The ATLAS superconducting magnet system for the ATLAS detector in the LHC at CERN comprises a Barrel Toroid, two End Cap Toroids and a Central Solenoid, with overall dimensions of 20 m diameter by 26 m length. Though this magnet system is used for a particle detector, its size, geometrical arrangement and coil technology is comparable to what is usual for fusion machines. The manufacturing has started in the industry and integration of the magnets will start in the year 2001. Assembly in the cavern is planned in the year 2003, starting with BT installation. In this paper the Magnet System is presented as well as the common infrastructure and services

Miele, P

ten Kate, H H J

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Paola Miele, CH-1211 Geneva 23, Paola.Miele@cern.ch, Tel. +41.22.7678432, Fax +41.22.7678350THE SUPERCONDUCTINGMAGNET SYSTEM FOR THE ATLASDETECTOR AT CERNPaola Miele and Herman H J ten Kate, on behalf of the ATLAS CollaborationCERN, CH-1211 Geneva 23, SwitzerlandAbstractThe ATLAS superconducting magnet system for the ATLAS detector in the LHC at CERN comprises a BarrelToroid, two End Cap Toroids and a Central Solenoid, with overall dimensions of 20 m diameter by 26 mlength. Though this magnet system is used for a particle detector, its size, geometrical arrangement and coiltechnology is comparable to what is usual for fusion machines. The manufacturing has started in the industryand integration of the magnets will start in the year 2001. Assembly in the cavern is planned in the year 2003,starting with BT installation. In this paper the Magnet System is presented as well as the commoninfrastructure and services.1 INTRODUCTIONThe superconducting Magnet System for the ATLAS Detector for the Large Hadron Collider atCERN to analyze proton-proton collisions at 14 TeV level has already passed the design phaseand is now under construction in industry. The realization of the detector and correspondinginfrastructure is organized by the ATLAS Collaboration, a joint enterprise of about 145institutes in 34 countries world-wide including CERN. The challenge of ATLAS to reachoptimum physics performance requires the particular magnetic field configuration made up bythe axial field created by the Central Solenoid and the tangential field created by the ToroidSystem.The ATLAS Magnet System [1,2] comprises the 4 superconducting magnets, the power supply,the cryogenics, the vacuum and the control and safety systems. The magnet system has overalldimensions of 26 meters in length and 20 meters in diameter, Fig. 1. The superconductingmagnets comprise the Toroidal Field (TF) magnet system with the Barrel Toroid (BT) and the 2End Cap Toroids (ECT) and the Central Solenoid (CS). Each of the 3 toroids consists of 8 coilsconnected in series, with flat pancake type of windings, assembled around the beam axis withan offset angle of 22.50 between the BT and ECT systems, in order to optimize the bendingCERN-OPEN-2001-06414/06/200121ST SOFT – Madrid 11-15 September 20002power in the available space. A model coil of reduced length of BT, the B0 model coil, is underconstruction to qualify the manufacturing cycle and related techniques. The Central Solenoid isa single layer coil wound internally in a supporting cylinder and enclosed by the commoncryostat of the Liquid Argon Calorimeter. Aluminum stabilized NbTi/Cu superconductor cooledat 4.5 K by liquid Helium forced flow and glass fibers-epoxy resin insulation are the maincomponents of the coils. A 21 kA common TF power system and an 8 kA CS power systemwith correspondent quench protection systems are implemented.The 4 main phases for the realization of the Magnet System comprise design, construction, testand installation. A co-operation between CERN/ATLAS and 5 magnet laboratories is the basisfor the design and construction of the ATLAS Magnet System. CEA-Saclay (F) and INFN-LASA (I) are responsible for the BT, RAL (UK) and NIKHEF (Nl) for the ECT and KEK (J)for the CS. CERN is mainly responsible for the overall project management and co-ordination,as well as the infrastructure for assembly and testing, the installation underground and thecommon cryogenics, power, vacuum, control and safety systems.The main parameters of the magnets are listed in Table 1.2 CONDUCTOR ANDWINDINGThe superconductor used is a composite consisting of a flat NbTi/Cu Rutherford cable co-extruded with aluminium stabiliser with rectangular cross section. For the BT and ECT thestabiliser is high purity aluminium while in the case of the CS high-strength aluminium wasdeveloped by KEK. The production of the conductor for the CS is completed by Furukawa(6 km) and Hitachi (6 km), while it is planned to be completed by the end of the year 2000 forthe BT and ECT by Vacuumschmelze (D) and LMI (I)-ACS/ETH (CH).Table 1 shows the main characteristics of the conductors.The qualification of the conductor is fully established, as well as reproducibility andreliability in long unit lengths. Crucial for acceptance of produced conductor units is the criticalcurrent, the aluminium RRR and the bond quality between aluminium and copper. This to fulfil21ST SOFT – Madrid 11-15 September 20003the requirements of high mechanical shear strength and low electrical and heat transferresistance, which guarantee proper cooling, stability and protection against conductor burn-out.The coils are vacuum impregnated with glass fibers-epoxy resin insulation. The indirect coolingof the coils through pipes glued to the Al coil casing implies that good thermal contact and lowstress interface is achieved between the different interface materials. This requires a huge effortin terms of design and technological development of the techniques for glass-resin impregnationand pre-stressing of such coils.3 BARREL TOROIDThe Barrel Toroid [3], Fig. 1, is composed of 8 flat racetrack coils each consisting of twodouble pancake windings housed in a common aluminium alloy casing that guarantees cooling,rigidity and transfer of the magnetic forces. Every coil has its own vacuum vessel which isconnected one to the other through the cryogenic ring for electrical and cryogenicinterconnection. 2 X 8 supporting rings complete the warm structure which must guarantee bothmechanical stability under static and dynamic loads and a light and open structure. The wholeassembly is supported on the feet. Full redundancy is implemented by two cooling circuits at4.5 K liquid He and 60 K high-pressure He gas for the cold masses and thermal shieldsrespectively.Manufacture of the double pancakes consist of winding the conductor with dry glass tapewrapping on a specific mandrel and impregnation in an Al mould at 2.5 MPa pre-compression.Manufacture of the Al coil casing is based on connecting long bars, so as to minimize thenumber of welded connections and to decrease the thickness to be welded. The crucialrequirements to be met are the deformation level of such long bars, and the welding quality ofsuch thickness of material. Extensive development and testing of the welding procedure wasmade by industry on electron beam and TIG/MIG welding. Integration of the impregnateddouble pancakes into the coil casing is made by gluing with pre-preg layers using the bladdertechnique to pre-compress the coil. Long Al tubes are inserted between the double pancakes and21ST SOFT – Madrid 11-15 September 20004the coil casing filled with glass microballs and impregnated with epoxy resin, then pressurizedand cured under a pressure p=120 bar. This gives a safety margin to the shear stress at theinterface between the double pancake and the coil casing. Integration of the cold mass andrelated components into the vacuum vessel requires a large effort in terms of assemblyrequirements and coordination/responsibility sharing. This operation includes gluing of thecooling pipes onto the cold mass, assembly of the cold mass supports, i.e. the tie rods and thecryogenic stops, assembly of the Al thermal shields and welding of the cooling pipes to it,assembly of the superinsulation and closing of the vacuum vessel with 160 m longitudinalwelding, Fig. 2.The model coil B0, with a reduced length of 9 m and the same cross section, aims to validatethe technologies and manufacturing processes of the BT. The fully integrated coil will bedelivered to CERN by October 2000 ready for testing. B0 was proven to be fundamental tooptimize the production cycle of BT. Following B0 experience many improvements havealready been implemented in the winding and impregnation procedures, in the coil casingtechnology, in the pre-compression technique, in the cold mass support sliding system as well asin the integration scenario and procedure. The production of the coils has started in industry: thefirst double pancake was successfully impregnated at ANSALDO, I and the vacuum vesselproduction has started at Felguera CM, E.4 END CAP TOROIDSThe two End Cap Toroids [4], Fig. 3, are inserted in the Barrel Toroid at each end of the CentralSolenoid.Each ECT contains 8 racetrack coils mounted as a single cold mass in a vacuum vessel. Eachcoil module consists of 2 double pancake windings mounted in an Al coil casing. Impregnationof each coil is done in autoclave with a resin system developed by RAL with high toughness at4.5 K. An Al castellated thermal shield with its 60 K He gas cooling circuit and thesuperinsulation layers surround the cold mass. All services are connected through a single21ST SOFT – Madrid 11-15 September 20005service turret mounted on top of the vacuum vessel. Each ECT is mounted on the main railstructure to allow movement for access to the inner part of the detector. The electromagneticaxial force of about 240 tons pulling each ECT towards the center is reacted at 8 force transfersupports against the 8 BT vacuum vessels.Production of the cold masses and vacuum vessels is in good progress at HMA Power Systems(Nl) and Royal Schelde Exotech (Nl) and first delivery to CERN is planned in the year 2001ready for integration and testing.5 CENTRAL SOLENOIDThe Central Solenoid [5] with a length of 5.3 m and a bore of 2.5 m, Fig. 4, provides 2 T axialmagnetic field in the central tracking volume of the ATLAS detector. The CS is installed in acommon cryostat with the Liquid Argon Calorimeter in order to minimize the amount ofmaterial. For further improvement high-strength aluminium stabilized superconductor wasdeveloped by Furukawa and Hitachi in collaboration with KEK. A yield strength σy = 110 MPaat 4.2 K and RRR = 592 were achieved by using an additive of Ni (1000-ppm) in combinationwith a cold work of 21%. The support cylinder made of Al alloy 12 mm thick supports thesingle layer coil winding inside and is connected to the He cooling circuit outside. Thesuperconducting bus bars and cryo-lines connect the coil through a 10-m long chimney to theproximity cryogenics positioned on top of the detector.The CS is completed and will be tested at Toshiba at the end of the year 2000. The proximitycryogenics is also completed and has been successfully tested at Toshiba. Thereafter the coilwill be delivered to CERN for integration with the Liquid Argon cryostat and final testingbefore installation.6 THE SERVICESThe cryogenic system consists of:21ST SOFT – Madrid 11-15 September 200061. Internal cryogenics: the cooling circuit of the cold mass and thermal shield, with fullredundancy;2. Proximity cryogenics: the auxiliary cryogenic equipment necessary for the operation of eachmagnet, i.e. the toroids which are cooled as a single cold mass (600 tons) and the CS (6tons). The toroid system requires 1200 g/s of two-phase He at 4.5 K, which is circulated bymeans of 2 centrifugal pumps and related distribution unit, while the CS requires only 7 g/sof 4.5 K He taken directly from the refrigerator.3. External cryogenics: the He refrigerator plants and related distribution system, which arecommon to all magnets.The refrigeration system comprises two cryoplants: the shield refrigerator and the mainrefrigerator. The first one is used on the one hand for the cool-down phase of magnets between300 K and 100 K with a cooling capacity of 60 kW provided by a liquid nitrogen/helium heatexchanger and on the other hand, to cope in steady-state with the shield cooling which requiresa non-isothermal capacity of 20 kW between 40 K and 80 K. The main refrigerator is used forthe final cool-down phase from 100 K to 4.5 K and the steady-state operation of the magnetcold masses at 4.5 K. This main refrigerator has an equivalent cooling power of 6 kW at 4.5 K.The design of the cryogenic system is completed and contracts should be placed by the end ofthis year.To decrease the complexity of the distribution lines and increase the overall reliability of thesystem a common electrical and proximity cryogenic system for the toroids is implemented,Fig. 5,6. The toroids circuit comprises the 21000 A – 16 V power supply, two circuit breakers,some 300 m bus bars, He cooled current leads and the diode-resistor dump unit connectedacross the supply/breaker and the coils. The ramp-up time of the Toroids is 2-3 hours. The CShas a separate 8 kA power supply due to its different current rating.The Magnet Safety System (MSS) insures the magnet protection in stand alone mode withfast and slow discharge request and the personnel safety. The protection of the coils is based onthe internal dump of the stored energy through the use of a dump resistor of low value and the21ST SOFT – Madrid 11-15 September 20007initiation of quenches in all coils by means of quench heaters. In the slow and fast dischargemodes the stored energy is dissipated respectively in the dump circuit and in the coil itselfthrough the quench heaters. For safety reasons 48 VDC UPS (Uninterrupted Power Supply) isused to guarantee the functionality of the system in any faulty condition. Redundancy isforeseen for safety sensors and channels.The Magnet Control System (MCS) provides the process control for ensuring in fullautomatic mode the operating sequences of the magnets and the control, regulation andmonitoring of all the functional parameters related to the magnet and to the cryogenic, electricaland vacuum systems. Harmonization and standardization requirements are followed foroperational and maintenance easiness. It consists of sensors and actuators, programmable logiccontrollers and the magnet supervisor. High level communication is through a field-bus whileall sensor and actuator connections and safety system are hard-wired.7 TESTING AND INSTALLATIONPerformance and acceptance test on surface is performed at CERN before undergroundinstallation. For size reasons each BT coil will be tested with a magnetic mirror to simulate theelectromagnetic load induced by the entire system. Instead each of the 2 ECT magnets and theCS magnet will be fully assembled and tested on surface and then installed as complete systemsin the cavern. The test facility [6] is ready at CERN to receive the first coils. After installation inthe cavern, Fig. 7, the complete BT will be tested and then integrated with both the ECTs andCS ready for testing of the whole ATLAS Magnet System.8 CONCLUSIONSThe ATLAS Magnet System comprising a solenoid and three toroids is the largest integratedsuperconducting magnet ever built. It is under the constructing phase and all main contractshave been signed with industry. Production of the cold masses and vacuum vessels for thetoroids are in good progress and the first delivery is planned in the year 2001. The CS is nearly21ST SOFT – Madrid 11-15 September 20008completed and will be tested at Toshiba at the end of this year. Integration of the toroids andtesting is performed at CERN, starting with the test of the B0 model coil in the beginning of theyear 2001. The design of the cryogenic, electrical, control and vacuum systems has beenfinalised and contracts will be placed soon. The present schedule is compatible with the start ofunderground installation in summer 2003 and commissioning of the whole system in 2005.Manufacturing and testing of the coils are crucial steps to demonstrate the feasibility andreliability of the ATLAS Magnet System, which for its exceptional size represents a uniquechallenge in magnet technology.ACKNOWLEDGEMENTSThis work is part of the ATLAS detector construction and is financed by the ATLAS Collaboration, about 145Institutes in 34 countries. In particular the members of the magnet teams at CEA (F), RAL (UK), KEK (J), INFN-LASA (I), NIKHEF (Nl) and CERN are acknowledged.21ST SOFT – Madrid 11-15 September 20009REFERENCES[1] ATLAS Magnet System Technical Design Report – 4 Volumes, CERN document CERN/LHCC/97-18+19+20+21 (1997).[2] H H J ten Kate, ‘The Superconducting Magnet System for the ATLAS Detector at CERN’, Proc. MT-16, PonteVedra Beach, Florida, USA, 1999.[3] A Dael et al., ‘Synthesis of Technological Developments for the B0 Model Coil and the ATLAS Barrel ToroidCoils’, Proc. MT-16, Ponte Vedra Beach, Florida, USA, 1999.[4] D E Baynham et al., ‘Engineering Status of the Superconducting End Cap Toroid Magnets for the ATLASExperiment at LHC’, Proc. MT-16, Ponte Vedra Beach, Florida, USA, 1999.[5] A Yamamoto et al., ‘Progress in ATLAS Central Solenoid Magnet’, Proc. MT-16, Ponte Vedra Beach, Florida,USA, 1999.[6] P Miele et al., ‘Status and Commissioning of the ATLAS Magnet Test Facility at CERN’, Proc. ASC 2000,Virginia Beach, Virginia, USA, 2000.

The Superconducting Magnet System for the ATLAS Detector at CERN

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