New concepts in transverse field magnet design - IEEE Xplore

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 13, NO. 2, JUNE 2003

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New Concepts in Transverse Field Magnet Design Andrew V. Gavrilin, Mark D. Bird, Victor E. Keilin, and Alexey V. Dudarev

Abstract—The National High Magnetic Field Laboratory (NHMFL) in Tallahassee, Florida, USA, continues research and development of transverse field magnets (with the field perpendicular to the access tube). Presently, the emphasis is on a novel approach with concentric nested coils tilted at an angle to the central axis; current flows in opposite directions within the coils at opposite tilt angle, generating a transverse dipole field. Superconducting tilted coils using wire-wound technology and resistive tilted coils using advanced technology are being examined. Some very preliminary, conceptual designs and magnetic field calculations are presented. Related problems, including behavior under the Lorentz forces are discussed briefly. Index Terms—Dipole magnet, nondestructive repetitively pulsed magnet, superconducting winding, transverse field.

I. INTRODUCTION

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UPERCONDUCTING magnets generating transverse dipole magnetic fields, i.e., perpendicular to the magnet bore, are typically of either curved saddle or race-track type, i.e., consist of mirror-like symmetrical halves [1], [2]. When experiments do not require a uniform transverse field and a very small part of the bore is only used, a split pair of circular solenoids is a good option virtually having a mirror-like symmetrical configuration too. A relatively new, alternative, “dissymmetrical” option for transverse field magnets is concentric cylindrical nested tilted coils where the plane of conducting turns is rotated an angle with respect to the central axis. This results in the induced field being oriented this angle off the axis. In a set of two concentric nested coils, oppositely tilted and energized with opposite polarity, the axial components of the field cancel each other out, leaving a uniform transverse magnetic field, Fig. 1(a). The main disadvantage of a tilted coil magnet is that in it a considerable proportion of power and conductor turns out to be lost due to the axial field canceling. Obviously each regular turn within a tilted winding is roughly aspect ratio. To obtain a of elliptical shape with the , with tilted coils, higher uniformity of the transverse field, more pairs of such coils can be used, Fig. 1(b). At a fixed total number of coils of fixed length, a higher uniformity of the transverse field should be observed with thinner coils, i.e., the field uniformity has to increase as the difference in average radius Manuscript received August 5, 2002. A. V. Gavrilin and M. D. Bird are with the National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310 USA (e-mail: [email protected]; [email protected]). V. E. Keilin is with the Institute for Superconductivity and Solid State Physics of Russian Research Center “Kurchatov Institute,” Moscow, 123182 Russia (e-mail: [email protected]). A. V. Dudarev is with CERN, ATLAS Magnet Project, EP Division, Geneva 23, CH-1211 Switzerland (e-mail: [email protected]). Digital Object Identifier 10.1109/TASC.2003.812636

Fig. 1. Sketch of tilted coil magnets. Cross-sectional view: (a) the simplest two-coil configuration, B is the field from the inner coil, B —from the outer coil, B B is the resultant transverse field; (b) advanced, more practical multi-coil configuration with a higher field uniformity and strength, two pairs of tilted coils are shown. is the tilt angle.

=

between adjacent coils decreases, all other factors being equal. At the same time, at a fixed radial thickness of the coils, the field uniformity can be increased due to an increase of the coils’ length. II. SUPERCONDUCTING TILTED COIL MAGNET A. Conceptual Design The highest uniform transverse fields in a large volume of space are traditionally provided by superconducting curved saddle dipole magnets usually used in accelerators [1], [2]. We are not ready yet to say that a superconducting tilted coil magnet should be considered as an alternative for a traditional accelerator magnet. However, a comparison of the former with the latter can give an idea about advantages and disadvantages of both the configurations. In Table I, characteristics of a four tilted coil superconducting magnet are given as an example. Such a small number of tilted coils makes the magnet practical. The magnet (Fig. 2) is assumed to have the same inner diameter and central field as the LHC main dipole magnet prototype is planned to have at CERN [1]. We used a NbTi/Cu

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TABLE I CHARACTERISTICS OF THE 4 TILTED COIL SUPERCONDUCTING MAGNET

Fig. 3. Axial distribution of dipole, B , and axial, B , magnetic fields of the four tilted coil superconducting magnet. Transverse dipole field is given along the central axis (X 0, Y = 0, solid line) and along the parallel axis (X = 0, Y = 27 mm, dashed) in the vicinity of the magnet inner surface at a radius of 27 mm. Solenoidal (undesired) field (dash-dot) is given along the parallel axis only; on the central axis the solenoidal field is lower.

=

Fig. 2. Sketch of four tilted coil optimized superconducting magnet with high field uniformity.

composite superconductor of 1.065 mm diameter for the tilted coil winding, as this conductor is one of the candidates for the LHC Rutherford cable, in so doing the average current density in our tilted winding is close to that in the LHC dipole winding [1]. It turns out that an influence of the length of the four tilted coil magnet on its field characteristics is practically excluded, if the magnet is sufficiently long ( 1.5 m). In the magnet, the innermost and outermost coils (1&4) differ from the inner coils (2&3) in the total number of winding layers only; the inner coils (2&3) have roughly twice the number of layers as 1&4 do that represents a peculiar kind of optimization, Fig. 2.

across the whole aperture along roughly 60% of the winding , is at length. Another transverse component of the field, least 10 orders of magnitude lower. The solenoidal (axial) component of the field is suitably small, and it can be reduced by 2–3 times through an additional optimization of the coils. By such an optimization of radially grading the superconductor, its total amount can be considerably reduced. Of course, such a uniform transverse field is obtained under the assumption that each turn has the ideal elliptical shape and all the turns are perfectly 45 degrees oriented and evenly distributed with equal spacing over the coils, not to mention that the coils are ideally adjusted. We do not know yet how minor natural errors in the conductor positioning within a tilted winding and the conductor helicity can affect the field uniformity. It will be the subject of a future study, together with stress and quench analyses. However, we are inclined to believe that statistically such errors would be leveled to some extent due to the large number of turns, and tilted coils could be used even in accelerators. At the same time, the transverse field uniformity that could be obtained with the tilted windings would be certainly sufficient for many experiments in solid state physics, neutron scattering or for material tests. C. Superconducting Tilted Coils Manufacturing Problems

B. Magnetic Field Calculations A Fortran code has been developed to compute a three-dimensional magnetic field from a set of tilted coils of arbitrary length, radius, total number of layers and turns in each layer. Each tilted turn of winding is assumed to be an ideal flat ellipse, the helicity effect (helical path of conductor) has been neglected as yet. This seems to be a minor simplification for multi-layer closely packed windings with a large number of turns and small pitch. However, influence of the helical path of conductor on the resultant field may be important for single layer tilted coils and requires further analysis. The results for the four tilted coil magnet obtained with the code are given in Fig. 3. As can be inferred from Fig. 3, the , is very uniform desired transverse component of the field,

Within traditional wire-wound technologies presently used for circular conventional long solenoids, it is very difficult to wind a wire in solenoidal shape at angle other than 90 degrees to the bore tube. As the wire is pulled tight, it has a tendency to take the shortest path: a circle lying in a plane perpendicular to the tube instead of the inclined ellipse. To wind at some other angle requires winding without tension (pre-load) and/or holding turns in an unstable position. Since winding with no tension is seemingly out of practical interest and a wire pre-load ought to be used, holding turns becomes important and hence tilted solenoidal winding is a technological problem [3]. Such a problem can be solved by development of nonstandard coiling apparatus that seems to be more complicated than those used for conventional solenoid winding [3].

GAVRILIN et al.: NEW CONCEPTS IN TRANSVERSE FIELD MAGNET DESIGN

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TABLE II CHARACTERISTICS OF THE FOUR TILTED LAYER CUT PULSE MAGNET

Fig. 4. Sketch of the four tilted cut layer nondestructive pulse magnet. Cross-sectional view. Configuration A is shown.

There exists another solution. It consists of flat ellipse-shaped pancake type windings instead of solenoidal ones. A long tilted coil can be assembled from a proper number of elliptical flat pancakes tilted and stacked at 45 degrees (or even less angle) off the central axis instead of layerwise winding the coil in a long solenoidal shape [3]. The idea is directly derived from the one of tilted Bitter coils described in work [4], where also the concept itself of a transverse field magnet with tilted coils was first put forward. All the pancakes are electrically connected in series. Every elliptical flat pancake representing a flat spiral can be wound with a reasonable pre-load on an ellipse-shaped bobbin, using a rather traditional pancake winding technology. Obviously, this technology can only be used for manufacturing of radially thick tilted coils. For a tilted multi-pancake coil, a rectangular conductor looks preferable to a round one (albeit theoretically the latter can be used too with a side support structure). A superconducting cable-in-conduit conductor (CICC) may be a good candidate for a tilted superconducting coil, as the conduit can provide the whole ellipse-shaped pancake with needed strength and stiffness.

III. TRANSVERSE FIELD PULSE MAGNET

Fig. 5. Configurations A and B of the four tilted cut layer nondestructive pulse magnet.

The idea of tilted coils can be also applied to resistive pulse magnets providing the highest fields. Non-destructive pulse magnets with field over 20 T and ability to withstand several millions of shots are of our interest [5]–[7]. Some of these so-called repetitively pulsed magnets are made with wire EDM technology (or other advanced cutting technology) by which a tilted coil turns can be precisely cut from a cylindrical metallic tube at any angle. In Table II, characteristics of a four-coil cut magnet are given. Each coil (Fig. 4) represents a single conducting layer—elliptic helicoid of 4 mm radially thick, cut from a copper alloy hollow cylinder. The magnet parameters are based on the Repetitively Pulsed Magnet for neutron scattering being presently under development at the NHMFL [7]. The conducting layers—helicoids are divided by inter-layer reinforcement-insulation, and the system as a whole has an outer reinforcement and inner support as well. Structural support is complicated, as turns of the innermost helicoid will

exhibit torque in the -plane about the -axis under Lorentz force [4], but we believe that solutions can be found. Various configurations of the tilted layers are possible. The simplest configuration A is shown in Fig. 5 (upper). In it, each layer has the opposite angular orientation and current polarity as its neighbors. In another configuration B, Fig. 5 (lower), we start with an inner layer of a particular orientation and then place pairs of layers outside it with each pair in alternate orientation to preceding pair. A final layer is installed at the outer perimeter. We find that configuration B provides a more uniform transverse and a lower axial field than configuration A for layers field of equal current density and length, Fig. 6. As our experience shows, the rule for positioning of tilted coils used in configuration B, and described above, also works well for tilted coil magnets with a large total number of tilted coils—even greater than 50.

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are rather demonstrative than really practical, the innovation looks promising and worth further considering. ACKNOWLEDGMENT The authors are deeply indebted to Prof. H.-J. SchneiderMuntau and Prof. S. W. Van Sciver for fruitful discussions, stimulation and unflagging support of this work. REFERENCES

Fig. 6. Magnetic field distribution within the aperture of the four tilted cut layer nondestructive pulse magnet (at the peak current). The undesired 0, = 12 mm) only, solenoidal field is shown along a parallel axis ( i.e., at a radius of 12 mm—in the vicinity of the inner surface. The transverse dipole field is shown along the central axis, on the parallel axis it is almost the same, though in config. B it is slightly more uniform than in config. A (the difference is not visible at the scale used).

X= Y

IV. SUMMARY Innovative option for superconducting and resistive magnets with tilted coils for transverse field generation in a large volume of space is considered. Despite the fact that the magnet designs

[1] A. Artoos et al., “Design, manufacturing status, first results of the LHC main dipole final prototypes and steps toward series manufacture,” IEEE Trans. Appl. Supercond., vol. 10, no. 1, pp. 98–102, March 2000. [2] M. N. Wilson, Superconducting Magnets. Oxford: Clarendon Press, 1983, ch. 3. [3] V. E. Keilin, A. V. Gavrilin, M. D. Bird, and A. V. Dudarev, “Tilted transverse field magnets,” US patent pending, 2002. [4] A. V. Gavrilin et al., “Conceptual design of high transverse field magnets at the NHMFL,” IEEE Trans. Appl. Supercond., vol. 12, no. 1, pp. 465–469, March 2002. [5] M. Motokawa et al., “Cu–Ag alloy bitter type magnet for repeating pulsed field,” IEEE Trans. Magn., vol. 32, pp. 2534–2537, July 1996. [6] Y. M. Eyssa et al., “25–30 T water cooled pulsed magnet concept for neutron scattering experiment,” in Proc. of the 15th Intern. Conf. on Magnet Technology (MT-15), Beijing, China, 1997, pp. 1254–1257. [7] M. D. Bird et al., “Design of repetitively pulsed magnet for neutron scattering,” in 17th Intern. Conf. on Magnet Technology (MT-17), Geneva, Switzerland, Sept. 2001.