~6 PHILlPS TECHNICAL REVIEW VOLUME 25

A DYNAMO FOR GENERATING A PERSISTENT CURRENTIN A SUPERCONDUCTING CIRCUIT

by J. VOLGER *).

In various fields of research a considerable de-mand has arisen in recent years for stationary mag-netic fields of exceptional strength. In solid-state,research, for example, it would be useful to be ableto study the Hall effect and various resonances infields of 5 'to 10 Wb/m2 (50000 to 100000 gauss).Nuclear physicists require strong fields for aligningatomic nuclei by the "brute force" method. Andfinally, investigations in the field of plasma physics,which it is hoped will one day lead to controllednuclear fusion and to the building of thermonu-clear power stations, are confronted with theproblem of confining extremely hot, highly ionizedgases (plasmas) in a space where they are not incontact with a material wall. It is hoped to achievethis with what are called "magnetic bottles"; inview of the elevated temperature of the gas (of theorder of 107°C) the magnetic fields required for thispurpose must be exceptionally strong, perhapsgreater than 10 W1m2• In some of the cases men-tioned the problem of generating such a strong fieldis increased because it is required in a large volume,e.g. of several cubic decimetres.

In order to generate such fields using normallyconducting coils - coils of course without an ironcore - enormous power is needed and hence an,enormous cooling capacity, for the power is almostcompletely converted into Joule heat 1). The use ofsuperconducting coils, to reduce the power requiredand ease the cooling problem, was not possible untilrecently because the superconducting state in thematerials then known was destroyed byeven afairly weak magnetic field; a coil made of such ma-terial therefore reverts spontaneously to normalconductivity if the current through it exceeds, acertain value.The discovery of "hard" superconductors has

changed this situation. These materials can beexposed to a very strong field, and in the form ofwires they can carry extremely high currents 2).

A difficulty of working with a superconductingcoil is the problem of' generating the current, the

") Philips Research Laboratories, Eindhoven.1) The highest power which can at present be supplied to a

normally conducting magnet coil is in the region of 10megawatts. This can produce, for example, a field of 20WbJm2 in a coil having an inside diameter of no more than4 cm.

2) A survey will be found e.g, in R. H. Kropschot and V.Arp, Superconducting magnets, Cryogenics 2, 1-15, 1961.

621.313.291:537.312.62

reason being that the coil has, of course, to be con-tained in a cryostat. Normally the current sourcewill be outside the cryostat and the current has tobe supplied through cables that are not supercon-ducting. This raises cryogenic problems. If relative-ly thin supply cables are used, too much heat isgenerated in them; if, to avoid this, thick cables areused, the result is an impermissible leakage in thethermal insulation of the cryostat. It is obvious,therefore, to look for a means of generating thecurrent inside the cryostat. If one used for thispurpose a current source which is itself super-conducting, a "persistent current" can then flow inthe circuit. The current source then does no morethan set this current in motion, after which it canbe switched off.

In the Philips Research Laboratories, Eindhoven,an experimental version of such a current sourcewas recently successfully put into operation. It isin fact a kind of dynamo which works on a newprinciple 3). Referring to fig. 1, we shall brieflydescribe the essentials of its construction, afterwhich we shall explain the operation.A major part of the dynamo is the thin lead disc

D, which forms part of the superconducting circuitin which the persistent current is generated. Therest of this circuit is formed by the wire W (for thetime being we can ignore the coil L in this wire),which is fixed to the rim of the disc at a and to thecentre at b. The current is generated by turning theshaft S, thereby causing the bar magnet M - onepole of which is immediately under the disc -to describe a circular path. For the dynamo tofunction properly, the pole should be close enoughto D to enable the magnetic field of M to destroy thesupérconducting,state of the disc in the immediatevicinity of the pole. Further, part of the flux of Mmust pass through this zone of normal conductivi-ty. This part above M is represented schematicallyin the figure as a hole (H). As M revolves, the "hole"must revolve with it. The current that now flowsin W is in the first instance proportional to thenumber of revolutions that the shaft S has made.When the shaft is stopped, the current retains thevalue it has reached at that instant. Fig. 2 gives arough sketch of the actual construction.

3) J. Volger and P. S. Admiraal, A dynamo for generating apersistent current in a superconducting circuit, PhysicsLetters (Amsterdam) 2, 257-259, 1962 (No. 5).

1963/64, No. 1 DYNAMO FOR PERSISTENT CURRENT 17

L

w

------ -----, \-\+11('- (-:nr -:-:-:;H"-- -- __~ __ -_:~>HHI----·o

Fig. 1. Schematic diagram of the dynamo for generating apersistent curren t in a superconducting circuit. D lead disc.~17circuit of "hard" superconducting material, connected toD at the rim (a) and in the centre (b). The shaft 5 mountedbelow the centre of D carries on an arm the bar magnet M.The upper pole of M is so close to D that the field is strongenough to destroy the superconductivity in the zone H. Partof the flnx of M passes through the zone H. The rotation of Scauses H, together with the flux, to describe a circular path.Consequently a current flows, as expl ained in the text, throughthe circuit formed by D and W. L coil forming part of rv.

To explain the operation of this device, we canbest take as a starting point the property that it isimpossible to cause any change in the magneticflux enclosed by a superconducting ring. If one triesto do this, for example by bringing a magnet near toit, a current starts to flow in the ring that has thecffect of exactly cancelling the change of flux pro-duced by the change of position of the magnet. (It istacitly assumed here that the field of the magnet isnot so strong as to interrupt the superconductingcircuit. )It is, however, possible to alter the spatial field

distribution inside a superconducting ring. Supposewe have a complicated ring circuit as sketched in

Fig. 2. Sketch showing the construction of the new dynamo andits positioning in a cryostat. The letters have the same meaningas in fig.1.0ther symbols are: 1 Dewar vessel of inner cryostat.2 surface of the liquid helium. 3 Dewar vessel of outer cryostat;the space between 1 and 3 is filled with liquid nitrogen. 4tubular rods screwed to the cap, carrying the bearing 5 andthe disc D.

For measurement of the current in W, this circuit is coupledto a ballistic galvanometer via a transformer 6 (of which L isthe primary). If the current in W is interrupted, the galvano-meter gives a deflection proportional to that current. Thecurrent is interrupted by means of a heating element 7, withwhich the superconductivity of W can be removed locally. 8cerium-glass plate (explained later).

.fig. 3a, and that it has been sufficiently cooled inthe presence of the magnet M to become supercon-ductive. Owing to the eccentric position of M, theflux enclosed by the ring passes mainly through thepart x; for simplicity we suppose that the entireflux passes through x. If we now constrict the hole

n

5 ],_

18 PHILlPS TECHNICAL REVIEW VOLUME 25

a cFig. 3. Illustrating the properties of a superconducting ring circuit of complicatedshape, in one part of which a flux is enclosed.

in x, for example by gradually closing it with asuperconducting stop, the flux then shifts to party of the ring (fig. 3b). If on the other hand weincrease the flux at x by an amount LI <P, e.g. bymore strongly energizing the magnet M, the currentinduced in the ring is such that yencloses a flux- Ll <P; the total flux through the ring remains equalto (/J (fig. 3a). It should be noted here that in asoft superconductor, such as lead, these currentsflow in a surface layer. The interior of such a super-conductor is always free from currents and magneticfields.A further extension of the property of magnetic-

flux conservation is that it holds not only for ringsbut equally for the holes in e.g. a triply or quadruplyconnected body. We have found in fact that theflux through each hole remains constant when sucha body is subjected to a change of shape, evenwhen the holes become unrecognizably deformed inthe process.

The operation of the dynamo can now easily beexplained by applying the latter property to thecomplicated superconducting body sketched infig. 4a. This body, in which the circuit of fig. liseasily recognized, contains two "holes". The fluxcontained in both holes is invariant. According tothe theory of superconductivity this must be for-mulated thus: magnetic fluxes are invariant whichare contained within the contours 1 and 2 runningwithin the (currentless) interior of the supercon-

ductor. Where exactly these contours are drawn isimmaterial. For contour 1 the enclosed flux has afinite value, which we again call <P, and for contour2 the value is taken to be zero.If we now displace the hole contained by 1 in the

same way as in the dynamo with the zone H ofnormal conductivity (fig. I), then contour 2, inorder to pass through superconducting materialcarrying no current, must alter in shape (fig. 4b).Owing to the required invariance, the moving holecontinues while in motion to enclose the flux <P.When the hole has completed one revolution, wehave the situation drawn in fig. 4c. This differs fromthe starting situation in that the flux (/J is now alsocontained within contour 2. Since, however, thetotal flux inside 2 must remain zero, this contourwill now have to contain elsewhere a flux - <P,which calls for the flow of a current in the circuitformed by Wand D. After two revolutions the latterflux is - 2(/J, and the current must now be corre-spondingly larger, and so on. If a coil is included inthe circuit W (fig. I), a field is then generated init 4).

4) In this article we have not mentioned variants of the me-thod described. In one variant the plate in which the flux isrotated is smaller than the disc D discussed here, in such away that during its revolution the flux alternately leavesand re-enters the plate. For the dynamo to work well it isonly necessary that the magnetic flux during its passagethrough the plate should not cause any complete inter-ruption of the superconducting path between the connec-tion points of the coil.

--------- w(c==l'"I II II II II II I\..._--------_/

2

(~----------\\I - i> II II II II _ II 2 ././~(~ I~..._ -- -, _ ..,1

Fig. 4. Illustrating the operation of the new dynamo.

b

1963/64, No. 1 DYNAMO FOR PERSISTENT CURRENT 19

A practical difficulty is encountered if, with adynamo of the type in fig. 1, it is desired to feed acoil of a large magnet, the difficulty being the highself-inductance of such a coil. In general, when aconstant voltage E is applied to a superconductingcircuit having a self-inductance L, a current i willflow which increases with time in accordance withthe equation i = EtfL. The time which, for a givenL, elapses before i reaches the required value isthus inversely proportional to E. In order tobe used for energizing coils with which strong fieldsare to be generated, a dynamo working on the newprinciple must therefore be capable not only ofdelivering the required current, but should alsohave not too small an e.m.f, In a set-up as shown infig. 1, this e.m.f, is proportional to the speed ofrevolution of the shaft S and to ([J. With our experi-mental set-up it is not yet possible to achieve avoltage high enough to produce a current of 10 Ain a coil of, say, 1 H within an acceptable time. Thereason is that the proportionality between the e.m.f.and the speed of revolution is not in practice un-limited: if the shaft speed exceeds a certain value,the e.m.f. gradually moves towards a maximumvalue. In circuits having a small L, on the otherhand, very strong currents (more than 100 A) havebeen generated in a short time.

The reason why the e.m.f. shows a maximum isthat the zone of normal conductivity does notfollow the movement of the magnet at unlimitedspeed. This was observed with the aid of a plate ofcerium glass with a reflecting surface on one side,which was mounted immediately under the disc Din the set-up in fig. 2. At low temperatures cerium

glass shows a marked Faraday effect (rotation of theplane of polarization of light transmitted through amagnetized medium). When polarized light isdirected onto the plate and the reflected light ispassed through an analyser, the part immediatelyunder the zone of normal conductivity - which istherefore exposed to the magnetic field of M -is seen to he darker (or lighter) than the rest. Whenthe shaft is rotated fairly fast, the dark patch isseen to acquire a "tail". Upon very fast rotation, thetail fills the complete circumference of a circle andthe zone of normal conductivity becomes ring-shaped. With a thin disc this effect is less strong thanwith a thick one.

The maximum current that can be produced in acoil by this new method of current generation isdetermined either by the current-carrying capacityof the coil (the "hardness", see above), or by thatof the dynamo itself, or by the quality of thejunctions. One should therefore try to bring thesethree roughly into correspondence with one another.

Summary. The extremely strong magnetic fields (of theorder of 10 Wb/m2) needed in various fields of research cannow in principle be generated, without requiring enormouspower, by making coils from a "hard" superconductor. Thecurrent generation in such a coil should preferably take placeinside the cryostat. A description is given of the principle of asuperconducting dynamo which can generate a persistentcurrent in a superconducting circuit, and an experimentalversion of such a dynamo is discussed. This consists of alead discto which the remaining part of the circuit is connectedin the centre and at a point on the periphery. One of the polesof a bar magnet is situated eccentrically under the disc, pr!>.ducing in the disc a small zone of normal conductivity. Part ofthe flux of the magnet passes through this zone. If the latteris rotated around the centre of the disc, a current is producedin the circuit. The e.m.f. (which governs the speed at which thecurrent can grow in a circuit possessing finite inductance) is inprinciple proportional to the speed of rotation.