MAGNET TECHNOLOGY AT THE MULLARD CRYOMAGNETIC LABORATORY

OF THE CLARENDON LABORATORY, OXFORD.

P .A. Hudson

Clarendon Laboratory, University of Oxford

Oxford, UK

Abstract

The original high field installation of theClarendon Laboratory is based on a 2000 kW dcgenerator. This machine now serves 7 water­cooled solenoids, most of them mobile bothvertically and horizontally, giving maximum fieldsranging between 4.5 T (102 mm internal diameter)and 12 T (25 mm internal diameter). The instal­lation has been used for a variety of experimentscomprising nuclear orientation and nuclear cool­ing, magneto-optical spectroscopy, research onthe properties of semiconductors and semi-metals,measurements on high field superconductors andbiochemical applications, and a wealth of tech­nology particularly of the design and constructionof resistive solenoids has been accumulated.

With the advent of high-field, high currentdensity superconducting materials, our magnettechnology has developed in 2 directions.

1) To relieve the pressure on the single powersupply, resistive solenoids are being replacedwherever possible by superconducUve coils.These range from simple coils built into theexperimental helium cryostat to a coil with aroom temperature bore of 51 mm which is to allintents and purposes identical with our standard51 mm Ld. 9 T resistive solenoids.

2) In order to increase the available fields toabout 16 T a hybrid magnet is being constructed.It comprises a 9.3 T, 51 mm bore 2 MW resis­tive solenoid, surrounded by a 6.7 T super­conducting solenoid. In addition a 25 mm boreliquid helium immersed 15 T superconductingcoil is being designed.

The High Field Installation

The high field installation of the ClarendonLaboratory which dates from 1948 now occupiesthe whole of the Mullard Cryomagnetic Laboratorywhich opened in 1963. The facility is centredon a 2000 kW (4500 A, 450 V) dc motor-generatorlocated in a hall adj acent to the magnet laboratorywhich has two levels with three and four magnet

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stations respectively grouped around a centralcontrol area. The third level houses purificationequipment and pumps for circulating deionisedwater used as the

2cooling fluid delivere~at

0.7 MPa (lOO/in. ) with a flow of 2.5 m Imino(550 gal/min.). An air to water heat exchangerof the forced draught type is included in thesystem making possible continuous operation atfull power. Each magnet station serves up tofour fixed experimental gantries in addition tobeing available for experiments more of a tem­porary nature. Mobility is achieved by mountingthe solenoid on jacks on a wheeled trolley andmaking connections to it using flexible reinforcedhose and water cooled power cables which allowfreedom of movement of about 3 metres horizon­tally and 1 metre vertically. Figure 1 showsthe general arrangement of a magnet station.

Fig. 1. General arrangement of a magnet stationin the cryomagnetic laboratory.

Magnets

Development of magnets designed to matchthe impedance of the power supply and to meetthe needs of expanding research programmes hasproceeded along two main lines. In the first

1largely due to Tsai and described in the literaturehard-drawn copper strip is wound under tensioninto a spiral coil on to a central bush of a chrom­ium copper alloy. Pairs of coils of opposite handare assembled with a series connection made bya driving fit between the central bushes. Duringwinding the strip is wound with nylon monofilamentof usually 0.25 mm diameter and at a pitch of3 mm to space the turns in a radial direction andprovide axial cooling passages. Axial supportbetween coils is prOVided by spacers of height2.5 mm machined from a glass-reinforced-plasticlaminate and coated with polyurethane. Two coilpairs are connected in series to produce a coilpackage of simple construction and high reliabi­lity. Improvement over the Fabry factor for arectangular coil of uniform current density is madeby grading the thickness of copper strip betweencoils or sections of coil. Adjustment to the homo­geneity of magnetic field over the central region(e. g. by cancellation of the second derivative ofthe field) is made by alteration to the height ofmedian plane spacer. The coil pairs and axialsupport spacers are assembled over the bore tubeand located on a base plate before being clampedinto position in the outer case also fabricatedfrom glass reinforced plastic, see Fig. 2.

Fig. 2. Components of 9.5 T magnet.

TABLE 1. Characteristics of some standard "Tsai" Magnets

Useful Coil Geometry Overall height Max. working PowerBore 2a

12a

22b of magnet field kW

mm mm mm mm mm

102 115 285 174 450 4.5 T 1200

51 58 260 126 295 9.ST - 9.9T 2000

25 33 172 100 250 10. 5T -10. 7T 2000

a 1 ' a Z inner and outer radii of solenoid

b half length of solenoid

TABLE II. Characteristics of two "polyhelix" Magnets

UsefulBoremm

Number ofsub-coils

Coil Geometry2a

12a

22

bmm mm mm

Overall heightof magnet

mm

Max. workingfield

PowerkW

83

25

7 93 370 205 620 • 7.3 1600

10 33 172 trapezoidal 250 11.2 - 12.0 2000Winding

a 1 ' a2

inner and outer radii of solenoid

b half length of solenoid

• •Thls includes four additional coils for axial profile control of the field in a region330 mm from the central zone.

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The second commonly used design of magnet 02"which much early development is due to Woodand colleagues at the Clarendo~Laboratory anddescribed on several occasions is the "poly­helix", which with concentric sub-coils connectedin series is ideally suited to the high voltagepower supply of the Cryomagnetic Laboratory.Because of the complicated interdependence offactors influencing the design of such high per­formance composite coil electromagnets optimi­sation techniques based on the incremental fieldper watt concept are adopted but are only possibleusing a digital computer, the ICL 1906 A of theUniversity Computing Laboratory. Sub-coils areeither edge wound from hard drawn copper strip ormachined from the solid. Adjacent turns arespaced and bonded with epoxy resin following aphenolic pre-treatment before finish machining tofinal dimensions. Axial cooling channels usuallyO. S mm in height are formed by bonding strips ofglass reinforced plastic to membranes of stainlesssteel coated with an insulating resin which areinserted in annular gaps between the sub-coils OD

assembly. Series connections are made with linksof copper which are either clamped or soldered tothe sub-coil ends.

The installation has been widely used for avariety of experiments including nuclear orienta­tion and nuclear cooling, research on the proper­ties of semiconductors and semi-metals, magneto­optical spectroscopy and biochemical researchand also measurements of the properties of highfield superconductors.

Superconductive Maqnets

The successful development of intrinsicallystable multifilament conductors of niobium­titanium4following early experience with solidcore materialS has resulted in the expansion of aconstruction programme of superconductive mag­nets aimed at the relief of some pressure on the2000 kW power supply and the seven operationalsolenoids dependent on it.

Many superconductive magnets have beenconstructed having a wide range of relativelysimple geometries and incorporated in experimen­tal helium cryostats for operation at 4.2 K andtemperatures below the A.-point to meet the re­qUirements of such applications as nuclearorientation, Raman spectroscopy, Faraday rotationand nuclear magnetic resonance. Typical of sucha system is the nuclear orientation cryostat shownin Fig. 3 which includes a ST coil of 76 mm dia­meter bore wound from 3 kg of superconductingwire of 0.36 mm overall diameter containing 61filaments of Nb 44 wt'f" Ti in a matrix of copper(manufactured by Imperial Metal Industries Ltd.).

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Fig. 3. Cryostat for nuclear orientation studies.

Sixth order correction for homogeneity is achievedby an inside notch machined from a sleeve castfrom filled epoxy resin onto the coil former duringconstruction. Mechanical stability of the wind­ings is ensured by vacuum impregnation with epoxyresin using standard potting techniques and re­sults in reliable operation of the coil on eachsuccessive cooldown at a level close to shortsample performance of the conductor. The con­ductor for this particular magnet was supplied inseveral lengths. The joins made by careful lap­ping and soft soldering are inside t~1J winding andhave a resistance of the order of 10 (2 each.Cryostat design has been largely determined by anexisting experimental assembly and is fairly sim­ple. The annular magnet chamber of stainlesssteel, sealed with demountable indium gasketshas a reservoir of 2.2 litres capacity and is sus­pended from five thin walled (0. 1S mm) germansilver tubes I 0 mm in diameter and 530 mm longthrough which the evaporating helium is returned.The current leads made from brass tube run inside

these german silver tubes. They were optimisedfor a total voltage dro!' of 165 mV and present asurface area of 60 cm per watt dissipated in thelead to the counterflowing gas stream. Brass waschosen as the material for the current leads sinceoverall economies in refrigeration requirementscan be made when the leads are to remain con­nected at the low temperature end yet are usedonly intermittently to carry the full magnetcurrent.

Fig. 4. Split coil for 1 .5 K operation.

The split coil shown in Fig. 4 also woundfrom multifilament conductor of NbTi with an in­dividual filament diameter of less than 30 u. iscapable of stable operation at 1 . B K with a corres­ponding enhancement in performance over operationat 4.2 K6 ,7. This technique is now usefully em­ployed to increase the economic limit of operationof coils made from currently available conductorsof niobium titanium to field generation around lOT.

Currently under construction in our workshopsis a superconductive magnet system designed forlOT operation at 4.2 K with a room temperatureaperture of 51 mm diameter (Fig. 5) which can beused as a substitute for one of the resistivesolenoids described earlier. The magnet is de­signed to operate at 150 amperes and consists ofa three section outer coil of 105 mm bore woundfrom intrinsically stable multifilamentaryconductor of Nb 44 wt. % Ti graded to suit the

local field. The inner coil which contributes 2 Tat 4. 2 K is constructed with double pancakemodules wound from Nb

3Sn ribbon. The design

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of the cryostat is based on a room temperatureaperture system previously constructed in thelaboratory which proved highly satisfactory in use.In order to maintain compatibility with the resis­tive solenoids the overall height of the cryostatwas made small. The coil chamber is suspendedwithin the liquid nitrogen shield by means of two50 mm diameter thin walled (0.25 mm) stainlesssteel tubes with a length to room temperature of590 mm. It is thus expected that the liquidhelium evaporation rate will be less than 1 litreper hour, the loss rate being accounted for byradiation and the thermal loading of the optimisedhigh current leads. The coil chamber is held inposition against sideways movement within theliquid nitrogen shield by six stainless steel wiresO. 5 mm in diameter. The liquid nitrogen shieldis similarly supported within the outer vacuumcase.

Fig. 5. General scheme of room temperatureaperture system.

Higher Field Magnets

There is a growing need in the CryomagneticLaboratory for continuously maintained magneticfields in the range 15 T to 20 T. It is consideredimpracticable at present to meet this need with anincrease in power and an extension to cooling

capacity. An additional 5 MW to 7 MW would berequired by resistive magnets for the brute-forceproduction of fields in the above range in a boreof 50 mm diameter.

Two other possible ways of producing fieldsabove 15 Tare:

1) To construct an all superconductive magnet;

2) To construct a combined supercondugtige18ndresistive system or "hybrid" magnet. I I

Both methods are being adopted.

For a whole range of experiments a 25 mmbore at 4.2 K is adequate. Furthermore thegeneration of 15 T in such a bore is within thepresent capability of the technology of supercon­ductive magnets and can be economically achieved.Thus as a valuable complementary facility a smallbore Nb Sn solenoid of s~iral wound modular de­sign is Eeing constructed with the above para­meters.

As a larger bore facility the hybrid magnetwill make efficient use of the 2000 kW installationand be compatible with existing 51 mm bore resis­tive solenoids in the Cryomagnetic Laboratorysuch that experimental equipment already develop­ed can be readily aC99mmodated. The design o,fthe combined system is aimed at the generatlOnof 16 T and this will provide a significant increasein large volume field for a minimum of cost. Themagnet consists of an inner water cooled solenoidwith a working bore of 51 mm diameter generating9.3 T I surrounded by a superconducting solenoidhaving a room temperature bore of 250 mm andproducing 6.7 T. All water and power connectionsare located at the bottom end and the inner sole­noid can be removed as a unit without disturbingthe cryostat and warming up the superconductingsolenoid. Thus any other compatible insert canbe introduced or the outer supercon/ductive sole­noid can be independently used as a large borefacility.

The maximum central field produced by theouter section alone is 0.2 T less than its contri­bution to the hybrid combination because the stableoperating current is reduced by an effectivelyhigher peak field at the superconductor.

A major problem in the design of a hybridmagnet system is the support of forces resultingfrom the magnetic interaction between the principlesections. Under normal running conditions theaxial force is a restoring force and the equilibriumposition is stable. However I consideration must

• In collaboration with Intermagnetics GeneralCorporation.

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be given to an asymmetric failure of the high­power water-cooled solenoid which would resultin a relative displacement of magnetic centres andthe introduction of a large accelerating force. Asmall radial displacement gives rise to an unstablesituation with an increasing radial force. Be­cause of cryogenic considerations and the need toreduce the refrigeration load I the superconductivesolenoid and the resistive solenoid cannot bestrongly coupled mechanically. It is thereforedesirable to minimise the forces which can poten­tially act between the two individual sections.

Design of the inner room te~I2erature solenoidis a development I due to Carden I of the "poly­helix" scheme described earlier and consists ofnine concentric coils connected in six seriesgroups. There are two particular advantages ofchoosing concentric helical coils for the innersolenoid over the simpler Bitter type of con­struction. Firstly I by isolating the individualcoils both mechanically and electrically the pro­p,agation of a local failure is limited to a singlecoil or pair of coils and the maximum force thatcan develop under fault conditions is considerablyreduced.

Secondly I because the large clamping struc­ture required for axial compression in a discsolenoid is not needed for the coils of the poly­helix I the bore of the superconductive magnetcan be made smaller. Axial location of thehelical coils is provided by cooling water pressure.

Modular construction of the superconductiveouter magnet using rectangular section conductorspiral wound into double pancakes enables bestadvantage to be taken of variations in criticalcurrent between the individual lengths of con­ductor and also means that damaged sections canbe replaced with relative ease should this becomenecessary. In addition to satisfying the con­ditions for it to be intrinsically stable I the con­ductor is also designed to have cryostatic stabi­lity. This should thus ensure reliability andpredictable operation of the superconductingsolenoid even under conditions such as rapidshut-down of the inner solenoid which could re­sult in the adiabatic stability criterion beingviolated.

Acknowledgement s

For their assistance with work reportedthanks are due to colleagues of the ClarendonLaboratory; also to P.E. Hanley and The OxfordInstrument Company and p.O. Carden of theAustralian National University and Research Tech­nology I Canberra, for their valuable contributionto the hybrid magnet project.

References

1. J.M. Daniels, Proc. Phys. Soc. B. LXIII,1028 (1950).

2. M.F. Wood, High Magnetic Fields (MIT Press,Cambridge, Mass, 1962), 387.

3. see, e.g. N. KurU, J. Inst. Metals~, 5(1964-5) •

4. P.F. Smith, M.N. Wilson, C.R. Walters,J•D. Lewin, Proc. Summer Study on Super­conducting Devices and Accelerators, (Brook­haven National Laboratory, 1968), 967.

5. J.A. Good and P.A. Hudson, Cryogenics~,

64(1969).

6. R. Hampshire, J. Sutton and M.T. Taylor,Conference on Low Temperatures and ElectricPower, London, 1969, 69.

7. M.N. BUtcliffe, P.E. Hanley, J.B. McKinnonand P. Roubeau, Cryogenics li, 44 (1972).

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8. see, e.g. M.F. Wood and D.B. Montgomery,Les Champs Magnettgues Intenses leur pro­duction et leurs applications, Grenoble, 1966(editions du Centre National de la RechercheScientifique, Paris, 1967), 91 .

9. see, e.g. D.B. Montgomery, I.E.C. Williams,N •T. Pierce, R. Weggel and M. J. Leopold,Advances in Cryogenic Engineering U, 88.

10. P .A. Hudson, A Hybrid Magnet in The Claren­don Laboratory (1970) (an unpublished CLstudy) .

11. P.E. Hanley, Applied Superconductivity Con­ference, Annapolis, 1972 t p. 302.

12. P.O. Carden, J. Phys. E.~, 654 (1972).