists have contributed significantly during the past decades, has treated more than 300,000 patients to date. As collaborations and projects have grown over time, new methods aimed at improving and democratising this type of cancer treatment have emerged. Among them, therapy with proton beams from circular accelerators stands out as a particularly effective treatment: protons can obliterate tumours, sparing the surrounding healthy tissues at higher rates than conventional electron or photon therapy. Unfortunately, present proton- and ion-therapy centres are large and very demanding on the design of buildings, accelerators and gantry systems.

A novel proton accelerator for cancer treatment based on CERN technology is preparing to receive its first patients in the UK. Advanced Oncotherapy (AVO), based in London, has developed a proton-therapy system called LIGHT (Linac Image-Guided Hadron Technology) – the result of more than 20 years of work at CERN and spin-off company ADAM, founded in 2007 to build and test linacs for medical purposes and now AVO’s Geneva-based subsidiary. LIGHT provides a proton beam that allows the delivery of ultra-high dose rates to deep-seated tumours. The initial acceleration to 5 MeV is based on radio-frequency quadrupole (RFQ) technology developed at CERN and supported by CERN’s knowledge transfer group. LIGHT reached the maximum treatment energy of 230 MeV at the STFC Daresbury site on 26 September. Four years after the first 16 m-long prototype was built and tested at LHC Point 2, this novel oncological linac will treat its first patients in collaboration with University Hospital Birmingham at Daresbury during the second half of 2023, marking the first time a proton linear accelerator is used for cancer therapy.

LIGHT operates with components and designs developed by CERN, ENEA, the TERA Foundation and ADAM. Components of note include LIGHT’s RFQ, which contributes to its compact design, as well as 19 radio-frequency modules composed of four side-coupled drift-tube accelerating cavities based on a TERA Foundation design and 15 coupled accelerating cavities with industrial design by ADAM. Each module is controlled to vary the beam energy electronically, 200 times per second, depending on the depth of the tumour layer. This obviates the need for absorbers (or degraders), which greatly reduce the throughput of protons and produce large unwanted radiation, therefore reducing the volume of shielding material required. This design allows the linear accelerator to generate an extremely focused beam of 70 to 230 MeV and to target tumours in three dimensions, by varying the depth at which the radiation dose is delivered much faster than existing circular accelerators.

“Our mission is simple: democratise proton therapy,” says Nicolas Serandour, CEO of AVO. “The only way to fulfill this goal is through the development of a different particle accelerator and this is what we have achieved with the successful testing of the first-ever proton linear accelerator for medical purposes. Importantly, the excitement comes from the fact that cost reduction can be accompanied with better medical outcomes due to the quality of the LIGHT beam, particularly for cancers that still have a low prognosis. I cannot over-emphasise the importance that CERN and ADAM played in making this project a tangible reality for millions of cancer patients.”

Superconducting detector magnets for the future

The Superconducting Detector Magnets Workshop, co-organised by CERN and KEK, was held at CERN from 12 to 14 September in a hybrid format. Joining were 90 participants from 36 different institutes and companies, with 57 on-site and 33 taking part remotely.

The workshop aimed to bring together the physics community, detector magnet designers and industry to exchange ideas and concepts, foster collaboration, and to discuss the needs and R&D development goals for future superconducting detector magnets. A key goal was to address the issue of the commercial availability of aluminium-stabilised Nb-Ti/Cu conductor technology.

Fifteen physics-experiment projects, which had either been approved or are in the design phase, presented their needs and plans for superconducting detector magnets. These experiments covered a wide range of physics programmes for existing and future colliders, non-colliders and a space-based experiment. The presented projects showed a strong demand for aluminium-stabilised Nb-Ti/Cu conductor technology. Other conductor technologies that were featured during the workshop included cable-in-conduit technology (CICC) and aluminium-stabilised high-temperature-superconducting (HTS) technology.

Presentations by leading industrial partners showed that the industrial capability to produce superconducting detector magnets does exist, as long as a suitable conductor is available. It was also shown that aluminium-stabilised Nb-Ti/Cu conductors are currently not commercially available, although an R&D effort is currently on-going with IHEP in China. In particular, the co-extrusion process needed to clad the Nb-Ti/Cu Rutherford cable with aluminium is a key missing ingredient in industry. At the same time, the presentations showed that other ingredients, such as Nb-Ti/Cu wire production, the cabling of strands into a Rutherford cable, the high-purity aluminium stabiliser itself and the technique for welding-on of aluminium-alloy reinforcements for high-strength conductors, are still available.

The main conclusion of the workshop was that, given the need for aluminium-stabilised Nb-Ti/Cu conductors for future superconducting detector magnet projects, it is important that the commercial availability of this conductor is re-established, which would require a leading effort from international institutes through collaboration and cooperation with industry. This world-leading effort will advance technologies to be transferred openly to industry and other laboratories. Of particular importance is the co-extrusion technology needed to bond the aluminium stabiliser to the Rutherford cable. Hybrid-structure technology through electron- beam welding or other approaches to maximise the performance of an Al-stabilised superconductor combined with high-strength Al-alloy is needed for high-stress detector magnets. Back-up solutions such as copper-coated and soldered aluminium stabilisers, copper-based stabilisers and CICC should also be considered. In the long term, aluminium-stabilised HTS technology will be important for specific detector-magnet applications.

The workshop was received with strong interest and enthusiasm, and it is expected that another will be organised in one to two years, depending on the progress being made.

The power of polarisation for FCC-ee physics

Evolution of beam energy at LEP — **High tides** The evolution of the beam energy at LEP due to Earth tides, showing the measurements from resonant depolarisation (red points), and the predictions of a model. At FCC-ee the Earth tides, if uncorrected, will induce energy changes that are an order of magnitude larger. Source: J Wenninger

The FCC-ee, a proposed 91 km future circular collider at CERN foreseen to begin operations in the 2040s, would deliver enormous samples of collision data at a wide range of energies, allowing for ultra-precise studies of the Higgs, W and Z bosons, and the top quark. For example, when running at the Z resonance the FCC-ee will produce – in little more than one minute – a data set the same size as that the LEP collider accumulated in the 1990s during its entire period of operation. For this reason, unlocking the full potential of FCC-ee data will require exquisite systematic control at a level far beyond that achieved at previous colliders.

A beautiful and unique attribute of circular e⁺e^– colliders is that the beams can naturally acquire transverse polarisation, and the precession frequency of the polarisation vector divided by the circulation frequency around the ring is directly proportional to the beam energy. This property allows the energy to be determined with very high precision through applying an oscillating magnetic field which, when in phase with the precession, depolarises the beams. This technique of resonant depolarisation underpins the precise knowledge of the mass and other properties of many particles that now serve as “standard candles”.

A key example is the measurement of the mass and width of the Z boson, and associated electroweak observables, which was the major achievement of the LEP programme. FCC-ee offers the possibility of improving the precision of these measurements by a factor of around 500 – a gigantic advance in precision that will allow for ultra-sensitive tests of the self-consistency of the Standard Model, and provide excellent sensitivity to new heavy particles that may affect the measurements through quantum corrections or mixing. Achieving the best possible knowledge of the collision energy is essential to accomplish this programme, and was the focus of the second FCC Energy Calibration, Polarization and Mono-chromatisation (EPOL) workshop held at CERN from 19 to 30 September, which was a follow-up to the first workshop that took place in 2017.

The two-week workshop was attended by more than 100 accelerator physicists, particle physicists and engineers from around the world; some remote and others participating in person. Presentations focused not only on the challenges at the FCC-ee, but also encompassed activities and initiatives at other facilities. The first week highlighted the plans for polarimetry measurements at the future Electron Ion Collider in the US. Complementary projects were presented from SuperKEKb in Japan, where the accelerator is stress-testing many aspects of the FCC-ee design, CEPC in China and other machines around the world.

Earth tides

The collision-energy calibration is a central consideration in the design and proposed operation strategy of the FCC-ee, in contrast to LEP where it was essentially an afterthought. At LEP, resonant depolarisation measurements were performed in dedicated calibration periods a few times per year. At FCC-ee these measurements will take place continually. This is essential, as a hard-learned lesson from LEP is that the beam energy is not constant, but varies throughout a fill, and also evolves over longer timescales. The gravitational pull of the moon distorts the tunnel in “Earth tides”, and modifies the relative trajectory of the beam through the quadrupole magnets, leading to energy changes that at LEP were around 10 MeV over a few hours during Z running, but will be 20 times larger at FCC-ee. Seasonal changes in the water level of Lac Leman lead to similar effects. At FCC-ee these distortions will be combatted by continuous adjustment of the radio frequency (RF) cavities, as is now routinely done in the LHC.

Additional challenges that were discussed in the workshop included the requirements on the laser polarimeters that will monitor the polarisation levels of the e⁺ and e^– beams, the shifts in collision energy that will occur at each interaction point through the combined effect of synchrotron radiation and the boost provided by the RF system, as well as spurious dispersions folded with collision offsets. Here the project will benefit from the considerable progress achieved since LEP in both the reliability and precision of beam position and dispersion measurements. A particular highlight of the discussions was an agreement that it will be feasible to perform resonant depolarisation measurements at higher energies for use in the determination of the mass of the W boson, which was not possible at LEP, allowing this important parameter of nature to be measured around a factor 20–40 times better than at present.

The workshop concluded with a list of future tasks to be tackled and open questions. These questions will be addressed as part of the ongoing FCC Feasibility Study, with updates planned for the mid-term review, scheduled for the middle of 2023, and the final report in 2025.

Crystal collimation brings HL-LHC into focus

The start of LHC Run 3 in 2022 marked an important milestone for CERN: the first step into the High-Luminosity LHC (HL-LHC) era. Thanks to a significant upgrade of the LHC injectors, the Run 3 proton beams are more intense than ever. Together with the raised centre-of-mass collision energy from 13 to 13.6 TeV, Run 3 offers a rich physics programme involving the collisions of both proton and heavy-ion beams. This is made possible thanks to several important upgrades involving HL-LHC hardware that were carried out during Long Shutdown 2 (LS2), ahead of the full deployment of the HL-LHC project during LS3, around four years from now.

The HL-LHC aims to operate with 2.3 × 10¹¹ protons per bunch (compared to the goal of 1.8 × 10¹¹ protons per bunch at the end of Run 3), producing a stored beam energy of about 710 MJ (compared to 540 MJ in Run 3). Lead–ion beams, on the other hand, will already reach their HL-LHC target intensity upgrade in Run 3. This is thanks to the “slip stacking” technique currently implemented at the Super Proton Synchrotron, which uses complex radio-frequency manipulations to shorten the bunch spacing of LHC beam trains from 75 to 50 ns. Equating to a stored beam energy of up to 20.5 MJ at 6.8 TeV (compared to a maximum of 12.9 MJ achieved in 2018 at 6.37 TeV), the full HL-LHC upgrade needed to handle these more intense ion beams must be available throughout Run 3.

When the LHC works as a heavy-ion collider, many specific challenges need to be faced. Magnetically, the machine behaves in a similar way as during proton–proton operation. However, since the lead–ion bunch charge is about 15 times lower than for protons, a number of typical machine challenges – such as beam–beam interactions, impedance, electron-cloud effects, injection and beam-dump protection – are relaxed. Mitigating the nuisance of beam halos, however, is certainly not one of the tasks that gets easier.

Absorber collimators — **Multi-stage collimation** An optimised arrangement of primary, secondary and absorber collimators, located in the LHC’s ~500 m-long warm betatron cleaning insertion region (left), is needed to intercept primary beam losses and safely dispose of the higher order halos and energy that they produce following the interaction with the collimator materials. In an ideal crystal-based collimation scheme (right), the channelling process suppresses the nuclear interaction of halo particles compared to that of the standard primary collimators. Credit: S Redaelli

These halos are formed by particles that stray from the ideal beam orbit. More than 100 collimators are located at specific locations in the LHC to ensure that errant particles are cleaned or absorbed, thus protecting sensitive superconducting and other accelerator components. Although the total stored beam energy with ions is more than 30 times lower than it is for protons, the conventional multi-stage collimation system at the LHC (see “Multi-stage collimation” figure) is about two orders of magnitude less efficient for ion beams. Nuclear fragmentation processes occurring when ions interact with conventional collimator materials produce ion fragments with different magnetic rigidities without producing transverse kicks sufficient to steer these fragments onto the secondary collimators. Instead, they travel nearly unperturbed through the “betatron” collimation system in interaction region 7 (IR7) responsible for disposing safely of transverse beam losses. This creates clusters of losses in the high-dispersion regions, where the first superconducting dipole magnets of the cold arcs act as powerful spectrometers, increasing the risk of quenches whereby the magnets cease to become superconducting.

The ion-collimation limitation is a well-known concern for the LHC. Nevertheless, the standard system has performed quite well so far and provided adequate cleaning efficiency for the nominal LHC ion-beam parameters. But the HL-LHC targets pose additional challenges. In particular, the upgrade does not allow sufficient operational margins without improving the betatron collimation cleaning. Lead–ion beam losses in the cold dipole magnets downstream of IR7 might reach a level three times higher than their quench limits, estimated at their 7 TeV current equivalent.

Various paths have been followed within the HL-LHC project to address this limitation. The baseline solution was to improve the collimation cleaning by adding standard collimators in the dispersion-suppressor regions that would locally dispose of the off-momentum halo particles before they impact the cold magnets. To create the necessary space, two shorter dipoles with a stronger (11 T) field would replace a standard, 15 m-long 8.3 T LHC dipole. This robust upgrade, which works equally well for proton beams, was planned to be used in Run 3. However, due to technical issues with the availability of the new dipoles, which are based on a niobium-tin rather than niobium-titanium conductor, the decision was taken to defer their installation. The HL-LHC project now relies on an alternative solution based on a crystal collimation scheme that was studied in parallel.

Crystals in the LHC

The development of crystal applications with hadron beams at CERN dates back to the activities carried out by the UA9 collaboration at the CERN SPS. Crystal collimation makes use of a phenomenon called planar channelling: charged particles impinging on a pure crystal with well-defined impact conditions can remain trapped in the electromagnetic potential well generated by the regular planes of atoms. If the crystal is bent, particles follow its geometrical shape and experience a net kick that can steer them with high efficiency to a downstream absorber. Crystal collimation was tested at the Tevatron, and in 2018 a prototype system was used for protons at the LHC in a special run at injection energy. The scheme is particularly attractive for ion beams as it was demonstrated that the existing secondary collimators can serve as a halo absorber without risking damage.

**Silicon swerve** Top: a silicon-strip crystal (built by INFN Ferrara) used for LHC beam collimation, which is bent to an 80 m-curvature radius to produce a 50 μrad kick. Bottom: the vacuum tank in the LHC housing the bent crystal and interferometer head used during beam tests to control the crystal’s orientation. Credit: CERN BE/CEM & INFN-Ferrara

At the LHC, a total of four bent crystals are needed for the horizontal and vertical collimation of both beams. During Run 2, a test stand for crystal-collimation tests was installed in the LHC betatron cleaning region of IR7 with the aim of demonstrating the feasibility of this advanced collimation technique at LHC energies. Silicon crystals with a length of just 4 mm were bent to a curvature radius of 80 m to produce a 50 μrad deflection – much larger than the few-μrad angles typically experienced by proton interaction with the 60 cm-long primary collimators (see “Silicon swerve” image). Indeed, to produce such a kick with conventional dipole magnets would require a field of around 300 T in the same volume of the crystal. The crystals were mounted on an assembly (see “On target” image) that is a jewel of accelerator technology and control: the target collimator primary crystal (TCPC). This device allows the crystal to be moved to the desired distance from the circulating beam – typically just a few millimetres at 7 TeV – and its angular orientation to be adjusted to better than 1 μrad. While the former is no more demanding than the control system of other LHC collimators, the angular control demands a customised technology that is the heart of LHC crystal collimation.

Crystal channelling can only occur for particles impinging on the crystal surface with well-defined impact conditions. For a 6.8 TeV proton beam, they must have an angle of 90° with angular deviations of at most ±0.0001° (around ±2 μrad) – which is similar to aiming at a 10 cm-wide snooker pocket from a shooting distance of 25 km! If this tiny angular acceptance is not respected, the transverse momentum is sufficient to send particles out of the potential well produced between the planes of the crystal lattice, thus losing the channelling condition. Both the beam-impact conditions and the accuracy of the crystal’s angle must therefore be kept under excellent control.

The crystal collimators are steered remotely using a technology that is unique to the CERN accelerator complex. It relies on a high-precision interferometer that provides suitable feedback to the advanced controller, and a precise piezo-actuation device that drives the crystal orientation with respect to impinging halo particles with unprecedented precision. During Run 2, the system demonstrated the sub-microradian accuracy required to maintain crystal channelling at high beam energy (see “High precision” figure, top). A recent feature of the newly installed devices is that the interferometer heads (which enable the precise control of the angle) are located outside the vacuum with the laser light coupled to the angular stage by means of viewports. This means that any fibre degradation due to motion or radiation, which was observed on the prototype system, can be corrected during routine maintenance. Using this setup in 2018, an improvement in ion-collimation cleaning by up to a factor of eight was demonstrated experimentally with the best crystal, paving the way for crystal collimation to become the baseline solution for the HL-LHC.

The test devices used during Run 2 served their purpose well, but they do not meet the standards required for regular, high-efficiency operation. An upgrade plan was therefore put in place to replace them with a higher performing new design. This has been developed in a crash programme at CERN that started in November 2020, when the decision to postpone the installation of the 11 T dipoles was taken. Two units were built and installed in the LHC in 2021 (see “On target” image) and another four are nearing completion: two for installation in the LHC at the end of 2022 and the others serving as operational spares. The first two installed units replaced the two prototype vertical crystals that showed the lowest performance. The horizontal prototype devices remain in place for 2022, since they performed well and were tested with a pilot beam in October 2021.

Improved ion-collimation cleaning has paved the way to adopt crystal collimation as the baseline of the HL-LHC

The start of Run 3 in April this year provided a unique opportunity to test the new devices with proton beams, ahead of the next operational ion run. One of the first challenges is to establish the optimal alignment of the crystals, to make sure stray particles are channelled as required. While channelled, the impinging particles interact with the crystal with the lowest nuclear-interaction rate: halo particles travel preferentially in the “empty” channel relatively far from the lattice nuclei. Optimum channelling is therefore revealed by the orientation that has the lowest losses, as measured by beam-loss monitors located immediately downstream of the crystal (see “High precision” figure, bottom). Considering the large angular range possible (more than 50 μrad, compared with the full angular range of 20 mrad), establishing this optimum condition is a bit like finding a needle in a haystack. However, following a successful campaign in dedicated operational beam tests in August 2022, channelling was efficiently established for both the new and old crystals, allowing the commissioning phase to continue.

Looking forward

The LHC collimation system is the most complex beam-cleaning system built to date for particle accelerators. However, it must be further improved to successfully face the upcoming challenges from the HL-LHC upgrade which, for heavy-ion beams, begins during Run 3. Crystal collimation is a crucial upgrade that is now being put into operation to improve the betatron cleaning in preparation for the upgraded ion-beam parameters, mitigating the risks of machine downtime from ion-beam losses. The collimation cleaning performance will be established experimentally as soon as Run 3 ion operation begins. Initial beam tests with protons indicate that the newly installed bent crystals perform well. The first measurements demonstrated that the crystals can be put into operation as expected and showed the specified channelling property. We are therefore confident that this advanced technology can be used successfully for the heavy-ion challenges of the HL-LHC programme.

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