High Energy Physics experiments have frequently adopted cryogenic versions of their apparatus to achieve the desired performance. Among the four new experiments for the CERN Large Hadron Collider (LHC) the two largest, ATLAS and CMS, include spectrometers using 4.5 K superconducting magnets and detectors filled with liquid argon at 87 K, respectively for particle momentum and energy measurements. These detectors are of unprecedented size and complexity and the definition of the associated cryogenic systems is the result of a collaboration between CERN and several external institutes all around the world. A review of the various systems is presented with particular emphasis to the basic cooling principles, the special cryogenic features and the operation scenarios

Bremer, J

Delikaris, D

Delruelle, N

Haug, F

Passardi, Giorgio

Perinic, G

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsCRYOGENICS FOR THE LARGE HADRON COLLIDER EXPERIMENTSJ. Bremer, D. Delikaris, N. Delruelle, F. Haug, G. Passardi and G. PerinicHigh Energy Physics experiments have frequently adopted cryogenic versions of their apparatus toachieve the desired performance. Among the four new experiments for the CERN Large HadronCollider (LHC) the two largest, ATLAS and CMS, include spectrometers using 4.5 K superconductingmagnets and detectors filled with liquid argon at 87 K, respectively for particle momentum and energymeasurements. These detectors are of unprecedented size and complexity and the definition of theassociated cryogenic systems is the result of a collaboration between CERN and several externalinstitutes all around the world. A review of the various systems is presented with particular emphasis tothe basic cooling principles, the special cryogenic features and the operation scenarios.LHC-ECRPresented at the 6th IIR International Conference "Cryogenics 2000"10-13 October 2000, Praha, Czech RepublicGeneva, Large Hadron Collider ProjectAdministrative SecretariatLHC DivisionCERNCH - 1211 Geneva 23SwitzerlandLHC Project Report 458Abstract11 December 20001CRYOGENICS FOR THE LARGE HADRON COLLIDER EXPERIMENTSJ. Bremer, D. Delikaris, N. Delruelle, F. Haug, G. Passardi, G. PerinicCERN, European Organization for Nuclear Research, Geneva, CHABSTRACTHigh Energy Physics experiments have frequently adopted cryogenic versions of theirapparatus to achieve the desired performance. Among the four new experiments for theCERN Large Hadron Collider (LHC) the two largest, ATLAS and CMS, includespectrometers using 4.5 K superconducting magnets and detectors filled with liquid argon at87 K, respectively for particle momentum and energy measurements. These detectors are ofunprecedented size and complexity and the definition of the associated cryogenic systems isthe result of a collaboration between CERN and several external institutes all around theworld. A review of the various systems is presented with particular emphasis to the basiccooling principles, the special cryogenic features and the operation scenarios.INTRODUCTIONThe design of detectors for High Energy Physics experiments is based on large magnetsgenerating the field required by particle momentum measurements. Both resistive andsuperconducting (s.c.) magnets are used and the choice is dictated by economy and/ornumber of radiation lengths (a parameter indicating the "transparency" of the mechanicalstructure along the path of the elementary particles). Following these design criteria two,ATLAS and CMS [1],[2], of the four experiments for LHC (Fig.1) have adopted 4.5 Ks.c. magnets whilst the others, ALICE and LHC-B, have retained the resistive version.The CMS experiment is built around a single large s.c. solenoid [3] (length 13 m, innerdiameter 5.9 m, uniform field of 4 T), whilst the magnetic configuration of ATLAS is basedon a "thin" central solenoid [4] (length 5.3 m, inner diameter 2.4 m, uniform field of 2 T)surrounded by a large s.c. toroid [5] consisting of three separate magnets, the Barrel andtwo End-Caps. They generate a toroidal field in a cylindrical volume (length 26 , externaldiameter 20 m) covering the entire ATLAS detector. At the exception of the ATLASsolenoid operated at 8000 A, all the others are powered up to 20000 A. The total storedelectromagnetic energy is 4.4 GJ (60% in CMS and the remaining in the ATLAS magnetstogether).Particle energy measurements are carried out by means of detectors called calorimeterstransforming the energy into measurable signals (ionisation charge or scintillation light). Theionisation principle was chosen for ATLAS and liquid argon selected among the possibleionisation media as it is the best suited fluid, fully compatible with the LHC radiationenvironment, to satisfy the detector requirements [6]. This choice implies cooling in therange 84 to 89K of the calorimeter made of three large d tectors: the Barrel and two End-Caps, their respective cryostats are filled with 45 m3 and 2x19 m3 of liquid argon. Alltogether they cover a cylindrical inner structure of a length of 13 m and an external diameterof 9 m.All these detectors are in advanced stage of technical specification and of construction.1. CRYOGENIC SYSTEMSThe LHC experiment cryogenics can be sub-divided into three parts: internal, proximity andexternal.Internal cryogenics means any cryogenic device like heat exchangers, thermal shields,supports, feedthroughs located inside the cryostat housing a magnet or detector.2Proximity cryogenics means the links, from a conceptual and an operational point of view,to a given cryostat. It includes phase separators, expansion volumes, current leads turrets,cryogenic pumps, distribution valve boxes, transfer lines and safety devices. All these linkslocated on top of the corresponding detector or near-by are installed in the main caverns(typically 100 m underground) housing the experiments.Figure 1: Schematic layout of the Large Hadron Collider (LHC) with its four experimentsExternal cryogenics means all devices needed to generate the cooling capacity at differenttemperature levels and in the various operating modes. Included are compressors,refrigerators, cryogens to boost the cooling capacity and all auxiliaries. These externalcomponents are located in service caverns near the main ones and in surface buildings at thevertical of the underground areas.ATLAS and CMS detectors, in opposite location on the 27 km ring of the collider, haveindependent cryogenic systems in turn separated from the very large 1.8 K system of LHC,however unified control systems will be adopted to allow operation and maintenance of allLHC cryogenics by a team sharing common resources.1.1 Internal cryogenicsCooling of all magnets is achieved by the indirect method which greatly simplify the cryostatdesign. For the two solenoids (CMS and ATLAS [7],[8]), the helium flow in the coolingpipes is driven by the hydrostatic pressure difference between the pure liquid supply and thetwo-phase mixture in the return pipes (thermosyphon). The CMS solenoid uses a typical Ushaped geometry with several parallel return channels, whilst the ATLAS solenoid has a3single pass serpentine cooling pipe running parallel to the axis on each side of the magnet.For the ATLAS toroid, where the thermosyphon could not be applied because of theunfavourable geometry and the complex helium internal distribution, the two-phase heliummixture is forced by centrifugal pumps (total flow of 1200 g.s-1). Both systems(thermosyphon and pumps) provide the high flow rate that should ensure hydraulic stabilityin the two-phase helium by combining a mass flow per unit area higher than 4 g.s-1.cm-2 with a vapour mass fraction at the outlet of the coils of less than 10% [9]. In allcases helium is supplied to the coils circuits at 4.5 K, hydrostatic or pumps heads will addsubcooling at inlet of 0.2 to 0.4 K. For the CMS solenoid and the ATLAS Barrel toroiddouble (one redundant) cooling piping is implemented since, for these systems, dismountingor access for repair of the internal cryogenics after final underground assembly cannot beenvisaged.Cooling of calorimeters [10] is achieved by evaporation of two-phase nitrogen flowingthrough pipes placed in the liquid argon volumes. Nitrogen circulation is driven again bypumps (total flow of 100 g.s-1) providing a flow rate corresponding to the thermal loads plus10 % to ensure wet operating conditions of the heat exchangers over all their length.Pressure and, therefore, temperature of the cooling pipes can be varied between 90 and84.1 K as function of operating conditions. They will set the temperature of the calorimetersat about 87 K creating subcooled conditions by 5 to 8 K depending on the height of thecryostat. Thus argon gas bubbling, being detrimental for the functioning of the detector, isprevented. Furthermore, the cooling pipes are located in such a way to control thetemperature gradients over the overall calorimeter volumes within 0.6 K for mi imisingpossible non uniformity in the detector response due to differences of the liquid argondensity.All magnets have thermal shields cooled by a high pressure flow of helium at about 60 Kwhilst no thermal shields are foreseen for the calorimeters. The ATLAS solenoid is a specialcase since the Barrel calorimeter is housed in the same cryostat and externally shields at 87K the solenoid. This complex solution was imposed by the requirement of minimumradiation length along the path of elementary particles.Cooling from ambient to the operating temperature is carried out via the same pipes usedduring normal operation. In case of the magnets, by means of high pressure helium at atemperature 40 K lower than that one of the cold mass and, in the case of the calorimeters,by means of liquid nitrogen in all the cool-down temperature range. In the first case, heat isextracted by conduction through the magnet structure made of aluminium and, in the secondcase, by convection and/or conduction through the argon gas gaps and the structure of thedetector made of various materials (copper, lead, kapton). To improve the overall heattransfer and the corresponding cool-down time of the calorimeters (a factor 2 to 3 ), theargon gas can be temporarily replaced by helium or condensation of argon allowed below120 K.1.2 Proximity cryogenicsThe proximity cryogenics takes the cooling capacity provided by the external cryogenics anddistributes it to the various sub-systems. By means of centrifugal pumps (ATLAS toroidsand calorimeters) or of thermosyphon (CMS and ATLAS solenoids) the two-phase mixtureis forced through the internal cooling pipes. In the first case, the excess of liquid is returnedto phase separators placed on the wall of the main cavern near-by the detector and, in thesecond case, is returned to phase separators placed on top of the detectors to favourthermosyphon effect.In case of failure of the external cryogenics, the proximity systems are designed to have asufficient autonomy to allow: a) the magnets slow dump of the stored electromagnetic4energy in a few hours; b) the continuation of the cooling of the calorimeters for at least oneday. This is achieved by a sufficient liquid capacity of the phase separators with theirassociated back-up dewars: 15000 litres of liquid nitrogen for the calorimeters and 12000and 6000 litres of liquid helium for respectively ATLAS and CMS magnets. Furthermore,the liquid argon volumes of the three calorimeters are linked to correspondingexpansion/phase separator vessels (volume 10% of the total) since no gas is allowed insidethe detectors.ATLAS experiment requires longitudinal movement of several meters of the magnets andcalorimeters End-Caps for periodic (a few times per year) access to the electronics placednear the central part. The proximity systems include several flexible transfer lines with acomplex system of guidance to perform these interventions with both the calorimeters andthe magnets kept at their nominal cryogenic operating condition.Only if required by major problems or hazard, the liquid argon contained in the calorimetercryostats can be emptied in two dewars (each of 50000 litres capacity) installed in the maincavern and equipped with internal condensers for a closed circuit operation.1.3. External cryogenicsTo cope with the static and the dynamic thermal loads (ramp up and down of the magnetscurrent and operation of the pre-amplifiers immersed in the liquid argon), ATLAS will usetwo helium and one nitrogen refrigerators independent from each others [10],[11] providingrespectively:- 3.6 kW isothermal capacity at 4.5 K and 15 g/s liquefaction for current leads- 20 kW at 40 to 80 K for thermal shield cooling of the magnets- 20 kW isothermal capacity at 84 K for the calorimeters. and CMS will use a single helium refrigerator providing:- 800 W at 4.5 K, 4 g/s liquefaction for current leads and 4.5 kW at 60 to 80 K for thermalshield cooling.No liquid nitrogen supply from the surface is foreseen to achieve these performances and allthe indicated cooling capacities include a 40 to 50 % margin.The above refrigerators will also cope with the thermal recovery from the temperature(about 60 K) reached after a fast dump of the electromagnetic energy stored in the magnets.Both for ATLAS and CMS, the recovery time is expected to be less than 3 days.Furthermore, in the case of the calorimeters the demand of uninterrupted cooling isextended over several years since emptying/refill of liquid might affect the detectorcalibration during a physics run. To satisfy this special requirement, nitrogen storage dewarsinstalled at the surface can take over the supply of liquid during failure and annualmaintenance work of the 84 K nitrogen refrigerator.The cold mass of all the detectors is 1500 tons (85% in ATLAS) and, in total, 200 GJ (82%in ATLAS) must be extracted to cool them from ambient to the operating temperature. Thehelium refrigerators, designed for base-line operation, include additional heat exchangers  tocool the high pressure helium flow to the magnets by means of liquid nitrogen supplied fromthe surface dewars (two for ATLAS and one for CMS each of 50000 litres capacity). Theseheat exchangers, integrated for CMS in the refrigerator cold box and for ATLAS in thethermal shield refrigerator, have a cooling capacity of respectively 30 and 60 kW in therange 300 to 100 K. For the calorimeters, no external heat exchangers are used sincecooling-down is carried out directly by the liquid nitrogen flow through the internal coolingpipes operated in a way to provide 30 kW capacity. In all cases, the time required to reachfrom ambient the operating temperature is less than one month.5CONCLUSIONAll the LHC detectors will pass surface tests prior to their final installation. The CMSsolenoid will be fully assembled and tested with its final proximity and external cryogenicsystems in a surface building at the vertical of the underground area. For ATLAS, a largetest area with a dedicated cryogenic facilities [12] is now being prepared where the threecalorimeters, the two End-Caps toroids and the central solenoid will be tested as single unit.For the Barrel toroid, the eight coils making the magnet will be tested in sequence becausethe very large dimension of the fully assembled magnet does not allow its mounting at thesurface with further transfer underground. All these facilities are designed for providingbasic cryogenic test conditions identical to those foreseen in the final configuration to fullyvalidate the cooling principles.Large experiments for High Energy Physics are the result of world-wide collaborations and,for the LHC detectors, this is also valid for the cryogenics. In total 10 external institutesfrom Europe, USA and Japan are collaborating with CERN in designing, constructing andtesting the various systems of the internal, proximity and external cryogenics. REFERENCES[1] ATLAS collaboration, ATLAS Technical Proposal, CERN/LHCC/94-43, Geneva,1994.[2] CMS collaboration, CMS Technical Proposal, CERN/LHCC/94-38, Geneva,1994.[3] CMS Magnet Project, Technical Design Report, CERN/LHCC 97-10, Geneva,1997.[4] ATLAS Central Solenoid, Technical Design Report, CERN/LHCC 97-21, Geneva,1997.[5] ATLAS Magnet System, Technical Design Report, CERN/LHCC/97-18, Geneva,1997.[6] ATLAS Liquid Argon Calorimeter, Technical Design Report, CERN/LHCC/96-41, Geneva,1996.[7] D.Delikaris, J.P.Dauvergne, G.Passardi, J.C.Lottin, J.P.Lottin, Ch. Lyraud, The Cryogenic System of theSuperconducting Solenoid Magnet of the CMS Experiment, MT-15 Proceedings, pp.1230-1233, Beijing,October 1997.[8] Y.Doi, A.Yamamoto, T.Haruyama, Y.Makida, K.Tanaka, H.Yamaoka, T.Kondo, Proximity CryogenicSystem for the ATLAS Solenoid, KEK, Tsukuba Japan, April 1999.[9] Yu.P.Filippov, Chracteristics of horizontal two-phase helium flows, Part I-flow patterns and voidfraction, Cryogenics vol.39, pp.59-68,1999.[10] J.Bremer, The Cryogenic System for the ATLAS Liquid Argon Detector, Paper presented at ICEC-18,Mumbai India, February 2000.[11] N.Delruelle, F.Haug, G.Passardi, H.Ten Kate, The Helium Cryogenic System for the ATLASExperiment, MT-16 IEEE Transactions on Applied Superconductivity vol.10, pp.1511-1513, Florida,1999.[12] F.Haug, A.Cambon, N.Delruelle, J.P.Orlic, G.Passardi, J.Tischhauser, The CERN Cryogenic TestFacility for the ATLAS Barrel Toroid Magnets, MT-16 IEEE Transactions on Applied Superconductivityvol.10, pp.1514-1517, Florida,1999.

Cryogenics for the Large Hadron Collider Experiments

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