Following a record year of proton–proton operations at the LHC in 2016, which was followed by a successful proton–lead run, on 5 December 2016 the machine entered a longer than usual winter shutdown. Since then, hundreds of people from CERN’s technical teams have been working to repair and upgrade equipment across the whole accelerator chain and also the LHC experiments themselves. The extended year-end technical stop (EYETS), which will be complete by the end of April, has enabled CERN and its users to perform important interventions including the upgrade of the CMS pixel detector.
As the EYETS officially got under way in early December, 10 days were dedicated to powering tests for the LHC magnets to investigate the feasibility of operating at its design energy of 7 TeV per beam. Although this is only 0.5 TeV higher than the energy of the LHC during Run 2, which began in 2015, the LHC’s 1232 dipole magnets must be trained at higher currents to allow the higher-energy beams to circulate. Powering tests were conducted in two of the LHC’s eight sectors during which the current was gradually increased: in sector 4–5, for example, the current reached 11.535 kA, corresponding to a beam energy of 6.82 TeV. The considerable amount of data collected during the powering tests will now be analysed to define the best strategy for reaching the LHCʼs full design energy.
The EYETS is now in full swing, with several activities taking place: maintenance of cryogenic, ventilation, vacuum, electrical and other systems; upgrades to the accelerators and injectors for the High-Luminosity LHC (HL-LHC) and LHC Injector Upgrade (LIU) projects; consolidation works; and other activities such as the replacement of two lifts that have been in use since the early days of LEP.
The entire LHC has been emptied of liquid helium, which normally keeps the superconducting dipoles at a temperature of 1.9 K, and the bulk of the machine is being held at a temperature of 20 K during the shutdown. This is to avoid wasting any of the precious gas due to unexpected electrical failures during EYETS activities, and also to allow important maintenance works to be carried out on the cryogenic system. Since it takes several weeks to refill, pump and “boil off” the cryogenics before the LHC can restart operations, the already busy EYETS schedule is extremely tight. Cryo-filling of the first sector is foreseen between the end of February and the beginning of March, with the final cool-down expected in early April.
Another major activity is the replacement of a dipole magnet in sector 1–2, which lies between ATLAS and ALICE. This meant that the sector had to be warmed up to ambient conditions, allowing several tests of its electrical quality and liquid-helium insulation at ambient temperature, which revealed no major issues. One of the major risks of warming up a sector is the deformation of the expansion bellows – the thin corrugated structures that compensate for the contraction and expansion of the quench recovery line for the helium distribution system as the machine is cooled and warmed – but X-ray scans performed on all 250 bellows in this sector show no such problems. In addition, the “ball test”, during which a ping-pong ball is fired along the LHC beam-pipe, has been carried out and no faults were found in the sector interconnects.
Regarding the injectors, which transport protons between the various accelerators in the LHC complex, the main EYETS activities concern the Proton Synchrotron Booster (PSB) and the Super Proton Synchrotron (SPS). Critical activities at the PSB include a major de-cabling and cabling campaign, which involves removing all obsolete cables identified during the previous technical stop to make way for the LIU project. Many works are also being carried out on the surface of the PSB to install all the required LIU components.
The SPS is also undergoing a de-cabling and cabling campaign. Other key activities here concern the installation of the cryogenic modules and related infrastructure for the HL-LHC’s superconducting crab cavities (see “On the trail of the HL-LHC magnets”), in addition to civil engineering works to prepare for the replacement of the SPS internal beam dump. The poor functioning of this dump last year limited the number of proton bunches that could be injected from the SPS to the LHC, and the new beam dump will be installed during Long Shutdown 2 beginning the end of 2018.
Despite the extensive works taking place and many technical challenges faced, the EYETS schedule is on track with no major disruptions. Once complete, the LHC will be prepared for its 2017 run, for which commissioning will begin in May.
Fig. 1. Mass spectrum of beauty baryons decaying into a proton and three pions (red).
The LHCb experiment has uncovered tantalising evidence that baryons made of matter behave differently to those made of antimatter, violating fundamental charge-parity (CP) symmetry. Although CP-violating processes have been studied for more than 50 years, dating back to the Nobel-prize winning experiment of James Cronin and Val Fitch in 1964, CP violation has only been observed in mesons – that is, hadrons made of a quark and an antiquark. Until now, no significant effects had been seen in baryons, which are three-quark states, despite predictions from the Standard Model (SM) that CP violation also exists in the baryon sector.
Searching for new sources of CP violation, which is one of the main goals of LHCb, could help account for the overwhelming excess of matter over antimatter observed on cosmological scales. Since this excess is too large to be explained by CP violation as described in the SM, other sources must contribute.
The new LHCb result is based on an analysis of data collected during Run 1 of the LHC, from which the collaboration isolated a sample of Λ0b baryons (comprising a beauty, up and down quark) decaying into a proton plus three charged pions. The analysis also selected events in which the antimatter Λ0b baryon decays into an antiproton and three pions. Both of these processes are extremely rare and have never previously been observed. The high production cross-section of beauty baryons at the LHC and the specialised capabilities of the LHCb detector allowed a pure sample of around 6000 such decays to be isolated (figure 1).
The LHCb data revealed significant non-zero asymmetries in certain bins (figure 2) and the general pattern of asymmetries across all bins was found to be inconsistent from that which would be expected in the CP-conserving case with a statistical significance of 3.3σ.
The results, published in Nature Physics, will soon be updated with the larger data set collected so far in Run 2. If this signal of CP violation is reproduced and seen with greater significance in the larger sample, the result will be an important milestone in the study of CP violation.
The large mass of the top quark means that the top-quark sector has great potential for gaining a deeper understanding of the Standard Model (SM) and for revealing new physics beyond it. With the large statistics available at the LHC, very precise measurements of the top-quark properties are possible. Two recent analyses performed by ATLAS based on proton–proton collisions recorded at an energy of 8 TeV have allowed the collaboration to probe the angular distributions of the top quark and its decay products in unprecedented detail.
The first analysis concerns the polarisation of W bosons produced in the decays of top-quark–antiquark pairs, which is determined by measuring the angle between the decay products of the W and the b-quark from the top decay. Both leptonic and hadronic W decays were identified, and the fractions of longitudinal, left- and right-handed polarisation states fitted from the angular distributions. The results from ATLAS are the most precise to date and are in good agreement with the SM predictions. This measurement is also used to probe the structure of the Wtb vertex, which could be modified by contributions from new-physics processes and thus allows new constraints to be placed on anomalous tensor and vector couplings.
The goal of the second analysis was to completely characterise the spin-density matrix of the top-quark–antiquark pair production. This required the measurement of 15 independent variables, 10 of which were never previously measured. Specifically, ATLAS measured the polarisation of the top quark and the spin correlation between the top and anti-top along three different spin-quantisation axes: the helicity axis, the axis orthogonal to the production plane created by the directions of the top quark and the beam axis, and a third axis orthogonal to the former two. Using this scheme, the collaboration was able to measure new “cross-correlation” observables for the first time, based on the angular distributions of the leptons from the top-quark decays. The distributions were corrected back to generator-level to allow the results to be interpreted in terms of new physics models, and so far all results are in agreement with the SM expectations.
These studies of the angular distributions of top-quark decays will benefit from the larger data sample collected at 13 TeV, allowing stronger constraints to be placed on potential new-physics contributions or opening new opportunities to observe deviations from the SM.
In high-energy nucleus–nucleus collisions, heavy-flavour quarks (charm and beauty) are produced on a very short time scale in initial hard-scattering processes and thus they experience the entire evolution of the collision. Such quarks are valuable probes to study the mechanisms of energy loss and hadronisation in the hot and dense matter, the quark–gluon plasma, formed in heavy-ion collisions.
To investigate these effects, proton–proton (pp) and proton–lead (p–Pb) collisions are measured as a reference. While the former allows the study of heavy-flavour production when no medium is formed, the latter gives access to cold nuclear matter effects, namely parton scattering in the initial state and modifications of the parton densities in the nucleus.
The excellent electron identification capabilities and track impact parameter resolution of the ALICE detector enable measurements of electrons from heavy-flavour hadron decays at mid-rapidity. To study the predicted quark mass dependence of the parton energy loss, the contributions of electrons from charm- and beauty-hadron decays are statistically separated using the different impact parameter distributions as a proxy for their decay length and empirical estimations of the background.
The measurement of electrons from heavy-flavour hadron decays in p–Pb collisions shows no indication of a modification of the production with respect to pp collisions at high transverse momentum (pT), indicating that cold nuclear matter effects are small. The observed reduction in yield at high pT in central Pb–Pb collisions relative to pp interactions can thus be attributed to the presence of the hot and dense medium formed in Pb–Pb collisions. This implies that beauty quarks interact with the medium.
The larger suppression of electrons from both charm- and beauty-hadron decays compared with the beauty-only measurement is consistent with the ordering of charm and beauty suppression seen previously in the comparison of prompt D mesons (measured by ALICE) and J/ψ from B meson decays (measured by CMS). The larger samples of Pb–Pb collisions in Run 2 will improve the precision of the measurements and will make it possible to determine if beauty quarks participate in the collective expansion of the quark–gluon plasma.
Recently, the CMS collaboration performed an updated search for a neutral Higgs boson decaying into two τ leptons using 13 fb−1 of data recorded during 2016. Although the existence of the Higgs has been established beyond doubt since its debut in the CMS and ATLAS detectors in 2012, the vast majority of Higgs bosons recorded so far concern its decay into pairs of bosons. Observing the Higgs via its decays into pairs of fermions further tests the predictions of the Standard Model (SM). In particular, τ leptons have played a major role in measuring the Yukawa couplings between the Higgs and fermions, and thus proved to be an important tool for discovering new physics at the LHC.
CMS first reported evidence for Higgs to ττ decays in 2014. With a lifetime of around 10–13 seconds and a mass of 1.776 GeV, τ leptons present a unique but challenging experimental signature at hadron colliders. Their very short lifetime means that τ particles decay in the LHC beam pipe before reaching the inner layers of the CMS detector. Approximately 35% of the time, the τ decays into two neutrinos plus a lighter lepton, while 65% of the time it decays into a single neutrino and hadrons. τ decays yield low charged and neutral particle multiplicities: more than 95% of the hadronic decays contain just one or three charged hadrons and less than two neutral pions. The primary difficulty when dealing with the τ is the distinction between genuine τ leptons and copiously produced quark and gluon jets that can be misidentified as taus.
To identify the dominant τ decay modes, CMS has developed a powerful τ reconstruction algorithm, which makes use of the single-particle reconstruction procedure (called particle flow). Charged hadrons are combined with photons from neutral pion decays to reconstruct τ decay modes with one or three charged hadrons and neutral pions (figure 1). The algorithm also pays particular attention to the effects of detector materials in converting photons into electron–positron pairs. The large magnetic field of CMS causes secondary electrons to bend, resulting in broad signatures in the phi (azimuthal) co-ordinate, and “strips” are created by clustering photons and electrons via an iterative process. In a new development for LHC Run 2, the strip size is allowed to vary based on the momentum of the clustered candidates.
Applying the latest τ algorithm, along with numerous other analysis techniques, CMS finds no excess of events in which a Higgs decays into two τ leptons compared to the expectation from the SM. Instead, upper limits were determined for the product of the production cross-section and branching fraction for masses in the region 90–3200 GeV, and the results were also interpreted in the context of the Minimal Supersymmetric SM (MSSM) (figure 2). The LHC is now operating at its highest energy and an increase in instantaneous luminosity is planned. The next few years of operations will therefore be vital for further testing the SM and MSSM using the τ lepton as a tool.
The SNOLAB laboratory in Ontario, Canada, has received a grant of $28.6m to help secure its next three years of operations. The facility is one of 17 research facilities to receive support through Canada’s Major Science Initiative (MSI) fund, which exists to secure state-of-the-art national research facilities.
SNOLAB, which is located in a mine 2 km beneath the surface, specialises in neutrino and dark-matter physics and claims to be the deepest cleanroom facility in the world. Current experiments located there include: PICO and DEAP-3600, which search for dark matter using bubble-chamber and liquid-argon technology, respectively; EXO, which aims to measure the mass and nature of the neutrino; HALO, designed to detect supernovae; and a new neutrino experiment SNO+ based on the existing SNO detector.
The new funds will be used to employ the 96-strong SNOLAB staff and support the operations and maintenance of the lab’s facilities.
Researchers at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany, have demonstrated the feasibility of using carbon ions to treat cardiac arrhythmia, in which abnormal electrical patterns can lead to sudden heart failure or permanent damage as a result of stroke. Conventional treatments for certain forms of cardiac arrhythmia include drugs or “catheter ablation,” in which catheters are guided through blood vessels to the heart to destroy certain tissue. The GSI team, in conjunction with physicians from Heidelberg University and the Mayo Clinic in the US, have now shown that high-energy carbon ions produced by a particle accelerator can in principle be used to perform such treatments without catheters.
The non-invasive procedure induces specific changes to cardiac tissue that prevent the transmission of electrical signals, permanently interrupting the propagation of disruptive impulses. Following promising results from initial tests on cardiac cell cultures and beating-heart preparations, the researchers developed an animal study. Further detailed studies are needed, however, before the method can start to benefit patients.
A crucial advantage of the new method it that the ions can penetrate to any desired depth. Irradiating cardiac tissue with carbon ions appears as a promising, non-invasive alternative to catheters, and ultimately ion-based procedures are expected to take a few minutes compared with a few hours. “It is exciting that the carbon beam could work with surgical precision in particularly sensitive areas of the body,” says Paolo Giubellino, scientific managing director of FAIR and GSI and former spokesperson of the LHC’s ALICE experiment at CERN. “We’re proud that the first steps toward a new therapy have now been taken.”
Although the night sky appears dark between the stars and galaxies that we can see, a strong background emission is present in other regions of the electromagnetic spectrum. At millimetre wavelengths, the cosmic microwave background (CMB) dominates this emission, while a strong X-ray background peaks at sub-nanometre wavelengths. For the past 50 years it has also been known that a diffuse gamma-ray background at picometre wavelengths also illuminates the sky away from the strong emission of the Milky Way and known extra-galactic sources.
This so-called isotropic gamma-ray background (IGRB) is expected to be uniform on large scales, but can still contain anisotropies on smaller scales. The study of these anisotropies is important for identifying the nature of the unresolved IGRB sources. The best candidates are star-forming galaxies and active galaxies, in particular blazars, which have a relativistic jet pointing towards the Earth. Another possibility to be investigated is whether there is a detectable contribution from the decay or the annihilation of dark-matter particles, as predicted by models of weakly interacting massive particles (WIMPs).
Using NASA’s Fermi Gamma-ray Space Telescope, a team led by Mattia Fornasa from the University of Amsterdam in the Netherlands studied the anisotropies of the IGRB in observations acquired over more than six years. This follows earlier results published in 2012 by the Fermi collaboration and shows that there are two different classes of gamma-ray sources. A specific type of blazar appears to dominate at the highest energies, while at lower frequencies star-forming galaxies or another class of blazar is thought to imprint a steeper spectral slope in the IGRB. A possible additional contribution from WIMP annihilation could not be identified by Fornasa and collaborators.
The constraints on dark matter will improve with new data continuously collected by Fermi
The first step in such an analysis is to exclude the sky area most contaminated by the Milky Way and extra-galactic sources, and then to subtract remaining galactic contributions and the uniform emission of the IGRB. The resulting images include only the IGRB anisotropies, which can be characterised by computing the associated angular power spectrum (APS) similarly to what is done for the CMB anisotropies. The authors do this both for a single image (“auto-APS”) and between images recorded in two different energy regions (“cross-APS”).
The derived auto-APS and cross-APS are found to be consistent with a Poisson distribution, which means they are constant on all angular scales. This absence of scale dependence in gamma-ray anisotropies suggests that the main contribution comes from distant active galactic nuclei. On the other hand, the emission by star-forming galaxies and dark-matter structures would be dominated by their local distribution that is less uniform on the sky and thus would lead to enhanced power at characteristic angular scales. This allowed Fornasa and co-workers to derive exclusion limits on the dark-matter parameter space. Although less stringent than the best limits achieved from the average intensity of the IGRB or from the observation of dwarf spheroidal galaxies, they independently confirm the absence, so far, of a gamma-ray signal from dark matter.
The constraints on dark matter will improve with new data continuously collected by Fermi, but a potentially more promising approach is to complement them at higher gamma-ray energies with data from the future Cherenkov Telescope Array and possibly also with high-energy neutrinos detected by IceCube.
This 11 m-high structure with thick steel walls will soon contain a prototype detector for the Deep Underground Neutrino Experiment (DUNE), a major international project based in the US for studying neutrinos and proton decay. It is being assembled in conjunction with CERN’s Neutrino Platform, which was established in 2014 to support neutrino experiments hosted in Japan and the US (CERN Courier July/August 2016 p21), and is pictured here in December as the roof of the structure was lowered into place. Another almost identical structure is under construction nearby and will house a second prototype detector for DUNE. Both are being built at CERN’s new “EHN1” test facility, which was completed last year at the north area of the laboratory’s Prévessin site.
DUNE, which is due to start operations in the next decade, will address key outstanding questions about neutrinos. In addition to determining the ordering of the neutrino masses, it will search for leptonic CP violation by precisely measuring differences between the oscillations of muon-type neutrinos and antineutrinos into electron-type neutrinos and antineutrinos, respectively (CERN Courier December 2015 p19). To do so, DUNE will consist of two advanced detectors placed in an intense neutrino beam produced at Fermilab’s Long-Baseline Neutrino Facility (LBNF). One will record particle interactions near the source of the beam before the neutrinos have had time to oscillate, while a second, much larger detector will be installed deep underground at the Sanford Underground Research Laboratory in Lead, South Dakota, 1300 km away.
Insertion of the 3 × 1 × 1 m3 technology demonstrator in the cryostat of the dual-phase protoDUNE module. Image credit: M Brice/CERN.
In collaboration with CERN, the DUNE team is testing technology for DUNE’s far detector based on large liquid-argon (LAr) time-projection chambers (TPCs). Two different technologies are being considered – single-phase and double-phase LAr TPCs – and the eventual DUNE detectors will comprise four modules, each with a total LAr mass of 17 kt. The single-phase technique is well established, having been deployed in the ICARUS experiment at Gran Sasso, while the double-phase concept offers potential advantages. Both may be used in the final DUNE far detector. Scaling LAr technology to such industrial levels presents several challenges – in particular the very large cryostats required, which has led the DUNE collaboration to use technological solutions inspired by the liquified-natural-gas (LNG) shipping industry.
The outer structure of the cryostat(red, pictured at top) for the single-phase protoDUNE module is now complete, and an equivalent structure for the double-phase module is taking shape just a few metres away and is expected to be complete by March. In addition, a smaller technology demonstrator for the double-phase protoDUNE detector is complete and is currently being cooled down at a separate facility on the CERN site (image above). The 3 × 1 × 1 m3 module will allow the CERN and DUNE teams to perfect the double-phase concept, in which a region of gaseous argon situated above the usual liquid phase provides additional signal amplification.
The large protoDUNE modules are planned to be ready for test beam by autumn 2018 at the EHN1 facility using dedicated beams from the Super Proton Synchrotron. Given the intensity of the future LBNF beam, for which Fermilab’s Main Injector recently passed an important milestone by generating a 700 kW, 120 GeV proton beam for a period of more than one hour, the rate and volume of data produced by the DUNE detectors will be substantial. Meanwhile, the DUNE collaboration continues to attract new members and discussions are now under way to share responsibilities for the numerous components of the project’s vast far detectors (see “DUNE collaboration meeting comes to CERN” in this month’s Faces & Places).
Inside the IB3 Tech Building at Fermilab on the outskirts of Chicago, a heavy-duty machine several metres long slowly winds a flat superconducting cable. Watching the bespoke coil winder – called the Spirex and manufactured by Italian firm SELVA – in action, and the meticulous attention to detail from the coil’s specialist operators, is mesmerising. Their task is to fabricate the precision coils that will form the core of novel magnets for CERN’s High-Luminosity LHC (HL-LHC) project, scheduled to begin operation in the early 2020s. “It has to make 50 turns in total, 22 on the inner layer and 28 on the outer,” explains Fred Nobrega, of Fermilab’s magnet-systems department. The main challenge is the niobium-tin (Nb3Sn) material, he says. “Bend it and it breaks like spaghetti.”
The HL-LHC magnets will be built from Nb3Sn, a new conductor used for the first time in an accelerator. Unlike copper, however,Nb3Sn is extremely brittle. Winding turns around the ends of the coil is particularly difficult, says Nobrega, and new chemical and heat treatments are being developed in the current R&D phase of the project at Fermilab to address this issue. The aim is to move from the prototype stage directly to the mass production of 45 long coils that are uniform and of high quality. A further 45 coils will be manufactured more than 1000 km away at Brookhaven National Laboratory (BNL).
Fermilab’s Giorgio Apollinari in the former assembly hall of the CDF experiment, where preparations for HL-LHC magnets are under way.
The HL-LHC relies on a number of innovative magnet and accelerating technologies, most of which are not available off-the-shelf. Key to the new accelerator configuration are powerful superconducting dipole and quadrupole magnets with field strengths of 11 and 12 T, respectively (for comparison, the superconducting niobium-titanium dipoles that guide protons around the existing LHC have fields of around 8.3 T. The new quadrupoles will be installed on either side of the LHC collision points to increase the total number of proton–proton collisions by a factor 10, therefore boosting the chances of a discovery. Although the project requires modifications to just 5% of the current LHC configuration (see article on p28), each one of the HL-LHC’s key innovative technologies pose exceptional challenges that involve several institutes around the world.
Magnets of choice
Fermilab has a glorious history in superconductivity. It was here that the first large superconducting magnet accelerator was built, for example. “But more than that, it was shown that [superconducting magnets] could be reliably employed in a collider experiment for hours and hours of stable beams,” says physicist Giorgio Bellettini, who was spokesperson of the CDF experiment at Fermilab’s Tevatron collider during the mid-1990s at the time the top quark was discovered there. “The LHC experience is built upon this previous large endeavour.”
Flat Nb3Sn cables, coloured white after treatment with glass fibre to insulate each spire, are slowly fed from the coil winder to form prototype dipole magnets for the HL-LHC
The plan is to develop and build half of the focusing magnets for the HL-LHC in the US. These have the specific project labels Q1 and Q3, and are a collaboration between three laboratories: Fermilab, BNL and Lawrence Berkeley National Laboratory in California. Nb3Sn technology, whose development has been supported by the US Department of Energy, was not applicable to accelerator magnets until around a decade ago. Now, Nb3Sn magnets are the technology of choice. The prototypes being developed here are 4 m long, and once assembled with the surrounding “cold mass” to keep them below the superconducting operational temperature of Nb3Sn, they will grow to around twice this length.
The innovative feature of these magnets is their very large aperture – 150 mm in diameter – which is necessary to focus the proton beams more tightly in the interaction points. It also allows greater control of the stress on the magnets and the coils induced by the large magnetic field, explains Giorgio Apollinari, who joined Fermilab in the early days and is now director of the US LHC Accelerator Research Program (LARP). No magnet today can achieve fields of 12 T with such a big opening, which is three times larger than that of the existing LHC dipoles. This is a new development introduced by the LARP team, explains Apollinari, and it took several years to go from 70, then 90 to 120 and now 150 mm required by the HL-LHC. “And then you have to have all the infrastructure necessary to build the magnets, test the magnets, make sure they work, measure the field quality and hopefully send them to CERN for installation in the beamline in 2025.”
The clean room at ANL.
Fermilab and the other LARP laboratories have successfully built 1 m-long short models to demonstrate that the technology meets the technical requirements, and the components are working exactly as expected. Now the teams are building longer prototypes with the correct length, aperture and all other design features. The next step is to build a full prototype with four coils, to complete the quadrupole configuration of the magnets, this coming spring. Similar magnets are being prototyped at CERN with a more ambitious length of 7.5 m. The final product from the US will be a 60 cm-diameter 4 m-long basic magnet containing a hole for the HL-LHC beam pipe. Twenty of these structures will be built in total, 10 in the US and 10 at CERN, of which 16 will be installed and the rest kept as spares. “This is collaboration in physics at its best,” explains Apollinari. “Everybody is trying to go faster, but we are looking at what each other does openly and learning from each other.”
Focus on cavities
Over at Fermilab’s sister laboratory, Argonne National Laboratory (ANL) some 40 km away, the other substantial part of the US contribution to the HL-LHC project is gathering pace. This involves novel “crab”-cavity technology, which is needed both to increase the luminosity and reduce so-called beam–beam parasitic effects that limit the collision efficiency of the accelerator. Unlike standard radiofrequency cavities, which accelerate charged particles in the direction along their path, crab cavities provide a transverse deflection of the beam which causes it to rotate.
The view from the clean-room control room at ANL, where researchers are developing pure niobium structures for the HL-LHC’s superconducting crab cavities.
The cavities are made from pure niobium and therefore require strict control from contamination via chemical processing. ANL specialises in superconducting cavities with a wide range of geometries, and a joint facility for the chemical processing of cavities is in place. ANL’s extensive experience with superconducting cavities includes the Argonne Tandem Linac Accelerator System (ATLAS). Built and operated by the physics division, this is the world’s first superconducting linear accelerator for heavy ions, working at energies in the vicinity of the Coulomb barrier to study the properties of the nucleus. It is for this machine that niobium was used for the first time in an accelerator, in 1977, and for which “quarter-wave” superconducting cavities were developed. “We developed superconducting cavities for a whole variety of projects, for the ATLAS accelerator, Fermilab, BNL, SLAC and of course for the HL-LHC at CERN,” says ANL accelerator scientist Michael Kelly. We meet in the lobby of the ANL physics division, next to a piece of the laboratory’s history: Enrico Fermi’s original “chopper”, a mechanical rotating shutter to select neutrons built in 1947 as part of ANL’s original nuclear-physics programme. “Today we process crab cavities for the HL-LHC, trying to achieve the highest possible accelerating or crabbing voltages, by making a very very clean surface on the cavity,” he explains. Chemical processing taking place at a separate Argonne facility.
ANL’s chemical processing facility has recently been enlarged to accommodate new buffer chemical polishing and electro-polishing rooms. Wearing a complete set of clean-room garments as we enter the facility, electronic engineer Brent Stone explains the importance of surface processing. “A feature of niobium is that a damaged layer is formed as it is mined from the ground and goes through all different processes, so when the niobium is transformed into cavities we need to remove a 120–150 μm-thick damaged layer,” he says. “Inside these layers you can have inclusions that may affect their performance and it is critical to remove them.”
Several steps, and journeys, are required to process the cavities. After the application of acids to remove material from the surface, the cavities undergo two cycles in ultrasonic tanks before being rinsed at high pressure and returned to Fermilab to be degassed in vacuum at high temperatures. They are then taken back to ANL for final chemical treatment, cleaning and assembly in the clean room. Finally, the cavities processed at Argonne are sent to BNL were they are cooled down to liquid-helium temperatures to test if they meet the crabbing voltage required for the HL-LHC. “One of the cavities processed has just very easily achieved its design goal,” says Kelly proudly, before we take leave of the laboratory.
Next stop CERN
The crab cavities are less advanced than the magnets for the HL-LHC, both at CERN and at Fermilab. But efforts are progressing on schedule on both sides of the Atlantic. Two different designs have been developed for the HL-LHC interaction points: vertical plane for ATLAS and horizontal plane for CMS. Both cavity designs originated from LARP, the LHC accelerator R&D programme created by the DOE in 2005 while the LHC was nearing its completion. “Without that foresight we wouldn’t have the HL-LHC today,” says Apollinari.
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