Direct Double-Helix Magnet Technology

Proceedings of PAC09, Vancouver, BC, Canada

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DIRECT DOUBLE-HELIX MAGNET TECHNOLOGY R.B. Meinke, J. Lammers, P.J. Masson, G. Stelzer, Advanced Magnet Lab, Inc., FL 32905, U.S.A. Abstract Magnets for beam steering, focusing and optical corrections often have demanding requirements on field strength, field uniformity, mechanical robustness and high radiation strength. The achievable field strength in normal conducting magnets is limited by resistive heating of the conductor. A break-through magnet technology, called “Direct Double-HelixTM” allows operation at current densities in excess of 100 A/mm2 with conventional water cooling. The conductive path generating the magnetic field is machined out of conductive cylinders, which are arranged as concentric structures. Geometrical constraints of conventional conductors, based on wire manufacturing, are eliminated. The coolant, typically water or air, is in direct contact with the conductor and yields very high cooling efficiency. Based on Double-HelixTM technology the conductor path is optimized for high field uniformity for accelerator magnets with arbitrary multipole order or combined function magnets. Advanced machining technologies, enable unprecedented magnet miniaturization. These magnets can operate at temperatures of several hundred degrees Celsius and can sustain high radiation levels.

INTRODUCTION

Figure 1: Left: 2-layer quadrupole winding. Right: 2-layer dipole winding. Both windings produce transverse magnetic fields.

Figure 2 : Two concentric cylinders of a DDH coil in a sextupole configuration Machining the current path out of a cylinder with a given wall thickness produces a conductor with rectangular cross section that changes with azimuth angle around the coil axis as shown in Figure 3.

Currents described by Eq. 1 produce simultaneously axial and transverse magnetic fields. sin

(1)

cos sin Superimposing two such currents with appropriate direction allows cancelation of either the axial or the transverse magnetic field. Pure transverse magnetic fields of arbitrary multipole order can be produced. Coils based on this technology have high field uniformity without the use of field forming spacers as required in other winding configurations. Conductors surround the coil aperture giving intrinsic mechanical stability to the coils. The resulting winding configurations are called DoubleHelixTM coils [1]; examples of a dipole and quadrupole are shown in Figure 1.

Direct Double-Helix Technology Double-helix coils (DH) can be produced by machining the current path out of conductive cylinders, which are then arranged as concentric structures (see Figure 2). This patented technique, called “Direct Double-Helix” (DDH) technology offers significant advantages over conventional coil winding, which uses wire, cable or tape conductors.

6

Strip Width [mm]

∑

6.5

5.5 5 4.5 4 3.5 3 2.5 -200

-150

-100

-50

0

Angle [deg]

50

100

150

200

Figure 3: Variation in conductor width as a function of azimuth angle for a single turn of a DDH coil. The resulting resistance of the machined conductor is significantly smaller than a round wire that would fit in the thickness given by the cylinder. Already a square has about 25% more area than its inscribed circle. But more important is the effect that the machined conductor is significantly wider in sections that are perpendicular to the coil axis and do not contribute to the generation of a transverse field. The machined conductor also eliminates any constraints from the wire or cable manufacturing process, and arbitrary bending radii of the conductor are possible. As for the DH coil configurations DDH coils have high intrinsic mechanical robustness, since the conductor completely surrounds the coil aperture as in solenoidal windings.

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Material Choice C Dependingg on the appplication a large choicce of materials is possible for the conductorrs and the suupport structures neeeded for DDH H coils. For normal n conduucting coils coppeer or aluminnium are naatural choicees as conductors. If the coil has to operrate at very high temperaturess, in the rangge of severall hundred deegrees Celsius, carrbon or tunggsten can bee used. Advaanced composite conductors c likke carbon naanotubes in a Cu matrix, whicch are currenttly under devvelopment [2]] will lead to coills with unprrecedented peerformance. These T conductors can c have a reesistivity that is 10 times lower l than Cu, andd enable perfoormances that are normallyy only possible witth superconduuctors. The support strucctures needed for some s DDH cooils can be made m from cerramic materials, whhich allow higgh operationaal temperatures and yield very high radiation hardness. h

Field Unifo formity Based on double-helixx technology, which prodduces almost pure multipole fieelds away froom the coil ends, DDH coils show s excellennt field uniform mity. Furtherm more, computer conntrolled machhining of the conductor pathh with possible variiation of condductor cross section allow ws for unique fieldd optimizationn potential. No field forrming spacers are required, andd high field uniformity u caan be achieved witthout cost penaalty. Possible fiield configuraations includee, dipoles for beam steering, quaadrupoles forr beam focuusing, higher--order multipole maagnets for beaam optics corrrections, combbined function maagnets (typiccal dipole with w superimpposed quadrupole), twisted dipolles for polarizzed beams, tw wisted quadrupoles for multi-diirectional foccusing. If neeeded, bent magnetss can be produuced or those with w flared ennds.

Figu ure 4: Left: Teemperature profile along a single turn off DH coil, shoowing reducedd temperaturees, where thee a DD ductor width is largest. Right: Currrent densityy cond distrribution near a conductor beend.

mensional Sccaling Dim range off w The DDH Technoology is appllicable to a wide m machiningg coil dimensions. Laser or eelectron beam a coils withh bles unprecedeented coil minniaturization and enab gure 5 (Left)) dimeensions of a few mm aree feasible. Fig ws small dipoole coils with apertures of about 1 mm.. show mensions, andd H coils also leend themselves for large dim DDH sectionn c hining the conductor avvoids the cross mach m bending prefabricatedd ortions that result from disto cylinders for largee coil aperturee ductors. The required r cond and seam welded. are formed f

Conductorr Cooling As seen inn Figure 3, thee conductor cross c section varies v along each turn of the helical h windinng and is larrgest, where it doees not contribbute to the trransverse maggnetic fields that form f dipoles, quadrupoles and higher--order multipole fields. As a result r the reesistive lossess are automaticallyy small whenn a conductor segment doees not contribute too the required magnetic fielld. Figure 4 shows s the temperatture distributiion along onne conductor turn, indicating the reduced tem mperature in thhe wider part of o the conductor. The space between the concentric c cyllinders forminng the DDH coils are a used as coooling channels with large cross section. In contrast c to conventional Cuu coils made with hollow condductor, very liittle pressure is needed to force the coolant through a DDH coil, and very small temperature gradients exisst along the path p of the cooolant. Additionally, the machined groove betw ween adjacent turns can be used as a a cooling chhannel and the conductor can be cooled from m 3 sides. Coooling efficienncy can be fuurther improved byy using heat coonducting suppport structuress.

ure 5: Left; Miiniature DDH coil for medical Figu w an appliications. Righht: Larger quaddrupole coil with m. apertture of 100 mm

hievable Currrent Densities Ach steeringg b Small dipoles forr horizontal aand vertical beam 6. The ttwo DDH coiil pairs shownn in Figure F are presented p i 20 mm, thee nto each otherr. The inner coil diameter is fit in g m outerr diameter is 40mm. A special manufacturing g a DDH coill nique allows for the two laayers forming techn m them b to bee built with only one suppoort structure between mbly with thee (see Figure 6). Thhe complete magnet assem wateer containmennt vessel is shoown in Figuree 7. As shownn II, the coils produce the neededd I and Table T in Table T Magnets

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fields of aboout 950 Gauss for horizontaal steering andd 250 Gauss for verrtical steeringg, respectivelyy.

Figure 6: Tw wo sets of DD DH coils that fit into each other forming onee unit for horizontal h annd vertical beam steering.

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ontal steeringg Tablle 2: Operatioonal parametters of horizo dipole Inom (A) (

P (W)

Jc Peak (A/mm2)

TInlet (C)

TPeak (C)

Field (Gauss)

20 2 25.0 2 30 3 35 3 40 4 45 4 55 5 60.0 6

174 268 387 515 692 792 578 714

38 48 58 67 77 86 106 115

35 35 35 35 35 35 10 10

41.1 44.4 47.8 52.2 57.5 57 62.5 77.8

308 385 463 540 617 694 848 925

Ass can be seen from the dataa, the coils op perate reliablee 2 at peeak current deensities of moore than 100 A/mm A . Whilee the maximum m meeasured tempeerature at the conductor iss 77.8 C, the wateer temperaturee rise at the outlet of thee magn net is less than 10 C, andd even higherr currents andd field ds are possiblee.

CONCLUSIONS

Figure 7: Exploded view w of magnet assembly foor the DDH coils of o Figure 6. The T SS housing with inlet and outlet water cooling c tubes are shown. O paarameters off vertical steeering Table 1: Operational dipole Inom (A)

P (W W)

Jc Peak (A/mm m2)

TInlet (C)

TPeak (C)

Field d (Gausss)

5.5 7.0 8.5 10 11 13

773 120 181 251 309 369

39 49 60 70 78 92

35 35 35 35 35 10

339.6 4 42.7 4 46.3 51 55 44

83 1055 1288 1500 1655 1955

Table I andd Table II also show the opperational currrents, the power consumption,, the calculaated peak cuurrent density in thhe conductor, the inlet wateer temperaturre (35 C and 10C),, and the meaasured peak temperature t a the at conductor.

A novel magnnet technologyy has been developed inn whicch the field geenerating curreent path is maachined out off cond ductive cylindders. This teechnique offfers completee contrrol over the coonductor cross section alon ng its path andd the usual u constraints caused bby wire manu ufacturing aree elim minated. The unnprecedented cooling efficiiency in thesee coilss enables currrent densitiess in water cooled copperr cond ductor in excess of 100 A/m mm2. Such currrent densitiess havee been achieveed in long term m tests with DC D operation. Baased on douuble-helix coiil configurations arbitraryy multtipole orders of magnetss and combined functionn magn nets with highh field uniform mity are possib ble. A computer controlled c m manufacturing process onn conv ventional CNC C machines, and the lack k of magnet-specific tooling makes thesse magnets highly cost-hniques, usingg effecctive. With addvanced manuufacturing tech laserrs or electronn beams, unprrecedented miniaturization m n of coils is feasibble. DDH coils also allow w operation att temp peratures of seeveral hundredd degrees Cellsius and highh ionizzing radiation.

REFERENCES [1] R. B. Meinkee et al., “Supeerconducting Double-Helix D Accelerator Magnets”, M IEE EE Proceeding gs of PAC 03, 2003, Vol.3, pages p 1996-19998. C. Goodzeit et e al., IEEE Prroceedings of PAC 07, Vol.1, pages 561-563. 5 [2] D. Elcock, “P Potential Impaacts of Nanotechnology onn Energy Trannsmission A Applications and a Needs”,, Argonne Techhnical Reportt, No. ANL/E EVS/TM/08-3,, 2008, DOE-Innformation Brridge.

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