A LIGHTWEIGHT SUPERCONDUCTING MAGNET FOR A TESTFACILITY OF MAGNETIC SUSPENSION FOR VEHICLES

K.Hirai, S.Akiyama, H.Fujino and K.OnoderaCentral Research Laboratory, FUji Electric Co., Ltd.

Japan

Conductor

Table 1. Specifications of the conductor

II. Superconducting coils

Usually the large superconducting coils arecompletely stabilized with the large quantity ofcopper. The overall current densities of co~

pletely stabilized coils are relatively small.Therefore, the weight of conductor is one offactors increasing the weight of magnet.

To increase the stability of conductor andreduce the weight of it, we have taken account ofthe criteria of the intrinsic stabilization bymultifilament composite wires and of minimumpropagation current.

161100 mm

6.3140

1.8 mm x 3.2 mm80 ~ diameter Nb-Ti

178~1910 A at 3 T

990 A at 5 TPVF :20 ~ thick700 m

The conductor consists of 161 filaments of80~ diameter Nb-Ti alloy embedded in OFHC coppermatrix, and is coated with polyvinylformal sothat the thickness of film is 20~ thick. Thecopper ratio of the conductor is 6.3, and thestable current is about 10% larger than thenominal current 855A.

The diameter of a superconducting filamenthas been determined from the criterion2 for theadiabatic stabilization. The insulation thicknesshas been determined to make the heat transfer co­efficient greater than 0.lW/cm2K.

The superconducting wires were produced bythe Furukawa Electric Co., Ltd.

Figure 1 and Table 1 show the cross sectionand the specifications of the conductor.

structure and mUltifilamentary superconductingcomposites as the conductor. However, as thismagnet has required the special arrangementof superconducting coils, the design of thecryostat has been modified partially. Thecryostat is ring-shaped and its cross section isarch-shaped, and a pair of C-shaped modified racetrack coils are installed in it. The magnet isdesigned to be operated in the persistent mode.

The experiments was performed by using thefacility of the Railway Technical ResearchInstitute of JNR.

InsulationUni t length

Cross sectionSuperconducti ngfilamentNumber of filamentsTwist pitchCopper ratioHesidual resistanceratio of OFHCCritical current

For the magnetically suspended high speedtrain a light weight superconducting magnet isrequired. As one of the preliminary studies wehave constructed such a light weight magnet fora magnetic suspension test facility and haveperformed magnetic suspension experiments. Thedesign concept of the cryostat has been based on atubular structure, but modified partially for therequirement of the facility. The cryostat includesa pair of C-shaped modified race-track coils withthermally operated persistent switches. Supportsto carry the lift forces induced on the coils arecomposed of FRP tUbes with hinge connectors. Theconductor of coils is the twisted multifilamentarysuperconducting composite.

The main features of this magnet areConfiguration: A pair of C-shaped coilsConductor: 1.8 x 3.2 mm2 Nb-Ti copper

composites having 161 fila­ments of 80~ diameter (twisted)

Current andAmpere turns: 855 A and 200 kATCryostat dimensions: 1540 mm in a.d.

260 mm in i. d .560 mm in height

Weight: conductor 29.5 kg,coil and cryostat 650 kg

Field: maximum 2.3 T, center 0.7 TStored energy: 4.5 x 104 J

The magnet \IDrked very stably in the exter­nally excited mode and the persistent mode. Themaximum levitating height was 8Omm.

I. Introduction

Abstract

Since several years ago, magnetically sus­pended trains have been proposed for high speedpassenger trains by many investigators. TheJapanese National Railways is developing thesuperconducting magnetic suspension system drivenby linear motors for the train with the averagerunning speed of 450km/h. In this system thesuperconducting magnets have to be designed underthe aspect of high safety, small size and lowweight.

Several possible magnet structures have beenproposed by Powell and Danbyl, and others. Wethink, however, that the pipe structure whichPowell and Danby have proposed is better one. Asto structural materials, the stainless steel whichis used for conventional magnets should be re­placed by aluminium or titanium to reduce theweight of the magnets.

We have constructed the magnet for a testfacili ty of magnetic suspension. This magnethas been designed considering to reduce the weightof the cryostat by applying the concept of tubular

41

Fig. 1. Cross section of the superconductingcomposite

Coil structure

The coils have been v.ound solenoidally, andeach layer of windings is separated by the insu­lated pieces of 1.5mm thickness and of 4mm width,which are placed at 30mm intervals along theconductor, so as to make the coolant channelvertical. The heat transfer of the liquid heliwn­conductor interface may be influenced l::y thedesign of cooling channels, and the verticalcoolant channels is expected to make the perfor­mance of the heat transfer better.

The coils have been v.ound as rigidly aspossible on the stainless frames with reinforcingribs, and have been tightened with the circularframes and the end sustaining arch-shaped framesfrom the outside. The spacers of the insulatedcopper have been fastened on the conductors bythe cryogenic adhesive, so as not to displace.

The coils structure is shown in Fig.2 andFig.3, and the specification in Table 2.

Fig. 2. Superconducting coil

Table 2. Specifications of the Coil

Ampere turns 200 kAT

Nominal current 855 A

Number of layers 18

Number of turns in a layer 13

Cross section 58 mm x 43 mm

Maximum field strength 2.3 T

Center field strength 0.7 T

Weight of conductors 29.5 kg

I~II

IoEIE,---- 58 ... 1

Fig. 3. Sketch of the cross section of thesuperconducting coil

42

Stability test of coil

The stability has been tested by a model coilwhich has the same cross section with that of thelevitating magnet. Figure 4 shows the predictedstable region (under the solid line) and theobserved values (circles denote stable and crossesunstable) .

(Aj2000J-----l----I--~-+---1

750011------'-=.:..f':.:....:::..::~-+--__+_----'!~+

5001-----t-;tL---+---+--_+_

7 2 Jfvfognetic Field rTj

Fig. 4. Minimum propagation current and loadlines of the superconducting magnet forlevi tation and the model magnet(1) Critical current(2) Minimum propagation current(3) Load line of the magnet for

levitation(4) Load line of the model magnet

Thermally operated persistent switch

Two thermally operated superconductingswi tches have been provided so that the magnet canbe operated in the persistent mode. They havebeen located between the coil ends of two coilsso as not to be influenced by magnetic field ofcoils. The switches have consisted of the bundlesof superconducting filaments and the heaters, andhave been assembled in the case with the 60rnmouter diameter and the 4lmm height. The resis­tance at the normal state is 60 ~ 70mQ at 4.2K.Figure 5 shows the persistent switches.

The response of the persistent switches hasbeen so quick as to v.ork wi thin few seconds afterswitching off heater. The decay of the persistentcurrent has been too slow to be detected in aboutone hour. The evaporation rate of liquid heliumdue to the heater. of a persistent switch is about5 liters/h, but the quantity of liqUid heliumevaporated by the heater over a operating cycleof the magnet is less than one liter per swi tchbecause the v.orking period of the heater has beenabout 9 minutes which is the period to excite themagnet.

43

Fig.5. Thermally operated persistent switch

II I. 'Cryostat

GeneralThe superconducting magnet for the high speed

train is required to be light in weight, short inheight and small in the consumption of liquidhelium. Considering these requirements, we havedesigned conceptUally the cryostat to be installedon the high speed train, as shown in Fig.6. Thefeatures of this cryostat are as follows:a) By adopting the tubular structure, the thickness

of the wall of cryostat is reduced as comparedwith the cylindrical or rectangular structure,and the reduction of the weight is expected.

b) The inner vessel is placed eccentricallyagainst the outer vessel and the supercon­ducting coil is located on the lower part ofthe inner vessel to increase the field at thelevel of lifting coils and the lift force.

Structure of the cryostat

The magnet has been def'igned on the basis ofthe above concept. However, some modificationsof design were necessary to apply the magnet tothe experimental facility. Figure 7 shows thecross section of the cryostat and the coilarrangement in it and Table 3 gives the specifi­cations of the cryostat. The cross section ofthe outer vessel has the semicircUlar upper partand the rectangUlar lower part to reduce thedistance between the superconducting coils andthe lifting coils. The cross section of the innervessel is rectangUlar. The outer vessel is madeof 1.6mm thick stainless steel plate, and the innervessel is made of 2.0nun thick plate. The radi­ation shield is located between the inner andouter vessels, and made of O.8mm thick copperplate with the exception of the bottom plate andcooled by liqUid ni trcgen flowing in the coppertubes soldered on the shield. The bottom plate ismade of 3.2mm thick aluminium plate to shield thealternating field from the lifting coils.

Fig.6. Conceptual design of the magnet for highspeed trains

1

r7 ______

3C) 2~Lr)

11I~

Fig.? Cross section of the superconductingmagnet

1No. Names1 Superconduct ing Coil

2 Liquid Helium

3 Inner Vessel

4 Liquid Nitrogen

5 Radiation Shield

8 6 Vacuum Space

6 7 Support

5 8 Outer Vessel

4J

44

Table 3. Specifications of the cryostat IV. Magnet performance

Fig.S. Support of FRP

Supports of lift force

/Diode

The maximum excitation current experiencedwas 875A (2.35T) which was about 90% of theminimum propagation current 960A. During theexcitation, the flux jumps were not observed inthe records of the terminal voltages of the super­conducting coils and the search coils beneath thesuperconducting coils. Even when the currentsupply was interrupted at 855A, the coils did notquench but discharged in about one minute.

A typical process of the persistent modeoperation is as follows:a) exciting the coils up to 855A by the current

supply at the rate of 100A/min.b) switching off the heaters of the persistent

switches to make the switch conductors super­conductive.

c) reducing the supplied current with the rate of100A/min from S55A to 450A, 40A/min from 450Ato 200A and 20A/min from 200A to OA.

In the process to reduce current we chosethree steps of the rate to protect the switchesfrom the destruction of switch conductors causedby excessive current change, because the switchconductor was not stabilized with the copper sub­strate. It needed about 30 minutes to complete

5upercmducting coil

The exciting circuit is shown in Fig. 9.The diode in the circuit has a role to protect thecoils and the persistent switches. Even if thecurrent supply is accidentally interrupted, theterminal voltage of coils is limited to the for­ward blocking voltage of diode and then the coilcurrent is prevented to change rapidly. At thesame time it is prevented for the excessivecurrent to flow into the persistent switches inthe resistive state.

Excitation and persistent mode operation

Fig.9. Exciting circuit for the superconductingmagnet

650 kg

140 liters

560 mm

1540 rom

260 rom

Outer diameter

Diameter of bore

Height

Weight (coils and cryostat)

Liquid helium

To~r of bellows structure

The cryostat has two to~rs with the lid.The tov.er wall introduces heat from the outervessel to the inner vessel. Excepting the heatinleak through the power leads, a major portionof the heat inleaks into the cryostat depends onthe tower wall. The bellows has the thin walland prolongs the heat conduction path, and so ~have adopted the bellows structure as the tov.erto reduce the heat inleak. Furthermore,to improvethe reduction of the heat inleak, the bellows hasattached a heat sink controlled at the nitro-gen point.

The rUbber O-ring has been used for thevacuum seal in the tower. The sealing part hasbeen designed so as to be maintained at the roomtemperature by separating thermally the flange forO-ring from the flanges for ports, through whichcold gas flows, by the steel bellows.

SUperconducting coils and the inner vesselmust carry the magnetic force caused by theinteraction between the magnet and lifting coils.The FRP tubes have been used for supports becauseof the large ratio of the mechanical strength tothe thermal conductivity. The supports have hingestructures at the both ends to prevent the stresscaused by the thermal contraction.

Figure 8 shows the supports with hingestructures.

45

the process. The excess helium loss due to thepersistent switches in the excitation process wasabout 0.8 liters per switch.

The above process is reversed for the de­excitation. When the persistent mode was switchedoff, the discrepancy between the supplying currentand the persistent current was aJ.lowed up to 6Awi thout any trouble.

When the magnet was levitated, the coilcurrent in the persistent mode might increase soas to maintain the total magneti c flux constantagainst the reaction field from the lifting coils.We observed the current increase of 1-2% for thetotal current of coils.

Magnetic suspension test

The test facility is shown in Fig.lO. Themagnet is suspended over the six lifting coils onthe rotating disk and free to displace vertically.The rotation of the magnet is persisted by thestoppers on which the load cells are installed tomeasure the drag forces. The lifting coils arefastened on the rotatable disk with the revolu­tion velocity up to 600rpm which corresponds tothe linear velocity of 100km/h of the liftingcoils. Figure 11 shows a test scene.

The magnet was levitated to the height of80mm at the lifting coils velocity 100km/h and thesuperconducting coil current 800A in the persistent

Measuring apparatusfor levitation height

Fig.IO. Test facility of magnetic suspension

46

mode. An example of the test data is shown inFig.12 where the solid lines are the predictedvalues and the broken lines are the observedresults.

The ac component of the reaction field fromthe lifting coils was detected by the search coilsinstalled beneath the superconducting coils. Theampli tUde and the frequency of the ac componentwere respectively 50 gauss at most and 60 Hz.Any trouble by such a reaction field was notobserved as to the superconducting coils. Theincrement of the evaporation rate of helium wasabout 25- 37 Iiters/h, most of which could beevaluated as the JOUle loss (20 liters/h) causedby the eddy current in the copper substrate.

Evaporation rate of helium

The vacuum space of the cryostat has beenevacuated by the 1200 liters/sec diffusion pumpand maintained at a pressure of 3 x Hj"7 Torr.The radiation shield was supplied the liquidni trogen at the rate 26 Iiters/h. The evaporationrate was measured by the float type gas flowmeterand the helium level gauge 3 which consisted of asuperconducting wire. Figure 13 shows theprinciple of the level gauge. The mean value ofthe rate was 15 liters/h in the persistent mode.

The major portion of the heat inleak may becarried through t\\O pairs o,f short power leads.

Load cell for Iiff force

c it

z

Ie

Protecting Pipe

Heater

- calculated- - - measured

Ht---Superconducting Wire

Fd

o 0 20 60 80 J 0(km/h)

5peedof LIfting Coil

50

10

200 I,()O

Fig.13. Schematic structure of liquid heliumlevel gauge

Fig.12. Levitation height, current of the liftingcoil and drag force versus speed of thelifting coilZ Levitation heightFd Drag forceIe Lifting coil current

Fig.ll. Scene of the magnetic suspension test

AI though the heat inleaks through various partscould not be measured separately, calculatedvalues are listed in Table 4. The application ofthe FRP tube for the supports and the bellowsstructure for tov.ers might be able to reduce theevaporation rate of helium by 5 or 7 Ii ters/h.

Table 4. Calculated heat inleaks in the cryostat except two power leads

Parts Materials Temperaturedifference

Heat inleaks(kcal/h)

Helium loss(liter/h)

4 supporting columns3 supports for radiation shields2 tov.ersLeads for measurementHelium gas in the tov.ersThermal insulation plugsGuide for helium transferRadiationVacuum space

Total

FRPFRPsuseuHeurethanformteflonvacuumrarefied gas

80 to 4.2K80 to 4.2K81 to 4,2K

300 to 4.2K300 to 4.2K300 to 4.2K300 to 4.2K80 to 4.2K80 to 4.2K

1.034.48 x 10-2

5.02 x 10-2

0.2770.4910,6030.1073.21 x 10- 2

8.5 x 10- 2

2.98

1.660.0720,0810.4470.790.970.1720.0520.137

4.38

47

V. Conclusion

The magnet which we have built on the basisof the design concept of pipe structure has beenable to reduce the wall thickness of the cryostat,and has been useful to reduce weight and size.The magnet to be installed on the train as theextension of this design is able to make its crosssectional configuration simple as shown in Fig.6,and we can design the magnet more effectively thanthi s magnet.

The multifilamentary superconducting compo­site used to this magnet has shown the verystable performance, and this composite is usefulto reduce the weight of conductor.

The thermally operated persistent switch hasworked satisfactorily in this experiment, and sothis is usable to operate the magnet for the highspeed train without heavy power sources,

It has been confirmed that the superconductingmagnet is applicable to the high speed train fromour experience of this time,

48

Acknowledgment

The authors wish to thank the RailwayTechnical Research Institute of JNR for theexamination of the magnet, Mr.Y.Kyotani, Jap~ese

National Railways, and Prof,K.Yasukochi, NipponUniversity, for the valuable advice and theencouragement, and Mr,Y.Ishizaki, Tokyo Univer­sity, for the recommendation of insulated metalspacers, Acknowledgments for the permission topublish the paper are due to Mr.R,Ueda, ManagingDirector, FUji Electric Company Limited.

References

1. J.R.Powell and G.T.Danby, Cryo. Ind. Gases,19, <Oct, 1969),

2. M.N. Wilson et al.J. Phys. D, 3, 1517 (1970),3, M.Ishizuka and K,Yasukochi, Cr:Yo. Eng, (Japan),

.l, 131 (1968).