The LHC Conceptual Design Report issued on 20th October 1995 [1] introduced significant changes to some fundamental features of the LHC standard half-cell, composed of one quadrupole, 3 dipoles and a set of corrector magnets. A separate cryogenic distribution line has been adopted containing most of the distribution lines previously installed inside the main cryostat. The dipole length has been increased from 10 to 15 m and independent powering of the focusing and defocusing quadrupole magnets has been chosen. Individual quench protection diodes were introduced in magnet interconnects and many auxiliary bus bars were added to feed in series the various families of superconducting corrector magnets. The various highly intricate basic systems such as: cryostats and cryogenics feeders, superconducting magnets and their electrical powering and protection, vacuum beam screen and its cooling, support and alignment devices have been redesigned, taking into account the very tight space available. These space constraints are imposed by the desire to have maximum integral bending field strength for maximum LHC energy, in the existing LEP tunnel. Finally, cryogenic and vacuum sectorisation have been introduced to reduce downtimes and facilitate commissioning

Brunet, J C

Cruikshank, P

Erdt, W K

Genet, M

Parma, Vittorio

Poncet, Alain

Rohmig, P

Skoczen, Blazej

Van Weelderen, R

Vlogaert, J

Wagner, U

Williams, L R

CERN Document Server

LHC Division, *EST DivisionEUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsMECHANICAL DESIGN AND LAYOUT OF THE LHC STANDARD HALF-CELLJ.C. Brunet, P. Cruikshank, W. Erdt, M. Genet, V. Parma, A. Poncet, P. Rohmig, B. Skoczen*,R. Van Weelderen, J. Vlogaert, U. Wagner, L.R. WilliamsThe LHC Conceptual Design Report issued on 20th October 1995 [1] introduced significant changes to somefundamental features of the LHC standard half-cell, composed of one quadrupole, 3 dipoles and a set of correctormagnets. A separate cryogenic distribution line has been adopted containing most of the distribution lines previouslyinstalled inside the main cryostat. The dipole length has been increased from 10 to 15 m and independent poweringof the focusing and defocusing quadrupole magnets has been chosen. Individual quench protection diodes wereintroduced in magnet interconnects and many auxiliary bus bars were added to feed in series the various families ofsuperconducting corrector magnets. The various highly intricate basic systems such as: cryostats and cryogenicsfeeders, superconducting magnets and their electrical powering and protection, vacuum beam screen and its cooling,support and alignment devices have been redesigned, taking into account the very tight space available. These spaceconstraints are imposed by the desire to have maximum integral bending field strength for maximum LHC energy, inthe existing LEP tunnel. Finally, cryogenic and vacuum sectorisation have been introduced to reduce downtimes andfacilitate commissioning.Presented at 1997 Particle Accelerator Conference, Vancouver, 12-16 May 1997.Geneva, Large Hadron Collider ProjectCERNCH - 1211 Geneva 23SwitzerlandLHC Project Report 111Abstract03/06/97MECHANICAL DESIGN AND LAYOUT OFTHE LHC STANDARD HALF-CELLJ.C. Brunet, P. Cruikshank, W. Erdt, M. Genet, V. Parma, A. Poncet, P. Rohmig, B. Skoczen,R. Van Weelderen, J. Vlogaert, U. Wagner, L.R. Williams, CERN, Geneva, Switzerland1  INTRODUCTIONThe Large Hadron Collider (LHC) to be installed in theLEP tunnel at CERN will produce two proton beams of7 TeV energy for head on collisions in 4 points aroundthe circumference. The machine is subdivided into 8octants, each one comprising a standard arc composed of54 optical half-cells housed in a common cryostat (Arccryostat), flanked on each side by specific insertion opticsfor the various experimental areas and functions of themachine (injection, ejection, cleaning, RF, etc.).The standard 914 mm diameter arc cryostat, of anoctant 2.7 km long, is bounded at each extremity byelectrical current feedboxes (DFB) allowing the numerousfamilies of superconducting magnets to be powered inseries. The cryogenic distribution line (QRL) housingvarious headers servicing the main stream of cryomagnetsruns parallel to the arc cryostat, and as for the mainelectrical supplies is fed at the 4 even points of themachine from cryoplants installed at ground level. Thebasic repetitive segment of the arc is the half-cell of 53.4m length providing 90°  phase advance for the beams. It iscomposed of 3 two-in-one 8.3 T dipoles of 15 m length,one two-in-one quadrupole and various sets of correctingmagnets. The so-called Short Straight Section (SSS)housing the quadrupole, sextupole, octupole, dipole orbitcorrectors, and beam position monitors (BPM) is flankedby a cryogenic service module housing service piping forquench discharge into the QRL, phase separation for1.9 K heat exchanger cooling, etc.The half-cell length occupancy has been optimised toprovide maximum bending field length, taking intoaccount the various functions and elements of themachine.2  CRYOSTATThe string of superconducting magnets operates in a static1.9 K superfluid helium bath pressurised at 1 bar absolute[2]. The heat load budget to 1.9 K is composed of staticheat losses through cryostat components (radiation,conduction through support posts, vacuum barriers, etc.)and dynamic loads resulting from resistive heating insplices between cables, beam losses into the 1.9 K bathand synchrotron radiation and RF image current powerdeposited on the beam screen. To minimise the static heatload to the 1.9 K level at which the cost of refrigerationis 3 to 4 times higher than at 4.5 K, two actively cooledaluminium screens operating at 5-10  K (radiative shield)and 50-75 K (thermal shield) are interposed between the293 K and 1.9 K levels. These shields are made ofpolished aluminum sheets welded onto 15 m longstructures, extruded together with the cooling channels.dipolevacuumtank  (914mm)50-75 Kthermalshieldcold postsupportjacksalignmenttarget5-20 Kradiativeshieldmultilayerinsulationaluminumextrusions(headers)Figure 1: LHC Dipole CryostatThe radiative shield carries 10 layers of MLI tothermally protect the system in case of degraded vacuum.The thermal shield is covered by 30 layers of MLI pre-fabricated blankets to reduce thermal radiation load.The calculated static thermal loads are given inTable 1.The transverse dimensions of magnet cold masses withtheir shields have been minimised to fit in an insulatingvacuum vessel of diameter 914 mm (36” standard pipedimension).3  SHORT STRAIGHT SECTIONCRYOGENIC MODULE AND JUMPERCONNECTIONThe Short Straight Section which houses the quadrupole,chromatic correction elements, a set of orbit correctors andbeam position monitors, also incorporates a cryogenicservice module which connects the cryogenic distributionline to the main cryostat out every cell [3].An intricate set of components finds its place in areduced volume: the phase separator of the 1.9 K heatexchanger cooling a full cell, the connecting pipeshousing the bus bars with manifolds tubes to the QRLallowing filling and discharge of a stretch of cells, thebeam position monitors, beam pipes, sectorisation valves,and quadrupole protection diodes, accessible for repair viathe interconnect. A set of 50 A current leads installed inthe cryogenic module feeds the closed orbit dipolecorrector magnets. Low heat in-leak insulating vacuumseparation barriers are installed between the quadrupolecold mass and the vacuum enclosure every two cells. Thisfacilitates vacuum pump down, leak testing by confiningleaks. The vacuum hardware is attached to the SSS.To permit an easy and precise alignment of the SSS, itmust be mechanically decoupled from the QRL. For thispurpose, a set of interconnecting bellows in the jumperconnection gives the necessary flexibility andcompensation for forces due to atmospheric pressure.4  ELECTRICAL DISTRIBUTIONApart from the orbit correctors, all superconductingmagnets of the Arc are powered in series from the mainelectrical feed box installed at the even points. Thefocusing and defocusing quadrupole pairs of 12.5 kA busbars run through the two upper slots in the magnet yokes.The main dipole 12.5 kA circuit occupies a third slot, anddipoles have two different types of electrical connectionsbuilt into the cold masses, lodging alternatively (everyhalf-cell) the magnet inductance on the go and return busbars in order to limit voltages to ground during adischarge. 40 to 80 auxiliary superconducting cables ratedat 600 A run through the magnet cold masses and areelectrically connected at each magnet intergap. Thisrepresents a total of more than 80’000 electricalconnections for the whole machine, which must be carriedout within a tight electrical specification concerninginsulation breakdown voltage (5 kV) and connectionresistance (10-8 Ohms).To minimise erroneous connections and to reducemanpower costs at installation, special multichannelconnectors and distributors are envisaged and developed.Furthermore, the electrical connection of auxiliarymagnets in the interconnect gap is being studied, as thiswould specialise SSS only at the installation stage.Alternative designs currently being studied consider therouting of uninterrupted cable bundles through the coldmasses of a half-cell to feed the chromatic magnetcorrectors installed in the SSS.Bus bars are routed through 4 slots and 4interconnecting tubes in and between the cryomagnets.5  MAGNET SUPPORT AND ALIGNMENTThe superconducting magnets of the arc are positioned bycolumn-type supports. The stringent positioning precisionfor magnet alignment and the high thermal performancefor cryogenic efficiency are the main conflictingrequirements which have lead to a trade-off design. Anadditional function requested of the support system is thesuspension of the thermal and radiative screens in thecryostat.A three point support is chosen for the 15 m long, 30ton dipoles, where the support spacing limits themaximum vertical sagitta to 0.25 mm; only two supportpoints are needed for the shorter (6 m) and lighter(6 ton) cold mass of the SSS to keep the maximumvertical sagitta below 0.23 mm.The support is made of a main composite columnbolted to stainless steel pads previously welded on themagnet cold masses. The fixations on the vacuum vesselsallow sliding of the supports to free the thermalcontractions and to leave free the required degrees offreedom.The thermal performance of the supports is improvedby intercepting most of the residual heat conduction attwo intermediate temperature levels (one in the 50-75 Kand the other in the 4.5-20 K range). These interceptsconsist of aluminum plates, glued to the compositecolumn and welded to the thermal and radiative shields viaflexible aluminium straps.The heat loads reported in table 1 are estimated frommeasurements made on previous supports.Temperature levelWatts 1.9 K 4.5-20 K 50-75 KCryomagnetsupport posts0.5 9 44Vacuum barrierInstrumentationcapillaries, BPM3.1 0.2 18.6Cryostat 0.3 3.2 165.4Total for LHCstandard half-cell3.9 12.4 228   Table 1: Estimated half-cell static heat loads (Watts)  The composite column of the support is made of a4 mm thick tube with integrated top and bottom flanges.A glass fibber/epoxy composite system was chosen for itshigh stiffness-to-thermal-conduction ratio, allowingminimisation of the cross section and, as a consequence,of conduction heat loads. A long-fiber, woven fabric lay-up technology is chosen to achieve the highest stiffness.The initial positioning precision of the magnet coldmasses is – 0.5 mm with respect to the cryostat axis and– 1 mm in the longitudinal direction. This precision isachieved by a tight chain of mechanical tolerances. Underthe worst hypothesis of residual mechanical loads, thepresent design of the support system for magnets gives astability of – 0.2 mm in the radial and vertical plane.6  INTERCONNECTIONSThe interconnections between cryomagnets contain anumber of structural components which are designed toassure a continuity and provide maximum security todifferent systems integrated within the accelerator :vacuum systems [4], cryogenics , and electricaldistribution. In order to extend the magnetic length of theLHC magnets as much as possible the interconnectionspace is reduced to a strict minimum. This requirementimposes longitudinal and radial limitations on theinterconnect components and implies a compact designcompatible with the existing tunnel as well as theestablished transport and installation procedures [5].The combination of a relatively high pressure of 20bar (which may develop during the cool-down procedure ora quench) with a necessity of assuring leak tightness forall the superfluid helium transfer systems leads to a choiceof all welded assemblies.In order to keep up with the LHC project requirementsin terms of system reliability, automatic welding, cuttingand control procedures were proposed. The stringent spacerequirements for the welding and cutting tools wasrestricted to 50 mm. To reduce the helium consumption at1.9 K the electrical connections will be brazed and theohmic resistance limited to some 10-8  . The large numberof bus bar connections (80,000) imposes a high level ofreliability as well as strict procedures for manufacture,installation and control.The interconnect space is determined to a large extentby the superconducting cable overlap length of 120 mmand the necessary length of the bellows expansion jointswhich compensate for the thermal contraction of themagnets (around 50 mm). In order to reduce the length ofthe expansion joints and their axial stiffness a specialoptimisation program has been developed. It aims atreducing the global forces between the magnets(transmitted to the support posts) in the installation phaseand during the LHC operation.Protection diodes constitute some of the criticalcomponents of the LHC machine. The present designprovides an easy access to the diodes in case they need tobe replaced. The access to the box containing the diodeswill be possible through the interconnection space, afterthe removal of the large diameter bayonet sleeve of thevacuum vessel and of the thermal shields.7  SECTORISATION OF SUB-SYSTEMSThe 2.7 km LHC arc cryostat has a cold mass weight ofmore than 6000 tons. One cryoplant at the correspondingeven point will be able to cool-down and warm-up such amass via the QRL in about 44 days.Taking advantage of a separate parallel cryolinenaturally decoupling the feeding of cryogens from themain stream of magnets, the arc is cut in four sub-sectors.Insulating vacuum barriers separate the QRL from themain cryostat at each jumper connection and sectorise themain cryostat every two cells [6]. High vacuum valvesand cryogenic separation valves are installed at each sectorboundary and additional cryogenic valves complete theseparation of the QRL from the main cryostat at eachjumper connection. Finally, so-called bus bar plugs areinstalled every two cells in magnet gap interconnects tosectorise the 1.9 K cold mass cryogenic volume. Thissectorisation allows the partial and independent warm-upand cool-down of small stretches of a few cells. This willyield a factor 2 reduction in the time required for mostinterventions for repair.8  ACKNOWLEDGEMENTSA large design, development and testing effort is beingactively pursued to integrate all LHC intricate systems inthe LHC arc cryostat. Technical choices are regularlytested on the LHC prototype test String [7].Particular credit must be attributed to R.Saban (LHCtest string), W.Cameron,, M.Genet, Th.Renaglia,Ph.Trilhe and to many other people in the LHC team,andCEA and CNRS teams.REFERENCES[1] The Large Hadron Collider, conceptual design,CERN/AC/95-05 (LHC)[2] Ph.Lebrun, Superfluid Helium Cryogenics for theLHC, ICEC 15, 1994, CERN AT/94-18[3] L.Tavian et Al, A. simplified cryogenic DistributionScheme for the LHC to be published toCEC/ICMC’97, Portland, USA, August 97.[4] O.Gröbner, The LHC Vacuum System, thisconference.[5] J.C.Brunet, Space Management at the LHC DipoleMagnet Ends and in the Interconnections, LHCProject Note 33[6] Cryogenic and Vacuum Sectorisation of the LHCArcs, M.Bona et Al, LHC Project Report 60[7] R.Saban et Al, Experiments and Cycling at the LHCprototype half-cell, this conference.[8] The Short Straight Sections for the LHC, M.Peyrotet Al, this conference

Mechanical design and layout of the LHC standard half-cell

Mechanical design and layout of the LHC standard half-cell

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