The Next European Dipole (NED) activity is developing a high-performance Nb$_{3}$Sn wire (aiming at a non-copper critical current density of 1500 A/mm2 at 4.2 K and 15 T), within the framework of the Coordinated Accelerator Research in Europe (CARE) project. This activity is expected to lead to the fabrication of a large aperture, high field dipole magnet. In preparation for this phase, a Working Group on Magnet Design and Optimization (MDO) has been established to propose an optimal design. Other parallel Work Packages are concentrating on relevant topics, such as quench propagation simulation, innovative insulation techniques, and heat transfer measurements. In a first stage, the MDO Working Group has selected a number of coil configurations to be studied, together with salient parameters and features to be considered during the evaluation: the field quality, the superconductor efficiency, the conductor peak field, the stored magnetic energy, the Lorentz Forces and the fabrication difficulties. 2-D magnetic calculations have been performed, and the results of this comparison between the different topologies are presented in this paper. The 2-D mechanical computations are ongoing and the final stage will be 3-D magnetic and mechanical studies

Devred, Arnaud

Felice, H

Fessia, Paolo

Loveridge, P W

Regis, Federico

Rochford, J

Sanz, S

Schwerg, Nikolai

Toral, F

Védrine, P

Völlinger, Christine

CERN Document Server

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 17, NO. 2, JUNE 2007 1117Comparison of 2-D Magnetic Designs ofSelected Coil Configurations for the NextEuropean Dipole (NED)F. Toral, A. Devred, H. Felice, P. Fessia, P. Loveridge, F. Regis, J. Rochford, S. Sanz, N. Schwerg,P. Vedrine, and C. VollingerAbstract—The Next European Dipole (NED) activity is devel-oping a high-performance Nb3Sn wire (aiming at a non-coppercritical current density of 1500A mm2 at 4.2 K and 15 T), withinthe framework of the Coordinated Accelerator Research in Eu-rope (CARE) project. This activity is expected to lead to the fab-rication of a large aperture, high field dipole magnet. In prepara-tion for this phase, a Working Group on Magnet Design and Op-timization (MDO) has been established to propose an optimal de-sign. Other parallel Work Packages are concentrating on relevanttopics, such as quench propagation simulation, innovative insula-tion techniques, and heat transfer measurements. In a first stage,the MDO Working Group has selected a number of coil configura-tions to be studied, together with salient parameters and featuresto be considered during the evaluation: the field quality, the su-perconductor efficiency, the conductor peak field, the stored mag-netic energy, the Lorentz Forces and the fabrication difficulties.2-D magnetic calculations have been performed, and the results ofthis comparison between the different topologies are presented inthis paper. The 2-D mechanical computations are ongoing and thefinal stage will be 3-D magnetic and mechanical studies.Index Terms—Accelerator magnets, electromagnetic analysis,electromagnetic fields, superconducting magnets.I. INTRODUCTIONAWorking Group on Magnet Design and Optimizationhas been established within the NED collaboration [1],with participants from four Institutes: CEA/Saclay, CERN,CIEMAT and RAL. This Working Group has been chargedwith addressing the following questions:1) How far can we push the conventional, layer, designin the aperture vs. central field parameter space (especially,when relying on strain-sensitive conductors)?2) What are the most efficient alternatives in terms of perfor-mances, manufacturability and costs?Manuscript received August 29, 2006. This work was supported in part by theEuropean Community-Research Infrastructure Activity under the FP6 “Struc-turing the European Research Area” program (CARE contract RII3-CT-2003-506395).F. Toral and S. Sanz are with CIEMAT, Madrid, 28040 Spain (e-mail: fer-nando.toral@ciemat.es; s.sanz@ciemat.es).A. Devred is with the CEA, Saclay, France. He is also with the CERN,Geneva, Switzerland.H. Felice and P. Vedrine are with CEA, Saclay, France (e-mail: arnaud.de-vred@cern.ch; hfelice@dapnia.cea.fr; vedrine@dapnia.cea.fr).P. Fessia, F. Regis, N. Schwerg, and C. Vollinger are with CERN, Geneva,Switzerland (e-mail: arnaud.devred@cern.ch; paolo.fessia@cern.ch; federico.regis@cern.ch; nikolai.schwerg@cern.ch; christine.vollinger@cern.ch).P. Loveridge and J. Rochford are with RAL, Abingdon, England (e-mail:P.Loveridge@rl.ac.uk; J.H.Rochford@rl.ac.uk).Digital Object Identifier 10.1109/TASC.2007.898418A number of configurations have been selected (see Fig. 1):slotted , slotted motor-type, toroidal motor-type, andcommon coil (studied at CIEMAT); ellipse-type (studied atCEA); double helix dipole (studied at RAL, not included in thispaper as it is a pure 3-D problem); and, finally, the conventionallayered (studied at CERN and RAL).Table I shows the common starting parameters for all thecoil configurations. The strand is the one defined in the NEDcable specifications, whose diameter is 1.25 mm. A wide cableis useful for a high field magnet, as the self-inductance is lower,and the engineering current density increases. The upper limitis 40 strands in each cable, given by the available cabling ma-chines in Europe. The cable dimensions are derived from em-pirical formulae according to previous experiences [2]. All thedesigns use rectangular Rutherford-type cable, except the lay-ered , which uses a slightly keystoned one.II. 2-D MAGNETIC CALCULATIONS FOR SELECTED DESIGNSA. Layered DesignThe conventional layered configuration has been opti-mized at CERN [3]. Table II reports outstanding figures of meritof the 88 mm aperture dipole with round iron yoke. Special carehas been taken to place the conductors in the radial directionto ease winding and mechanical support. On the other hand, aprevious study [4] concluded that stresses on the coil mid-planewere above 150 MPa for the 130 and 160 mm aperture dipoles,which is excessive.As part of the optimization, an elliptical iron yoke is proposedto decrease the large variation of along the load line. Thechoice of an elliptical iron has one slight drawback comparedto a round iron: to maintain the same bore field, it requires afew percent increase in current. In addition, ferromagnetic shimshave been introduced in the coil cross-section to compensatethe effect of the persistent magnetization currents. Meanwhile,the harmonic optimization has also been done with Opera2D atRAL, using a different optimization algorithm. Results are ingood agreement with those obtained using Roxie at CERN.In short, the advantages of this topology are the low peak-to-bore-field ratio, the good superconductor efficiency, the goodmagnetic field quality, the low stored magnetic energy and thesmall overall magnet size. The only disadvantage is the highcoil mid-plane stress. One must notice that the stresses havebeen computed as the sum of the block forces averaged overthe corresponding cable dimension. Therefore, the actual localpressure can be higher, typically up to 20%.1051-8223/$25.00 © 2007 IEEE1118 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 17, NO. 2, JUNE 2007Fig. 1. Magnetic field (T) map in the studied coil configurations. From left to right, up to down: Layered cos , ellipse-type, slotted motor-type, slotted cos ,common coil and toroidal motor-type design.TABLE ICOMMON STARTING PARAMETERS FOR THE MAGNET OPTIMIZATIONB. Ellipse-Type DesignIt is well-known that an elliptical coil with a uniform cur-rent density creates a uniform dipole field in a round aperture.An ellipse-type dipole has been designed at CEA/Saclay (seeFig. 1) [5]. Tables II, III, and IV show the salient results for the88, 130 and 160 mm apertures, respectively, and a comparisonwith other arrangements. The peak-to-bore-field is low and thefield homogeneity is fine, but the stored magnetic energy and,therefore, the self-inductance per unit length, are greater thanones, because the upper layers conductors have a poorefficiency. The horizontal component of the Lorentz forces ishuge and, besides, an internal support is necessary to prevent thecoils from bending inwardly, which decreases the useful aper-ture for a given inner coil radius. Some further investigations areneeded to check the feasibility of the non-planar coil ends (bothmechanical 3-D computations and winding technique).C. Slotted Motor-Type DesignThe conductors are now placed in slots cut out in stainlesssteel collars, surrounded by a round iron yoke. It resembles thewinding of a conventional electrical machine. From the point ofview of fabrication and mechanical analysis, the coil end designis challenging, although some small NbTi models have beenmade some time ago at CEA/Saclay [6]. The cable bending ra-dius can be small, and some bending is forced over the cablenarrow face. However, it is a very efficient magnet, similar tothe layered one, but the mid-plane stresses are lower, asthe collar noses help to withstand the Lorentz forces of the upperblocks.D. Slotted DesignThis design is a particular case of the conventional de-sign, but the inner and outer layer spacers have the same angularpositions. Therefore, both spacers can be clamped together tothe collars, or even coil winding can be done directly onto thecollars, as for the slotted motor-type design. The cable is rect-angular, to better fit both layers, but a keystoned cable wouldenable a better placement of the turns in the radial direction.In any case, it seems a simple way to match the nice layeredperformance with affordable stresses on mid-plane ca-bles. It is worth studying this design from the mechanical pointof view, and to analyze the winding techniques as well.E. Common Coil DesignAn inherent problem to the common coil configuration is thatsome of the most effective ampere-turns—those close to theTORAL et al.: 2-D MAGNETIC DESIGNS OF SELECTED COIL CONFIGURATIONS FOR THE NED 1119TABLE IICOMPARISON OF 88 mm APERTURE DESIGNSTABLE IIICOMPARISON OF 130 mm APERTURE DESIGNSbeam tube—must be replaced by spacers to enhance bore fieldquality. The coil geometry is a hybrid between two theoreticalcurrent distributions which provide a uniform field: an infinitelylong current sheet and a cosine-type winding. The multipole1120 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 17, NO. 2, JUNE 2007TABLE IVCOMPARISON OF 160 mm APERTURE DESIGNSFig. 2. Toroidal motor-type assembly with iron yoke split as a number of solidblocks. Additional anti-dipole coils are longer than dipole ones.is not so low, as we have limited the number of spacers for thesake of simplicity and ease of fabrication. The stored magneticenergy is high, and the iron size is large. Obviously, the mainadvantage of this magnet is that coil winding is straightforward.Another specific feature of this design is the two-in-one ironyoke. The distance between both apertures ranges from 600 to700 mm (increasing with the aperture diameter) to weaken thecross-talk. This is also the reason for the coil up-down asym-metry. Furthermore, the even multipoles arise fromthe cross-talk. The peak-to-bore-field ratio is poor compared toother designs, and it would be even worse without the verticaliron poles (see Fig. 1). These poles make the assembly more dif-ficult, but the aforementioned ratio would be about 5% higherwithout them [7], because the field between the left and right coilblocks is even higher than in the aperture itself, as the closestconductors to aperture have been replaced by spacers. The ironpoles are deeply saturated, but they are still able to change thefield lines direction.F. Toroidal Motor-Type DesignThis magnet design resembles an iron-cored toroid. In a pre-liminary design [7], the large additional anti-dipole coils werenot present (see Fig. 2). The main advantages were the sim-plicity of the coil geometry and the low mid-plane coil stress.The most outstanding drawbacks were the strong fringe fieldsand the high number of turns, due to the anti-dipole field createdby the outermost coil blocks. Therefore, additional coil blockshas been included at the outer radius, but with opposite currentpolarity with respect to the adjacent coil blocks. It addressesboth issues: the overall number of turns is now half than before,as it cancels the anti-dipole field created by the outer coil blocks,and the fringe field is also reduced, because the magnetic mo-ment is getting lower. However, the fringe field is still high inthe vicinity of the coils and cannot be reduced by means of aniron screen, which even has the deleterious effect of enhancing itby providing a parallel flux path. These additional “anti-dipole”coils can be wound with a lower critical current density cable.The magnet assembly becomes complex, as the iron is splitinto a number of solid blocks, and the coil end design is still achallenging problem due to the different coil lengths. Finally,one must point out the high stored magnetic energy due to thelarge high field region, and that the peak-to-bore-field ratio isnot as low as in other designs. However, the Lorentz forcesare smaller. The field quality is not completely optimized as itcannot be done automatically, due to the novel geometry. TheTORAL et al.: 2-D MAGNETIC DESIGNS OF SELECTED COIL CONFIGURATIONS FOR THE NED 1121TABLE VCOMPARISON OF 88 mm APERTURE GRADED DESIGNScross section of the slotted motor-type dipole is the result ofeliminating the outermost coil blocks.III. 2-D MAGNETIC CALCULATIONS FOR GRADED DESIGNSSome of the selected configurations allow the use of differentsize or “graded” cables, as some of the coil blocks see a lowermagnetic field and, therefore, the current density can be in-creased in the superconductor while keeping a low workingpoint on the load line. The superconductor saving can reachup to 25% for a given bore field. Another possibility is to usea cable with the same size, but a lower critical current den-sity. Table V shows the main features of the layered ,the slotted (see Fig. 3) and the common coil designs.The layered design has an elliptical iron yoke, while theslotted one is round. All designs rely on two cable types:a large one with 40 strands, and a smaller one with 26.IV. CONCLUSIONSA number of alternative dipole magnet designs for very highfields and large apertures have been studied by the WorkingGroup on Magnet Design and Optimization, within the NEDframework. Common starting parameters and figures of meritFig. 3. Graded slotted cos  magnet.for a fair comparison have been identified. This paper summa-rizes the 2-D magnetic field calculations. For the 88 mm aper-ture, the conventional layered design is still the bestone. However, for large apertures (130 160 mm), the coilmid-plane stresses become too high for this topology. The mostpromising configuration for large apertures is the slotteddesign, as the others are less efficient and have obvious fabrica-tion issues. The next step of the analysis, that is, the 2-D me-chanical calculation, becomes very crucial, since the Lorentzforces are huge in all cases. A first estimate of the stresses hasbeen done by averaging the forces on the broad cable face, butnumerical computations are necessary to determine the actuallocal pressure, and the feasibility of the clamping structure. Fi-nally, graded designs have been also studied whenever possible,as they allow a superconductor saving by the use of a low crit-ical current density cable in the low field areas of the coil.REFERENCES[1] A. Devred et al., “Status of the Next European Dipole (NED) activityof the coordinated accelerator research in Europe (CARE) project,” Su-perconductor Science and Tech., vol. 19, pp. 67–83, March 2006.[2] R. M. Scanlan, D. R. Dietderich, and H. C. Higley, “Development of ca-bling techniques for Nb3Sn and Bi-2212 wires,” in Proc. Workshop onAccelerator Magnet Superconductors, Archamps, France, March 2004.[3] N. Schwerg et al., 2D Magnetic Design and Optimization of a 88 mmAperture 15 T Dipole for NED to appear in this Proceedings.[4] D. Leroy and O. aaaVincent-Viry, Preliminary Magnetic Designs forLarge-Bore and High-Field Dipole Magnets Geneva, CERN ReportCERN/AT/2004-22, 2004.[5] H. Félice, P. Védrine, and A. Mailfert, “Magnetic and Mechanical De-sign of a 130 mm Aperture Nb Sn Dipole Magnet,” to appear in thisProceedings.[6] A. Patoux, J. Perot, and J. M. Rifflet, “Test of new accelerator super-conducting dipoles suitable for high precision field,” IEEE Trans. onNuclear Science, vol. NS-30, no. 4, August 1983.[7] F. Toral, “Progress in comparison of different high field magnet designsfor NED,” in Proc. Workshop on Magnet Design and Optimization,CE7RN, April 2006.

Comparison of 2-D Magnetic Designs of Selected Coil Configurations for the Next European Dipole (NED)

Comparison of 2-D Magnetic Designs of Selected Coil Configurations for the Next European Dipole (NED)

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