Ambrosio, G

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DESIGN AND DEVELOPMENT OF Nb3Sn SINGLE-LAYERCOMMON COIL DIPOLE MAGNET FOR VLHC*G. Ambrosio†, N. Andreev, E. Barzi, P. Bauer, D. Chichili, K. Ewald, S. Feher, L. Imbasciati,V.V. Kashikhin, S.W. Kim, P. Limon, I. Novitski, J.M. Rey, S. Yadav, R. Yamada, A.V. Zlobin,FNAL, Batavia, IL 60510, USAR. Scanlan, LBNL, Berkeley, CA 94720, USAAbstractCommon coil dipole magnets based on Nb3Snconductor and the react-and-wind technology are apromising option for the next generation of hadroncolliders. The react-and-wind technology has potentialcost benefits related to less expensive cable insulation,structural materials and magnet fabrication. A commoncoil design allows the use of pre-reacted Nb3Snsuperconductor with low critical current degradation afterbending. Fermilab in collaboration with LBNL isinvolved in the development of a single-layer common-coil dipole magnet with maximum field of 11 T and 40-50mm aperture for a future VLHC. This paper reports thecurrent magnetic and mechanical designs of the dipolemodel, the magnet parameters and the status of theprogram.1 INTRODUCTIONThe design study of a staged Very Large HadronCollider (VLHC) has been recently accomplished atFermilab [1]. The goal is to build a 40 TeV collider (stage1), in a 233 km long tunnel, that will become, in a secondphase, the injector to a 175 TeV collider (stage 2)installed in the same tunnel. The stage 2 collider willrequire arc dipoles with a nominal operating field of 10 Tin a bore aperture ≥ 40 mm, operating at 4.5 K. The studyforesees the use of single-layer common coil dipoles withcoils made of Nb3Sn superconductor. The use of pre-reacted Nb3Sn allows cost savings in the insulatingmaterials, in the structural materials and in the fabrication.A single-layer design has a lower inductance than similartwo-layer designs and allows the adoption of longermagnets with the same maximum voltage during aquench. The design developed at Fermilab, introducingcollars reinforced by bridges, was presented in [2,3]. Thecurrent magnet design, the assembly procedure and thestatus of the program are presented. Quench protectionissues are discussed in [4].2 MAGNETIC DESIGNA 3D view of the single-layer common coil dipolemagnet for VLHC is shown in Figure 1. The cable, 22mm wide, consists of 60 strands with 0.7 mm diameter.The coil and yoke geometry were re-optimized withrespect to the previous version [2], in order to provide theFigure 1: 3D view of the single-layer common coildipole magnet.required field quality within 0-11 T field range with amodified collar structure. In particular, the number ofturns per coil was increased from 56 to 58 in order toincrease the maximum field and optimize the block sizes.The large 90 mm diameter holes in the magnetmidplane were introduced for correction of the skewquadrupole caused by the iron yoke saturation.Simultaneously they serve as helium and electrical buschannels. The small holes close to the coil are used forcorrection of the sextupole component. Optimizing thehole size and position along with bore separation distanceand yoke outer diameter made it possible to restrict skewquadrupole and sextupole deviations in 0-11 T field range.The main magnet parameters are listed in Table 1. Acritical current density (Jc) of 2400 A/mm2 at 12 T and 4.5Table 1: Magnet parametersParameter Unit ValueAperture mm 40Bore separation distance mm 290Number of turns per coil 18+22+18Conductor area per aperture cm2 27.7Bore quench field T 10.85Quench current kA 25.6Transfer function T/kA 0.424Inductance per aperture  @ 11 T mH/m 1.475Stored energy per apert. @ 11 T kJ/m 496___________________________________________* Work supported by the U.S. Department of Energy.†giorgioa@fnal.gov0-7803-7191-7/01/$10.00 ©2001 IEEE. 3409Proceedings of the 2001 Particle Accelerator Conference, Chicago-0.6-0.300.30.60.90 2 4 6 8 10 12B0, Ta2, b3, 10-4a2b3Figure 2: Field multipoles as function of bore field.K was used for quench field calculation, assuming astrand Jc equal to 3000 A/mm2, minus 20% for totaldegradation. Copper to non-copper ratio was 1. Themaximum field of 10.85 T meets current VLHCrequirements [1] providing 8.5% critical current marginwhich could be further augmented with an increase of thecritical current density of the Nb3Sn superconductor.Table 2 summarizes geometrical and random fieldmultipoles, calculated at low field and Figure 2 showslow-order multipole deviations due to the yoke saturationeffect. The estimated value of the persistent current effectat 1 T field is –1.5⋅10-4 of sextupole [5], which isacceptable and does not require any special correction.Table 2: Relative geometrical field multipoles @ 1 cm(10-4 unit). Random block displacements within ± 50 µm.SystematicOrdern an bnRMSσan,bn2 -0.0058 - 1.0403 - -0.0008 0.3604 -0.0034 - 0.1415 - 0.0011 0.0466 0.0002 - 0.0177 - 0.0031 0.0068 0.0120 - 0.0039 - -0.0282 0.00210 0.0003 - 0.0003 MECHANICAL DESIGNThe large magnetic forces at maximum field (Fx=2949kN/m, Fy=-1466 kN/m per aperture quadrant) require astrong mechanical structure in order to avoid an excessivestress in the coils and to restrain coil displacements withinthe acceptable limit for field quality.  The laminatedstainless steel collars reinforced by horizontal bridges,and a 10 mm thick stainless steel skin welded under apress are used for this purpose.  The current mechanicalstructure with modified (with respect to the previousversion [6]) collar laminations is shown in Figure 3. Thecollar and bridge laminations form a frame withrectangular windows for the coil blocks and round holesfor the beam pipe and the cooling channels. This allows aredistribution of pre-compression during coil assemblyand reduces the risk of damaging the brittle Nb3Sn.During operation the frame intercepts a significant part ofthe horizontal component of the magnetic forces, reducingthe force transferred to the yoke and skin. It also preventsan accumulation of the vertical component of themagnetic forces from one coil block to another.The stress analysis [3] confirmed an acceptablemaximum stress in the coils at all conditions (70 MPaafter cooling down and 130 MPa at 11 T).  All thestructural elements operate in an elastic regime, and thecoil displacements in the operating field range do notexceed 65 µm.ALIGNMENT KEYCOOLING CHANNELNb  Sn  COILIRON YOKE584 mm3COLLARSS SKINCORRECTION HOLEBRIDGES40 mm APERTURECOLLARFigure 3: Magnet cross section in the straight sectionAlso in the ends each coil is divided into three blocks asis shown in Figure 4 for the return (non-lead) end. Eachcoil block is wound into a U-shaped groove in thestainless steel blocks that contain the horizontalcomponent of the magnetic forces. The end blocks arereinforced by side plates in order to increase the stiffnessof the end section.Figure 4: End parts (return end).3410Proceedings of the 2001 Particle Accelerator Conference, Chicago4 ASSEMBLY PROCEDURE(i) Heat treatment and insulation: The cable for eachcoil is wound on a single-turn metallic spool resulting in apancake-like winding. A mica-glass tape is woundtogether with the cable, in order to prevent sinteringduring the heat treatment. After the heat treatment,performed in an Argon atmosphere inside a retort, thecable is insulated using a 50 µm thick 12.5 mm wide E-glass tape with 50% overlap. Horizontal rollers set veryclose to the insulation application point prevent strandsfrom popping out and protect the reacted cable. Duringinsulation and winding the cable is straightened but neverbent in the direction opposite to the bending during theheat treatment. The bending strain is minimized by usinga reaction spool with a diameter that is twice theminimum diameter in the coil ends.(ii) Winding and collaring: Coils are woundsimultaneously into the collars in order to insert thebridges during winding. The collars consist of 2 mm thicklaminations, spot welded to form 10 cm thick packages.Two independent tensioners are used to apply a tension of15 kg to each cable. The ground insulation consists of twolayers of Kapton with a layer of fiberglass in between.The layers are shaped in such a way that the fiberglasslayer provides a channel for epoxy during impregnation.Mold release is applied on the Kapton layers in order toprevent bonding of the coils to the collars. After windingthe innermost block of both coils, the collaring fixture isassembled around the coils. The first bridge and the firstend part are inserted and the collaring fixture is used tolock them in the nominal position by means of keys. Thefixture is then removed in order to wind the second block.The second bridge, the outermost part of the collars andthe other end parts are inserted following the sameprocedure.(iii) Impregnation and yoking: The collared coils arevacuum impregnated with CTD-101K epoxy.Impregnation is performed in a bath by slowly filling abox, slightly inclined, which contains the coils. Aftercleaning the collared coils of extra epoxy the yoke isassembled and the 10 mm thick skin is welded under apress.5 STATUS OF PROGRAM AND PLANSThe first part of the R&D was centered on the cabledevelopment for the react-and-wind technology, andresulted in the choice of 0.7 mm wire, in the use ofsynthetic oil during heat treatment, and in the rejection ofa stainless steel core (total degradation ≤ 20%) [7].Further cable R&D was focused on the 60 strand cable.ITER leftover wire was used to optimize cable design andproduction and 450 meters of cable were made in acontinuous run without any problem. Wire from OxfordSuperconducting Technologies was then used and thecabling degradation was measured to be about 6%. Aracetrack magnet consisting of two flat coils with a 5 mmseparation was assembled and is being tested. Its designincludes many features of the single-layer common coil:reacted and wound Nb3Sn cable (wire fromIntermagnetics General Corporation), winding of coilsinside the mechanical structure and in-situ impregnation.The engineering design of the first short common coilmodel and its tooling has been completed andprocurement is on course. The fabrication will start thissummer. Meanwhile a mechanical model, using cablemade with ITER wire, is being assembled and tested. Itconsists of two different collar designs, and will be usedto choose the best collar (for rigidity and simplicity ofassembly), to check assembly procedures (excluding endparts) and to compare stress distribution (after assemblyand cooling down at 4.2 K) with finite element analyses.A practice winding, using the parts procured for the firstshort model and a cable made with ITER wire, will beassembled (but not impregnated) in order to test windingand assembly procedures including the ends.The present design has inherent capabilities for furtherimprovement: bore size may be increased up to 50 mm byslightly bending the cables of the outer blocks around thebeam pipe, and bore separation could be reduced,decreasing magnet outer diameter and weight. These andother issues will be addressed during the short modelR&D phase.6 CONCLUSIONThe design of a single-layer common coil dipole usingNb3Sn conductor and the react-and-wind technology wascompleted at Fermilab. This design meets the currentVLHC requirements (10 T nominal field, aperture ≥ 40mm) and has inherent capabilities for furtherimprovement. An intense R&D program is underway andthe production of the first short model is close to start.REFERENCE[1] “VLHC design study report”, Fermilab, May 2001.[2] V.V. Kashikhin and A.V. Zlobin, “Magnetic designsof 2-in-1 Nb3Sn dipole magnets for VLHC”, IEEETrans. on Applied Superconductivity, Vol. 11, No. 1,2176, March 2001.[3] I. Novitski et al., “Design and Mechanical Analysis ofa Single-Layer Common Coil Dipole for VLHC”,IEEE Trans. on Applied Superconductivity, Vol. 11,No. 1, 2276, March 2001.[4] L. Imbasciati et al., “Quench protection of high fieldNb3Sn magnets for VLHC” this conference.[5] V.V. Kashikhin and A.V. Zlobin, “Correction of thepersistent current effect in Nb3Sn dipole magnets”IEEE Trans. on Applied Superconductivity, Vol. 11,No. 1, 2058, March 2001.[6] G. Ambrosio et al., “Development of React & WindCommon Coil Dipoles for VLHC” IEEE Trans. onApplied Supercond, Vol. 11, No. 1, 2172, March 2001.[7] P. Bauer et al., “Fabrication and Testing ofRutherford-type Cables for React and WindAccelerator Magnets”, IEEE Trans. on AppliedSuperconductivity, Vol. 11, No. 1, 2457, March 2001.3411Proceedings of the 2001 Particle Accelerator Conference, Chicago

Design and Development of \Nb_{3}Sn Single-Layer Common Coil Dipole Magnet for VLHC

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