The ATLAS collaboration achieved a milestone in February when it applied the finishing touches to the measurement of the luminosity for proton–proton (pp) data recorded in 2011 at 7 TeV in the centre of mass. With a relative uncertazinty of ±1.8%, the understanding of the luminosity delivered to ATLAS exceeds the accuracy expected before running at the LHC began and opens up exciting possibilities for precision measurements.
The absolute scale for luminosity – a measure of how many particles pass through a given area in a given time – is calibrated by combining simultaneous precision measurements of the bunch currents in the LHC and of the convolved transverse size of the colliding bunches. Using a technique pioneered by Simon van der Meer nearly 50 years ago at CERN’s Intersecting Storage Rings, the inelastic pp collision rate is monitored as the beams are separated first in the horizontal and then in the vertical direction. This “vdM scan” provides a measurement of the beam-overlap area, which when combined with the numbers of protons in each bunch, determines the absolute luminosity produced in head-on collisions.
The success of the procedure for vdM scans at the LHC resulted from close co-operation between the LHC accelerator team and the four large experimental collaborations. The scans are performed in special fills with carefully tailored beam conditions. These fills are optimized for the accuracy of the luminosity measurement while remaining within acceptable operational parameters for the accelerator complex. One key input, the understanding of the number of protons per bunch in the LHC, is determined from several different beam instrumentation measurements, as well as from additional supporting measurements by each of the four LHC collaborations. This effort, led by the LHC Bunch Current Normalization Working Group, has reduced the uncertainty on this key component of the luminosity calibration from around 10% in early 2010 to 0.5% for the final 2011 result.
ATLAS uses two main detectors to monitor the luminosity delivered during physics collisions. LUCID is a segmented Cherenkov detector wrapped around the forward beam pipe; it has been designed specifically for luminosity measurements. The beam conditions monitor (BCM) is a set of small sensors made from synthetic (CVD) diamonds, which also provide fast-abort signals to protect the inner tracking-detectors from radiation damage. LUCID and the BCM both deliver individual luminosity measurements for each of the 3564 possible colliding-bunch slots in the LHC’s fill pattern.
The vdM scans provide a direct calibration of these detectors at a single point in time. The accuracy of that calibration in 2011 was determined to be ±1.5%. The dominant uncertainties in this calibration are linked to the reproducibility of the result from one scan to the next and among different colliding bunches in the same scan, as well as to the understanding of the numbers of protons mentioned above.
To verify that the luminosity calibration determined during vdM scans is stable over an entire year of LHC operation, ATLAS relies on the consistency between several different detectors and algorithms. In addition to LUCID and the BCM, the electrical current flowing through the liquid argon gaps of the forward calorimeter, as well as the photomultiplier currents in selected cells of the hadronic calorimeter, have proved to be remarkably good luminosity monitors. Additional measurements, such as the rate of primary collision vertices reconstructed by the ATLAS tracking system, provide additional cross-checks. Altogether, the agreement among the different luminosity methods has limited any possible variation of the luminosity scale to less than ±1% over the entire year.
The story of the 2011 luminosity measurement has come to a close with the submission of an ATLAS paper on the topic. However, each year brings new challenges and past performance does not guarantee future returns. Considerable machine time was devoted to vdM scans in 2012 to provide the data necessary for a successful luminosity calibration, this time at 8 TeV. This analysis is ongoing, but the accuracy established in 2011 has set a high standard for future luminosity measurements at the LHC.
Three years after resuming operation at a centre-of-mass energy of 7 TeV in 2010 and ramping up to 8 TeV last year, the LHC is now taking a break for its first long shutdown, LS1. During the long period of highly successful running, the CMS collaboration took advantage of the accelerator’s superb performance to produce high-quality results in a variety of physics analyses, the most significant of which being the joint discovery with ATLAS of a new, Higgs-boson-like particle in July 2012.
Now, as the LHC teams prepare the machine for running from 2015 onwards at a higher centre-of-mass energy (13–14 TeV) and with increasing luminosity, the collaboration will continue to be busy maintaining and consolidating the CMS subdetectors and making sure that they can handle the collider’s improved performance. For several systems, this will involve making provision for upgrades to be implemented later in the detector’s lifetime. Point 5, the home of the CMS detector and control room, will see a busy LS1.
Tracker climate control
Perhaps the biggest priority for CMS is to reduce the effects of radiation damage on the performance of the Tracker. The CMS tracking system forms the innermost subdetector and fits snugly round the LHC beampipe. It must withstand an onslaught of some 1010 particles a second and the aggressive field of mixed radiation that this produces. The only way to mitigate against the progressive effects of this irradiation is to operate the Tracker at a lower temperature than the present few degrees Celsius – perhaps as much as 30° C lower. It is crucial that the Tracker will run under these conditions over the next decade, during which a replacement will be designed and built. The issue here is two-fold: on the one hand, the Tracker coolant must run at a lower temperature; on the other, there can be no condensation on the cooling circuits and detectors, which will be much colder than before, and that is a matter of controlling humidity.
Because the Tracker will not be in an hermetically sealed environment, despite an intensive programme of improvement, the humidity inside it will have to be controlled by blowing in dry gas to force out all of the water vapour. In addition to the Tracker itself, the nearby coolant pipes – which will also be at low temperature because of the coolant – are not well insulated. The collaboration will have to make sure that the detector and nearby pipework are dry, to avoid condensation and the growth of ice, which can inflict major damage.
CMS will require substantially more dry gas (nitrogen during operation, air during maintenance) than previously (up to a few hundred normal cubic metres per hour are envisaged) making it no longer cost-effective to purchase liquefied nitrogen. The collaboration has therefore procured an on-site plant that extracts the water vapour and, optionally, the oxygen from air, outputting a dry atmosphere with (optionally) 95% nitrogen. This plant is a relatively large piece of equipment that requires integration, installation and commissioning. It will be deployed in a few months’ time, after the detector is opened up, to confirm that the improved sealing system works well enough to allow the Tracker to run at a much reduced temperature after LS1 and beyond. This is the number one priority for CMS for the shutdown.
During the normal year-end technical stop of 2016–2017, the collaboration will install the Phase 1 upgrade of the CMS pixel tracker, which is the closest physics detector to the collision point. This will feature an additional, fourth layer, among other improvements. To get the first layer as close to the collision point as possible, a smaller-diameter beampipe will be installed during LS1, with an outer diameter of 45 mm – compared with the current 59.6 mm. The additional pixel layer will improve the CMS experiment’s ability to tell where a track comes from, which vertex it comes from or if, indeed, it comes from a primary vertex at all. Running under conditions of high pile-up, resolving which tracks and clusters belong to which vertices is absolutely crucial for the physics analyses.
The additional pixel layer will improve the CMS experiment’s ability to tell where a track comes from
Although replacing the pixel tracker will require a shutdown of only three to four months, installing a new beampipe will take significantly longer – more than a year – so this has to take place during LS1. It is a delicate operation that requires the detector to be in its most open condition with the pixels removed. Once the new beampipe is in place, the collaboration will conduct a dry run by installing a “3D print” of the new pixel detector: a shell that represents the volume of the detector. This is to make sure that the operation can be performed rapidly with the real object, that it does not jam anywhere and that the adjustment systems all work.
More for muons
Another major element of the CMS plans for LS1 features work to improve the muon detectors. The original design for the endcap part of this system had four triggering and measurement stations for muons but the fourth layer was not considered essential for initial operation. However, to function effectively in the future, the fourth layer is now needed to provide more discriminatory power between interesting muons and fake signatures from mismeasurement or background. Hundreds of detector components have to be built and installed. The biggest assembly site is in Building 904 on CERN’s Prévessin site, where teams from CERN and around the world, including the US, China, Russia, Korea, Pakistan and Italy, are halfway through the detector-construction project. Meanwhile, preparations are well advanced for a consolidation of the barrel part of the muon system; some key on-board electronics will be moved from the underground experimental cavern to the neighbouring service cavern, thus taking advantage of the accessibility of this latter cavern for maintenance activities even during LHC operation.
Associated with the installation of the fourth endcap layer is the refurbishment of chambers in the first layer. The inner wires of these chambers were read out in groups in the initial version of CMS. This was fine for lower collision rates but in future the full granularity of this detector layer will be required. In addition, the electronics are not optimal for the expected higher collision rates, so the collaboration is going to replace all of the on-board electronics. The electronics from the first layer will be reused to provide electronics for the outer layer, where it is easier to cope with the collision rate. A special operational support centre has been built at Point 5 specifically for this refurbishment task and for other detector activities, including cold-storage of the pixel tracker while the new beampipe is fitted. Because some elements to be stored or modified may have been activated by radiation, the centre includes a controlled workshop area.
New shielding discs, 10 cm deep, are to be installed outside the new fourth muon stations on the endcap yoke on either end of the detector. Each shielding disc is made of 12 iron sector-casings filled with a special concrete. Following manufacture and preassembly tests in Pakistan, these discs, whose preparation has taken five years, with the design finished only two years ago, are now being re-assembled and filled at CERN. The first has just been finished. The concrete, developed for this specific application by CERN’s civil engineers, is almost 50% denser than normal concrete – it is made using haematite (or ferric oxide) instead of the usual sand – and it is loaded with boron to absorb low-energy neutrons that would otherwise give rise to unwanted hits in the detector. The overall density of neutrons flying round the cavern will be decreased by having these massive 14-m-diameter shielding discs installed.
The new 100-tonne shielding discs represent the first large mechanical elements of CMS to be constructed entirely underground in the experimental cavern, because the heavy-duty cranes – used to lower each of the existing elements of CMS in their entirety – are no longer installed at Point 5. (The CMS experiment was unique in being constructed in massive “slices” above ground). Each disc will have to be taken apart into its 12 component sectors for lowering and then be rebuilt in a vertical position underground. The shielding discs will have an installed clearance to the new detector layer of around 10–20 mm, so it will be a delicate operation and the logical course of action is to install the discs before the detectors.
The magnet and other systems
The consolidation and upgrade programme aims to equip CMS for running well into the 2030s, and a key element of operating for another two decades will be the CMS magnet – a unique object that is impossible to envisage replacing. Changes are being made to ensure that the experiment is not vulnerable to a major breakdown of the supporting cryogenic system, which could prevent CMS from running for a long time, or to avoid unnecessary on–off cycles, which could prematurely age the magnet.
It is important to remember that the detector was designed for 10 years of operation, with a cycle of 7 months for operation and 5 months of shutdown, and a technical stop every three weeks. In practice, there has been three years of continuous operation with only short winter stops – not long enough to open the detector up for thorough servicing – and a technical stop every 6, 8 or 10 weeks. This is a radically different scenario from the one for which CMS was built. Although, the detector has performed well, there is a pressing need to consolidate it for the new regime. For the magnet consolidation, the obvious change is to install a duplicate compressor plant, to mitigate against the failure of the existing plant at Point 5, which has compressors that have run well beyond the recommended service intervals of 40,000 hours without maintenance.
The electrical system is going to be completely revised so that the two levels of the underground service cavern will be supplied through the UPS (uninterruptible power supply), system to give better protection against power glitches. There will also be cooling modifications, not only to make the magnet more robust but also to accommodate the new detectors of the fourth muon layer, the new operating conditions for the Tracker and the future pixel tracker. Many of these modifications have to be put in place during LS1 because there will not be adequate time to do so later.
All of the photo-transducers of the Hadron Calorimeter (HCAL) are to be replaced. Although the work will only be finished during subsequent shutdowns, it is important to begin now while the CMS teams have access to detector components that will not be accessible later. For example, it might not be possible to access the outer HCAL (HO) on the central yoke wheel during LS2, whereas this can be done in the current shutdown.
To be sure that all systems are running well, the collaboration will repeat the Cosmic Run At Four Tesla (CRAFT) (actually 3.8 T) exercise in late summer of 2014, after closing the yoke and testing the magnet. Although there will be no collisions, the detector will record valuable calibration and commissioning data from cosmic rays. If there is a problem with the new cooling systems or with the humidity control of the Tracker, for example, this should be detected promptly and should give the teams just enough time to open up the detector, do whatever needs to be done to fix it and close it again, before the designated end of the shutdown.
The schedule for 2013 is planned in fine detail with a list of hundreds of tasks that are currently being translated into day-to-day planning schematics, and with work packages that have to be understood, approved, checked for co-activity, possible radiological factors and so forth. In addition, amid all this important technical work, the CMS collaboration will attempt to welcome around 20,000 visitors to the site at Point 5 over the course of the year. The coming two years might be described as a shutdown period for the LHC and its experiments but life at Point 5 will be as busy as it has ever been.
In the last beam period before a two-year shutdown, the LHC began 2013 with a challenge: proton–ion collisions. Following a trial run in September, the machine went into full operation beyond its design specification, producing head-on collisions of protons with lead nuclei from mid-January to mid-February. At 5 TeV per colliding nucleon pair, the gain in collision energy is a factor of 25 above previous collisions of a similar type, making it one of the largest such gains in the history of particle accelerators.
Commissioning this new and almost unprecedented mode of collider operation was a major challenge for both the teams behind the LHC and its injector chain. The LHC configuration had to be modified quickly before and during the short run to achieve a number of physics goals.
Nonetheless, on 11 January, single bunches of protons and lead nuclei were injected into the LHC and successfully ramped to full energy. Over the following night the LHC-operations and beam-physics teams sprang into action to commission and measure the optics through a completely new sequence to squeeze the beams at collision. Interventions on the power and cryogenics systems slowed down the commissioning plan but by 20 January stable beams had been achieved with 13 bunches per beam.
In the next fill of the machine, the first bunch-trains were injected, leading to stable beams with 96 bunches of protons and 120 of ions. This important fill allowed the study of “moving”, long-range beam–beam encounters. Stationary long-range encounters occur in proton–proton or lead–lead runs, when bunches in the two beams “see” one another as they travel in the same vacuum chamber on either side of the experiments. The situation becomes more complicated with proton–lead collisions because the long-range encounters move as a result of the different revolution times of the two species – a key feature of proton–lead operation.
At injection energy, lead ions travel more slowly than protons and complete eight fewer turns a minute round the LHC (674,721 turns compared with 674,729 turns for protons). As a result, the two beams – and their RF systems – run independently at different frequencies. Once the energy has been ramped up, the frequency differences become small enough for the RF systems to be locked together in a non-trivial process known as “cogging”.
During the first cogging exercises, high beam-losses triggered beam dumps. This was later found to be caused by an improper synchronization of the two RF frequencies, and careful fine-tuning of the cogging process overcame the problem. After the cogging exercise and throughout the physics fill, the beams run “off-momentum” with opposite offsets to their orbits, requiring special corrections of the beam optics.
The full filling-scheme with 338 bunches in both beams was injected and successfully ramped on 21 January. In addition, the teams achieved a record lead-bunch intensity in the LHC thanks to the excellent performance of both the machine and the injectors. From 24 January onwards the machine was running routinely with stable beams of 338 bunches of protons in ring 1 (clockwise) and lead ions in ring 2. On 1 February, the beams were swapped so that ALICE, inherently an asymmetrical detector, could take data in both directions. A number of issues with cogging and squeezing made this beam reversal challenging, with the machine providing collisions between 192 ion bunches with 216 proton bunches for some days before the operators attempted to reach 338 bunches in each beam by the end of the run on 11 February.
Despite the short time-frame of this asymmetrical run, all seven LHC experiments were able to take data. On a good day, fills had peak luminosity at the beginning of the collisions of around 1029 s–1 cm–2 in ALICE, ATLAS and CMS. Integrated luminosity was well above expectations at around 2 nb–1 a day for each of these experiments. This bodes well for the experimental analysis that will continue to go from strength to strength as the LHC enters its first long shutdown to consolidate and improve this impressive machine.
Every so often the source of the lead ions has to be replaced. A small sliver of solid isotopically pure 208Pb is placed in a ceramic crucible that sits in an “oven” casing at the end of a metal rod. The metal is heated to around 800°C and ionized to become plasma. Ions are then extracted from the plasma and accelerated. Depending on the beam intensity, in stable running the accelerator chain consumes about 2 mg of lead every hour – a tiny amount, but 10 g costs some SwFr12,000 (approx US$13,000). In this image the position of the oven is being measured inside the source for Linac 3.
Despite first being described over three centuries ago, gravity remains one of the least understood of the fundamental forces. At CERN’s recently completed AEgIS experiment, a team is setting out to examine its effects on something much less familiar: antimatter.
Located in the experimental hall at the Antiproton Decelerator (AD), the AEgIS experiment is designed to make the first direct measurement of Earth’s gravitational effect on antimatter. By sending a beam of antihydrogen atoms through very thin gratings, the experiment will be able to measure how far the antihydrogen atoms fall and in how much time – giving the AEgIS team a measurement of the gravitational coupling. The team finished putting all of the elements of the experiment together by the end of 2012, but they will have to wait for two years for beams to return to the AD hall following the Long Shutdown (LS1), which has just begun.
To make progress in the meantime, the AEgIS team has decided to try out the experiment with hydrogen instead of antihydrogen. By replacing antiprotons with their own proton source, the team will be able to manufacture its own hydrogen beam to use for commissioning and testing the set-up. Surprisingly, carrying out the experiment with hydrogen will be more difficult technically than with antihydrogen. Another challenge will be in the production of the positronium that will be used in creating the hydrogen. The positronium needs to be moving fast enough to ensure that it does not decay before it meets the protons/antiprotons, but not so fast as to pass the protons/antiprotons altogether. The AEgIS team will be carrying out this commissioning during the coming months, opening up their set-up next month to make any necessary adjustments and to install a hydrogen detector and proton source.
The LHC has been delivering data to the physics experiments since the first collisions in 2009. Now, with the first long shutdown, LS1, which started on 13 February, work begins to refurbish and consolidate aspects of the collider, together with the experiments and other accelerators in the injections chain.
LS1 was triggered by the need to consolidate the magnet interconnections so as to allow the LHC to operate at the design energy of 14 TeV in the centre-of-mass for proton–proton collisions. It has now turned into a programme involving all of the groups that have equipment in the accelerator complex, the experiments and the infrastructure systems. LS1 will see a massive programme of maintenance for the LHC and its injectors in the wake of more than three years of operation without the long winter shutdowns that were the norm in the past.
The main driving effort will be the consolidation of the 10,170 high-current splices between the superconducting magnets. As many as 1000–1500 splices will need to be redone and more than 27,000 shunts added to overcome possible problems with poor contacts between the superconducting cable and the copper stabilizer that led to the breakdown in September 2008.
The teams will start by opening up the interconnections between each of the 1695 main magnet cryostats. They will repair and consolidate around 500 interconnections at a time, in work that will gradually cover the entire 27-km circumference of the LHC. The effort on the LHC ring will also involve the exchange of 19 magnets, consolidation of the cryogenic feed boxes and installation of pressure-relief valves on the sectors that have not yet been equipped with them.
The Radiation to Electronics project (R2E) will see the protection of sensitive electronic equipment optimized by relocating the equipment or by adding shielding. Nor will work during LS1 be confined to the LHC. Major renovation work is scheduled, for example, for the Proton Synchrotron, the Super Proton Synchrotron and the LHC experiments.
Preparations for LS1 started more than three years ago, with the detailed planning of manpower and other resources. For example, Building 180 on the Meyrin site at CERN recently became a hive of activity as a training centre for the technicians who are implementing the various repairs and modifications. The pictures shown here give the flavour of this activity.
A team of scientists from the Paul Scherrer Institute (PSI), CERN’s ISOLDE facility and the Institut Laue-Langevin (ILL) has published results from a preclinical study of new tumour-targeting radiopharmaceuticals based on the element terbium. The results demonstrate the potential of providing a new generation of radioisotopes with excellent properties for the diagnosis and treatment of cancer.
Radiopharmaceuticals in which a radioactive isotope is attached to a carrier that selectively delivers it to tumour cells are used in two main ways, for diagnosis and for treatment. Nuclear imaging for diagnostics involves either β+-emitting radioisotopes for positron-emission tomography (PET) or γ-emitting radioisotopes for use in single-photon-emission computed tomography (SPECT) and in planar imaging with gamma-cameras. By contrast, targeted radionuclide employs the short-range radiation (α-particles and electrons) emitted by radioisotopes to destroy cancer cells.
So-called “matched pairs” of diagnostic and therapeutic radioisotopes of the same chemical element are particularly useful because they allow the preparation of radiopharmaceuticals that are absorbed and distributed in identical ways in the body. Terbium is the only element in the periodic table to offer not just a pair but four clinically interesting radioisotopes with complementary nuclear-decay characteristics covering all of the options for nuclear medicine: 152Tb for PET, 155Tb for SPECT, 149Tb for α-particle therapy and 161Tb for therapy with electrons (β–, conversion and Auger electrons).
The team from the PSI, ILL and CERN has now made the first comprehensive preclinical study of this range of terbium radiopharmaceuticals. The neutron-deficient isotopes 149Tb, 152Tb and 155Tb were produced by 1.4 GeV proton-induced spallation in a tantalum target and separated with the ISOLDE online isotope separator at CERN. 161Tb was produced at the high-flux reactor of ILL and at the spallation neutron source SINQ at PSI. The isotopes were then purified using cation-exchange chromatography at PSI.
For this first in vivo proof-of-principle study the team developed a new delivery agent, which targets folate receptors in the body. These receptors are over-expressed in a variety of aggressive tumours, including ovarian and other gynaecological cancers as well as certain breast, renal, lung, colorectal and brain cancers, while their distribution in normal tissues and organs is highly limited. Folate vitamins have a rapid uptake in the body but they are also rapidly eliminated, so they do not remain long enough to reach all cancer cells. Hence, the team designed a new folate delivery agent called “cm09”, where folic acid is conjugated with an albumin-binding entity to prolong the circulation time in the blood.
For the study, the terbium radioisotopes were combined with the cm09 and then administered to tumour-bearing mice. Excellent tumour-to-background ratios 24 hours after injection allowed tumour xenografts in mice to be seen using small-animal PET (152Tb-cm09) and small-animal SPECT (155Tb-cm09 and 161Tb-cm09). In vivo therapy experiments using 149Tb-cm09 (α-therapy) and 161Tb-cm09 (β-therapy) resulted in a marked delay in tumour growth or even complete remission, as well as a significant increased survival in treated animals compared with untreated controls.
Future progress in these promising diagnostic and treatment options depends crucially on the regular availability of the terbium isotopes, in particular of 149Tb. At present ISOLDE at CERN is the world’s only source of this isotope.
The LHC, the largest scientific instrument ever built, will extend its discovery potential at the beginning of the next decade through a fivefold increase in luminosity beyond the design value, in a new configuration called the High Luminosity LHC (HL-LHC). This extraordinary technical enterprise will rely on a combination of cutting-edge 11–13 T superconducting magnets, compact and ultraprecise superconducting radio-frequency cavities for beam rotation, as well as 300-m-long, high-power superconducting links with zero energy dissipation. In addition, the higher luminosities will make new demands on vacuum, cryogenics and machine protection, and will require new concepts for collimation and diagnostics, as well as advanced modelling for the intense beams.
Now, as the LHC nears the end of its first long run – from March 2010 to March 2013 – preparation work for this major upgrade is gathering speed. The past year has seen major developments in some of the key superconducting technologies, in particular for the new high-field magnets and the high-power links. Meanwhile, important decisions have been taken within the HiLumi LHC Design Study, which was launched just over a year ago. Supported in part by funding from the Seventh Framework Programme (FP7) of the European Commission (EC), this is the first phase of the larger HL-LHC project.
Broad collaboration
Towards the end of 2012, two meetings provided the opportunity for people involved at these accelerator frontiers to review progress and plan future activities, not only within their institutes around the world but also with industrial partners. On 14–16 November, the INFN Frascati National Laboratory was host to the 2nd Joint HiLumi LHC–LARP Annual Meeting. This brought together some 130 experts from Europe, Japan, Russia and the US LHC Accelerator Research Program (LARP). Three weeks later, on 4–5 December, a workshop on “Superconducting technologies for next-generation accelerators” took place at CERN organized by the HiLumi LHC Design Study in conjunction with the Test Infrastructure and Accelerator Research Area (TIARA) project, which is also co-funded by the EC under FP7. The workshop attracted more than 100 specialists, half from industry and half from laboratories and institutes. The aim was to explore the technical challenges emerging from the design of new accelerators and to match them with state-of-the-art industrial solutions.
Superconductivity has been the most important enabling technology in particle accelerators for the past 30 years – since the time of CERN’s Intersecting Storage Rings (the first accelerator to employ superconducting magnets during operation) and Fermilab’s Energy Doubler. The latter, later renamed the Tevatron, was the first large-scale superconducting system and it paved the way for all of the subsequent superconductivity projects, including the HERA collider at DESY, phase II of the Large Electron–Positron collider at CERN, the TRISTAN electron–positron collider at KEK and the Relativistic Heavy-Ion Collider at Brookhaven National Laboratory. Today, superconductivity is the core technology of the LHC, which employs some 1700 large superconducting magnets (dipoles and quadrupoles) and nearly 8000 superconducting corrector magnets, all cooled by more than 100 tonnes of superfluid helium.
The LHC’s main dipoles are 8 T superconducting magnets made from coils of niobium-titanium (NbTi) alloy. To allow the installation of additional collimators to deal with the increased luminosity in the HL-LHC, in 2010 CERN’s Lucio Rossi suggested replacing some of the 8 T dipoles with shorter 11 T magnets based on niobium-tin (Nb3Sn), which is superconducting at a higher temperature than NbTi. This idea also interested Fermilab, which has a high-field magnet R&D programme aimed at developing magnets for future machines such as a muon collider. CERN and Fermilab began to collaborate and by the spring of 2012 they completed a 2-m-long Nb3Sn dipole. In summer it was tested at 1.9 K in the Fermilab Vertical Test Facility, reaching a current of 11.2 kA and a calculated field of 10.4 T.
Such developments feed directly into the HiLumi LHC Design Study, which covers six work-packages (WP) of the larger HL-LHC project. The work of the design study is overseen by project management (WP1), which has CERN’s Hermann Schmickler as its new technical co-ordinator. Various committees and bodies, in particular the newly formed HL-LHC Co-ordination Group, ensure the necessary link between the machine-upgrade and the detector-upgrade projects, under the supervision of CERN management. The recent Joint HiLumi LHC–LARP Annual Meeting reviewed their progress as well as the headway that has been made towards a final layout for the accelerator upgrade.
Good progress
The main target for the HL-LHC is to achieve an integrated luminosity of 250 fb–1 a year and a total of 3000 fb–1 over 12 years. A key step in reaching this target lies in reducing the β* function (related to the focal length) at collision. With this in view, the team working on accelerator physics and performance (WP2) has collaborated closely with members of the LHC injector upgrade project as well as the current LHC operation group. As a result, they have defined possible sets of machine optics (in relation to β* and the crossing angle) and beam parameters (emittance, bunch spacing, bunch charge) that can achieve their goal. A further important development in WP2 is the recent, successful test in the LHC of luminosity-levelling by varying β*.
The conceptual design of the new D1 dipoles for the IRs is being steered by the KEK laboratory in Japan
A reduced β* in turn requires a redesign of the magnets in the insertion regions (IRs) where the collisions occur, which is the task of WP3. One important decision, taken in July in collaboration with WP2 and WP10 of HL-LHC (energy deposition and absorber), was to opt for the maximum possible aperture for the quadrupoles of the inner triplets: 150 mm of coil-free bore. This choice was based on successful tests within US-LARP of a 4-m-long, 90 mm aperture quadrupole and a more recent 1-m-long structure with a 120 mm aperture, both based on advanced Nb3Sn superconductor. In light of this decision, the teams working on accelerator physics and magnets in US-LARP are adjusting their plans and preparing a construction project for 2015.
While the work of WP3 has focused on providing major input to the choice of the quadrupole aperture, a decision on shielding has been made to use tungsten elements and a beam screen. At the same time, the conceptual design of the new D1 dipoles for the IRs is being steered by the KEK laboratory in Japan, where teams have analysed the performance of three possible apertures. The proposal is to have an 8-m-long magnet operating at 5 T.
To make the decreased β* most effective, the HL-LHC will use superconducting “crab cavities” to rotate particle bunches before they collide. These special radio-frequency cavities, which are the focus of WP4, may also provide levelling of the luminosity during the beam spill. The conceptual and technical design of three compact cavities (“4-rod”, “double ridge” and “quarter-wave”) has now been completed successfully. The new Crab Cavity Technical Co-ordination Working Group will, after the first long shutdown of the LHC, oversee preparation for the integration of crab cavities in the LHC and the preliminary tests in the Super Proton Synchrotron in 2015. Laboratory tests of a prototype 4-rod crab cavity built from bulk niobium superconductor by Lancaster University and the Cockcroft Institute in the UK began in November at CERN, while a prototype of the double-ridge type is under final preparation by a team from SLAC and Old Dominion University (ODU) at Jefferson Lab in the US, with tests foreseen by the beginning of 2013. A prototype of the quarter-wave type is under manufacture at Brookhaven National Laboratory, also using bulk niobium.
The HL-LHC will require higher beam currents, so new collimators will be necessary to protect the magnets from the 500 MJ of stored energy in each beam. The collimation team (WP5) has made the first steps towards the design of new IR collimation, with close collaboration between teams at CERN and from US-LARP. Tracking simulation tools have been set up to calculate losses by performing multi-turn tracking of the collision products, which can induce significant losses in the matching sections and dispersion suppressors at Point 1, Point 2 (with ions) and Point 5.
A further challenge for the HL-LHC project is to relocate equipment such as power convertors away from the tunnel to avoid radiation damage to electronics as well as to ease installation and integration of new equipment near the high-luminosity IRs, which are already crowded. This will require superconducting links that can transport high currents (up to 150 kA DC per line) from power supplies at ambient temperature on the surface to components operating at 1.9 K in the tunnel, some 100 m below ground. Work on this “cold powering” has started well ahead of schedule in WP6, with a study made of possible powering layouts for the new quadrupole magnets in the IRs, based on input concerning features of the optics and magnets agreed with WP2, WP3 and WP7 of HL-LHC (machine protection). Preliminary studies of the integration of the cold-powering system in the LHC machine have also been performed.
Cables built from tapes of copper and either HTS or MgB2 have been built and tested at CERN
In the LHC, current leads that incorporate a high-temperature superconductor (HTS) supply currents of up to 13 kA to the magnets in the tunnel. For the upgrade, CERN has been working with the Italian company Columbus to develop new superconducting wires based on magnesium diboride (MgB2), which has a lower operating temperature than HTS – the former being superconducting at the operating current at up to 25 K rather than 35–50 K with the latter – but is considerably less expensive. Until now, only flat ribbons of MgB2 have been available but CERN and Columbus have jointly developed round wires that are more suitable for the higher currents required for the HL-LHC. Cables built from tapes of copper and either HTS or MgB2 have been built and tested at CERN, and multi-cable assemblies have also been designed and constructed. Tests of the first 20 kA superconducting link are now taking place at CERN in the new test-station that has been set up in building SM18.
New and more advanced superconducting devices lie at the heart of not only the HL-LHC but of other large projects, such as the Facility for Antiproton and Ion Research at GSI, the XFEL at DESY and the European Spallation Source (ESS) in Lund. The workshop held in December on superconducting technologies was therefore based on talks about the HiLumi LHC Design Study, TIARA and the ESS, interweaved with presentations by representatives from industry. Companies also had stands and meeting points to provide the opportunity to exchange ideas and information.
One point of discussion was the model for laboratory–industry relations. Both the approach based on “turnkey” contracts (where only the main characteristics of equipment are laid down in a functional specification) and an approach based on “built-to-print” contracts (where industry is responsible for a specific manufacture rather than for the full product) can be effective and yield the best value for money. For equipment that has been fully or even partly developed for previous projects, the turnkey model can probably be used, thus minimizing the human resources required at the laboratory. When R&D is long and based on new types of equipment, such as for the LHC upgrade, the built-to-print model is probably more suitable. However, in both cases, only a close laboratory–industry relationship during construction can avoid misunderstandings and painful extra costs. There were also discussions on how to improve the exchange of information on technologies, processes, materials, facilities, work organization and training of the next generation of engineers and technicians.
In addition to reviewing progress in superconducting technologies for the HL-LHC, the workshop looked forwards in considering items that will need to be procured once the project is approved by CERN Council; in principle in June, in the context of the updated European Strategy for Particle Physics. The requirements include: 20 large superconducting Nb3Sn quadrupoles rated for 12–13 T, 10 of which will be supplied by the US; five large 6 T superconducting dipoles (D1) in NbTi from Japan; five large 4–4.5 T superconducting dipoles (D2); five large superconducting twin quadrupoles (Q4) rated for 8 T; six to twenty superconducting 11 T twin dipoles in Nb3Sn; five large SC twin quads (Q7) rated for 7 T; five to six modules each of three superconducting crab cavities; 3 km of superconducting links rated for 50–150 kA; and a number, still to be defined, of corrector-magnet packages. These will all have their own cryostats and will need new cryogenic plants and vacuum requirements. In addition, there will be new collimators, some equipped with special wire to compensate inter-beam effects near collision, as well as other equipment that is under development.
The workshop was the first step in communicating with industry to find partners for new development and construction, with a goal of maximizing the industrial return and incrementing the industrial capability of the EU. The aim is to achieve full funding of the project, including design and prototyping, totalling around SwFr750 million by 2015, with an additional SwFr200–250 million from external collaboration with the US and Japan. Construction and testing would then take place between 2016 and 2020, ready for installation at the end of 2021.
• The next joint HiLumi LHC–LARP Annual Meeting is planned to take place at the Cockcroft Institute, Daresbury Laboratory, on 12–15 November 2013, while in May 2013 the collaboration will meet at the joint LARP–HiLumi LHC Annual Meeting in the US. For more about the workshop on “Superconducting technologies for next-generation accelerators”, see https://indico.cern.ch/conferenceDisplay.py?confId=196164. For more about HL-LHC, see http://cern.ch/hilumilhc.
Letting young scientists shine
A new section at the 2nd Joint HiLumi LHC–LARP meeting was the “Young Scientist Talk”, a session organized to showcase recipients of LARP’s Toohig Fellowship, which is awarded each year to two recent PhD recipients in physics or engineering. Toohig Fellows John Cesaratto and Valentina Previtali attended the meeting. Currently based at member institutions of LARP – Cesaratto at SLAC National Accelerator Laboratory and Previtali at Fermilab – they will also spend time at CERN as part of the fellowship. Cesaratto gave a talk on the control of beam instabilities in CERN’s Super Proton Synchrotron and Previtali presented first results from simulations of the hollow electron lens. They were joined by Meghan McAteer, a Marie Curie Fellow, who talked about optics measurements in the Boosters at Fermilab and CERN.
The section was convened by John Fox of SLAC and a member of LARP. He is chair of LARP’s Toohig Fellowship Committee and is keenly interested in promoting the work of young scientists. He believes that bringing young scientists to the conference is multi-valued, enabling the young Toohig Fellows to meet scientists at CERN and, conversely, allowing young scientists from CERN to meet members of LARP. Such opportunities to meet and strike up collaborations are important for young scientists at US labs, who get few chances to interact with the broader community.
The Toohig Fellowships are awarded in honour of the late Timothy Toohig, a physicist and Jesuit priest who devoted his life to promoting accelerator science and increasing understanding, communication and collaboration among scientists of all nations and religions. The fellowships are for two years, extendable to three, and are explicitly for postdoctoral research and development regarding the LHC.
Following tests in September, a short, dedicated run at the end of October provided “de-squeezed” beams to the ALFA and TOTEM experiments, allowing new measurements of the elastic proton–proton cross-section.
To squeeze the beam and so maximize the number of collisions, LHC beams at full energy typically have a value of β* – the distance to the point where the beam is twice as wide as it is at the interaction point – 0.60 m. However, squeezing to a small beam increases the angular beam divergence such that elastic proton–proton scattering at small angles cannot be observed.
The TOTEM experiment has measured the elastic proton–proton cross-section in previous dedicated runs, resulting in a determination of the total proton–proton cross-section using the optical theorem. To observe the contribution of electromagnetic interaction (Coulomb scattering) and its interference with the nuclear component to the elastic cross-section, scattering angles of the order of 5 μrad have to be reached. Since the Coulomb scattering cross-section is known theoretically, its measurement also gives access to an independent determination of the absolute luminosity of the LHC.
For this recent special run, a new record value of β* = 1000 m was reached, making the beams at interaction points 1 and 5 almost parallel. The angular divergence of the beams at the interaction points was reduced by a factor of 40 compared with low-beta (high-luminosity) operation. These special settings allowed the ALFA and TOTEM experiments – at points 1 and 5, respectively – to measure proton–proton scattering angles down to the microradian level. The experiments’ Roman Pots were moved as close as 0.87 mm to the centre of the beam, which contained three bunches of 1011 protons each. At that distance the beam halo is intense and had to be reduced by an optimized collimation procedure that allowed a reduction of the halo background by a factor of 1000. This configuration enabled data-taking in good conditions for about an hour and, for the first time, ALFA and TOTEM could measure the elastic scattering in the Coulomb-nuclear interference region.
For future runs at 13 TeV, optics with β* of around 2 km will have to be developed. This will require the installation of additional quadrupole power cables in the LHC tunnel.
It is a seemingly simple question: when an electron scatters off a proton, how many photons are exchanged? The obvious answer would be: just one. Whether nature really acts as simply as this is, however, far from clear. The venerable storage ring DORIS at DESY is dedicating the last few weeks of its nearly 40 years of operation to this simple yet fundamental question. After three consecutive lives at the forefront of physics and with a wealth of scientific achievements, DORIS will be shut down for good at the end of 2012.
When DORIS ceases operation, it will leave behind some degree of nostalgia but most of all an invaluable scientific legacy in many fields. DORIS has been a pioneer in several ways: in accelerator science; in particle physics; and, notably, in the application of synchrotron radiation, where the machine helped spark a whole new field of photon science. “Synchrotron radiation measurements played an important role right from the start and the people working at DORIS fostered a creative spirit that let this young scientific field thrive,” says DESY’s photon-science director, Edgar Weckert. “This led to DESY becoming a global magnet for research with extremely intense X-ray light.” Indeed, various methods that today are standard techniques in photon science were developed at DORIS.
When DORIS went into operation in 1974, it was one of the world’s first storage rings. The concept of keeping the accelerated particles for repeated head-on collisions within the ring was only just beginning to compete with the practice of shooting them at a target all at once. Experience with DORIS certainly helped to develop this technology further. With a circumference of 289 m, DORIS started out in fact as two storage rings (hence the German name DOppel RIng Speicher, double storage ring) for electrons and positrons, with a maximum beam energy of 3.5 GeV each. However, because of technical difficulties, the machine was converted after three years into a single storage ring with two circulating beams.
With its early particle-physics experiments, DORIS made important contributions to establishing the concept of quarks, which was still in question at the time. After the “November revolution” of 1974, when the unexpected J/ψ resonance was discovered at the Brookhaven National Laboratory and SLAC, DORIS helped to establish that the new particle was, indeed, a bound state of a new quark and its antiparticle, i.e. charm and anti-charm.
B-meson oscillations
In its second life, starting in 1982, an enhanced machine with a nearly doubled collision energy (compared with 1974) discovered and probed a host of new particles – and finally discovered spontaneous B-meson oscillations, probably the machine’s best known contribution to particle physics. The prolific ARGUS experiment at DORIS II observed that neutral B mesons spontaneously change into their antiparticles and vice versa. “The large mixing rate measured indicated that CP violation should also be observable in B-meson decays, which would be the second example of CP violation after the neutral K mesons,” explains DESY’s particle-physics director Joachim Mnich. CP violation is one of the pre-conditions to explain the observed dominance of matter over antimatter after the big bang. “The discovery at DORIS formed the foundation for further experiments, for instance with BaBar at SLAC and BELLE at KEK. But now we also know that the CP violation in the Standard Model is not sufficient to explain all of the matter in the universe. There have to be additional sources of CP violation beyond the Standard Model.” Today, B mesons and their oscillations are examined for instance by the LHCb experiment at CERN and soon also at the upgraded BELLE II experiment, in which DESY is participating.
In 1991, DORIS was reborn in its third incarnation – as one of Europe’s brightest hard X-ray sources, bringing synchrotron-radiation applications and photon science to full bloom at DESY and beyond. There had been synchrotron-radiation measurements at DORIS right from the start in 1974, and only one year later the European Molecular Biology Laboratory (EMBL) established an outstation on the DESY campus to use the intense light for the investigation of biomolecules.
In 1980 the Hamburg synchrotron-radiation laboratory (HASYLAB) was founded, and while synchrotron-radiation techniques were constantly improving and their applications gaining weight, they were still riding piggyback on a particle-physics machine. This changed with the proposal of a DORIS “bypass” in 1986, specially designed to improve its synchrotron radiation. The northern straight section of the racetrack-shaped storage ring was to be replaced – or bypassed – by a 74-m-long, gently curved arc that offered space for additional wiggler magnets to enhance the quality and intensity of the X-ray beams. After approval of the plans, work began in 1990 and DORIS III went into operation only about a year later.
Unfortunately, the alteration had unforeseen consequences for the luminosity at ARGUS. Although the machine operators tried hard to improve this, the highly successful experiment had to stop taking data early because it was no longer competitive. In 1993 DESY decided to dedicate DORIS III exclusively to photon science, leaving particle physics to DORIS’ bigger sisters PETRA and HERA. Today, ARGUS graces DESY’s main entrance as a scientific landmark.
X-ray beams for all
During its lifetime, DORIS has peered into nearly everything imaginable with its X-rays, from innovative alloys and magnetic nanostructures to biomolecules, viruses and corals. Even bronze-age axes, mediaeval palimpsests and hidden paintings by Dutch master Vincent van Gogh have been screened at DORIS. Thousands of guest scientists have used the facility every year. Eventually, there were more than 30 beamlines at DORIS, offering all sorts of X-ray techniques, with many results having a benefit for everyday life. Researchers there have investigated new kinds of electronics and routes towards better catalytic converters, evaluated new welding techniques and more effective luminescent materials for energy-saving lamps, developed medical and technical X-ray applications and studied the properties of clusters of atoms and even of the Earth’s interior.
Among the countless scientific highlights, one in particular stands out. In 1999 the team of Ada Yonath, who was leading a Max Planck research group founded on the DESY campus in 1986, decoded the structure of the ribosome with the help of DORIS and different machines at other centres. The ribosome is the protein factory of the cell and is of central importance to life. It is an incredibly complex structure that seemed almost impossible to decode. “DESY provided us very generously with beam-time even back in the 1980s, when our project met worldwide scepticism as it was widely assumed that the structure of the ribosome might never be determined,” recalls Ada Yonath, who in 2009 received the Nobel Prize in Chemistry for her groundbreaking research.
DORIS’s achievements are not only scientific. Over the years, the accelerator team maintained an innovative atmosphere and a remarkable collaborative spirit. Not only have several techniques that were pioneered at DORIS now become standards of photon science, the continued improvements also led to PETRA III, the world’s most brilliant X-ray source. Dedicated exclusively to photon science, PETRA III offers much more intense and much finer X-ray beams than DORIS, opening up new opportunities. “Some experiments, however, do not always require PETRA’s extraordinary brilliance,” says Wolfgang Drube of the DESY photon-science department. “Until now, these experiments have been located at DORIS.”
Profile of DORIS III
Type: storage ring
Particles: positrons
Circumference: 289.2 m
Beamlines: 33
Positron energy: 4.45 GeV
Initial positron beam current (5 bunches): 140 mA
Number of buckets: 482
Number of bunches: 1 (for tests), 2 and 5
Bunch separation (minimum): 964 ns (for tests), 480 ns and 192 ns
Horizontal positron beam emittance: 410 π nm rad
Vertical positron beam emittance: 12 π nm rad
Positron beam energy spread (rms): 0.11%
Curvature radius of bending magnets: 12.181 m
Magnetic field of bending magnets: 1.2182 T
Critical photon energy from bending magnets: 16.04 keV
To meet the continuing demand, DESY and its international partners are building two additional experimental halls at PETRA. The most successful of the DORIS III experiments will move into these extensions. “At several experimental stations, the beam will be up to 100 times more intense compared with DORIS,” explains Drube, who is leading the PETRA III extension project. This way, the best of DORIS will live on at PETRA III, which today is also complemented by DESY’s free-electron laser, FLASH. “The unique properties of our light sources are attractive for a multitude of research disciplines. Co-operation and exchange in these various disciplines stimulate research in the next generation,” stresses Weckert.
At the end of its lifetime, there is one more “half-life” in store for DORIS. Synchrotron-radiation operation ceased on 22 October but the rest of the year has been dedicated to a small but clever particle-physics experiment called OLYMPUS. The collaboration led by Richard Milner of Massachusetts Institute of Technology (MIT) will use it to compare in detail the scattering of electrons and positrons by protons to find out if more than one photon can be exchanged in this process. “Recent data suggest strongly that higher-order photon exchange is happening in certain situations,” says Milner. “The experiment is to measure the angular distribution of the scattering for electrons and positrons and compare it. If two-photon effects indeed are there, we should see a significant difference of the order of 5% at larger angles – that is around 60° – in this comparison.”
That this became possible is owed to a coincidence that Milner refers to as a miracle. His team had realized that the experiment that they wanted to do would be possible with a disused particle detector from MIT, and they were looking for a storage ring where intense electron and positron beams were readily available. “So, we came to DESY to discuss this and together we decided that we could attempt the experiment at the location of ARGUS. The miracle was that an experiment designed at MIT in the 1990s to do electron–proton scattering would fit exactly in the footprint of an experiment designed at DESY in the 1980s to look at things like B mixing. Essentially, you took one out and dropped the other one in. Everything fit on the rails of ARGUS!”
OLYMPUS may be DORIS’ last experiment but the legacy of this machine will live on for a long time.
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