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Physica B 346–347 (2004) 553–560

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doi:10.1016

First experiments in fields above 75T in the European‘‘coilin–coilex’’ magnet

Harry Jonesa, Paul H. Fringsb,*, Michael von Ortenbergc, Alex Lagutind,Luc Van Bockstale, Oliver Portugallf, Fritz Herlachd

aClarendon Laboratory, Oxford OX1 3PU, UKbVan der Waals-Zeeman Instituut, Universiteit van Amsterdam, Valckenierstraat 67, NL Amsterdam 1018XE, The NetherlandscMagnetotransport in Solids, Institute of Physics, Humboldt University at Berlin, Newtonstrasse 15, Berlin D-12489, Germany

dLaboratorium voor Vaste-Stoffysika en Magnetisme, Katholieke Universiteit Leuven, BelgiumeMetis Instruments and Equipment n.v., Leuven, Belgium

fLNCMP (Laboratoire National des Champs Magn!etiques Puls!es), 143, Avenue de Rangueil, Toulouse 31432 Cedex 04, France

Abstract

Magnetic fields above 75T were generated in a pulsed mode by energising simultaneously two concentric coils

utilising the 14MJ capacitor bank and a small 110 kJ ‘‘mobile’’ capacitor bank at the ‘‘Laboratoire National des

Champs Magn!etiques Puls!es’’ (LNCMP) in Toulouse. The aim is to develop a user-facility for magnetic fields up to

80T. The feasibility of this approach has been validated by optical and magneto-transport measurements up to 76T

and the equipment is now available to users.

r 2004 Elsevier B.V. All rights reserved.

PACS: 1.25; 7.55; 72.20.My; 73.43.Qt

Keywords: High magnetic fields; Pulsed field; Magnetic semiconductors; User facility

1. Introduction

1.1. History

In order to generate the highest possiblemagnetic fields in an economical way, an approachwith several coils and power or energy supplies isthe most promising. One of the earliest realisationswas based on three independent capacitor banks

onding author. Tel.: +31-20-5255744;

0-5255788. Present address: LNCMP Toulouse,

6-2172977; fax: +33-56-2172816.

ddress: frings@cict.fr (P.H. Frings).

- see front matter r 2004 Elsevier B.V. All rights reserve

/j.physb.2004.01.081

[1]. An original design based on two coils but onlyone power supply was proposed in Ref. [2]. AtOxford, a hybrid system based on a superconduct-ing outer coil and a pulsed inner coil was proposed[3]. The multi-coil system was applied at Felix inNieuwegein to generate very short pulses [4], andover 60T was produced in Amsterdam using atwo-coil system in combination with a pulsedmains power converter and a capacitor bank [5]. Along-pulse system based on the multi-coil designhas been successfully operated up to 60T at LosAlamos [6]. A mathematical optimisation for twocoils driven by capacitors banks was derived atToulouse [7]. In this paper the terms ‘‘coilin’’ (for

d.

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Table 1

Capacitor bank parameters

coil-ex bank coil-in bank

Maximum voltage (kV) 24 8

Maximum current (kA) 65 24

Minimal rise time (ms) 22 1.5

Maximum energy 14MJ 112 kJ

H. Jones et al. / Physica B 346–347 (2004) 553–560554

the inner coil) and ‘‘coilex’’ (for the outer coil)were introduced.

1.2. Motivation

A system with two (or more) coils gives inprinciple more design freedom. Since the power orenergy supply tends to be the most expensive partof a pulsed field installation, it is not surprisingthat this design freedom is used in the first place tobetter adapt the coils to the supply. Sources thatcan provide large energies tend to have rather longrise times (i.e. relatively low power) whereassmaller energy sources can have shorter rise times.The logical approach is thus to energise the coil-ex,which requires a lot of energy due to the extent ofits field and its large volume (because for a giveninner bore of the coil-ex the outer diameter shouldbe at least 2–3 times the inner diameter in order tomake a mechanical efficient coil), by a large andrelatively slow capacitor bank. The coil-in requiresonly moderate energy, thus it can be energised by asmall but fast capacitor bank. This approach hasseveral additional advantages: we can optimisestrength, pulse duration and heating along the coil.Moreover, the small volume of the coil-in permitsthe use of high-strength wire that normally is onlyavailable with short length and small cross-section(and could as such never be used for the entirecoil). Last, but not least, this approach limits therisk since the major volume of the coil (the coil-ex)can be designed with a larger safety margin,whereas, in order to produce the highest fields,one can experiment with the coil-in design andmaterials close to the limits.

2. The project

2.1. Project description

The ARMS project is based on the unique14MJ pulsed energy source at Toulouse [8]. Basedon previous calculations [7] it was chosen to aim ata coil-ex field of 45 T and a coil-in field of 35T inorder to build an 80T user magnet. A userinstallation means, among others, that the aimwas not to produce only a one-pulse record field

but a reliable set-up aiming at X50 coil-in pulsesand X200 coil-ex pulses at high field. Given thecoil-ex field, the available energy of 14MJ and theexperience with, and availability of, copper-stain-less steel (CuSS) wire, a coil-ex with a maximuminner bore was designed. The technical details ofthe coil-ex capacitor bank and the coil-in capacitorbank are given in Table 1.

2.2. Coil-ex wire production

The general principles and rationale behind thecopper/stainless-steel conductor have been de-scribed elsewhere [3,9–11]. But, briefly, a thick-walled tube of stainless steel is filled with roundcopper bar and this is then worked down by avariety of processes, using intermediate annealingheat treatments, to a predetermined diameter,beyond which it receives no further heat treat-ments. It is then cold drawn down to a final rounddiameter just over the final desired cross-sectionalarea. After that, the drawing process uses rectan-gular dies, which results in a rectangular cross-sectional wire of the chosen aspect ratio. Thepredetermined diameter of final anneal is chosenso that the wire ends up with a total area reductionof 80–85% with no more anneals. This introducesextreme cold work and this gives the wire its greatstrength.

In the case of the ARMS wire, the cross sectionchosen was a large as possible. The final size wasfixed at 5.6� 3.8mm2, (aspect ratio 1.47) andcorner radii of 0.5mm. At Oxford, the workingdown to the final anneal diameter has traditionallybeen done in industry with the finishing cold workbeing done in-house. This latter process is slowand tedious and requires very expensive specialisedlubricants. Because of the unprecedented quantityand size of this wire, a trial was conducted

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H. Jones et al. / Physica B 346–347 (2004) 553–560 555

wherein, after the final anneal size was reached, thecompany continued to draw on to B70% beforethey delivered the wire to the Clarendon Labora-tory for finishing. This proved successful so weadopted this procedure for all the wire that wasproduced for ARMS.

The raw material for the wire comprises a tubeof AISI 304 l stainless steel (OD 2 in (50.8mm), ID1.5 in (38.1mm)) and 1.5 in diameter bar of copper0.05% silver. In order to reduce the number ofjoints in the magnet, the starting length of thebillet needs to be as long as possible but it isdetermined by the capacities of the down-the-linedrawing equipment the company uses, which inthis case meant it was limited to 1.3m giving apiece weight of 23 kg. The copper was slightlyreduced in size using centre-less grinding so that itcould be fitted into the steel. The initial reductionis done using cold rotary forging. After that,straight drawing is carried out on a draw benchand finally a bull block is used to achieve thesmaller sizes.

The wire was insulated by an industrial1

company with a single wrap of Kapton at 50%overlap and a protective outer layer of a wrap ofpolyester impregnated glass fibre (total increase inthickness 0.3–0.35mm). Some 600 kg, equivalentto 26 full size billets, were produced in all.Processing logistics meant that it was not possibleto achieve 23 kg of wire from a billet as there weresacrificial processes, some breakages and theproblem of the excess from the stock lengths oftubes and rods (typically 7m), which wereprocessed for maximum economy. The largestwire length was about 20 kg equivalent. Theproduction of this wire was not without itsproblems. But finally sufficient wire was availablefor three coil-exes. In fact, this part of the projectwas a significant success as this was the first timesuch a quantity of such a large cross section ofcopper-stainless-steel wire was produced. Thetypical ultimate tensile strengths were >900MPaat 300K and 1.2GPa at 77K.

It has long been postulated that using a highnitrogen stainless steel may give beneficial in-creases in strength at 77K. This material is not

1http://www.buisin.com/

routinely available in thick walled tube, however.During ARMS, through the interaction with asister EU funded programme [12] a Dresden—Oxford collaboration [13] showed that using HighN steel does bestow benefits and may be the wayforward with this conductor.

2.3. The coils

2.3.1. Coil-in

The task of developing and manufacturing innercoils was shared by two partners in the project.The industrial partner Metis made user coils onbasis of proven technology, and the K.U.Leuvengroup developed coils on basis of advancedmaterials. Both partners used the same outer shelland the same contact terminals for compatibility(Fig. 1). All coil-ins have a protective outer steelshell with 8mm wall thickness, the outer diameteris 100mm. The wiring from the contacts to the coilis critical because these wires pass through the fieldof the outer coil and therefore are subject to a largemagnetic force; it was found that these had to beheavily reinforced by casting them in an epoxycomposite. Each partner made three differentcoils; the parameters of these are given inTable 2. All coils had a bore of 15mm, and the

Fig. 1. Coil-in, coil-ex assembly.

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Table 2

Data for coil-ins and coil-ex

Coil Length (mm) Layers L (mH) R77K (mO) I/B (A/T) Reinforcement Wire

Metis1 80 8 1.36 77.6 201 S-glass Soft Cu 1.8� 1.0

Metis2 78 8 1.25 75.5 208 S-glass Soft Cu 1.8� 1.0mm2

Metis3 77 8 0.56 37.8 310 S-glass Glidcop Al15 2.7� 1.0mm2

KUL1 100 12 3.51 362 139 Zylon CuNb 1.9� 1.0mm2

KUL2 70 8 0.41 26 373 Zylon CuNb 3.7� 2.2mm2

KUL3 75 10 2.1 99 156 Zylon Soft Cu 1.7� 0.9mm2

AMS2 270 26 125.5 193 231.6 — CuSS 5.6� 3.8mm2

H. Jones et al. / Physica B 346–347 (2004) 553–560556

outer diameter was 84mm in order to fit into theprotective shell.

The first two coils fromMetis, M1 and M2, weremade with the proven combination of soft copperand optimised reinforcement by glass fibre—epoxycomposite that has been employed for the usercoils at K.U.Leuven with an excellent record ofdurability [14]. M3 is under construction; it ismade with the combination Glidcop-glass fibrethat demonstrated excellent performance in coilsmade at Leuven for the NHMFL at Los Alamos.

For the K.U.Leuven coils, Zylon was used; forKUL1 and KUL2 in combination with Cu–Nbmicro-composite wire and for KUL3 with softcopper. As there is not yet much accumulatedexperience with these advanced materials, as a firststep several monolithic coils with these materialswere made and tested to destruction [15].

The desirable properties for the Cu–Nb wire hadbeen determined on basis of previous experienceand a series of design studies; these wires weremade to specifications at the Bochvar Institute inMoscow. These do not have the highestpossible UTS; we rather opted for ease of windingand for matching the strain range of Zylon. Thewire was designed to remain essentially elasticover the entire strain range of Zylon. The stress–strain relation is given in Ref. [16]. The Bochvarwires were insulated with a double Kaptonwrap that was not glued to the wire in order toavoid deterioration of the wire due to the heattreatment associated with the gluing procedure. Atthe outer radius, a layer of glass fibre wasadded; this was machined to fit precisely intothe steel shell that was shrink-fitted around thewinding.

Before delivery to Toulouse, all inner coils weretested for high voltage and up to the field level thatthey would contribute to the coil-ex/coil-in com-bination. Coil KUL3 was made with soft copper inorder to obtain higher field and longer pulseduration with the given coil-in capacitor bank.This coil is still on the shelf. According to stressand heating calculations, this coil ought to qualifyas an 80T user coil.

2.3.2. Coil-ex

The coil-ex was based on the experience atToulouse and Oxford in producing and usingCuSS wire and designed on basis of the Amster-dam design principles [17,18]. The coil was woundusing the 3.8� 5.6mm2 CuSS wire that wasproduced at Oxford. Layer to layer transitionswere carried out by means of filler pieces producedon a computer controlled milling machine. Thetotal amount of wire required for the entire coil-examounted to 145 kg, and the amount for theoutermost layer was approximately 8 kg. Since themaximum final batch size of the wire was 19 kg,several joints had to be incorporated into the coil.These joints were positioned at the crossover fromone layer to the next and thus resulted in anautomatic transition to the radius of the next layer.The ends of the wires from consecutive batcheswere placed over each other with an overlap ofapproximately 150mm, and then joined over thisentire length by arc welding the stainless steeljackets together. This part of the wire with twicethe thickness (in the radial direction) was pre-formed before welding in order to follow thewinding at the proper radius.The strength of thejoint obtained in this way was >50% of the UTS

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of the wire, and the increase in resistance could belimited by making the joints sufficiently long. Thecoil was reinforced on the outside by a steelcylinder of 25mm thickness. For this reinforce-ment, high strength steel combining good low-temperature properties with large plasticdeformation was required. The precipitation har-dened AISI 4340 fulfilled these requirements whenquenched at 855�C and tempered at a relativelylow temperature for a long time (4 h at 230�C).After this treatment the material showed anultimate tensile strength of 2.3GPa and anelongation of >10% at room temperature. Thecoil-ex was protected in order to maximise itssurvival chances in case of a coil-in incident. Theprotection consisted of stainless tube integratedwith the coil-in (this part was also used to fix thecoil-in with respect to the coil-ex) and of twoconcentric G10 tubes (wall thickness 1mm) thatwere part of the coil-ex. To provide an option toextract the coil-in from the coil-ex after explosion,a space of 2mm between the two concentric G10tubes was filled with grease. The idea is that at lowtemperature (and for short times) the solid greasewill not be reduced in thickness during a coil-inexplosion. After heating to room temperature orabove, the grease will become soft and assure,hopefully, sufficient clearance for coil-in extrac-tion. A detailed description of the coil-ex will begiven elsewhere.

2.4. Experimental setup

The coil-in and coil-ex were mounted togetherby means of the flange connected to the coil-incylinder and suspended in a liquid nitrogen dewar(see Fig. 1). This assembly was installed in atypical magnet test cell at the ‘‘LaboratoireNational des Champs Magn!etiques Puls!es’’(LNCMP) in Toulouse [8]. In the test cell thesmall generator for the coil-in and the dataacquisition equipment were installed as well. Carewas taken to avoid any galvanic connection to thecontrol room (outside the cell). The 14MJcapacitor bank is installed in the basement; it isconnected to the coil-ex via copper strips. Thepolarity of the 14MJ capacitor bank can bereversed and great care has been taken to assure

that the coil-in and coil-ex field had the samepolarity. An analogue delay circuit ensured thatthe coil-in was automatically energised at asufficient delay with respect to the coil-ex pulse(approximately 90ms).

Prior to the experiments, the coil-ex and variouscoil-ins were tested in a systematic way. Since thecoil-ex required a long time to cool down, astandard test consisted of slowly increasing thefield of the coil-in in a rather low coil-ex field up tothe maximum field for the coil-in and thengradually increasing the field of the coil-ex whilekeeping the coil-in field near maximum field. Thetotal number of pulses above 60T for two coil-insand one coil-ex amounts to more than 60, with 15pulses above 70T. The coil-in M1 failed at 68T ina background field of 39T without damaging thecoil-ex. KUL2 and the coil-ex are still in goodworking condition and do not show any signs ofimminent failure. The pulse shape of a maximumfield pulse is shown in Fig. 2.

2.4.1. Digital lock-in

The realisation of the coil-in/coil-ex concept waschosen in such a way that the pulse length of coil-in is sufficiently long to apply lock-in techniquesfor the signal detection. For the 5ms pulseduration of the coil-in, i.e. the high-field section,a modulation frequency of about 50 kHz seemsadequate, so that 125 full periods were probing thehigh-field range of the coil-in, resulting in about 4periods/T.

Any modulated response signal was recordeddigitally by a fast transient recorder with asampling frequency of 500 kHz, such that eachmodulation period was based on 10 sampledpoints. The total number of measured data pointswas 210� 1000 such that the maximum samplingtime with about 2 s exceeds well the pulse length ofthe coil-ex.

The evaluation of the modulated response signalwas performed by two different digital lock-intechniques. The first method was developed in Ref.[19] and applies the sliding average technique. Thesecond method is based on Fourier transforma-tions using filter-like apodisation.

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0

10

20

30

40

50

60

70

80

0 0.02 0.04 0.06 0.08 0.1 0.12 0.16 0.18 0.2

time [s]

B [T

]

40

45

50

55

60

65

70

75

800.102 0.103 0.104 0.105 0.106 0.107 0.108 0.109

time [s]

B [T

]

>200 ms

>3 ms

0.14

Fig. 2. A typical pulse at maximum field; full line is total pulse,

dotted line is enlargement at maximum field.

H. Jones et al. / Physica B 346–347 (2004) 553–560558

2.4.2. Liquid He and N2 cryostat

The cryogenic system of the set-up consists oftwo stages. The first stage provides cooling to 77Kby a simple foam-insulated dewar containing theentire coil-in/coil-ex system including the bathcryostat for cooling the sample to temperatures ofliquid He. This helium cryostat is fabricated in allparts of stainless steel in a standard design andreaches a minimum temperature of about 1.6K inthe double-wall vacuum insulated tail whenpumping on the He-vapour above the liquid He.The inner and outer diameter of the tail were 10and 14mm, respectively. The sample holder isinserted into the cryostat via a NW25 flange with ausable inner diameter of 24mm. The He-cryostatis well insulated from any conducting part of thecoil-in/coil-ex system.

3. The experiments

3.1. An optical experiment

A high-excitation luminescence experiment onInAs/GaAs self-assembled quantum dots up to73T was the first experiment successfully per-formed in the system. These results are reported ina separate paper at this conference [20].

3.2. A magnetogalvanic experiment

The performance of any magnetic fieldgenerator can best be characterised by a high-

sensitivity galvanomagnetic measurement. Espe-cially for pulsed magnets, this kind of experimentsprovides a good test of the practical limits for themeasuring sensitivity caused by unavoidable pick-up in contact loops, etc. That is why we havechosen a bulk HgSe:Fe sample providing amodulation amplitude of the transverse voltagedrop across the probing contacts of only0.24mV at zero magnetic field for a currentamplitude of 400mA [21]. As a matter of fact iswas shown that the actual transverse voltage dropbetween the probing contacts was orders ofmagnitude less than the induced pick-up voltageproduced by the coil-in despite most carefulminimising of the contact loop. Theoretically, thephase-sensitive detection could easily handle sucha problem, provided that the modulation signalitself is not distorted. The high pick-up amplitude,however, drives any amplifier in the measuringsetup into saturation, so that no response signal isdetected. Careful additional electronic compensa-tion of the pick-up signal and low-level amplifica-tion helps to minimise this shortcoming.A schematic of the experimental setup is given inFig. 3a.

The gauging of the magnetic field was per-formed by measurement of the pick-up voltage in acalibrated coil with subsequent numerical integra-tion; this agreed within 2% with the value deducedfrom the currents.

3.2.1. The experimental setup and first results up to

76 T

The bulk HgSe:Fe sample of about 3� 0.5�0.5mm3 was contacted by two current- and twoprobe-contacts (Fig. 3b) using Au-wires in trans-verse configuration. Special care was taken tominimise any remaining cross-section of the wireloops to decrease the pick-up voltage originatingfrom the coil-in. To suppress this pick-up voltageto a tolerable level, additional electric compensa-tion had to be applied. It should be noted that onthe one hand the maximum pick-up amplitudeshould not drive the preamplifier into saturation,and on the other hand the signal amplitude shouldnot yet be affected by the quantisation noise of the12 bit custom-made battery-driven ADC transfer-ring the digitised signal by light fibres to the data-

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LPADC 1

ADC 2

Sample

Mixer

Compensation Coil

Field Pick-up Coil

48 kHz 1:33DigitizerPreAmp Low Pass

Common

Compensation Coil

Field Pick-up Coil

Potential ProbesCurrent

Sample

(a)

(b)

Fig. 3. (a). Schematic set-up of the electronic measuring circuit,

(b). Schematic set-up of sample-holder.

Fig. 4. Magneto-galvanic results on the transverse magneto-

resistance of HgSe:Fe in a non-destructive field up to 76T. The

magnetic field ranges of coil-ex and coil-in are marked. The

amplitude of the measuring voltage at 0T is marked by

0.24mV.

H. Jones et al. / Physica B 346–347 (2004) 553–560 559

processing PC. Ideally, any galvanic couplingbetween the measuring system and the magnet/power-supply as well as with the data-processingsystem is excluded. During the first experiments,however, a connection between the two lattercomponents could not be avoided, such that alarge noise contribution of ground fluctuationsduring the pulse affected seriously the quality ofthe measured data. In Fig. 4 the data for theShubnikov–de Haas effect of the transversemagneto-resistance of the HgSe:Fe sample up to76T are reproduced for the down-sweep of thefield. To our knowledge these data show for thefirst time results obtained in a non-destructive coilsystem up to 76T. The field ranges correspondingto coil-ex and coil-in are marked. For theevaluation of the data the total of all 210� 1000data points was necessary and special routinesbeing able to handle such quantities of data had tobe applied.

We emphasise that by the choice of HgSe:Feas sample material we have intentionallydriven the requirements on the measuringelectronics to its limits for characterising the coil-in/coil-ex system as a user-magnet despitethe strong change of sweep-rates during theexperiment.

4. Conclusions and future plans

The system has performed up to now asexpected. Measurements in fields above 75T canbe performed in a reliable way. The number ofhigh-field pulses realised to date suggest that areasonable number of high-field pulses can beobtained with the current coils.

An IR-magneto-transmission set-up using thesame cryostat in connection with IR-transparentAgBr/AgCl light fibres for measurements of thecyclotron resonance with 10.6 mm radiation as wellas a magnetisation setup applying the compen-sated pick-up coil technique is under construction.The system will be included in the magnet stationsavailable for the users at the LNCMP.

In the immediate future some minor problemswill be addressed in order to improve theperformance of the system. These include (i) a‘‘soft start’’ of the coil-in pulses in order to reducevibration, noise and amplifier overload problems,(ii) increasing the coil-in pulse duration (since thecoil-in pulses are presently not heat limited this is a

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very economic way to increase the pulse duration),(iii) improvement of the grounding system to avoidground loop problems.

Acknowledgements

The authors wish to thank the European Unionfor their support under the 5th frameworkprogramme (grant: HPRI-CT-1999-50007). Wealso want to thank all our colleagues of the Vander Waals-Zeeman Instituut (Amsterdam), Insti-tute of Physics of the Humboldt University atBerlin, Metis Instruments & Equipment n.v.Leuven, Laboratorium voor Vaste-Stoffysika enMagnetisme, K.U.Leuven, HFML (Nijmegen),Clarendon Laboratory (Oxford), Inst. MASPEC(Parma) and the ‘‘Laboratoire National desChamps Magn!etiques Puls!es’’ (LNCMP, Toulouse)that were partner in the programme and contrib-uted with their valuable advice to the success of theproject. Special thanks go to Kris Rosseel (KU-Leuven) for help in winding and designing the coil-ins, Tony Hickman and Pete Richens (Oxford) forproducing the wire for the coil-ex and Hans-UlrichMueller (Berlin) for his help in correctly setting upthe data acquisition chain.

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