A new system of coil compression has been designed which uses iron laminations to transfer the pressure from an outer shrink ring to the coil. The laminations are simple circular discs around the coil with the peculiarity that the rim is slightly eccentric as compared to the coil. Successive laminations are mounted with different angular orientations to oppose their eccentricities. The shrink ring pushed these discs inwards against the coil creating compression by a scissor movement. Tests on mechanical models of single as well as multiple aperture magnets have shown it to work as expected. The system has already successfully been applied to several corrector magnets for LHC. The advantages are low cost (suppression of the usual collars), increased coil compression in particular from cooling down, and field enhancement from having the iron close to the coil

Ijspeert, Albert

Salminen, J

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SUPERCONDUCTING COIL COMPRESSION BY SCISSOR LAMINATIONSAlbert Ijspeert, Jukka Salminen,  CERN, Geneva, SwitzerlandABSTRACTA new system of coil compression [1] has beendesigned which uses iron laminations to transfer thepressure from an outer shrink ring to the coil. Thelaminations are simple circular discs around the coil withthe peculiarity that the rim is slightly eccentric ascompared to the coil. Successive laminations aremounted with different angular orientations to opposetheir eccentricities. The shrink ring pushes these discsinwards against the coil creating compression by ascissor movement. Tests on mechanical models of singleas well as multiple aperture magnets have shown it towork as expected. The system has already successfullybeen applied to several corrector magnets for LHC. Theadvantages are low cost (suppression of the usualcollars), increased coil compression in particular fromcooling down, and field enhancement from having theiron close to the coil.1  INTRODUCTIONCoils of superconducting magnets for acceleratorsneed to be compressed to avoid wire movements whichwould cause quenches. The prestress is in generalobtained by collaring the coils under a press or byshrinking rings around the coils. The iron yoke can helpincrease the pre-compression if pushed inwards by anouter shell but at the expense of subdividing the yokeinto different segments. Scissor laminations offer asimple and economic alternative to regular collarsystems.2  PRINCIPLEThe principle of the scissor laminations is shown infigure 1. The yoke is made of a single type of ring-shaped laminations stacked around the magnet coil.They have the peculiarity that the circular periphery isdesigned to be slightly off-centre as compared to themagnet centre (~0.5 mm). These laminations aremounted with different angular orientations. The outershrinking shell presses on the wider side of eachlamination only and the latter transmits the compressiveforce to the coil. The wide side of each laminationsimply acts as a local spacer between the shrink ring andthe coil and the rest of the circular lamination is free. Ingeneral  the laminations are oriented per pair ofopposing eccentricity each pair compressing the coilthanks to the “scissor” effect. Cooling the magnet tocryogenic temperatures enhances the pre-compression ofthe coil if the shrink ring is of a strongly shrinkingFigure: 1 Principle of Scissor laminations. Shown issextupole corrector.(eccentricity strongly exaggerated)material such as aluminium or stainless steel. If desired,one can introduce keys in the laminations to stop thescissor action beyond a certain cooling temperature.Another way to stop it is to choose such an eccentricitythat the peripheries of the laminations line up at a welldefined cooling temperature. This can be used to buildup a circular compression on the laminations that stiffensthe yoke; the Lorentz forces that would tend to move thelaminations outwards must first overcome that pre-compression before any elastic movement can occur.The scissor system is readily applicable to magnets withmore than one aperture, as shown in sections 4, 5.3  SINGLE APERTURE APPLICATIONSThe method has first been applied to some smallsuperconducting corrector magnets for LHC. Originallytheir design consisted of  a coil shrunk inside analuminium shrink ring and this assembly was centredinside an iron yoke by means of keys [2]. This has nowbeen simplified by packing the coil inside scissorlaminations and shrinking the ring around this assembly.The advantages are that the iron has been brought closeto the coil boosting the field, that the shrink ring actsnow at a larger diameter and is therefore easier to shrinkand also yields more prestress when cooling down, andfinally that it suppresses the expensive machining ofkeyways. Three different corrector magnet prototypeshave been built along these lines, one sextupole and onedecapole by INDAR, Spain and another sextupole byCERN [3]. Table 1 gives their data. All three magnetsuse the same enamelled wire of 0.61x1.13 mm  metalcross section. The first magnets are made with vacuumTable:1 Parameters of the three corrector prototypes  .INDAR INDAR CERNsextupole decapole sextupoleMagnet length mm 150 75 150Nominal current A 383 346 625Critical current (4.2K) A 765 945 993Peak field at crit. current T 5.2 4.1 3.26Coil Inner Diameter mm 56 56 56Coil Outer Diameter mm 76 71 61Number of radial layers 8 6 2Number of turns 112 48 26Diameter over coil insulation mm 85.5 78.9 66Scissors lamination I.D. mm 86.2 79 66.2Scissors lamination O.D. mm 115 95.2 89.4Largest lamination width mm 15.1 8.4 12Smallest lamination width mm 13.8 7.7 11.2Lamination thickness mm 0.8 0.8 1Angular orientations 6 10 6Aluminium shrink ring I.D. mm 115.4 95.3 89.88Shrink ring O.D. mm 140 106 100Interference on diameter mm 0.35 0.44 0.12Average coil prestress 300 K MPa -35 -46 -31Expected coil prestress 4.2 K MPa -53 -47 -36impregnated coils with several layers of winding. Thelast magnet is made in a more economical way with atwo layer coil wetted with epoxy during the windingprocess. The coils of these magnets are protected with a2.5 mm thick glassfiber epoxy layer and the laminationsare directly placed around this insulation layer. Thedesign did not incorporate a mechanical stop. Thesextupole laminations were oriented in six angularpositions. The circular coil assembly  will thereforeundergo a slightly hexagonal deformation calculated tobe less than 0.03 mm on the radius which does notcompromise the field quality. If necessary one can shapethe opening in the laminations to compensate for this.This also implies that the coil is supported by localforces repeated every 4.8 or 6 mm over the length. Thedecapole laminations were oriented in ten angularpositions and the pitch between every tenth lamination is8 mm. The magnets have been trained at 4.2 K atCEDEX in Spain (Fig. 2). The first two magnets showone quench at or below the nominal current. The thirdmagnet starts training well above the nominal current.Compared to other designs with similar impregnatedcoils the training is similar or even quicker which showsthat the laminations give a successful pre-stress.00.20.40.60.810 5 10 15 20 25Quench numberQuench current / Critical current8-layer sextupole (INDAR)6-layer decapole  (INDAR)2-layer sextupole (CERN)Fig: 2 Training and re-training of the three correctorprototypes4  MULTI APERTURE MODEL  The method can readily be applied to magnets with amultitude of apertures. A dummy assembly of a twinaperture model has been built and tested to measure theapplication to multi aperture magnets (Fig.3). In contrastto the single aperture case, one cannot turn the scissorlamination to any desired orientation. However, if themultiple apertures are symmetric in the X and the Yplanes a single type of lamination can be used in fourdifferent positions: a starting position, an opposingposition obtained by turning the plate over 180 degrees,and a third and fourth position by turning the first twoplates upside-down. The angle of eccentricity can bechosen to 45 degrees as we did in this model for equalprestress in all four directions or it can be chosendifferently to obtain a dominant pre-stress in oneparticular direction. The plate thickness was  5 mm andaluminium dummy coils were used as well as analuminium shrink ring. The stresses as found fromdiameter measurements show (Fig.4) that the coilcompression increases strongly during the cooldown andthe alignment of the plates occurs at about 130 K.Beyond this temperature the shrinkage of the shrink ringis not acting on the coil anymore and the coil is slightlyFig:3 Mechanical model of twin aperture magnetde-compressing. Several heat cycles showed that theresults are repeatable. The pressure on the coil beforeand after a heat cycle are practically identical (+/-10%) which means that frictional effects are verysmall. The model has several times successfully beendisassembled and re-assembled, once using a real coil.02000400060008000100001200014000Free(300 K)77KClamping force (N/mm)Shrink ringDummy coilsIf no mech. stopIf no mech. stopFig:4 Clamping force in the twin aperture model(corresponding to coil stress of 40 MPa at 300K and 60MPa at 77K for chosen interference)5  TWIN APERTURE DESIGNAs a possible application to a multi aperture yoke atentative design has been made for the LHC main dipole.Two questions turned up: Can one slip the laminationsover the not yet collared coil which has dimensions afew millimetres larger than the compressed dimensionsand how close can one bring the iron to the coil withoutlosing the field homogeneity due to saturation of the ironat the very high field of 8.4 Tesla? To satisfy thesedemands we use the fact that each lamination only needsto touch the coil over the 90 degree angle where thepressure is exerted. The shape of the rest of the hole inthe lamination can be at a larger radius to obtain the playnecessary for the assembly. Magnetic calculationsshowed that the multipoles caused by local saturation inthe iron can be reduced to less than 10-4 of the main fieldover the whole range of excitation by shaping the hole ofthe iron lamination elliptical and by adding holes to thelamination in the correct positions (Fig.5). Thecalculations showed that the field homogeneity is notinfluenced by the iron quality at least less than in thepresent design. A cost estimation showed that thesuppression of the usual collars may lead to aninteresting cost saving on such a magnet. The assemblycan be made by securing packages of laminations in theopen position using keys, slip the packages over thecoils, take the keys out, pre-compress the coilsmoderately under a press and maintain the compressedposition introducing corresponding keys. The skin canthen be welded around the magnet and the keyways aresuch that they accept the additional compression duringcooldown loosening the keys.6  CONCLUSIONSIt has been shown that the scissor laminations form apractical and elegant way to transmit the compressiveforce from the shrink ring to the coil body. The yoke isbuilt with a single type of lamination and does not needa subdivision in segments. The system allows to chosefreely the desired direction of the prestress on the coil byorienting the laminations accordingly. It can replace theusual collars leading to non-negligible savings. Itpermits to bring the iron close to the coil enhancing themagnetic field and allowing to reduce the outer diameterof the yoke. Finally it allows to “collar” the coil at arelatively low compression because an additionalcompression can be obtained from the differentialcontractions during the cooldown thus avoiding damageto the insulation by over-stress at room temperature andreducing the size of the necessary press. The system hasbeen tested on mechanical models and has alreadysuccessfully been applied to several types of correctormagnets.7  ACKNOWLEDGEMENTSWe would like to thank G. Laurent, H. Meyer and G.Trinquart for the design  work, J. Mazet, J. Perez and D.Regin for the development work and the fabrication ofthe CERN prototype. Thanks go to J. Etxeandia and J.Garcia from INDAR Spain for the fabrication of the twoother prototypes guided by E. Baynham, B. Coombs andD. Evans from DRAL England. Finally thanks go to L.Garcia Tabares, M. Bajko, J. Calero and M. Saenz deBuruaga of CEDEX Spain for the training tests at 4.2 K.REFERENCES[1] A. Ijspeert, ‘Compressing coils of single andmultiple aperture superconducting magnets with“Scissors” laminations: principles and results oftests on mechanical models’, LHC Note 292, CERN,October 1994.[2] D. Baynham, R, Coombs, A. Ijspeert, R. Perin,‘Design of superconducting corrector magnets forLHC’, IEEE Trans. on Magn., Vol.30, No 4,pp1823-1826, July 1994.[3] J. Salminen, A. Ijspeert, ‘Superconducting sextupolecorrector magnet for the LHC main dipoles’, thisconference.Fig:5 Elliptic lamination holes for field homogeneity

Superconducting Coil Compression by Scissor Laminations

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