US3869686A - Super-conductive coils incorporating insulation between adjacent winding layers having a contraction rate matching that of the super-conductive material - Google Patents

Abstract

A coil for producing a magnetic field is wound from a conductor made from a super-conductive material and is impregnated with a hardenable synthetic and preferably epoxy resin. Included within the coil structure prior to impregnation is a temporary inlay structure made from a solid material, e.g. Woods-metal which is thereafter removed, by heating, subsequent to hardening of the impregnating means in order to establish cooling ducts through which a coolant is passed while the coil is in operation in order to maintain the coil in its superconductive state. The coil structure also includes structural reinforcing elements in the form of fiber glass fabrics or netting having a thermally conductive metallic surface coating, and also metallic inserts in fabric or mesh form made from insulated copper or aluminum wire to assist in the removal of heat from the coil.

Description

United States Patent 11 1 1111 3,869,686 Benz Mar. 4, 1975 SUPER-CONDUCTIVE COILS 3.748.615 7/1973 Bogner et a1. 335/216 x Inventor: Hans Benz, Zurich, Switzerland Assignee: BBC Brown Boveri & Company Limited, Baden, Switzerland Filed: Oct. 31, 1973 Appl. No.: 411,219

Foreign Application Priority Data Nov. 6, 1972 Switzerland 16029/72 US. Cl. 335/216, 174/D1G. 6 Int. Cl. H0lf 7/22 Field of Search 335/216; l74/DIG. 6

References Cited UNlTED STATES PATENTS 1/1968 Brechna 335/216 12/1970 Albrecht 335/216 1/1971 Drautman 335/216 Primary E.\'amt'nerG. Harris Attorney, Agent, or Firm-Pierce, Schleffler & Parker [57] ABSTRACT A coil for producing a magnetic field is wound from a conductor made from a super-conductive material and is impregnated with a hardenable synthetic and preferably epoxy resin. lncluded within the coil structure prior to impregnation is a temporary inlay structure made from a solid material, e g. Woods-metal which isthereafter removed, by heating, subsequent to hardening of the impregnating means in order to establish cooling ducts through which a coolant is passed while the coil is in operation in order to maintain the coil in its superconductive state. The coil structure also includes structural reinforcing elements in the form of fiber glass fabrics or netting having a thermally conductive metallic surface coating, and also metallic inserts in fabric or mesh form made from insulated copper or aluminum wire to assist in the removal of heat from the coil.

7 Claims, 6 Drawing Figures PATENTED 4 I975 SHEET 1 0F 2 PATENIEDHAR M915 6; 869.686 sum 2 9g 2 SUPER-CONDUCTIVE COILS INCORPORATING INSULATION BETWEEN ADJACENT WINDING LAYERS HAVING A CONTRACTION RATE MATCHING THAT OF THE SUPER-CONDUCTIVE MATERIAL The present invention relates to a method for the preparation of an impregnated electrical coil, especially a coil made from a winding consisting of a superconductive material, which is used to generate electromagnetic steady or pulsating fields, and which is provided with at least one cooling duct for the passage of a liquid of gaseous coolant, as well as a coil manufactured in accordance with this method.

Heretofore, solid, thin wires of transition metal alloys, such as NbZr or NbTi, sheathed with a thin copper layer, or thin tapes, coated with the intermetallic compound Nb Sn, have been used primarily for the construction of super-conductive high frequency field coils with current densities in excess of 10,000 to 15,000 A/cm such as experimental solenoids, rayguiding and ray-focusing magnets.

However, all such coils which generate these magnetic fields, display instabilities during their operation, called degradation and training effects. They do not attain the critical current and field rates, as measured at short wire specimen within the transversal magnetic field, or only after repeated transitions from the superconductive to the normal-conductive state. It was found that this was caused partially by coupled thermal as well as magnetic instabilities, and partially by the shifting of wire position during the generation of the magnetic field.

The super-conductors discussed above have the additional disadvantage that their pulsating field losses become prohibitively high at a frequency of 1 Hz, and that they are unsuitable for the construction of rayguiding magnets for synchrotron-accelerators, to give one example.

Empirical tests and research based on theory indicated that the instabilities, the so-called flux-jump instabilities, as well as the pulsating field losses are influenced strongly by the diameter of the superconductors. This lead to the development of inherently stable conducting filaments, comprising a multitude of very thin super-conducting filaments, made, for exam ple, from NbTi, which are imbedded into, and strongly metallically connected to a standard-conducting matrix, consisting of Cu or Al. The magnetic instabilities and pulsating field losses are reduced further by other additional steps, for example, by a twisting of the filaments within the conductor, and by transposing several of the twisted conductors.

The use of the above-described conducting filaments per se will not insure a stable and degradation-free behavior of coils generating a pulsating field. Research and practical tests did disclose that the following additional requirements need to be met in order to obtain coils possessing the desired characteristics:

A. lnstabilities of mechanical origin, caused by a shifting of the conductor material, must be avoided. If a conductor within the coil is able to move under the force of the fluctuating magnetic field, the friction so generated, or the kinetic energy could heat up the conductor to such degree that it exceeds the critical temperature of the super-conductor, possibly aided by thermically triggered jumps in flux, thus restoring the coil partially or completely to a normal-conducting state. Since the specific heat of the super-conductor and of the stabilizing matrix has a very low rate, even a very slight movement will be sufficient to cause such reaction.

The attempt has been made to prevent such movements by producing unbroken layers of coils by keeping the conductor under heavy tension during the winding operation. lf this method is employed, the mcchanical state of stress within the conductor will change continuously throughout the already completed layers whenever a new layer is being added because the insulating material can deform easily and because the conductor as well as the winding supports possess resilient properties. As a result thereof, a fully completed winding will contain zones within the conductor, progressing from the first inner to the last outer layer, under tensile stresses, without stresses, under compression stresses, without stresses and under tensile stresses. Dangerous zones allowing movements by the conductor will exist in places where mechanical stresses within the conductor have disappeared, and this method will therefore not insure a mechanical stability.

Better results are obtained, as is well known, by a complete impregnation of the coil by suitable means, for example, an epoxy impregnating resin. When a thorough impregnation is accomplished, all interspaces will be filled and all windings are mechanically bonded to each other, provided the resin and the insulating materials used possess the proper adhesiveness.

However, several conditions must be met which are partially in conflict with each other:

1. The contraction of the materials involved differs widely when they are being cooled from room temperature to operating temperature, that is to 4 K. The conductor, consisting of NbTi filaments imbedded in Cu, has at 4 K a contraction of Al 300 -l0 while pure impregnating resin has a contraction of Al/1-- 600 to 1,200 10? Therefore, after the cooling-off process, a tension caused by the dissimilar contraction of conductor and resin will be geometrically superimposed onto the state of tension generated by the winding operation.

Furthermore, transient high temperature differences will arise during the cooling-off process, also leading to high mechanical stresses within the system.

When the coil is excited, the magnetic field will develop forces that act upon the current-carrying conductor, and which are likewise superimposed on the existing state of tension.

If the impregnating resin is unable to sustain these stresses, it will crack, and the mechanical tensional energy, transformed into heat, will drive the winding into the normal-conductive state, leading again to degrading and training effects.

Therefore, the contraction characteristics of the resin should conform to the contraction characteristics of the conductor and should possess high mechanical stability at low temperatures in its hardened state. This can be accomplished in principle by mixing the resin with suitable fillers.

2. In order to attain a high current density, the windings must be wound very tightly and solidly. In order to insure complete and thorough impregnation, the resin must possess a very low viscosity. Unfortunately, the filler will greatly increase this value and thus endanger a thorough impregnation. Furthermore, the very fine gaps and interspaces of the winding will act as a filter, preventing the filler from penetrating the winding at all, or in the quantities desired.

B. Even the best filament conductors, as known heretofore, are not free of losses in periodic pulsating field applications. These losses, functionally related to time and magnetic field, being converted into heat, will raise the temperature of the coil. Since the critical currents, fields and temperatures are functionally interconnected, the temperature increase should not exceed approximately O.2 K to avoid a sharp, inadmissible lowering of the critical current density. This will require an intensive cooling of the interior part of the coil.

Another need for maximum elimination of heat generated within the coil arises in the case of a transition from the super-conductive to the standard-conductive state. This transition begins at one point of the conductor'when the critical state of the super-conductor is surpassed. The conductor then develops a zone of relatively high electric resistance which will heat up due to the current flowing therein. This heat will spread rapidly, causing additional portions of the super-conductor to become standard-conductive. This complicated thermal and electromagnetic process will come to an end when the entire field energy has been transformed into heat. During this process, high temperatures can arise locally which might even destroy the coil. This occurrence cannot be completely avoided by proper heat distribution and elimination, but it can be attenuated and retarded.

The impregnation of a coil will have a counterproductive effect due to the impairment of the thermal conductivity.

The attempt has been made to improve the heat elimination by means of cooling tubes, placed inside the coil, with a coolant circulating through the tubes, and connected to cooling vanes possessing good heat conductivity. These devices will work satisfactorily if used in sufficient numbers and in proper distribution. However, they do have the disadvantages that they take up a relatively large winding space and will thus reduce the current density, and that additional eddy current losses are generated by them due to the pulsating field.

C. As mentioned above under (B), eddy currents are induced and additional sources of losses are created by any metal parts present within the zone of the pulsating field. This applies especially to structures carrying the winding, supporting parts, large-sized cooling aids, such as tubes, fins and the like, and cooling ducts consisting of metal.

D. In the case of electromagnetic coils for producing pulsating fields, there appear very high inner and outer forces at the desired high current density, and the amplitudes of these forces increase proportionally with the size of the coil and its field strength.

The mechanical stress generated within the coil can attain such a high value that this may exceed the yieldpoint of the matrix material. As already familiar, to increase the mechanical strength of the coil, antimagnetic stainless steel tape, or stampings should be laid in with the conductor or in the coil.

By the use of magnetic iron poles or under the effect of the pulsations with the adjoining stray fields, the outer forces can attain a very high value. To overcome the effect of such forces, rigid metallic supports for the coils and coil-bracings are used.

In both cases, the undesired eddy current-losses are caused-due to the periodic pulsating field.

'E. The desired high field-accuracy of the beamtransport and focussing magnets can only be attained by a suitable homogeneous and geometrically accurate coil. The principal object of this invention is to provide an improved electromagnetic coil which does not exhibit the aforementioned disadvantages.

The invention is characterized by the fact that between layers of the electrically insulated superconductor winding, a spacer of glass-fiber-reinforced material is laid, this material having such characteristics and properaties that it together with the impregnating medium form an eddy current-free, mechanically rigid system, the contraction factor of which corresponds to that of the super-conductor.

It is advantageous to use glass-fiber spacers woven or braided. It is also appropriate to stratify the spacers with materials having a good thermal conductivity, e.g., metals like copper or aluminum, or graphite powder. In doing so, it is of advantage to seee that the metalliclayer possesses a high electrical resistivity, in the case of a coil producing a pulsating magnetic field, sothat the eddy current loss can be eliminated. This can be achieved by applying these metallic layers in the form of small closely laid parallel tapes, each separated from the other. 1

A further object of the invention is to provide an improved process for fabrication of the improved coil having at least one internal cooling duct, the improvement being characterized by the fact that during winding of the coil, in addition to the glass-fiber-reinforced spacer, at least another layer is laid into the interior of the coil being formed, this layer being removed by a chemical or thermal process to form a coolant duct, after the coil has been wound and impregnated, or after a thermal treatment following impregnation. For this removable layer that is to form the coolant duct, it is advantageous to use Woods-metal, the melting point of which lies higher than the temperature at which the impregnating medium is cured, and lower than the softening temperature of the impregnating medium; For better thermal conductivity of the to be formed-coolant duct, it is of advantage to use an impregnating medium which is mixed with a material having a good thermal conductivity characteristic at least in the region of the coolant duct.

The foregoing objects of the invention will become more apparent from the following description of a preferred embodiment of the method and coil and the accompanying drawings wherein:

FIG. 1 illustrates the method for the manufacture of the coil,

FIG. 2 is a view in section of one species of a coil according to the invention,

FIG. 3 depicts a section of the coil along line Ill III in FIG. 2, and

FIGS. 4 to 6 show various partial sectional cuts through a species of the coil.

With reference now to the drawings, FIG. 1 shows a super-conductive coil, ready for impregnation and placed within a mold. The mold illustrated here consists of two inner, separable halves l and 2, into which are inserted, prior to winding of the coil, the outer disks 3 and 4 and the supporting sleeves 5 and 6. Upon the conclusion of the winding operation the band 7, made of fabric, is put in place, and the mold with its two outer halves 8 and 9 is closed. Seals 10 will keep the cover vacuum-tight.

Very compact windings with thin-super-conductors 11 of approximately 0.4 to 0.5 mm diameter are impregnated in the described closed mold most advantageously by means of an impregnating medium of low viscosity. After an evacuation at increased temperav ture, lasting for several hours, the impregnating medium is introduced through the inlet pipe 13, distributed over the periphery of the winding by means of the annular groove 14 and forced through the winding by an increase in pressure until it flows through the outlet pipe 15 free of gas bubbles. The outlet pipe can be closed off to subject the impregnating medium to a high static pressure so that it will penetrate even the smallest pores within the winding.

To make possible, or to facilitate a complete and thorough impregnation of the winding, it will be advantageous to use an impregnating medium oflow viscosity and without filler of such species that the system, formed by the impregnating medium and the material of the intermediate pieces 12, has a contraction characteristic which matches the contraction characteristic of the super-conductor 11. This can be accomplished, for example, by a system formed by a specially selected epoxy resin which is filled at 60 to 70% by volume with a composited fabric made from glass silk. A system of this type insures simultaneously a high mechanical strength, comparable with the strength of a high-grade alloy steel, thus serving to reinforce the coil unit mechanically.

The outer disks 3 and 4 are provided with radially arranged slots 16, FIG. 3, through which the inlays 17 are guided up to the inner rim of the mold, and which form together with the annular groove 14 apertures to admit the impregnating resin. The material of these prefabricated outer disks is again formed by a system of a tightly compressed fabric, for example, a glass silk fabric and a hardened impregnating resin, such as an epoxy resin with a contraction characteristic that matches the contraction characteristic of the superconductor.

The supporting sleeves 5 and 6 are formed from the same system of materials as the outer disks 3 and 4. They serve to maintain the precise cylindrical internal geometry of the coil, and to reinforce the unit mechanically.

The band 7 arranged at the outer circumference of the coil consists of the same glass silk fabric as the intermediate pieces 12, and serves additionally as a mechanical reinforcement of the winding unit.

Usually some hollow spaces will also be formed at the outer surface of the coil, in the case of the example given, within the annular grooves 14 and the slots 16 which would become filled with purely impregnating medium. In order to avoid the formation of cracks, it will be advantageous if immediately after the impregnation process an impregnating medium mixed with a substantial amount of filler is forced through, thus displacing the pure impregnating medium from these hollow areas with the mixture. If a suitable type of filler is employed, the contraction characteristic of these specific areas will also match those of the superconductor.

If the winding geometries are kept simple and if super-conductors of greater diameter are being used, it becomes possible to simplify the impregnating process by omission of the outer mold halves 8 and 9, and by impregnating the coil in its evacuated state by a dipping process, and possibly by utilizing a higher pressure, i the impregnating medium.

The winding shown in FIG. 1 is constructed as follows:

After the completion of each layer wound by use of the insulated super-conductor 11, there is inserted an intermediate layer 12, flush with the outer disks 3 and 4 and having one or more turns, and another layer of conducting wire under strong tractive force is then wound on top of it. The intermediate layers consisting of a highly tear-proof fabric, for example, a glass silk fabric, are provided prior to, or after their insertion with a layer or coating to improve their thermal conductivity in the impregnated and hardened state. This can be accomplished by powdering, or coating .by means of a binding agent, both sides of the fabric, completely or in parts, with a powder possessing good thermal conductivity, such as a graphite powder or a metallic powder, or by placing thin metal layers on the entire or on parts thereof.

The metallic layers can be applied by metal spraying, by vaporization, by a chemical process or any other suitable methods. In order to avoid eddy current losses in the magnetic pulsating field, these layers should possess a relatively high electric resistance. This can be accomplished, for example, by coating in the form of narrow strips, running in a meandering manner or parallel to each other and separated by small gaps so that the formation of closed electrical current circuits is avoided.

The intermediate layers can be impregnated prior to their insertion into the winding with a suitable impregnating medium, either dried only but not hardened, or also hardened. The powder possessing good thermal conductivity can, in this case, be used as filler for the impregnating medium and applied together with the medium to the fabric of the intermediate layer, or the methods described above can be followed. Preimpreganted but not hardened intermediate pieces offer the advantage that the impregnating medium will already be in the proper places during the winding operation, thus facilitating, or even making unnecessary the above described impregnating process.

The use of intermediate layers will not only improve the thermal conductivity of the coil but will also serve as a mechanical reinforcement of the coil, thus making it superfluous, in many instances, to provide any supports for the conductor or the coil in the form of antimagnetic, stainless steel inserts. The contraction characteristics of all materials used for the construction of the unit are chosen in such manner that they will match the contractioncharacteristic of the super-conductor, with the result that no additional mechanical stresses will arise at operating temperatures.

In order to attain high current densities of 30,000 to 50,000 A/cm it will be expedient to employ preimpregnated glass fiber fabrics of approximately 50 um thickness.

The temporary inlays 17 which are to form the cooling ducts, are placed at the completely wound layers of conductors in the desired shape, number and location in such manner that their ends will touch the inner rim of the mold. In the case of the example shown by FIG. 1, they are inserefor this purpose through the radial slots 16 of the outer disks 3 and 4.

These inlays l7 consist of a material which allows their removal from themold by chemical means or by a melting-out after formation of the fully impregnated coil.

FIGS. 2 and 3 depict a completed, impregnated coil with the inlays removed. The inlays can consist of metal with a low melting point, such as Woods metal, its melting point being higher than the impregnating and hardening temperatures but lower than the softening point of the impregnating medium being utilized. When the coil has been formed and completed, it is heated to such degree that the inlays will melt, so that the molten metal can flow out, or be forced out by a heated gas, for example nitrogen. Any thin layers of resin which might have formed in front of the duct openings, are removed mechanically prior to this process.

By use of this method there are produced open and thorough-going cooling ducts 18 which will permit the cooling medium, for example, helium in the liquid or gaseous state, to circulate, thus cooling the coil to operating temperature and to eliminate the heat generated by the pulsating field losses intermittingly or continuously. Due to the specific arrangement of the cooling ducts, the coil mass is cooled off uniformly and only small temperature gradients will thus arise, with the result that only relatively low mechanical stresses are being generated. The presence of the large and effective heat exchange surfaces within the ducts ensures rapid cooling and efficient utilization of the enthalapy energy of the cooling medium. 7

FIGS. 2 and 3 depict how the dimensions, numbers and locations of the cooling ducts can be chosen as desired and in accordance with the specific requirements.

The effectiveness of the cooling ducts can be still improved by connecting paof adjacent ducts by means of thermally well conducting cooling aids 19. In order to reduce eddy current losses, these cooling sides are made preferably in the form of a mesh or fabric from thin metal wires of good thermal conductivity such as copper and aluminum. The metal wires are individually insulated to prevent the formation of closed current loops. FIGS. 4 to 6 show sections of one species of this specific design of a coil in accordance with the invention. This coil is prepared layer by layer by using a super-conductor 20 with a rectangular profile and an electric insulation 21. Upon completion of one layer an above described intermediate piece 12 is inserted and another layer of conductor 20 is wound upon it.

Cooling inserts are placed when and as desired, with the inlays forming alternatingly the cooling aids 19 and the cooling ducts 18. In FIGS. 4 to 6 these inlays are not shown since it is assumed that they have been removed already. FIG. provides a top view of a cooling insert. In order to keep the heat transfer resistance at a low value, the cooling aids 19 are placed in such manner that their lateral sides will touch the inlays. The

cooling layers can be equipped with additional intermediate pieces 12, but only if conditions warrant such measure.

If, at any one point of the winding a source of heat is generated, for example, due to pulsating field losses, or a conductor section becoming normal-conductive, any heat so generated will be dispersed more uniformly and conducted more rapidly to the cooled cooling duct surfaces due to the improved thermal conductivities of the intermediate pieces and cooling aids. Proper dimensioning and placement of the heat-conducting and cooling components makes it feasible to keep any increases in temperature being generated within desirable limits.

I claim:

1. A magnetic field-producing coil comprising a plurality of layers wound from a conductor made of superconductive material, said coil being impregnated with a hardened synthetic and preferably epoxy resin, and containing a system of cooling ducts, there being disposed between adjacent winding layers a reinforcing fiber layer such as will form in conjunction with the impregnating resin a system free from eddy currents and having a high degree of mechanical stability and which has a contraction rate at the operating temperature of the coil that matches the contraction rate of the superconductive material.

2. A magnetic field-producing coil as defined in claim 1 wherein said reinforcing fiber layer is comprised of a fabric made from glass fibers.

3. A magnetic field-producing coil as defined in claim 1 wherein said reinforcing fiber layer is comprised of a netting made from glass fibers.

4. A magnetic field-producing coil as defined in claim 1 wherein said reinforcing fiber layer has applied thereto a metallic coating having a good thermal conductivity characteristic.

5. A magnetic field-producing coil as defined in claim 4 wherein the metallic coating applied to said reinforcing fiber layer is chosen from the group consisting of copper, aluminum and graphite powder.

6. A magnetic field-producing coil as defined in claim 4 wherein said metallic coating applied to said reinforcing fiber layer has a relatively high electrical resistance thereby to avoid eddy current losses in the case where the magnetic field pulsates.

7. A magnetic field-producing coil as defined in claim 6 wherein the relatively high electrical resistance characteristic of said reinforcing fiber layer is attained by application of the metallic coating in the form of narrow, parallel and closely laid strips of the metal.

Claims (7)

1. A MAGNECTIC FIELD-PRODUCING COIL COMPRISING A PLURALITY OF LAYERS WOUND FROM A CONDUCTOR MADE OF SUPER-CONDUCTIVE MATERIAL, SAID COIL BEING IMPREGNATED WITH A HARDENED SYNTHETIC AND PREFERABLY EPOXY RESIN, AND CONTAINING A SYSTEM OF COOLING DUCTS, THERE BEING DISPOSED BETWEEN ADJACENT WINDING LAYERS A REINFORCING FIBER LAYER SUCH AS WILL FORM IN CONJUNCTION WITH THE IMPREGNATING RESIN A SYSTEM FREE FROM EDDY CURRENTS AND HAVING A HIGH DEGREE OF MECHANICAL STABILITY AND WHICH HAS A CONTRACTION RATE AT THE OPERATING TEMPERATURE OF THE COIL

2. A magnetic field-producing coil as defined in claim 1 wherein said reinforcing fiber layer is comprised of a fabric made from glass fibers.

3. A magnetic field-producing coil as defined in claim 1 wherein said reinforcing fiber layer is comprised of a netting made from glass fibers.

4. A magnetic field-producing coil as defined in claim 1 wherein said reinforcing fiber layer has applied thereto a metallic coating having a good thermal conductivity characteristic.

5. A magnetic field-producing coil as defined in claim 4 wherein the metallic coating applied to said reinforcing fiber layer is chosen from the group consisting of copper, aluminum and graphite powder.

6. A magnetic field-producing coil as defined in claim 4 wherein said metallic coating applied to said reinforcing fiber layer has a relatively high electrical resistance thereby to avoid eddy current losses in the case where the magnetic field pulsates.

7. A magnetic field-producing coil as defined in claim 6 wherein the relatively high electrical resistance characteristic of said reinforcing fiber layer is attained by application of the metallic coating in the form of narrow, parallel and closely laid strips of the metal.

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