Benda, V

Bézaguet, Alain-Arthur

Casas-Cubillos, J

Claudet, S

Erdt, W K

Lebrun, P

Riddone, G

Sergo, V

Serio, L

Tavian, L

Van Weelderen, R

Vullierme, B

Wagner, U

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsCONCEPTUAL DESIGN OF THE CRYOGENIC SYSTEM FOR THE LARGEHADRON COLLIDER (LHC)V. Benda, A. Bézaguet, J. Casas-Cubillos, S. Claudet, W. Erdt, Ph. Lebrun, G. Riddone, V. Sergo, L.Serio, L. Tavian, B. Vullierme, R. van Weelderen and U. Wagner *The high-field superconducting magnets to be installed on the 26.7 km circumference of theLarge Hadron Collider at CERN will operate below 1.9 K in pressurised superfluid helium,thus requiring the development and implementation of a cryogenic system unprecedented insize, refrigeration capacity and complexity. After recalling the basic features of LHCcryogenics, which allow to make full use of the peculiar properties of superfluid helium as atechnical coolant for accelerator devices, we present recent updates in performancerequirements and changes in architecture of the system, discuss their technical impact, andreview the status of development of the key technologies involved.LHC DivisionEPAC - 96,  Sitges (Barcelona)Geneva, Large Hadron Collider ProjectCERNCH - 1211 Geneva 23SwitzerlandLHC Project Report 12Abstract9/7/96 CoolingtowerCoolingtowerElect.SupplyElect.SupplyHPStorageHPStorageHPStorageComp. Comp. Comp. Comp.UCB UCB UCB UCBLCBCIBLHC Sector LHC SectorCIBCCB CCB CCB CCBLCB LCB LCBEven Point Even PointOdd PointEven OctantSector (~3.3km) Sector (~3.3km)Even OctantOdd OctantTunnelCavernShaftSurfaceTFBTFBTFBTFBTFBTFBFigure 1 : General architecture of the LHC cryogenicschemeCONCEPTUAL DESIGN OF THE CRYOGENIC SYSTEM FOR THELARGE HADRON COLLIDER (LHC)V. Benda, A. Bézaguet, J. Casas-Cubillos, S. Claudet, W. Erdt, Ph. Lebrun, G. Riddone,V. Sergo, L. Serio, L. Tavian, B. Vullierme, R. van Weelderen and U. WagnerCERN, LHC Division, 1211 Geneva 23, SwitzerlandAbstractThe high-field superconducting magnets to be installedon the 26.7 km circumference of the Large HadronCollider at CERN will operate below 1.9 K in pressurisedsuperfluid helium, thus requiring the development andimplementation of a cryogenic system unprecedented insize, refrigeration capacity and complexity. After recallingthe basic features of LHC cryogenics, which allow tomake full use of the peculiar properties of superfluidhelium as a technical coolant for accelerator devices, wepresent recent updates in performance requirements andchanges in architecture of the system, discuss theirtechnical impact, and review the status of development ofthe key technologies involved.1. INTRODUCTIONThe Large Hadron Collider is the next high-energyparticle physics instrument being built by CERN. Thismachine is basically a proton collider with a centre-of-mass energy of 14 TeV and with luminosity in the 1034cm-2 s-1 range, providing the experiments with highinteraction rates. In order to keep costs down the LHC isto be installed on the floor of the 26.7 km circumferenceLEP tunnel, and be fed by existing particle sources andpre-accelerators. The LHC uses the most advancedsuperconducting magnet, cryogenic and acceleratortechnologies ever employed and it is thoroughly describedin the so-called "Yellow Book" [1].The LHC employs high-field twin-aperturesuperconducting magnets operating in static baths ofpressurised superfluid helium below 1.9 K. This lowoperating temperature is dictated by the high-fieldoperation of the superconducting cable, based on the well-developed and mass-produced Nb-Ti alloy. Moreover itallows to benefit from the peculiar thermophysicalproperties of helium II (large effective thermalconductivity and heat capacity as well as low viscosity)for heat transport and conductor stabilisation.The technology of pressurised superfluid heliumchosen for the LHC was first pioneered by the"Commissariat à l'Energie Atomique" (CEA) in Grenoble(France) and implemented industrially for the Tore Suprafusion tokamak at Cadarache and a few other projects [2].It allows kilowatts of refrigeration to be transported overthe 3.3 km length and across elevation differences of up to46 m of an LHC sector with a temperature drop of lessthan 0.1 K. The total LHC refrigeration capacity isequivalent to over 150 kW at 4.5 K distributed around thering, circumference.2. GENERAL ARCHITECTUREAcknowledging that the even-numbered areas of LEPare much more accessible, developed and already equippedwith technical buildings and infrastructure, we decided togroup together, as much as possible, all the activecryogenic equipment at these four points, and to transportthe refrigeration power over the complete length of asector, i.e. 3.3 km, instead of the previous 1.7 km half-octant [3]. This results in the four-point cryogenic feedscheme that is sketched in Figure 1, instead of theprevious eight-point feed scheme. This requires largerdiameter pipes for distributing the cryogenic fluids,making impractical the previously designed integratedcryostat housing both the superconducting magnets andthe cryogenic lines. Therefore the cooling power is nowdistributed by a separate Cryogenic Distribution Line(CDL) running alongside the magnets cryostats, withcryogenic interconnections at every half-cell cooling loopof 53.5 m. Two split-coldbox (Upper and Lower ColdBoxes, UCB -  LCB) helium refrigerators of the LEP type[4] with an equivalent refrigeration power of 18 kW at4.5 K serving adjacent sectors are installed in each evenpoint. Partial redundancy is obtained by using a CryoplantInterconnection Box (CIB) allowing distribution of thecryogenic power of each sector to either or both plants.Refrigeration at 1.8 K is provided by two ColdCompressor Boxes (CCB) installed at the level of thetunnel, each requiring four to five stages of centrifugal HALF-CELL COOLING LOOP (53.5 m)Thermal Shield & Magnet SupportsMagnet CryostatMQ MB MB MBMagnet SupportsE(50 K, 20 bar)D(20 K, 1.3  bar)C(4.5 K, 3  bar)SRVSRVSRVSRVCFVCFVTCV1TCV1TCV2TCV2B(1.83 K, 16 mbar)A(2.2 K, 1.3  bar)F(75 K, 19 bar)CDL Thermal ShieldC' C'C'C'Cryogenic Distribution LineBeam screensFigure 2: Cryogenic flow-scheme of an LHC half-cellcompressors in series. Two monostage centrifugalcompressors, supplied by European industry and featuringnovel ideas in wheel design, as well as bearing and drivetechnology, are being investigated [5,6]. Studies are alsounder way for possibly using volumetric cold compressorsin the CCBs [7]. The string of magnets is terminated byTunnel Feed Boxes (TFB) on each end of a sector that isused mainly for electrical feed of the LHC, by means ofhigh-temperature superconductor (HTS) current leads [8].1000 m3 and 1500 m3 storage vessels at 2 MPa are usedin the odd and even points respectively for recovery ofgaseous helium after a generalised resistive transition ofthe superconducting magnets in the sector concerned. Thetotal helium inventory in the LHC amounts to 93500 kg,most of which is stored in the cold mass of the magnetsystem during operation. For periods of shutdown partialstorage in liquid form is envisaged in standard 11000gallon (about 40 m3) industrial containers compatiblewith liquid helium handling by European distributors.3. RING OPERATION AND HEAT LOADSThe cryogenic flow scheme of an LHC half cell isshown in Figure 2. The pressurised superfluid heliumbath at 1.9 K in which the superconducting magnets areimmersed is cooled by saturated two-phase liquid heliumflowing in heat exchanger tubes extending along thestring of magnets and supplied by line A through theexpansion valve TCV1. The performance and efficiency ofthis cooling loop, that are key factors in achieving therequired magnet performance, have been assessed in theLHC test string [9]. The low saturation pressure ismaintained by pumping the vapour through line B. Cool-down and warm-up is achieved by forced circulation ofhigh-pressure gaseous helium supplied at variabletemperature by line C, tapped through valve CFV andreturned to the refrigerator by valve SRV and line D. Incase of magnet resistive transitions, the resulting pressurerise is contained below the 2 MPa design pressure bydischarging the liquid helium inventory of a half-cell intoline D through the SRV valves. The low hydraulicimpedance of this 150 mm diameter pipe, normallymaintained at 20 K, proves very helpful in containing thehelium discharge and buffering the gas storage vessels.The temperature levels are:• thermal shielding between 50 K and 75 K as a firstmajor heat intercept, lines E and F;• distribution of supercritical helium at 4.5 K by line Cfor initial filling of the magnets; during operation itprovides the lower heat intercept and cryogen for thenon-isothermal cooling of the beam screen by usingthe control valve TCV2;• quasi-isothermal superfluid helium cooling thesuperconducting magnets at a maximum temperatureof 1.9 K and transporting the heat loads across thelength of a sector (3.3 km) to the CCBs operating at1.8 K;• saturated liquid helium (between 4.5 K and 4.7 K)cooling special superconducting magnets,superconducting acceleration cavities and the lowersections of HTS current leads;• gaseous helium cooling the resistive upper sections ofHTS current leads in forced flow of gaseous heliumbetween 20 K and ambient.As the heat loads depend strongly on the energy andintensity of the circulating beams, we use hereafter thevalues of the dynamic loads estimated for nominal(7 TeV, 2 x 0.536 A) and ultimate (7 TeV, 2 x 0.848 A)operating conditions. The estimated heat loads depositedin the magnet cold mass through several processes aregiven below, respectively for the nominal and ultimatecases:• synchrotron radiation mostly absorbed by the beamscreens amount to  0.41 W/m and 0.65 W/m;• resistive heating due to image currents result in a0.33 W/m and 0.83 W/m;• continuous loss of particles results in a distributedheat load in the helium II baths of 0.06 W/m. Thisvalue should decrease with time due to improvementof the vacuum by beam cleaning;• continuous loss of particles escaping the collimationsections may result in localised heat loads of 55 W and92 W over a length of a few tens of metres, at onelocation around the ring;• absorption of secondaries in the regions close to thehigh luminosity experimental areas:- at the 1.9 K level 500 W to 1250 W in the innertriplets of low- b  quadrupoles and 54 W to 134 W inthe dispersion suppressor magnets, for the wholemachine;- at the 4.7 K level 240 W to 600 W in the beamseparation and recombination dipoles for the wholemachine.Table 1 Refrigeration capacity demands of LHC sector           cryoplantsSector Temperature levels50-75 K[kW]4.5-20 K[kW]4.7 K[kW]1.8 K[kW]20-300 K[g/s GHe]High-load 31 4.3 0.80 2.80 35Low-load 30 4.3 0.65 2.45 23On the basis of the heat load estimates for the LHCsectors, and allowing for some spare capacity, therefrigeration capacity to be simultaneously fulfilled by thecryogenic plants in each sector are listed in Table 1. Thesectors which house high-luminosity insertions aresubject to the largest 1.9 K heat loads, while sectorswhich feature the strings of superconducting accelerationcavities, exhibit the largest 4.7 K heat loads. Apart fromfulfilling the demands listed in Table 1, the refrigerationplants have to be capable of operating in wide ranges ofvariation in the cooling power requirements at 1.9 K (upto two-fold variation) and 4.5 - 20 K (up to five-foldvariation) that  depend strongly in the operational mode ofthe LHC.4. TECHNICAL VARIANTS UNDER STUDYThe system described above fully meets the designconstraints and technical requirements established for LHCcryogenics, and may therefore be implemented withconfidence. However, several technical variants,compatible with the basic design principles and presentlyunder study, exhibit a strong potential for simplifying thelayout, improving efficiency and performance, as well asreducing costs. Examples of such variants are givenbelow.Operating the two-phase helium II heat exchanger tubein counter-current as well as in co-current flow of liquidand vapour would bring complete symmetry in thecryogenic layout of the half-cell cooling loop, independentof the tunnel slope. This mode of operation is beingtested in the second phase of the LHC prototype string[10], and its limits investigated in the thermohydraulictest loop at CEA in Grenoble (France).Grouping all cryogenic headers in the CDL separatelyfrom the magnet cryostats, opens the way for finersectorization of the LHC, an issue which could greatlyshorten time spans for warm-up and cool-down of limitedstretches of magnets to be replaced or repaired, and thuscontribute to improving the availability of the LHC forphysics.Developing technology and establishing theperformance and efficiency of several competing solutionsfor key cryogenic components, such as helium heatexchangers and compressors, over the useful ranges in thepressure and temperature domains, will allow theoptimisation of the thermodynamic cycles for large-capacity refrigeration at 1.8 K. This could lead tosolutions based on cold, warm, or mixed compressionschemes, which may prove more economical or easier tooperate and interface with the 4.5 K refrigerators. Thistopic constitutes the subject of an ongoing R&Dprogramme conducted in collaboration with CEA (France)[7].5. REFERENCES[1] 'The Large Hadron Collider, Conceptual design',CERN/AC/95-05 (LHC), 1995[2] 'Tore Supra and Helium II Cooling of Large High-Field Magnets',  by G. Claudet and R. Aymar, Adv.Cryo. Eng. 35A, 55-67 (1990)[3] 'Superfluid Helium Cryogenics for the Large HadronCollider Project at CERN' by Ph. Lebrun,Cryogenics 34, ICEC Supplement 1-8 (1994)[4] 'Four 12 kW/4.5 K Cryoplants at CERN' by S.Claudet, W.K., Erdt, P.K. Frandsen, Ph. Gayet, N.O.Solheim, Ch. Titcomb and G. Winkler, Cryogenics34, ICEC Supplement 99-102 (1994).[5] 'Helium Low-Temperature Compressor' by P.Schustr, M. Vins, I. Brunovski and L. Tucek, ProcKryogenika'96 Conference, Praha 78-82 (1996)[6] 'A Cryogenic Axial-Centrifugal Compressor forSuperfluid Helium Refrigeration' by L. Decker, K.Löhlein, P. Schustr, M. Vins, I. Brunovsky, L.Tucek, Ph. Lebrun and L. Tavian, presented atICEC16, Kitakyushu (1996)[7] 'Development of Large-Capacity Refrigeration at 1.8K for the Large Hadron Collider at CERN', by P.Lebrun, L. Tavian and G. Claudet, ProcKryogenika'96 Conference, Praha 54-59 (1996)[8] 'Potential of High-Temperature SuperconductorCurrent Leads for LHC Cryogenics, A. Ballanino, A.Ijspeert and U. Wagner, presented at ICEC16,Kitakyushu (1996)[9] 'The Superfluid Helium Cryogenic System for theLHC Test String: Design, Construction and FirstOperation', by A. Bézaguet, J. Casas-Cubillos, B.Flemsaeter, B. Gaillard-Grenadier, Th. Goiffon, H.Guinaudeau, Ph. Lebrun, M. Marquet, L. Serio, A.Suraci, L. Tavian and R. van Weelderen presented atCEC'95, Columbus (1995)[10]'Cryogenic Operation and Testing of the ExtendedLHC Prototype Magnet String', by A. Bézaguet, J.Casas-Cubillos, H. Guinaudeau, B. Hilbert, Ph.Lebrun, L. Serio, A. Suraci and R. van Weelderenpresented at ICEC16, Kitakyushu (1996)

Conceptual design of the cryogenic system for the Large Hadron Collider (LHC)

Conceptual design of the cryogenic system for the Large Hadron Collider (LHC)

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