The design of the beam vacuum interconnect is described in this paper. Features include a novel RF bridge design to maximise lateral flexibility during cryostat Cold arcs of the LHC will consist of twin aperture dipole, quadrupole and corrector magnets in cryostats, operating at 1.9 K. Beam vacuum chambers, along with all connecting elements require flexible 'interconnects' between adjacent cryostats to allow for thermal and mechanical offsets foreseen during machine operation and alignment. In addition, the beam vacuum chambers contain perforated beam screens to intercept beam induced heat loads at an intermediate temperature. These must also be connected with low impedance RF bridges in the interconnect zones.alignment and so-called 'nested' bellows to minimise the required length of the assembly

Brunet, J C

Gröbner, Oswald

Kleimenok, V

Lepeule, P

Nikitin, I N

Reymermier, C

Schneider, G

Skoczen, Blazej

Veness, R J M

CERN Document Server

BEAM VACUUM INTERCONNECTS FOR THE LHC COLD ARCSR.Veness@, J-C. Brunet, O.Grobner, P. Lepeule, C.Reymermier, G. Schneider, B. Skoczen, CERNV.Kleimenov#, I.Nikitine#, IHEP Protvino                                                          @ Email: raymond.veness@cern.ch# Scientific Associate at CERNAbstractCold arcs of the LHC will consist of twin aperture dipole,quadrupole and corrector magnets in cryostats, operatingat 1.9 K. Beam vacuum chambers, along with allconnecting elements require flexible 'interconnects'between adjacent cryostats to allow for thermal andmechanical offsets foreseen during machine operationand alignment. In addition, the beam vacuum chamberscontain perforated beam screens to intercept beaminduced heat loads at an intermediate temperature. Thesemust also be connected with low impedance RF bridgesin the interconnect zones.The design of the beam vacuum interconnect is describedin this paper. Features include a novel RF bridge designto maximise lateral flexibility during cryostat alignmentand so-called 'nested' bellows to minimise the requiredlength of the assembly.1  INTRODUCTIONThe Large Hadron Collider (LHC) is a 7 TeV proton-proton collider under construction at CERN, planned forstart-up in 2005. Each of the 8 arcs of the machine willconsist of repeating half-cells comprising three twinaperture dipole magnets, each 15.2 m long and a 6 m long'short straight section' (SSS) containing lattice quadrupoleand corrector magnets. Each arc is a continuous 2.5 kmlong cryostat operating at 1.9 K.All of the links between these magnets required forelectrical power, cryogenics and beam vacuum (thesubject of this article) must be connected at roomtemperature before the cryostat can be closed and cooleddown. Thus, all these 'interconnects' must allow for thethermal contraction of the magnet cold masses. Due to thetime and cost involved, the magnets will be warmed-up asseldom as possible. This means that the interconnectsmust have the flexibility to allow for re-alignments of themachine when cold or warm.The beam vacuum system is described in detail elsewhere[1], however, a particular requirement for the LHC willbe for a beam screen [2] inside the vacuum chambers tointercept beam induced heat loads at an intermediatetemperature. These beam screens must also be connectedwith low impedance RF bridges between magnets.2  DESIGN REQUIREMENTSDesign offsets required for the beam vacuum interconnectcan be divided into those expected during normaloperation of the machine, transient offsets duringmachine cool-down and warm-up and 'exceptional' valuesdue to possible faults, e.g. in the cryogenic system. Thetotals comprise mechanical tolerances, alignmenttolerances and ground movements between alignmentperiods in addition to thermal expansion and contraction.For longitudinal offsets this sum is made arithmetically asthe causes are principally thermal expansions. However,the lateral offsets are due to alignment and assemblytolerances, so the decision has been taken to assume anormal distribution of random errors and to calculate theoffset for 3 standard deviations.Following a review of similar RF bridge designs, inparticular those for LEP [3], KEKB [4] and the SSC [5], aminimum contact force of 35g per finger under alloperation conditions was specified. An upper value forthe contact force of 70g was imposed on the slidingcontact to avoid the possibility of particulate generation.Beam screens are cooled by supercritical helium flowingin two cooling tubes welded to each screen. Since theheat loads in the two beam apertures are not the same, thetubes must be swapped over between beam lines at eachinterconnect zone.The envelope available for the interconnect is restricted inall directions. The length is limited to 480 mm betweendipole magnets to minimise the loss of magnetic lengtharound the machine. The radial space available forbellows and welding equipment is also limited by theother interconnect lines. In addition the beam screen mustbe passed through the 50 mm cold bore, which preventsthe pre-mounting of equipment on one end.The interconnects on either side of the SSS haveadditional requirements. On one side a pumping port isrequired on the beam lines. On the other side, the inter-magnet zone contains a beam position monitor on eachaperture.3  GENERAL DESCRIPTIONThe layout of a dipole-dipole beam vacuum interconnectis shown in figure 1 with the main features labelled. Eachinterconnect zone contains two such beam lines in0-7803-5573-3/99/$10.00@1999 IEEE. 1339Proceedings of the 1999 Particle Accelerator Conference, New York, 1999parallel. A line contains two bellows; an 'RF bellows'around the RF bridge which allows for the thermalcontraction and lateral offset of the magnet cold massesand a 'beam screen bellows' which absorbs differentialthermal expansion between beam screen and magnet coldmass during thermal transients, preventing damage to thecooling tube feedthroughs. The required offsets of theseelements both for normal machine operation (includingthermal cycles) and for exceptional conditions (includingre-alignment) are given in table 1. The 'tolerances'column contains static mechanical and alignment values.Table 1:  Design Offsets of the Interconnect FlexibleElements in mmTransverse(All designs)Longitudinal(Dipole-Dipole)Longitudinal(SSS-Dipole)Longitudinal(Dipole-SSS)LongitudinalTolerancesBeam Screen BellowsNormalExceptional00.53475829347588RF-Bellows: NormalExceptional245262363834571212RF-bridges: NormalExceptional245262363834571212Each line has four feedthroughs for the helium coolingtube cross-overs. There are fixed mechanical linksbetween both ends of the beam screens and theinterconnect to prevent stressing of these feedthroughs.As explained in section 2, the SSS interconnects havesome additional requirements. However, the sameprincipals, and a majority of common components areused in all three designs.Details of the other principal components are given in thefollowing sections.4  RF BRIDGE DESIGNThe RF bridge design concept was developed to avoid thehigh shear stress in contact fingers associated with thelarge lateral offsets required to re-align the machine whenwarm. The solution adopted was to provide a clearance,rather than a contact when warm and to use an change inthe chamber wall diameter to apply the contact fingerforce only when the magnets are cold. This principle isshown in figure 2.One RF bridge form was developed for all interconnects,fulfilling the 'normal' offset requirements of table 1 with aforce per finger as defined in section 2. It was alsodesigned so as not to plastify when under the 'exceptional'offsets from table 1.Both beam screen ends have a transition from theirstandard 'racetrack' section to a circular section for the RFbridge. The profile of the fingers was optimised tominimise the variation of contact force over a range ofoffsets and to avoid buckling during magnet warm-up.Figure 1: View of one Dipole-Dipole Beam VacuumInterconnect.1340Proceedings of the 1999 Particle Accelerator Conference, New York, 1999The baseline material for the fingers is 0.2 mm CuBe2sheet which is electro-etched, formed and plated with 30Pm of silver. It is then rolled to shape and electron beamwelded to a copper retaining ring. Figure 2: Section Through One Interconnect Line,Showing the RF bridge with the Beam Line at 293 K(Installed Position) and 20 K (Operating Position).5  BELLOWS DESIGNThe minimised space in the interconnection zone imposesstrong limitations on the length of both bellows units. TheRF bellows is a hydroformed U-type profile, with offsetsas described in table 1. The size and axial stiffness wereoptimised for mechanical stability (buckling) and fatigue(thermo-mechanical cycles). The optimisation procedureis mainly based on the EJMA standards [6], extended tolow temperature applications [7].For the beam screen bellows a very compact 'nestedbellows' has been designed for this application tocompensate for the large transient axial offsets whichoccur during cool-down and warm-up (see table 1). Thebellows profile, which has a large local curvature of theconvolutions, has been analysed for local buckling, globalmechanical stability and low cycle fatigue. Although thisunit gives little lateral flexibility, it can accept 10-15 %more stroke for a given unit length than a U-type bellows.The alternative would be to use edge-welded bellows,however, this would imply more than 30 km of additionalweld length in the 1.9 K system. A more detaileddescription of the bellows design is given in [8].6  MANUFACTURE AND ASSEMBLYThe LHC will require a total of 3292 beam vacuuminterconnect assemblies (1232 cryodipoles and 410 SSSseach with two apertures) which will be manufactured inEuropean industry. All components except bellows andRF bridges will be manufactured in 316LN stainless steel.This grade has been chosen to ensure a fully austeniticstructure at the operating temperatures, even afterwelding.In order to maximise the reliability of the beam vacuumsystem, all sub-components will be manufactured withoutwelds. Welds between components will be made byautomated orbital TIG welding without the use of fillermetal. The only exception will possibly be in the capillaryexits, where laser welding is being considered to avoidthe possibility of over-penetration into the capillariescontaining high-pressure helium.The design has been optimised to maximise the pre-assembly of the connections, either in a clean workshopor magnet assembly environment. Mechanical fixedpoints and RF bridges will be screwed in place inside thebeam vacuum and held by tab washers. This leaves onlythe welds connecting the adjacent magnets, i.e. the RFbellows ends and cooling line cross-overs, to beperformed in the tunnel. A number of intermediatequality controls and leak checks can therefore beperformed to minimise the risk of interference to the verytight LHC installation schedule.7 WORK IN PROGRESSManufacture of a pre-series of beam vacuuminterconnects is under way. These will be installed in theLHC prototype string, consisting of 6 dipoles and 2 SSS'sduring 1999. This will allow a thorough testing ofassembly and operation under cryogenic conditions.Extensive RF tests are under way to show that the bridgeconforms to requirements.8  ACKNOWLEDGEMENTSThe authors would like to thank F. Caspers, D. Chauville,G. Favre,  H. Kos and F. Ruggiero for helpful discussionsand their significant contributions to the design.9 REFERENCES[1] O.Grobner. The LHC Vacuum System. PAC 97, Vancouver, May1997[2] P.Cruikshank et.al. Mechanical Design of the LHC Beamscreen.PAC 97, Vancouver, May 1997.[3] Specification for the Standard Bellows Assemblies of the LEPMain-Ring Vacuum system. CERN/I-1318/LEP. February 1985.[4] K.Kanazawa, Y.Suetsugu. Development of a Bellows Assemblywith RF-Shield for KEKB. EPAC 96, Sitges, June 1996.[5] Design Report on the SSCL Prototype Synchrotron RadiationLiner System. SSCL note SSCL-SR-1224, September 1993.[6] Standards of the Expansion Joint Manufacturers Association Inc.,6th Edition 1993, Tarrytown, New York 10591. USA.[7] B.Skoczen. Preliminary Optimum Design of the Expansion Jointsfor the LHC Dipole-Dipole Interconnects. CERN LHC ProjectNote 34, 1996[8] B.Skoczen et.al. Development and Optimisation of NestedBellows for the LHC Beam Line interconnection.CERNLHC/CRI Report 1-19981341Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

Beam Vacuum Interconnects for the LHC Cold Arcs

Beam Vacuum Interconnects for the LHC Cold Arcs

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