A common coil design for high field 2-in-1 accelerator magnets has been previously presented as a "conductor-friendly" option for high field magnets applicable for a Very Large Hadron Collider. This paper presents the mechanical design for a 14 tesla 2-in-1 dipole based on the common coil design approach. The magnet will use a high current density Nb/sub 3/Sn conductor. The design addresses mechanical issues particular to the common coil geometry: horizontal support against coil edges, vertical preload on coil faces, end loading and support, and coil stresses and strains. The magnet is the second in a series of racetrack coil magnets that will provide experimental verification of the common coil design approach. (9 refs)

Caspi, S

Chow, K

Dietderich, D R

Gourlay, S A

Gupta, R

McInturff, A

Millos, G

Scanlan, R M

A common coil design for high field 2-in-1 accelerator magnets has been previously presented as a 'conductor-friendly' option for high field magnets applicable for a Very Large Hadron Collider. This paper presents the mechanical design for a 14 tesla 2-in-1 dipole based on the common coil design approach. The magnet will use a high current density Nb{sub 3}Sn conductor. The design addresses mechanical issues particular to the common coil geometry: horizontal support against coil edges, vertical preload on coil faces, end loading and support, and coil stresses and strains. The magnet is the second in a series of racetrack coil magnets that will provide experimental verification of the common coil design approach

Caspi, S.

Chow, K.

Dietderich, D.

Gourlay, S.

Gupta, R.

McInturff, A.

Millos, G.

Scanlan, R.

UNT Digital Library

l LBNL #43481 SC MAG #668 MECHANICAL DESIGN OF A mGH FIELD COMMON COIL MAGNET· S. Caspi, K. Chow', D. Dietderich, S. Gourlay, R. Gupta, A. McInturff, G. Millos, R. Scanlan, Lawrence Berkeley National Laboratory, Berkeley, CA Abstract A common coil design for high field 2-in-1 accelerator magnets has been previously presented as a "conductor-friendly" option for high field magnets applicable for a Very Large Hadron Collider [1,2). This paper presents the mechanical design for a 14 tesla 2-in-1 dipole based on the common coil design approach. The magnet will use a high current density Nb}Sn conductor. The design addresses mechanical issues particular to the common coil geometry: horizontal support against coil edges, vertical preload on coil faces, end loading and support, and coil stresses and strains. The magnet is the second in a series of racetrack coil magnets that will provide experimental verification of the common coil design approach. 1 INTRODUCTION The superconducting magnet program at Lawrence Berkeley National Laboratory (LBNL) is focused on development of high field accelerator magnets using brittle superconductors. The current designs follow a "common coil" approach wherein a pair of racetrack coils is shared between two apertures to produce opposing fields in each aperture [I). A 6 tesla magnet Wire Wrap Iron Yoke Iron Yoke Side Rails Outer Layers Inner Layer Iron Inserts _w..-'L __ _ ____ _ Figure 1: Basic components in magnet cross section (only one quadrant shown). .Email: kpchow@lbl.gov Work supported by the U.S. Department of Energy under contract No. DE·AD03·76SFOOO98. using Nb,Sn conductor has been built and tested as a first step towards the ultimate goal of attaining IS T accelerator quality dipoles [3.4). This paper describes the mechanical design of a second magnet in the common coil series: a 14 T Nb,Sn magnet of limited field quality (field quality will be addressed in the next magnet). A preliminary design has been previously described elsewhere (2). The design has since been refined to allow a modular and flexible R&D program for systematic testing of different issues related to high field magnets that use brittle superconductor. The design also addresses several mechanical issues particular to a rugh field common coil geometry: horizontal support on coil edges, vertical preload on coil faces, end support, and coil stresses and strains. Figure 1 shows the basic components in a cross section of the magnet straight section. 2 MAGNETIC DESIGN The primary purpose of this magnet is to demonstrate the viability of high field dipoles based on the common coil geometry in a mechanical structure suitable for high field accelerator magnets. As such, field quality issues will not be addressed in this magnet but will be fully addressed in subsequent common coil magnets (5). Table I lists the major parameters of the present design. Table I: Major parameters of the present design. Bore diameter 35mm Main coil soacinl( (aperture) 40mm Computed quench field at 4.2 K 14.3 T Peak field, inner layer 15.1 T Peak field, outer layer 1O.3T Duench current 12.3 kA Wire Non-Cu Jo {4.2 K, 12 Il 2000 Nmm , CulNon-Cu ratio, inner & outer 0.7, 1.7 Strand diameter 0.8mm Number of strands, inner & outer 40,26 Cable width, inner & outer 17.2,11.3 mm Number of main coil layers 1+3 Straight section length inner&outer 5oo,6oomm Number of turns (half magnet) 5+48+48+48 Height of each main coil Ill}'er 80mm Minimum coil bend radius 70mm Bore spacing 220mm Yoke outer heil(ht and width 300mm Total wire wind thickness 30mm , Field Mog. (T) 0' m 1.7 .H • S.I • 1S.8 .8.S • 10.2 • 11 .9 • 1l.6 '" Figure 2: Field magnitude contours and flux lines in a quadrant of magnet cold mass (wire wind not shown). This design uses a total of six main coil layers and two auxiliary coil layers (Fig. 1). Computed short sample (quench field) in the magnet bore is 14.3 tesla at 4.2 K (this field value uses the strand short sample and does not include any cable strain degradation). Figure 2 shows the field lines and field magnitude in a cross section of the current design. 3 MECHANICAL DESIGN The mechanical design is governed by the need to support high Lorentz forces and manage coil stresses. The integrated horizontal Lorentz forces total 18.9 MN for the magnet, acting to push the windings apart. A rigid support structure is required to mmlmlze coil displacements and maintain magnet integrity. Within the coil winding, Lorentz forces compress the winding and result in accumulated stresses approaching 150 MPa (principal compressive stress). One method of reducing coil stresses being developed in a high field magnet design at Texas A&M University is incorporation of rigid structural elements within the winding [6,7]. For the current 14 T design: no stress management is considered. The high coil stresses are not in areas of highest field and a previous high field magnet (D20) behaved well with similar levels of stress [8,9]. In addition to structural concerns, thermal conductance is an issue because adequate heat transfer is required for magnet thermal stability. The current magnet design is based on design criteria for a flexible R&D program. In contrast to accelerator quality production magnets, the magnet is designed to be modular; the magnet is separaled into three types of components and each serves a dedicated function. The first component is the magnet core and is comprised of coil modules. The second component is the iron yoke, which serves as a flux return. The third component is an external wire wrap applied around the yoke to provide preload and support. Due to the modular nature of this design, the magnet can be reassembled with different coil modules for a flexible magnet system appropriate for conducting systematic R&D parametric studies . 3.1 Inner and Outer Coil Modules Vertical preload and support of magnet coil windings is provided completely by the coil module structure. The magnet core has three modules: one inner module sandwiched between two outer modules. Each outer module contains one double pancake winding, islands, end shoes, side rails, faceplates, and an iron insert. The inner module contains two pancake windings (each pancake has 48 turns for the main layer and 5 turns for the auxiliary layer), islands, end shoes, side rails, face plates, bore spacers, and iron inserts. Nter epoxy impregnation of the Nb,Sn windings the outer coils are assembled into the coil module structure. The windings are preloaded in the direction nonnal to the cable faces and the module faceplates are welded to the side rails to maintain preload. For the outer coil modules, application of 150 MPa of pressure on the cable faces yields 37 MPa of preload after the faceplates are welded and the complete structure is cooled to 4 K (Fig. 3a, 3b). This preload prevents the coil inner turn from separating from the island when the magnet is energized. Magnet training is expected to be reduced by preventing energy release due to coil movement. Each outer module supports 450 kN per meter of straight section length. The inner module is preloaded in a similar manner to the outer modules. For the inner module coil windings, 40 MPa of vertical preload is required to prevent coil separation at the islands. The inner module supports 245 kN per meter of straight section length. Magnet ends are also supported within each coil module. Preload and support is provided through the module end shoes and reacted against the faceplates and side rails. The coil has a higher thermal contraction than iron inserts at the racetrack centers and the iron inserts exert an axial force against the winding ends when the magnet is cooled to 4 K. In addition to this internal preload from the iron inserts, an external preload is also applied against the end shoes. A gap is designed into the iron inserts to balance the internal and external preload. 3.2 External Support Structure The external support structure provides all of the horizontal preload and support for the magnet. The coil modules are assembled between a set of yokes comprised of two rectangular sections and two half-elliptical sections (Fig. 1). Thirty layers of 1 mm thick stainless steel wire will be wound on a bobbin around the yoke perimeter with a tension of 220 MPa to preload the \ Energized Figure 3: Vertical and horizontal stresses in one quadrant of the coil module straight section (yokes not shown). yokes and provide rigid support for horizontal Lorentz forces. After the last wire layer is applied, tension in the innermost wire layer decreases to 125 MPa. When the magnet is cooled to 4 K the wire tension increases to 300 MPa in the first layer and 400 MPa in the last layer. This wire tension pretoads the iron yokes in compression to minimize coil horizontal displacements when the magnet is energized. The total horizontal force of 18.9 MN is supported by the preloaded yokes and wire winding. The coil modules are designed to stack 0.05 mrn wider than the rectangular yoke components. During the wire winding at room temperature, the coil modules are compressed horizontally to a maximum stress of about 30 MPa (Fig. 3d). When cooled down to cryogenic temperatures, the coil modules contract more than the iron yoke components and the horizontal compressive stress on the coils decreases to less than 10 MPa (Fig. 3e), (Correspondingly, the horizontal compression on the yokes increases as the magnet is cooled.) With less than 10 MPa of horizontal preload, the coil surfaces on either side of the magnet bores separate from the bore spacer at low field and are essentially free at high fields. When the magnet is energized, the stresses within the coil windings approach 150 MPa in both the horizontal and vertical directions (Fig. 3c, 31). However, the region of highest coil stress is not near the region of highest field. The region of highest horizontal stress coincides with the region of lowest magnetic field. The highest vertical stress exists in a region with a magnetic field magnitude of 9 T. A previous epoxy impregnated Nb,Sn magnet behaved well with similar coil stress levels [8,9) and we do not expect the high coil stresses to be a limiting factor. The epoxy impregnation increases the mean stiffness of the composite coil and provides support for the brittle superconductor. At field levels beyond 14 or 15 T the coil strength (epoxy impregnated) may become a critical issue and stress management may be necessary to prevent significant cable degradation. 4 CONCLUSIONS The design of a high field 2-in-1 dipole based on a common coil design has been presented. The magnet is designed to provide a modular platform for flexible R&D parametric studies of high field magnets using brittle superconductor. Coil modules may be replaced, added, or substituted to achieve new magnet configurations and improved field quality. An external wire winding structure that was successfully used in the past provides preload and supports the substantial horizontal Lorentz forces. The structure is also easily scaled for higher field magnets. Stresses within the coil winding when the magnet is energized approach critical levels; however, stress management has not yet proven necessary at -14 T field levels. Lastly, the common coil design uses flat racetrack coils with a large bend radius in the ends, enabling the future introduction of coils made with high temperature superconductors (HTS). 5 REFERENCES (IJ R. Gupta, "A Common Coil Design for High Field 2·in·l Accelerator Magnets," Proceedings of the 1997 Particle Accelerator Conference, Vol. 3, pp. 3344-3346. May 1997. [21 R. Gupta. et al., "A High Field Magnet Design for A Future Hadron Collider," Applied Superconductivity Conference, Palm Desert, CA, September 1998. [3] K. Chow, et aI., "Fabrication and Test Results of a Prototype, Nb)Sn Superconducting Racetrack Magnet," Applied Superconductivity Conference, Palm Desert, CA. September 1998. (4) S.A. Gourlay, et at., "Fabrication and Test Results of a Prototype. NblSn Superconducting Racetrack Dipole Magnet," 1999 Particle Accelerator Conference, New York. NY, March 1999. [5] R. Gupta. "Common Coil Magnet System for VLHC," 1999 Particle Accelerator Conference, New York, NY. March 1999. (6) N. Diaczenko. et al., "Stres~ Management in High Field Dipole~," Proceedings of the 1997 Particle Accelerator Conference, Vol. 3, pp. 3443-3445, May 1997. (7) T. Elliot, et aI •• "16 Tesla NblSn Dipole Development at Texas A&M University," IEEE Transactions on Applied Superconductivity, Vol. 7, #2, pp. 555~557. June 1997. [81 A.D. McInturff. et al., "Test Results for a High Field (13 T) NblSn Dipole," Proceedings of the 1997 Particle Accelerator Conference. Vol. 3. pp. 3212-3214, May 1997. (9) S. Caspi, et at .• "Development of High Field Dipole Magnets for Future Accelerators," Fifteenth International Conference on Magnet Technology. Beijing. China, October 1997. 

Mechanical design of a high field common coil magnet

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MECHANICAL DESIGN OF A HIGH FIELD COMMON COIL MAGNET*S. Caspi, K. Chow+, D. Dietderich, S. Gourlay, R. Gupta, A. McInturff, G. Millos, R. Scanlan,Lawrence Berkeley National Laboratory, Berkeley, CAAbstractA common coil design for high field 2-in-1 acceleratormagnets has been previously presented as a “conductor-friendly” option for high field magnets applicable for aVery Large Hadron Collider [1,2]. This paper presentsthe mechanical design for a 14 tesla 2-in-1 dipole basedon the common coil design approach. The magnet willuse a high current density Nb3Sn conductor. The designaddresses mechanical issues particular to the commoncoil geometry: horizontal support against coil edges,vertical preload on coil faces, end loading and support,and coil stresses and strains. The magnet is the second ina series of racetrack coil magnets that will provideexperimental verification of the common coil designapproach.1  INTRODUCTIONThe superconducting magnet program at LawrenceBerkeley National Laboratory (LBNL) is focused ondevelopment of high field accelerator magnets usingbrittle superconductors. The current designs follow a“common coil” approach wherein a pair of racetrackcoils is shared between two apertures to produceopposing fields in each aperture [1]. A 6 tesla magnetFigure 1: Basic components in magnet cross section(only one quadrant shown). _______________________ +Email: kpchow@lbl.gov *Work supported by the U.S. Department of Energy under contract No.DE-AD03-76SF00098. using Nb3Sn conductor has been built and tested as a firststep towards the ultimate goal of attaining 15 Taccelerator quality dipoles [3,4]. This paper describes themechanical design of a second magnet in the commoncoil series: a 14 T Nb3Sn magnet of limited field quality(field quality will be addressed in the next magnet).A preliminary design has been previously describedelsewhere [2]. The design has since been refined to allowa modular and flexible R&D program for systematictesting of different issues related to high field magnetsthat use brittle superconductor. The design also addressesseveral mechanical issues particular to a high fieldcommon coil geometry: horizontal support on coil edges,vertical preload on coil faces, end support, and coilstresses and strains. Figure 1 shows the basiccomponents in a cross section of the magnet straightsection.2  MAGNETIC DESIGNThe primary purpose of this magnet is to demonstrate theviability of high field dipoles based on the common coilgeometry in a mechanical structure suitable for highfield accelerator magnets. As such, field quality issueswill not be addressed in this magnet but will be fullyaddressed in subsequent common coil magnets [5]. Table1 lists the major parameters of the present design.Table 1: Major parameters of the present design. Bore diameter  35 mm Main coil spacing (aperture)  40 mm Computed quench field at 4.2 K  14.3 T Peak field, inner layer  15.1 T Peak field, outer layer  10.3 T Quench current  12.3 kA Wire Non-Cu Jc {4.2 K, 12 T}  2000 A/mm2 Cu/Non-Cu ratio, inner & outer  0.7, 1.7 Strand diameter  0.8 mm Number of strands, inner & outer  40, 26 Cable width, inner & outer  17.2, 11.3 mm Number of main coil layers  1 + 3 Straight section length, inner&outer  500, 600 mm Number of turns (half magnet)  5+48+48+48 Height of each main coil layer  80 mm Minimum coil bend radius  70 mm Bore spacing  220 mm Yoke outer height and width  300 mm Total wire wind thickness  30 mm0-7803-5573-3/99/$10.00@1999 IEEE. 3236Proceedings of the 1999 Particle Accelerator Conference, New York, 1999Figure 2: Field magnitude contours and flux lines in aquadrant of magnet cold mass (wire wind not shown).This design uses a total of six main coil layers and twoauxiliary coil layers (Fig. 1). Computed short sample(quench field) in the magnet bore is 14.3 tesla at 4.2 K(this field value uses the strand short sample and doesnot include any cable strain degradation). Figure 2 showsthe field lines and field magnitude in a cross section ofthe current design. 3  MECHANICAL DESIGN The mechanical design is governed by the need tosupport high Lorentz forces and manage coil stresses.The integrated horizontal Lorentz forces total 18.9 MNfor the magnet, acting to push the windings apart. A rigidsupport structure is required to minimize coildisplacements and maintain magnet integrity. Within thecoil winding, Lorentz forces compress the winding andresult in accumulated stresses approaching 150 MPa(principal compressive stress). One method of reducingcoil stresses being developed in a high field magnetdesign at Texas A&M University is incorporation ofrigid structural elements within the winding [6,7]. Forthe current 14 T design, no stress management isconsidered. The high coil stresses are not in areas ofhighest field and a previous high field magnet (D20)behaved well with similar levels of stress [8,9]. Inaddition to structural concerns, thermal conductance isan issue because adequate heat transfer is required formagnet thermal stability. The current magnet design is based on design criteriafor a flexible R&D program. In contrast to acceleratorquality production magnets, the magnet is designed to bemodular; the magnet is separated into three types ofcomponents and each serves a dedicated function. Thefirst component is the magnet core and is comprised ofcoil modules. The second component is the iron yoke,which serves as a flux return. The third component is anexternal wire wrap applied around the yoke to providepreload and support. Due to the modular nature of thisdesign, the magnet can be reassembled with differentcoil modules for a flexible magnet system appropriatefor conducting systematic R&D parametric studies.3.1  Inner and Outer Coil ModulesVertical preload and support of magnet coil windings isprovided completely by the coil module structure. Themagnet core has three modules: one inner modulesandwiched between two outer modules. Each outermodule contains one double pancake winding, islands,end shoes, side rails, faceplates, and an iron insert. Theinner module contains two pancake windings (eachpancake has 48 turns for the main layer and 5 turns forthe auxiliary layer), islands, end shoes, side rails, faceplates, bore spacers, and iron inserts.After epoxy impregnation of the Nb3Sn windings theouter coils are assembled into the coil module structure.The windings are preloaded in the direction normal tothe cable faces and the module faceplates are welded tothe side rails to maintain preload. For the outer coilmodules, application of 150 MPa of pressure on thecable faces yields 37 MPa of preload after the faceplatesare welded and the complete structure is cooled to 4 K(Fig. 3a, 3b). This preload prevents the coil inner turnfrom separating from the island when the magnet isenergized. Magnet training is expected to be reduced bypreventing energy release due to coil movement. Eachouter module supports 450 kN per meter of straightsection length. The inner module is preloaded in asimilar manner to the outer modules. For the innermodule coil windings, 40 MPa of vertical preload isrequired to prevent coil separation at the islands. Theinner module supports 245 kN per meter of straightsection length.Magnet ends are also supported within each coilmodule. Preload and support is provided through themodule end shoes and reacted against the faceplates andside rails. The coil has a higher thermal contraction thaniron inserts at the racetrack centers and the iron insertsexert an axial force against the winding ends when themagnet is cooled to 4 K. In addition to this internalpreload from the iron inserts, an external preload is alsoapplied against the end shoes. A gap is designed into theiron inserts to balance the internal and external preload.3.2  External Support StructureThe external support structure provides all of thehorizontal preload and support for the magnet. The coilmodules are assembled between a set of yokes comprisedof two rectangular sections and two half-ellipticalsections (Fig. 1). Thirty layers of 1 mm thick stainlesssteel wire will be wound on a bobbin around the yokeperimeter with a tension of 220 MPa to preload the3237Proceedings of the 1999 Particle Accelerator Conference, New York, 1999Figure 3: Vertical and horizontal stresses in one quadrantof the coil module straight section (yokes not shown).yokes and provide rigid support for horizontal Lorentzforces. After the last wire layer is applied, tension in theinnermost wire layer decreases to 125 MPa. When themagnet is cooled to 4 K the wire tension increases to 300MPa in the first layer and 400 MPa in the last layer. Thiswire tension preloads the iron yokes in compression tominimize coil horizontal displacements when the magnetis energized. The total horizontal force of 18.9 MN issupported by the preloaded yokes and wire winding.The coil modules are designed to stack 0.05 mm widerthan the rectangular yoke components. During the wirewinding at room temperature, the coil modules arecompressed horizontally to a maximum stress of about30 MPa (Fig. 3d). When cooled down to cryogenictemperatures, the coil modules contract more than theiron yoke components and the horizontal compressivestress on the coils decreases to less than 10 MPa (Fig.3e). (Correspondingly, the horizontal compression on theyokes increases as the magnet is cooled.) With less than10 MPa of horizontal preload, the coil surfaces on eitherside of the magnet bores separate from the bore spacer atlow field and are essentially free at high fields.When the magnet is energized, the stresses within thecoil windings approach 150 MPa in both the horizontaland vertical directions (Fig. 3c, 3f). However, the regionof highest coil stress is not near the region of highestfield. The region of highest horizontal stress coincideswith the region of lowest magnetic field. The highestvertical stress exists in a region with a magnetic fieldmagnitude of 9 T. A previous epoxy impregnated Nb3Snmagnet behaved well with similar coil stress levels [8,9]and we do not expect the high coil stresses to be alimiting factor. The epoxy impregnation increases themean stiffness of the composite coil and providessupport for the brittle superconductor. At field levelsbeyond 14 or 15 T the coil strength (epoxy impregnated)may become a critical issue and stress management maybe necessary to prevent significant cable degradation. 4  CONCLUSIONS The design of a high field 2-in-1 dipole based on acommon coil design has been presented. The magnet isdesigned to provide a modular platform for flexibleR&D parametric studies of high field magnets usingbrittle superconductor. Coil modules may be replaced,added, or substituted to achieve new magnetconfigurations and improved field quality. An externalwire winding structure that was successfully used in thepast provides preload and supports the substantialhorizontal Lorentz forces. The structure is also easilyscaled for higher field magnets. Stresses within the coilwinding when the magnet is energized approach criticallevels; however, stress management has not yet provennecessary at ~14 T field levels. Lastly, the common coildesign uses flat racetrack coils with a large bend radiusin the ends, enabling the future introduction of coilsmade with high temperature superconductors (HTS).5 REFERENCES[1] R. Gupta, “A Common Coil Design for High Field 2-in-1Accelerator Magnets,” Proceedings of the 1997 Particle AcceleratorConference, Vol. 3, pp. 3344-3346, May 1997.[2] R. Gupta, et al., “A High Field Magnet Design for A Future HadronCollider,” Applied Superconductivity Conference, Palm Desert, CA,September 1998.[3] K. Chow, et al., “Fabrication and Test Results of a Prototype, Nb3SnSuperconducting Racetrack Magnet,” Applied SuperconductivityConference, Palm Desert, CA, September 1998.[4] S.A. Gourlay, et al., “Fabrication and Test Results of a Prototype,Nb3Sn Superconducting Racetrack Dipole Magnet,” 1999 ParticleAccelerator Conference, New York, NY, March 1999.[5] R. Gupta, “Common Coil Magnet System for VLHC,” 1999 ParticleAccelerator Conference, New York, NY, March 1999.[6] N. Diaczenko, et al., “Stress Management in High Field Dipoles,”Proceedings of the 1997 Particle Accelerator Conference, Vol. 3, pp.3443-3445, May 1997.[7] T. Elliot, et al., “16 Tesla Nb3Sn Dipole Development at TexasA&M University,” IEEE Transactions on AppliedSuperconductivity, Vol. 7, #2, pp. 555-557, June 1997.[8] A.D. McInturff, et al., “Test Results for a High Field (13 T) Nb3SnDipole,” Proceedings of the 1997 Particle Accelerator Conference,Vol. 3, pp. 3212-3214, May 1997.[9] S. Caspi, et al., “Development of High Field Dipole Magnets forFuture Accelerators,” Fifteenth International Conference on MagnetTechnology, Beijing, China, October 1997.3238Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

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Mechanical design of a high field common coil magnet

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