CERN Accelerating science

HL-LHC project stimulates new collaboration
by Carsten Welsch (University of Liverpool)


View from the LHC tunnel (Credit: CERN)

A new multi-million-pound project between CERN, the Science and Technology Facilities Council (STFC) and six other UK institutions has been launched to contribute to the upgrade of the Large Hadron Collider (LHC) at CERN in Geneva. The world’s highest energy particle collider shall be upgraded to the High Luminosity LHC (HL-LHC) in the 2020s through international collaboration.

The challenges of this project are best tackled with input from the project partners from around the world. Several partnerships have already been established with the HL-LHC project and there is room for more potential partnerships in the future. It has now been announced that the UK will make contributions in four areas across the new HL-LHC-UK project among other contributions from UK universities1.

The full exploitation of the LHC is the highest priority in the European Strategy for Particle Physics, adopted by the CERN Council and integrated into the ESFRI Roadmap. The full HL-LHC project funding was approved by the CERN Council in June 2016. To extend its discovery potential, the LHC will need a major upgrade around 2025 to increase its luminosity (rate of collisions) by a factor of 10 beyond the original design value (from 300 to 3,000 fb-1). This will enable scientists to look for new, very rare fundamental particles, and to measure known particles such as the Higgs boson with unprecedented accuracy.

Upgrading the LHC calls for technology breakthroughs in areas already under study, and requires about 10 years of research to implement. HL-LHC relies on a number of key innovative technologies, representing exceptional technological challenges. Led by experts from the Cockcroft Institute, the HL-LHC-UK project has now been established to address these challenges.

Within HL-LHC-UK, the partner institutions will perform cutting-edge research and deliver hardware for the LHC upgrade in four areas: 1) proton beam collimation to remove stray halo protons, 2) the development and test of transverse deflecting cavities (“crab cavities”), 3) new methods to diagnose the stored beams including gas jet-based beam profile monitors and, 4) novel beam position monitors, as well as sophisticated cold powering technology needed for the cryogenic systems.

Lucio Rossi, Head of the High-Luminosity LHC project, commented: “In order to make the project a success we have to innovate in many fields, developing cutting-edge technologies for magnets, the optics of the accelerator, superconducting radiofrequency cavities, and superconducting links. We are very excited for the UK to be making key contributions and using their expertise to help deliver this upgrade.”

The HL-LHC-UK project comprises the University of Manchester (Cockcroft Institute), Lancaster University (Cockcroft Institute), the University of Liverpool (Cockcroft Institute), the University of Huddersfield (International Institute of Accelerator Applications), Royal Holloway University of London (John Adams Institute), the University of Southampton and the Science and Technology Facilities Council (STFC). The spokesperson is Rob Appleby (Manchester) and the project manager is Graeme Burt (Lancaster).

More information about the High Luminosity LHC project, its technology and design as well as the challenges ahead can be found in the recently released open access HiLumi LHC book “The High Luminosity Large Hadron Collider. The New Machine for Illuminating the Mysteries of the Universe”.

 

1. UK is currently also contributing with the technology for surface modification of metals in a collaboration between CERN, the University of Dundee and the Science and Technology Facilities Council.

 

Progress in the interaction region magnets of HL-LHC 
by Ezio Todesco (CERN)

During the past months, significant advancements have been done in the development of the interaction region magnets for HL-LHC.

In KEK, Japan, the short model of the separation dipole D1, that showed insufficient quench performance after the first test, has been disassembled. Significant movements of the coils (up to few mm) were observed in the heads, and a clear evidence of a lack of prestress in the straight part was found. The new assembly took place during winter, and a prestress increase in the straight part of about 35 MPa has been achieved. The magnet was tested in February, reaching nominal current after 2 quenches and ultimate after 5 quenches (see Figure 1). “The magnet performance is now in line with the project requirements – says T. Nakamoto, in charge of the D1 project – we will have a warm-up and cool-down to prove the magnet memory in the next weeks”.  The short model design is being updated in some features of the iron yoke, and to account for an unexpected contribution to field quality from the coil heads in the strong regime of saturation. A second model will be built in the second part of 2017, and tested in 2018.

Figure 1: Training of MBXFS1 in KEK: quenches (markers), nominal and ultimate current (solid lines) and short sample limit (dotted line). (Credit: HL-LHC WP3 collaboration)

In the US, the first 4-m-long coil has been tested in a mirror configuration, reaching 85% of short sample limit (see Figure 2). “This is the new world record for coil length in Nb3Sn accelerator magnets – says G. Ambrosio, in charge of the US contribution for the triplet - and paves the way to the assembly and test of the first 4-m-long quadrupole, to be done in the second part of the year”.  At the same time at CERN the first 7.15-m-long dummy coils are being produced to validate the assembly procedures (see Figure 3).

Figure 2: Training of mirror 4-m-long coil in BNL: quenches (markers), 70% and 80% of short sample (solid lines) and short sample limit (dotted line). (Credit: HL-LHC WP3 collaboration)

Figure 3: Winding of the first 7.15-m-long dummy coil of the triplet quadrupole at building 180 (CERN)

Furthermore, in CIEMAT, Madrid, the prototype for the nested orbit correctors is entering the construction phase. The concept of double collaring has been validated on a mechanical model with the final design of the collars and a dummy coil made of aluminum (see Figure 4). This is an important step of the validation of the mechanical concept of this magnet, where a mechanical lock between the horizontal and vertical dipoles is required to control the large torque.  In particular, the second collaring of the outer dipole on the inner one is critical. “Both collaring operations were in line with our expectations, and we managed to insert pins without any criticality – said F. Toral from CIEMAT, in charge of the Spanish contribution for the orbit correctors – we saw some asymmetries that need more investigations, but given the complexity of the design, this is a very encouraging first step towards construction”.

Figure 4: Double collaring of the nested corrector in CIEMAT

Finally, in LASA, Milano the activity on the high order corrector prototypes is at full speed. After the successful test of the sextupole, the first decapole coil in a single coil configuration has been tested successfully. The coil reached twice the ultimate current with negligible training, thus proving the assembly procedures and tooling concepts. LASA is working in parallel on two magnets: besides the first decapole coil, eigth octupole coils have been completed and will be assembled in the first prototype, and tested in April.

 

Civil engineering for the High-Luminosity LHC
by Jean Laurent Tavian (CERN), Peter Mattelaer (CERN)

The High-Luminosity LHC (HL-LHC) project at CERN will require large infrastructures and services for the powering and the cooling of the high-field superconducting quadrupole magnets constituting the new inner triplets and of the superconducting RF crab-cavities used for the luminosity levelling. These new LHC accelerator components will be integrated at Point 1 and Point 5 of the LHC accelerator where the two large LHC detectors ATLAS and CMS are located (see Figure 1). These new infrastructures and services consist mainly of power transmission, electrical distribution, cooling, ventilation, cryogenics, power converters for superconducting magnets and inductive output tubes for superconducting RF cavities. To house all these new infrastructures and services, civil engineering structures are required including buildings, shaft, caverns and underground galleries.

Figure 1. Underground civil engineering of LHC (Image credit: CERN)

At ground level, the civil engineering consists of five buildings, technical galleries, access roads, concrete slabs and landscaping (See Figures 2 and 3). Per Point, the total surface corresponds to about 20’000 m2, including 3’300 m2 of buildings. A cluster of three buildings is located at the head of the shaft and will house the helium refrigerator cold-box (SD building), the water-cooling and ventilation units (SU building) as well as the main electrical distribution for high and low voltage (SE building). Two stand-alone buildings complete the inventory and will house the primary-water cooling towers (SF building) and the warm compressor station of the helium refrigerator (SHM building). Buildings housing noisy equipment (SU, SF, SHM) are built with noise-insulated concrete walls and roofs. 

Figure 2. Point 1 ground-level civil engineering work (Image credit: CERN)

Figure 3. Point 5 ground-level civil engineering work (Image credit: CERN)

At underground level, the civil engineering work consist of a shaft, a service cavern, galleries, and vertical cores (See Figure 4). The total volume to be excavated corresponds to about 40’000 m3 per Point. The PM shaft (9.7-m diameter, 80-m height) will house a secured access lift and staircase as well as the services required at underground level. The service cavern (US/UW, 16-m diameter, 45-m long) will house cooling and ventilation units, a cryogenic box, an electrical safe room and electrical transformers. The UR gallery (5.8-m diameter, 300-m long) will house the power converters and electrical feed boxes for the superconducting magnets as well as cryogenic and service distribution. Two transversal UA galleries (6.2-m diameter, 50-m long) will house the RF equipment for the powering and controls of the superconducting crab-cavities. At the end of the UA galleries, evacuation galleries (UPR) are required for personnel emergency exits. Two transversal UL galleries (3-m diameter, 40-m long) will house the superconducting links powering the magnets and cryogenic distribution. Finally, the connection of the HL-LHC underground galleries to the LHC tunnel is made via 16 vertical cores (1-m diameter, 7-m long).

Figure 4. Underground civil-engineering work (Courtesy of LAP consortium)

The definition of the civil engineering for the HL-LHC has started in 2015. First integration studies have been performed in collaboration with the CERN SMB (Site Management and Buildings) Department, the equipment groups and the HL-LHC Project Office. In 2016, the completion of a preliminary study has allowed to issue a call for tender for two civil-engineering consultant contracts, which have been adjudicated in June 2016. These consultants are in charge of the preliminary, tender and construction design of the civil engineering works, as well as of the management of the construction including the defect liability. At Point 1 on the Swiss side, the consultant contract was adjudicated to a consortium, called ORIGIN, constituted of 3 companies: SETEC (FR) the consortium leader, CDS Engineers (CH) and Rocksoil (IT). At Point 5 on the French side, the consultant contract was adjudicated to a consortium, called LAP, constituted of 3 companies: Lombardi (CH) the consortium leader, Artelia (FR) and Pini Swiss (CH). In November 2016, the two consultants have completed the preliminary design phase including cost and construction-schedule estimates for the civil engineering work execution.

In parallel with the preliminary design, CERN, with the help of architects, has prepared the building permit applications which have been submitted to the Swiss and French Authorities in October and November 2016. Between 6 to 9 months will be required to get the building permit authorization, that is compatible with the start of the construction works scheduled by mid-2018. CERN has also performed geotechnical investigation in order to better identify the soil constituents. CERN has placed a contract with an independent engineer (Joint venture of ARUP (UK) and Geoconsult (AT)). This independent engineer will perform peer reviews of the consultant designs and will confirm that these designs have been performed with the appropriate skill, care and diligence in accordance with applicable standards. In addition, an adjudicator panel is constituted with lawyers, architects and civil engineers to resolve disputes in-between all parties.

The next important milestone will be the adjudication in March 2018 of the two contracts (one per Point) for the civil-engineering construction works. The tendering process has started with the issue of a market survey in December 2016 including relevant selection criteria requirements. It will be followed by calls for tenders, which will be sent to the qualified companies by June 2017. The main excavation works, producing harmful vibrations for the LHC accelerator performance, must be performed during the second long-shutdown of the LHC accelerator scheduled in 2019-2020. The completion of the civil-engineering with the hand-over of the last building is scheduled by end-2022. The vertical cores connecting the HL-LHC galleries to the LHC tunnel will be burrowed during the first semester of the third long-shutdown of the LHC accelerator, which is expected to start beginning of 2024.

New collaboration board for HL-LHC
by Isabel Bejar Alonso (CERN) and Panos Charitos (CERN)

The HL-LHC Collaboration Board [HLCB] is the official forum for information exchange and dialogue between the HL-LHC collaborators, HL-LHC project management and CERN management (Image: CERN)

The first session of the new HL-LHC Collaboration Board took place in Paris on 14th November 2016. The HL-LHC project moves from its initial conceptual design phase into the constructive design phase, which marks the beginning of construction for some HL-LHC components.

Moving into the new phase is reflected not only by the change of the composition of the Collaboration Board, but also in the relations with the institutions working for the HL-LHC.

Lucio Rossi, HL-LHC Project Leader points to the increasing number of Member States that contribute through their universities and research centres. Finland, Poland and Sweden have joined, in addition to a strengthened relationship with the States which were already part of the design study, such as France, Italy, Spain and the United Kingdom. 

“We are particularly proud of the UK contribution, where not only the number of universities is increasing, but also the domains of competence,” notes Lucio Rossi, HL-LHC Project Leader.

Contributions to HL-LHC are also not limited to Europe. Canada is represented by Triumf laboratory, which is also a new member of the Collaboration Board. The SLAC National Accelerator Laboratory, located in California, joins BNL, LBNL, Fermilab and Old Dominion University (Virginia) in the effort of US contribution. In addition, Asia is represented in the collaboration by Japan, while China may soon join the collaboration. 

A new general framework contract based on a multi-party memorandum of understanding (MoU) that allows a more flexible exchange of personnel between partners has been agreed upon. Additional contributions can be added by the laboratories via simple addenda.

Collaboration partners include laboratories and institutes who have either signed directly a HL-LHC collaboration agreement or are part of an overreaching general collaboration agreement, will provide either significant in-kind contributions or studies and personnel for the HL-LHC project.

Institutes that have signed the MoU but do not provide an explicit in-kind contribution to HL-LHC are to be considered observers.

The present HL-LHC Collaboration Board has 21 members and 10 observers and is chaired for the next two years by Robert Appleby from the University of Manchester in the UK.

A year of successes for HL-LHC
by Isabel Bejar Alonso (CERN)


180 HL-LHC project members participated to the 6th Annual Meeting in Paris from the 14th-16th November, co-organized with CEA at the “Espace Saint Martin” premises (Image: CERN, HL-LHC collaboration)

2016 has been a very busy year for the Hi-Luminosity team. In March, the HL-LHC was declared as an ESFRI landmark, and in June the project received the formal approval of the CERN Council. Finally, after an international review, the 2nd HL-LHC Cost and Schedule review took place in October; where a group of international experts scrutinized the status of the project. The reviewers gave very positive feedback and pointed out risks of which the management team were already aware.

On the technical side, it is difficult to determine the three main technical milestones of the project due to the sheer volume of achievements.

For Lucio Rossi, HL-LHC Project Leader, 2016 was the year of the consolidation of the civil engineering and technical infrastructure design. In particular, in 2016 we optimized the so called “double decker” solution, which was selected in summer 2015 as best solution for hosting the technical services that will feed the new insertion regions in a gallery a few meters above the main LHC tunnel.

This definitive design pushed to a re-baselining exercise which aimed to resolve the excess in civil engineering costs that emerged in spring 2016. The results of this exercise were presented and validated at the Cost and Schedule review after being approved by the CERN Executive Committee in August. The results have also been integrated into the latest version of the Technical Design Report (TDR), that will be published soon.

In addition, Lucio Rossi pointed that the HL-LHC technical teams have achieved many things, such as the production of full cross-section models of the HL-LHC’s future quadrupole magnets. The first short model (MQXFS1) of the future quadrupoles was produced by a collaboration between US-LARP and CERN. The magnets (MQXFS1) achieved the ultimate gradient and retained it after a thermal cycle showing an optimal memory. Had the magnet been a prototype rather than a model, would have been qualified for installation.

Furthermore, the SPS testing of the crab cavity cryo-assemblies has also made significant progress. After being considered in a critical state and with challenging planning just one year ago, the schedule is back on track and has been adhered to, without any further delay.

Beyond SPS testing, the industrialization plan for the HL-LHC production was validated by the external reviewers and considered extremely solid.

HL-LHC has had a successful year both in terms of project management and technical milestones. Project members gathered for the 6th HL-LHC Collaboration meeting held in Paris in November to discuss the past year and look forward to the next steps. 

 

 

A novel optics scheme to meet the HL-LHC targeted performance
by Stephane Fartoukh and Panos Charitos

Beam sizes [mm] along a quarter of the LHC ring. The interaction point (IP) of the ATLAS experiment is exactly in the centre. In the end of the Tele-squeeze the spot size is no larger than 5 microns at the IP, thanks to the ATS scheme transforming 7km of machine into a giant final focus system with natural embedded chromatic correction sections in Arc81 and Arc12. (Image: Stephane Fartoukh)

Upgrading the luminosity of a circular collider means increasing the number of interactions between the two counter-rotating particle beams. The goal is to maximise the potential of observation (or discovery) of rare (or unexpected) events, and to improve the measurement accuracy of already known phenomena.

An important ingredient to reach this goal is to increase the density of particles colliding at the interaction point (IP), in particular decrease the transverse beam spot size at the IP, that is quantified in accelerator science with the so-called beta* parameter. Like light rays, however, a strong focusing assumes a certain lever arm, that is also and mainly a certain distance, between the incoming beam and its focus point. When this distance is fixed, increasing the number of lenses (the role of the quadrupole magnets in particle accelerators) and increasing their strength is the only way forward. But this presents obvious limits in terms of integration and maximum possible field for new magnets.

This is the case for the long straight sections of the LHC, which host the machine experiments. Here the geometry is fixed by the existing LEP tunnel, and no modifications are expected for the LHC luminosity upgrade project (HL-LHC) or its possible energy upgrade (HE-LHC), one of the scenarios explored under the FCC design study.

In order to address this challenge, Stephane Fartoukh, who works in CERN’s beam department came up with the idea of a novel optics scheme inspired by the classical principle of a telescope in light optical systems. The LHC is a ring with a 27 km long circumference where 8 long-straight sections, or “insertions”, are evenly distributed along the ring. Half of the insertions are for special services (such as collimation) and the others host high-energy physics experiments, in particular the two high luminosity insertions ATLAS and CMS.

In the new scheme designed by Stéphane, the beam focusing is achieved in two stages. The first stage, called “Pre-squeeze”, is confined to the high luminosity insertions proper (see Fig. 1 left) until limits of strength are reached in the matching quadrupoles. This is the common approach followed by modern colliders. In order to gain an additional beta* squeezing factor, which is of vital importance for the HL-LHC program, the second stage, called “tele-squeeze”, involves quadrupole magnets which are located in the two insertions 3.5 km downstream and upstream on either side (see Fig.1 right). The two stages are part of a scheme called ATS, short for “Achromatic Telescopic Squeeze”.

The term “Achromatic” reflects the second novelty of the technique. As is the case in light optics, particles with slightly different energies are focused differently, inducing chromatic aberrations, in particular chromatic distortions of the beam spot at the IP, which increase violently for very strong focusing, rendering it inefficient after a certain point. Special magnets called sextupoles are located in the LHC arcs between two consecutive insertions in order to compensate for this effect. Stephane explains: “Contrary to conventional optics squeezing techniques, these magnets are run at constant strength and therefore cannot exceed any field limits in the second, telescopic, part of the ATS, because they are made more efficient when the beam becomes bigger in the arcs.” He adds, “Otherwise it would have been quite a copious effort to build and replace more than 500 lattice sextupoles for the HL-LHC”.

This concept required fully new optics deployed in all parts of the LHC ring, from injection to collision energy. A detailed study program has been setup with dedicated beam time allocated in the LHC schedule. The first stage of the ATS has been successfully tested, and first collisions with nominal specific luminosity have been established.

“The heart of the HL-LHC is already beating in the LHC. We are now ready to enter in what one could call the telescopic era of the LHC, and prove with beam the reliability of the overall scheme, with the High Luminosity LHC and High Energy LHC as medium and long term objectives for the ATS scheme,” Stephane concluded.

For more information on the ATS scheme, please see a detailed report from June 2016 (submitted to the ICFA Beam Dynamics Newsletter of Vol. 70).

 

by Marta Bajko (CERN) and Panos Charitos (CERN)  
 


Overview of CERN's Large Magnet Hall (Image Credit@CERN)

 
Within the HL-LHC project about 100 superconducting magnets will be produced in the next years up to 2024. Magnets of different type and size will use NbTi or Nb3Sn technology. The design of the superconducting magnets is well advanced at CERN, and other collaborating institutes and for most of them plan to build a model and a prototype magnet during the period 2016-2018. From13-14th of June CERN hosted the 1st International Workshop on Superconducting Magnet Test Facilities where the status of different facilities was presented along with the needed steps to meet the needs and the rising interest on superconductors.

For the design of new high-field magnets for HL-LHC and for the scenarios explored under the FCC study, there are several institutes together with CERN. working on the testing of the models, prototypes and even the series magnets. Some institutes are equipped with adequate infrastructure while others are completing their installations during the next coming years.

The test of a superconducting magnets is part of the QA process to assess the soundness of the construction and the suitability for machine operation. In addition, tests are important also during construction/building of magnets, as an integral part of the construction chain. One has to get timely feedback  to take corrective actions during the construction process. In that sense test is a key milestone for triggering acceptance and passage of responsibility between firms and institutes (in case of industrial orders) or among institutes (in the case of in-kind contribution). For the readiness of the test stands and for achieving a a good coherence between the facilities and planned test, it is essential to coordinate the activities with the magnet production managed by WP3, WP6 and WP11 of HL LHC (MSC group at CERN and collaborating Institutes).

The SMTS Workshop contributes to the above mentioned coordination and assessment of the needs and readiness. The main goal of the SMTS workshop is to verify that all test stand will be ready in time and able to perform measurements that will meet the HL-LHC standards. Through regular meetings we to create an active network between the test stands allowing them to exchange methods, techniques, and experience on equipment, data and finally expertise when and where needed. Presently test stations for the HL-LHC magnets exist in the following laboratories: FNAL (USA), BNL (USA), CERN (Switzerland), KEK (Japan), KEK (Japan), CEA Saclary (France), Freia (Sweded), INFN (Milano), LBNL (USA) and Nafassy (Italy) 

Most of the test stands for superconducting magnets are in collaboration with CERN for the HL-LHC project but we would like to welcome those working in this area as well independently if they will or not test magnets for HL-LHC or future collider projects. Therefore the above mentioned list has been extended and now also includes GSI (Germany), PSI (Switzerland), JINR (Russia). 

This workshop provided a forum for potential users of the Trans National Accesses (TNA) supported by FP7 Eucard2 project and Aries (presently under evaluation). More than 60 participants were registered for the one and a half days of presentations followed by an interactive visit to  CERN test facility in SM18. OnAt that occasion 20 experts, mainly from CERN, were sharing their knowledge with the workshop participants during 2 h.

 

Further events will cover subjects linked to specified areas of measurements as magnetic or mechanical measurements while protection and detection systems will also be treated in detail. The connection to the final IT STRING test will be covered, too. The safety and the interlock systems used by the different test stands will be presented with the goal to optimise the stands and improve where possible the safety of the personnel and the equipment.
 

 

By T. Nakamoto (KEK) & E. Todesco (CERN) 
 

Figure 1: Collaring test of the final cross-section of the D1 in KEK. Yoking press is in the background 

A key element of the High Luminosity LHC interaction region is the superconducting separation dipole D1, which will replace the resistive dipole presently installed in the LHC. Thanks to the superconductive technology of Nb-Ti, this magnet will increase the field from the present 1.28 T to 5.6 T, thus reducing the overall length from 25 m to 7 m (A first layout for the High Luminosity Upgrade) and providing at the same time a 30% stronger kick. This large saving in terms of space along the beam line is crucial to allow the installation of longer triplet and of the crab cavities, which are among the main pillars of the luminosity upgrade.

The D1 upgrade was one of the first HL LHC magnets to be studied in collaboration with KEK. Activities started in 2011, led by T. Nakamoto, with the contribution of a young scientist (Q. Xu, now in IHEP, Bejing) focusing on the conceptual design. After the selection of the triplet aperture of 150 mm, KEK has developed an engineering design in 2013-2014: the construction of a short model has been carried out during 2015. The magnet is not trivial due to its large aperture, the relatively high field for Nb-Ti technology, the tight requirements on field quality, and the constraints imposed on the mechanical structure. Since one needs to maximize the iron dimension to limit saturation and fringe fields outside the magnet, the KEK team opted for a concept where the collars are thin spacers and forces are kept by the massive iron: this is the same mechanical structure that was used for the triplet magnet MQXA presently installed in the LHC. Since forces are kept by the iron, the assembly of laminations around the coils and the control of the compression given under the press (see Fig. 1) is critical and entailed in the iteration in the design.

The first model has been completed in March 2016, and test has been done in April and June. The test started at 4.5 K (see Fig. 2) since the test station underwent an upgrade and some additional cross-checks were required.  At 1.9 K, the magnet had a first quench at 9.5 kA, i.e. ~60% of the short sample, and approached the 12 kA nominal current (75% of short sample) after ~10 quenches. After a thermal cycle, the magnet showed good memory (i.e. it started from the same values reached before the warm-up).


Figure 2: Training of the first D1 short model in KEK (Image credit: M. Sugano)

The magnet eventually reached 500 A more than nominal current in the second run, but exhibited an erratic behaviour around these current values. At the same time, the strain gauges gave a clear indication of a lack of support in the coil already at around 8 kA (see Fig. 3). This suggested not to push the training towards the goal of ultimate current, take all the information about field quality and quench protection, and possibly have a second assembly with an increase of prestress (a similar step is being done for the first short model triplet). “We will soon have a review in KEK to present the test result and define the next steps”, says A. Musso, who follows the collaboration from the CERN side. “A second model is foreseen in the HL LHC planning for the year 2016-2017, and the construction of the long prototype will take place from 2018.”


Figure 3. Stress variation during the energization of the magnet, with flattening at 8 kA indicating coil unloading (Image credit: M. Sugano)

 

 

 

 

by Rama Calaga and Ofelia Capatina (CERN)
 

The crab cavities for HL-LHC and their validation tests in the SPS are well into the fabrication phase. The compact superconducting cavities are made from 4mm high purity (RRR) Niobium sheets which are operated to at 2 K to sustain the very high surface electromagnetic fields at the required performance. The cavities are immersed in bath of super-fluid Helium contained in specially designed Titanium tank. This vessel also provides necessary stiffness to the cavity body to withstand the strong external forces applied to the cavity in its lifetime.

To minimize undesired stresses during assembly to the RF cavity, a novel concept of a bolted helium vessel is used. Superficial welds are applied to ensure vacuum integrity and minimize the stress induced during welding. To validate this approach, a full scale prototype was built and thoroughly tested. The prototype was machined from six thick Titanium plates, mounted and welded using the same procedure envisioned for the real system. The dimensions of the tank were monitored using 12 different position sensors between the different steps of assembly and welding which showed only minor deformation after the full assembly. A pressure test of up to 2.6 bar and a several cool-down tests performed by immersing the tank in a bath of liquid nitrogen.  Leak tightness was verified during these tests. The prototype showed no degradation, plasticity, leaks or unloading of the screws during the assembly and testing steps.

The minor deformations during the welding steps are being analyzed to further compensate them for the real system. Extensive simulations of the steps showed close agreement with the measurements which was also an important validation step.

An internal magnetic shield was proposed as a solution to suppress the residual trapped flux during cool-down. This became essential due to the numerous interfaces of the cavity which cannot be easily shielded by only an internal shield and therefore compromise the cavity performance. The shields are made of Cryophy which after special thermal cycle, exhibits excellent shielding properties. However, their properties can easily be compromised due to shock impact and handling. These shields were developed in collaboration with UK-STFC and UK-industry which were successfully fabricated and assembled. Final shielding measurements show perfect agreement with the expected shielding from simulations and therefore qualified for their assembly into the Helium vessel.

by Panos Charitos (CERN)
 

A new furnace for the heat-treatment of superconducting coils is currently commissioned at CERN’s Large Magnet Facility. It is the last large equipment that will allow the production of superconducting coils needed for the HL-LHC upgrade and future colliders explored under the FCC study. 

HL-LHC targets to increase the integrated luminosity by a factor of 10, resulting in an integrated luminosity of 3000 fb−1. The higher luminosities will allow higher-precision measurements and enable scientists to collect data at a much faster rate. HL-LHC calls for the development of new 11 Tesla dipole magnets with a total length of 11 meters to replace some of the existing 8 Tesla LHC dipole magnets. The development of these magnets was launched at the end of 2010 in the framework of a collaboration between the magnet groups of CERN and the US laboratory FNAL. Another major improvement for HL-LHC is the reduction of the beam size near the collision points. This requires the development of 150 mm single aperture quadrupoles to be installed in the interaction regions around the ATLAS and CMS experiments. The new quadrupoles, to be operated at a peak field of nearly 12 Tesla, are developed in a joint collaboration between CERN and the US-LHC Accelerator Research Program (LARP).

The FCC study has launched an R&D programme for the development of 16 Tesla magnets. In order to keep the protons on a circular track at the record-breaking energies of 100 TeV foreseen for a future hadron circular collider, scientists have to design and demonstrate very powerful magnets accelerator. The FCC relies on the use of magnets with a nominal field of 16 Tesla, almost double compared to the nominal 8.3 Tesla of the superconducting magnets used in the LHC.

To attain the goals posed by the HL-LHC and the FCC study, the use of new superconducting materials is needed. The niobium-titanium alloy that is currently used for the LHC superconducting magnets doesn’t allow to go higher than 9 T at 1.9 Kelvin in accelerator magnets. A new superconductor, based on the metallic compound niobium-three-tin (Nb3Sn) is at present the only practical option to reach such a high magnetic fields. Nb3Sn coils can sustain the required current densities to create magnetic fields of up to 16 T. Therefore it could fulfil the requirements of the HL-LHC upgrade as well as allow to realize a future circular hadron collider like the one explored by the FCC study.

In order to use Nb3Sn for new magnets one should understand in depth its properties and have the manufacturing processes at hand at a reasonable cost. Nb3Sn is much more complicated to work with compared to niobium titanium. A high temperature heat treatment (HTHT) is needed to form Nb3Sn superconductor via a solid state reaction of the primary components. This is a long process that lasts about two weeks. During the HTHT, the coils reach different temperature plateaus up to 665 oC.

After this process the material becomes brittle as ceramic. This poses a challenge for the manufacturing processes of the Nb3Sn superconducting coils. “Nb3Sn has been chosen for the next generation of superconducting magnets. The field achieved with this material can reach up to 16T. The production of such coils is complex as we must first wind the coils and then perform the heat treatment - a technique called "wind and react" - that will allow the formation of Nb3Sn” explains Friedrich Lackner, the project engineer who supervises the long quadropule coil production for HL-LHC. He continues: "Submitting an entire coil of many meters length, with its content of insulators and residuals of organics, to a high temperature treatment is a complicated process. Therefore, the oven to accommodate the heat treatment of Nb3Sn coils requires latest technology to achieve temperature uniformity and process stability."

Friedrich Lackner, project engineer who supervises the long quadropule coil production for HL-LHC, explains the special features of the new furnace.

In 2010 CERN launched the procurement of industrial furnaces to perform the in-house thermal treatment of the new coils. Before the construction of the long coils for future high-field dipole and quadrupole magnets, experts study and develop the process based on a short-model programme based on 2.5 meter magnets. This is a necessary step before starting the construction of the 5.5 m coils for the dipole magnets (that will be installed in the LHC tunnel during the Long Shutdown 2 in 2018) and the 8 meter long coils needed for new quadrupoles.

CERN’s Large Magnet Test facility has been equipped with different furnaces for treating the new superconducting materials forming a full production chain for the new coils. The first piece of this chain is the so-called HB160 high-temperature furnace arrived at CERN in 2011 soon followed by the installation of GLO750.

In 2014, the installation of GLO2000 in the Large Magnet Test hall allowed the treatment of 6.5 m long coils. The first 5.5 m long coils were wound in the CERN Large Magnet to build the HL-LHC 11 Tesla dipole magnets successfully tested this year. The results fulfilled the requirements that were specified for the furnace proving an excellent collaboration between CERN, the US laboratories in developing Nb3Sn coils that can meet the requirements of the HL-LHC project.

On May, the last missing piece of this chain arrived at CERN. The new furnace, called GL010000, will allow the heat treatment of coils with length of up to 11 meters while it can reach temperatures up to 900 oC providing a sufficient margin for future challenges. The new furnace will allow CERN to lead a rich R&D effort in the production of superconducting coils and the development of high-field magnets. Moreover, what makes the new oven unique is the high temperature homogeneity that can be achieved with a tolerance of +/- 3 oC for up to two weeks in an Argon atmosphere. The whole gas guiding system is optimized to achieve the nominal temperature as fast as possible and to achieve the best possible uniformity during the heat treatment.

GL010000 will be mainly used for treating Nb3Sn quadrupole coils magnets for the HL-LHC while it can also be used for prototyping the 16 Tesla magnets for FCC. The first copper practice coil for the quadrupole magnets (MQXF) will be wounded in early 2016 and a full RHT based on this coil will be performed in summer 2016.

“The completion of this project that started as an initiative for HL-LHC but will be also useful for the new magnets needed for FCC, puts CERN and the global HEP community in a unique position in the development of new powerful accelerator magnets” says Lucio Rossi, leader of the HL-LHC project.

The installation of the new furnace at CERN’s Large Magnet Facility (LMF) will help scientists researching and developing the new materials needed for future colliders to understand the superconductor development based on this Nb3Sn alloy, and allow CERN to lead the production of superconducting coils and the development of high-field magnets.

The successful test of 11 Tesla magnets for the HL-LHC upgrade and the developments for the 16 Tesla magnets programme of the FCC study proves that the future is bright for developing high-field accelerator magnets. The realization of more powerful magnets will allow extending the luminosity and energy frontiers and pave the way to answer some of the fundamental questions that lie open after the Higgs discovery.

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