ELSEVIER Physica B 201 (1994) 560-564

PflYSICA

Seventy-two tesla non-destructive pulsed magnetic fields at AT&T Bell Laboratories

G.S. Boebinger*, A. Passner, J. Bevk

AT&T Bell Laboratories, 600 Mountain Avenue, Room 1D-208, Murray Hill, NJ 07974, USA

Abstract

We review the pulsed magnetic field capabilities at AT&T Bell Laboratories, with particular attention paid to a recently developed series of non-destructive, multiturn magnets. This new design has resulted in a pair of magnets surviving pulses of 71.8 and 72.6 T, respectively. The magnet bore is 9.5 mm diameter and the pulse duration ranges from 12 to over 80ms, depending whether a sinusoidal or exponential pulse decay is selected. Experience to date suggests that these magnets are reliably reproduced and long-lived.

1. Introduction

Successful generation of pulsed magnetic fields [1] requires consideration of the intense magnetic stresses generated, within limitations due to resistive heating of the conductors and total energy available for generation of the magnetic field. There are a variety of successful destructive techniques [2,3], both implosive and explos- ive, which essentially sidestep the problems of stress and heating. These strategies accept the resulting destruction in order to realize the highest achievable magnetic fields. The disadvantages of these destructive techniques in- clude relatively short pulse durations (typically ~< 10 Its at the highest fields), long delays between pulses (typically many hours), and higher costs per pulse. Additionally, data can usually only be taken during the upward mag- netic field sweep and, with implosive technologies, the sample is destroyed with each pulse.

Non-destructive magnet technologies must directly confront the stress and heating problems. One strategy is the machining of helical coils from a high strength mater-

* Corresponding author.

ial, such as Cu-Be [4] or maraging steel [5]. Machining technology and high resistivity tend to limit the total number of turns in the coil. By virtue of the resulting low inductance of the helical magnet, the pulse duration remains relatively short (typically 100 Its).

Multi-turn non-destructive coils use a more ductile conductor (typically copper or a copper-based alloy or composite) which, exhibits a much higher conductivity than steel, although often at a cost of lower tensile strength. The higher conductivity allows many more turns in the coil, resulting in a much longer pulse dura- tion (typically 10ms). Historically, the peak field of these multi-turn magnets is determined by the strength of the conductor. Copper magnets have achieved 40-50T [1], while a magnet made of high strength, high conductivity Cu-Nb composite [6] has reached 68.4 T [7].

Although external reinforcement of magnets can im- prove performance, a more effective strategy for improv- ing the mechanical strength of a magnet is to better distribute the peak stresses within the coil. A recent ad- vance in multi-turn magnet technology from the Leuven group [8,9] uses insulating reinforcement (fiberglass windings) between each layer of the coil to individually support the stresses imposed on each layer of conductor.

0921-4526/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 1 3 6 - J

G.S. Boebinger et al. / Physica B 201 (1994) 560-564 561

COAX CABLES ! i

TO 12kV POWER SUPPLY

SAFETY ABORT SAFETY RESISTORS SOLID STATE INDUGTORS r "

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INTERLOCKED CAPACITOR BANK ENCLOSURE

~O MAGNET

l I I

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Fig. 1. A schematic of one of the four identical subbanks in the 12 kV, 518 kJ capacitor bank at AT&T Bell Laboratories.

For adequate mechanical support in the first few layers, where the stresses are the highest, about 40% of the total magnet volume is occupied by fiberglass windings. A magnet of this design made with soft copper wire has survived a 66T in a 12mm bore with a pulse time- to-peak-field, tveak, of 9ms. Such performance clearly indicates that magnet stresses can be successfully shifted from the conductor to a neighboring insulating reinforce- ment.

At AT&T Bell Laboratories, we have developed a new series of multiturn magnets which are designed to accom- modate virtually all of the high magnetic field stresses using the conductor itself. These magnets have achieved fields of 72 T in a 9.5 mm bore with tpeak ~ 5 ms, using as little as 270 kJ of stored energy.

2. The capacitor bank

A schematic of the capacitor bank at AT&T Bell Laboratories is given in Fig. 1. This schematic depicts one of four identical subbanks, yielding a total capacitance of 7.2mF. The capacitor bank can be charged in one minute to the full energy of 518kJ at 12 kV. This voltage is higher than that commonly used to drive multiturn coils. It was selected as the highest practi- cal voltage for economical capacitor bank construction and it allows higher-inductance magnets to devote more energy to peak magnetic field rather than pulse duration. The capacitor bank switch consists of 24 thyristors (eight parallel stacks of three thyristors). This sizeable switch can survive a discharge of the full bank energy into

a shorted load. It is thus well suited even for initial testing of untried magnet designs.

The capacitor bank and magnet form a simple LC circuit for sinusoidal pulses of order 10ms duration. A diode/crowbar resistor assembly with variable resist- ance over a range of 0 2.1 f~ allows the experimenter to impose a tuneable exponential tail on the downward sweep of the magnet. The capacitor bank is contained in the lower level of a two-level shielded room and the magnet and experimental detection circuitry are located on the upper level. The capacitor bank is controlled remotely through a fiber optic link to a control panel external to the shielded room.

3. The magnets

The design philosophy of the magnets is focussed on accommodating the high magnetic field stresses above 70 T using the conductor itself. Only a minimal quantity of insulation is utilized as necessary for wire insulation [10]; no significant structural strength arises from the insulation. High magnetic fields are made possible prim- arily through the use of three different conductors of large cross-sectional area within a magnet. Each conduc- tor is tailored to the specific local requirements of ductil- ity, tensile strength, and conductivity. The inner layer of the magnets is a Cu-Be alloy [11]: cross-section of 0.100 x 0.150in (2.45 x 3.81 mm); tensile strength of a = 120ksi (0.83 GPa); and conductivity of 65% IACS (Inter- national Annealed Copper Standard). This material is highly ductile, allowing it to easily bend around the 3/8 in

562 G.S. Boebinger et al. / Physica B 201 (1994) 560-564

60

q so W 1"7 40

3o ~ 2o

~ o

. . . . . . . . . . . . . . o 03_

0 1 0 20 30 40 50 60 70 80 TIME (msec)

Fig. 2. Profile of the 71.8T pulse from magnet CN93-1. The time to peak is tpeak = 4.9 ms; the time constant of the roughly exponential tail is te~p ~ 18 ms. A second coil (magnet CN93-2) reaches 72.6T with tpeak = 6.4ms and t~xp ~ 26ms.

(9.53 mm) diameter bore. The next eight layers of the magnet must be able to reliably handle extreme stress. For these layers we use a large cross-section Cu-Nb composite [12]: cross-section of 0.122 x0.140in (3.10×3.56mm); tensile strength of tr = 160ksi (1.10GPa); and conductivity of 60% IACS. The outer five layers must contribute to the magnetic field without unduly increasing the total magnet resistance. For these layers we use a Cu-Alumina alloy [13]: cross-section of 0.100 x 0.150 in (2.54x3.81mm); tensile strength of tr = 90ksi (0.62GPa); and conductivity of 80% IACS. This combination of materials in a coil of 4in (10cm) total height (magnet CN93-1) has successfully achieved a peak magnetic field of 71.8T with a tpe~k = 4.9ms. A profile of the pulse shape from magnet CN93-1 is contained in Fig. 2. A second 14-layer coil (magnet CN93-2) of 6in (15 cm) total height successfully reached 72.6 T with t p e a k = 6.4 ms. In this magnet, due to limita- tions on the supply of high quality Cu-Nb conductor, only six layers were of Cu-Nb and the outer seven layers were of Cu-Alumina.

One attractive feature of these magnets is the reliable prediction of imminent stress-induced failure provided by monitoring the coil inductance during initial magnet training. Fig. 3(a) contains the training history of magnet CN93-1, with peak magnetic field and percent increase of coil inductance plotted versus capacitor bank voltage. Note the relative absence of plastic deformation within the coil even at magnetic fields as high as 65 T. When the coil inductance begins rising dramatically with increasing magnetic field, a decision can be made whether to pre- serve the coil for use in pulsed experiments or to continue trying to reach higher fields at an increasing risk of coil failure. Fig. 3(b) contains the training history of magnet CN93-2. In this magnet, with only six layers of Cu-Nb, the first few Cu-Alumina layers are subject to more extreme stresses than in magnet CN93-1. Fortunately,

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12

Fig. 3. Initial training histories of (a)magnet CN93-1 and (b) magnet CN93-2. Note the relative absence of plastic defor- mation within the coils even at magnetic fields as high as 65 T for magnet CN93-1.

Cu-Alumina is quite ductile and can deform to pass this stress on to the outermost layers. In Fig. 3(b), this addi- tional deformation is evidenced as a larger inductance increase which begins at a lower magnetic field. Note also in Fig. 3(b) that the longer pulse duration for magnet CN93-2 has resulted in increased resistive energy losses, indicated by the non-linear dependence of the magnetic field on capacitor bank voltage at magnetic fields above 55T.

The magnets exhibit several other attractive features. The pulse repetition rate, limited by the cooling time of

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G.S. Boebinger et al./ Physica B 201 (1994) 560-564 563

f / - f ' / - ~

f f f f A

f ~ . f j - j

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" f f / /

Fig. 4. Cross-section of the magnet structure. The coil is wound between epoxy endwheels (gray areas) and contained within a VascoMax steel assembly (diagonal lines). The volume im- mediately around the coil (dotted regions) is filled with beads of stabilized cubic zirconium.

the magnet, is relatively short (<20min between 50 T pulses, ,%< 30 min between 70 T pulses). Also, the mechan- ical destruction in failed magnets is often confined to the precise point of failure. These two features result in part from the design of the structural reinforcement around the magnet coils.

A cross-section of the entire magnet structure is con- tained in Fig. 4. The coil is wound between epoxy end- wheels (gray areas in Fig. 4) and the magnet is placed inside an assembly constructed of a titanium- strengthened 18% nickel maraging steel [14] (diagonal lines). This particular steel was chosen because it main- tains good toughness even at liquid nitrogen temper- atures and is, thus, particularly suitable for pulsed mag- net applications. The volume immediately around the coil (dotted regions) is filled with 1 mm diameter polished spheres of stabilized cubic zirconium. This ceramic was chosen for its high Young's modulus and exceedingly high ultimate compressive strength of 440 ksi (3.0 GPa). The beads are confined by steel rams driven by a series of bolts. This configuration places a rigid insulating support

structure around the magnet which conforms to any irregularities in the outer surface of the coil. It also allows liquid nitrogen to come in contact with the entire outer surface of the magnet, which, coupled with the large conductor cross-section, greatly enhances the cooling rate of the magnet after a pulse.

In addition to any contribution to the realization of higher magnetic fields, the support structure serves to dramatically contain the destructive effects of magnet failure. The steel cylinder provides excellent protection for experimental dewars and other equipment: no debris has ever escaped the steel cylinder. Additionally, the overall rigidity of the assembly has confined the mechan- ical destruction in failed magnets to the precise point(s) of failure. This allows a layer-by-layer post-failure analysis of the coil. As an example, the failure of magnet CN93-1, at a magnetic field above 72 T, resulted in simultaneous and isolated damage to both layers 2 and 5. This suggests empirically that we have distributed the maximum bear- able stress to different portions of the magnet. According to our computer analysis of the coil, the failure in layer 5 is expected: layer 5 is the first layer to undergo unac- ceptable plastic deformation from the distributed mag- netic field stresses. Although layer 2 may have failed as a direct consequence of the failure of layer 5 (or vice versa) we believe that layer 2 failed because it is the Cu-Nb layer which is most severely deformed during the winding of the magnet. Cu-Nb is much less ductile than either Cu-Be or Cu-Alumina: CuNb wires of this dimen- sion have been observed to break during winding when forced around a diameter of < 0.5 in (12.5 mm). As a final note on the relatively quiet failure of magnet CN93-1, we mention that even the field-detecting pickup coil in the bore of the magnet was undamaged by this stress failure above 72 T!

4. Conclusion and outlook

We have pursued a pulsed magnet design philosophy which seeks to accommodate the high stresses by opti- mizing the use of the mechanical properties of the con- ductor itself. These efforts have resulted in a pair of magnets which survive 72T pulses. In these magnets there is no reinforcing insulation in the volume of the magnet turns. Rather, three different conductors of large cross-sectional area are utilized within a magnet. Each conductor is tailored to the specific local requirements of ductility, tensile strength, and conductivity. Based on past experience and future designs, we believe that mag- nets built from existing conductors will establish still higher non-destructive magnetic field pulses of tens of milliseconds duration.

564 G.S. Boebinger et al./ Physica B 201 (1994) 56~564

Acknowledgements

GSB would like to t hank the many people in the field(!) of pulsed magnets who assisted him as the pulsed field l abora tory was being designed and set up. Special thanks to F. Herlach, H. Jones and S. Fone r for their hospital i ty and generous dona t ion of t ime and facilities dur ing visits to their laborator ies in 1988.

References

[1] For a review of recent efforts in magnetic field generation, see other papers in this chapter of these proceedings; also see Proc. 3rd Int. Symp. on Research in High Magnetic Fields, eds. F.R. de Boer, P.F. de Chatel and J.J.M. Franse, Physica B 177 (1992); also see Physical Phenomena at High Magnetic Fields, eds. E. Manousakis, P. Schlot- tmann, P. Kumar, K. Bedell and F.M. Mueller (Addison- Wesley, Redwood City, MA, 1992).

[2] N. Miura, in: Physical Phenomena at High Magnetic Fields, eds. E. Manousakis, P. Schlottmann, P. Kumar, K. Bedell and F.M. Mueller (Addison-Wesley, Redwood City, MA, 1992) p. 589 and references therein.

[3] J.E. Crow, N.S. Sullivan and D.M. Parkin, Physica B 177 (1992) 16.

[4] S. Foner and H.H. Kolm, Rev. Sci. Instr. 28 (1957) 799. [5] A. Yamagishi, K. Tokumoto, K. Taniguchi, O. Kondo and

M. Date, Physica B 177 (1992) 27. [6] J. Bevk, J.P. Harbison and J.L. Bell, J. Appl. Phys.

49 (1979) 6031; J. Bevk, Ann. Rev. Mater. Sci. 13 (1983) 319.

[7] S. Foner, Appl. Phys. Lett. 49 (1986) 982. [8] Fritz Herlach, M. van der Burgt, I. Deckers, G. Heremans,

G. Pitsi, L. Van Bockstal, S. Askenazy, R.G. Clark, H. Jones and J. Mallett, Physica B 177 (1992) 63.

[9] L. Van Bockstal, Guido Heremans and Fritz Herlach, Meas. Sci. Technol. 2 (1991) 1159.

[10] The wire is wrapped in Kapton/Teflon tape. Kapton and Teflon are trademarks of the E.I. duPont de Nemours & Co., Inc. The coils are wet wound using Stycast 2850 FT epoxy. Stycast is a trademark of Emerson & Cuming, Inc., Woburn, MA, USA.

[11] The Cu Be wire is Cu Be Alloy 3 from Brush Wellman, Inc., Cleveland, OH, USA.

[12] The Cu-Nb wire is from Supercon, Inc. Shrewsbury, MA, USA.

[13] The Cu-Alumina wire is Glidcop. Glidcop is a trademark of SCM Metal Products, Inc., Research Triangle Park, NC, USA.

[14] The maraging steel utilized is VascoMax T-200. Vas- coMax is a registered trademark of Teledyne Vasco, Mon- roe, NC, USA.