Electric Magnet With Superconductive Windings

Abstract

A superconductive magnet coil having an elongated cylindrical dipole superconducting winding is supported on a carrier cylinder, the winding comprising two winding components shaped as cylinder segments which are symmetrical with respect to a plane of symmetry which goes through the longitudinal axis of the carrier cylinder and which is perpendicular to the magnetic field of the coil. The winding components each consist of a plurality of superimposed winding layers each having axially extending sides approximately parallel to the longitudinal axis of the carrier cylinder and circumferentially extending winding heads, the mutually corresponding winding layers of these winding components cooperatively forming cylindrical layers. On at least one side each of the winding layers has bare uninsulated surfaces contacting a liquid permeable layer through which a coolant can pass in axial, tangential and radial directions in such a manner that each winding layer can be wetted by the coolant on at least that side. Support bars extend between the respective axially extending sides of the two winding components to support them circumferentially apart, and these bars have relatively large coolant passages formed in them and extending radially with respect to the coil winding from the liquid permeable layers to the outer periphery of the coil. The winding components are held together and on the winding carrier cylinder by high-tensile strength wrappings and suitably shaped cores are positioned within the components' windings, and smaller coolant passages are formed through these parts so as to extend radially from the liquid permeable layers to the outer coil periphery. The magnet coil may be operated horizontally as well as vertically, immersed in a liquid coolant bath.

Description

BACKGROUND OF THE INVENTION

Superconductive magnet coils for magnetohydrodynamic generators are described by Z.J.J. Stekly in "IEEE Transactions on Magnetics," September 1966, vol. Mag-2, No. 3, pgs. 319 to 322, and K. Koyama et al. in "Proceedings of the Third International Cryogenic Engineering Conference," Berlin, May 25 to 27, 1970, pgs, 351 to 354.

Such a superconductive magnet coil has an elongated cylindrical dipole superconducting winding supported on a carrier cylinder, the winding comprising two winding components shaped as cylinder segments which are symmetrical with respect to a plane of symmetry which goes through the longitudinal axis of the carrier cylinder and which is perpendicular to the magnetic field of the coil. The winding components each consist of a plurality of superimposed winding layers each having axially extending sides approximately parallel to the longitudinal axis of the carrier cylinder and circumferentially extending winding heads, the mutually corresponding winding layers of these winding components cooperatively forming cylindrical layers. Support bars may extend between the respective axially extending sides of the two winding components to support them circumferentially apart. The winding components may be held together and on the winding carrier cylinder by wrappings and suitably shaped cores may be positioned within the components' windings.

The individual winding layers consist of ribbon-shaped, so-called stabilized conductors in which wires of a high-field superconductive material, such as niobium-titanium or niobium-zirconium are embedded in a ribbon of metal such as copper or aluminum having high normal electrical conductivity at the operating temperature of the coil of about 4.2 Kelvin, and also high thermal conductivity.

Such conductors composed of superconductive material and electrically normal conductive metal must be adequately cooled so that the heat which is released in the normal conductive metal in the event of a possible transition of the super-conductive material to the normal conducting state, can be removed quickly and the conductor does not warm up above the transition temperature of the superconductive material. The conductors are edge wound and adjacent to the winding layers, liquid coolant ducts are provided through which a liquid coolant can pass to contact edges of the ribbon-shaped conductors at least on one side. For cooling purposes such a superconducting magnet coil is positioned in liquid helium contained in the tank of a cryostat, this being called bath cooling because the coil is completely immersed in the bath of liquid helium. The liquid helium can get to the individual winding layers through the coolant ducts but the latter have another function; namely, to remove helium vapor bubbles generated at the surface of the winding conductors from the interior of the coil as quickly as possible.

In the prior art magnet coils of the type described, the coolant ducts lead through the entire coil comprising the two components, in the axial direction of the carrier cylinder, this requiring the coil to be built into the helium tank of the cryostat with the longitudinal axis of the carrier cylinder oriented vertically. The helium vapor bubbles generated at the winding conductors with this vertical orientation can rise through what is in this case vertically extending coolant paths. In one known coil as described by the paper by Stekly the coolant ducts are in the axial or lengthwise direction of the carrier cylinder exclusively. In the coil described by Koyama et al. the coolant can move also tangentially with respect to the coil, but in this case also the helium vapor bubbles can escape from the coil only in the axial direction.

In the above prior art constructions there is the disadvantage that the paths for the removal of the helium vapor bubbles, and for the supply of fresh liquid helium, are relatively long. In some cases the helium vapor bubbles may have to flow through the entire vertical coil from the bottom to the top. Furthermore, the magnet coil is limited to operation with the longitudinal axis oriented vertically because, if otherwise, helium vapor bubbles generated during the operation of the electro-magnet do not rise at all in the coolant ducts and leave the coil, this introducing the danger that gas cushions might develop which greatly impair the effective cooling of the coil. In the event of violent and rapid development of helium vapor within the coil winding, for instance, due to a disturbance in the operation of the magnet, it may be necessary to discontinue the operation of the magnet.

SUMMARY OF THE INVENTION

An object of the present invention is to improve on the cooling of a superconducting magnet coil of the design or type referred to hereinabove, and, particularly, to obtain a rapid removal of even larger quantities of vaporized helium, or other coolant, as well as to provide for a more rapid supply of liquid coolant, through paths as short as possible. In addition, operation of the superconducting magnet coil should desirably be made possible in positions other than vertical without appreciable impairment of the operation of the coil.

With the above in mind, the present invention involves the formation of coolant passages extending through the support and mounting elements of the described design or type of magnet coil. These passages extend radially with respect to the coil, to the latter's outside which is exposed to the liquid coolant bath, and within the coil they connect with the coolant ducts through which the coolant passes to contact the edges of the ribbon-shaped conductors.

Through these radially extending or directed coolant passages, bubbles of helium vapor generated in the coolant ducts adjacent to the winding layers can travel out of the coil rapidly even if the orientation of the longitudinal axis of the coil deviates from the vertical, such as being horizontal. At the same time, fresh liquid helium, or other coolant, can pass through these radially directed passages into the coolant ducts which are adjacent to the winding layers, rapidly and over short paths, regardless of the position of the magnet coil.

Furthermore, because the radial passages are formed in the windings support and mounting members, they are located outside of the winding layers themselves and do not pass through them. This avoids a reduction of the packing factor of the windings such as would result in an accompanying increase in the structural length of the winding due to the then necessary lengthening of the winding heads.

Further, according to the invention, the coolant ducts through which the coolant can pass to contact the winding layers, are in the form of cylindrical elements which encompass the conductor surfaces of the entire winding layers. These cylindrical coolant duct elements provide a uniform coolant supply to all parts of the winding layers including their winding heads.

As previously noted, it is possible that support bars may extend between the axially extending sides of two of the winding components to support them circumferentially apart, and that the winding components may be mounted on the winding carrier cylinder by wrappings, and cores may be positioned within the components' windings. Such possibilities are used by the present invention, the bars having a series of relatively large holes formed through them to form portions of main radial coolant passages, which are completed by corresponding holes formed through the cylindrical duct elements and wrappings so the passages extend radially from the innermost cylindrical duct element to the coil's outside without passing through the windings themselves. These duct elements are constructed to be permeable in all directions to the coolant. Also, smaller holes are distributed throughout the cores with registering holes formed through the wrappings and duct elements.

The support bars are between the winding components' horizontally extending winding portions which extend approximately parallel to each other and therefore are within or in the immediate vicinity of the previously referred to plane of symmetry of the magnet. Such bars permit simplified bracing of the winding layers of any two components against each other and permit simple fabrication of the radial coolant passages formed in them. The innermost turns of the windings of the individual winding component layers are provided with the cores which function as fillers for what would otherwise be empty space, and these cores are provided with radial coolant passages as previously indicated.

As mounting members for the respective winding components, wrappings of material of high tensile strength surround these shells and are provided with the openings for the coolant passages. Preferably between the carrier cylinder and the innermost following windings, a cylindrical duct element is provided through which the liquid coolant can pass in all directions.

Although the superconducting magnet coil of the present invention can be operated in any position, it is particularly advantageous if it is arranged in the coolant tank of the associated cryostat in such a manner that the longitudinal axis of the carrier cylinder is approximately horizontal and the plane of symmetry approximately vertical; that is to say, in the direction of the force of gravity.

The winding components' axially or longitudinally extending sections are connected at their ends by the circumferentially extending portions which are referred to herein as winding heads. At these winding heads particularly high magnetic fields occur. With the arrangement described hereinabove, there is the particular advantage that the coolant vapor bubbles generated at these winding heads largely rise vertically tangentially upwardly in the coolant ducts adjacent to the winding layers at those locations, and can escape from the winding heads to the outside through the relatively large coolant passages located within or in the immediate vicinity of the plane of symmetry.

It is of particular advantage if the passages which extend through the support bars are made with a larger cross section that those at other locations, because these bars are located at the lowest and highest points of the magnet coil and therefore form the main discharge passages for the helium in vapor form as well as the main inlet passages for the liquid coolant.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings schematically show a preferred embodiment of the present invention, in which:

FIG. 1 is a broken-away perspective view of a cross-sectioned portion of this embodiment;

FIG. 2 is a segment of the cross-sectioned end of FIG. 1, on an enlarged scale permitting details to be shown which cannot be seen in FIG. 1; and

FIG. 3 shows an arrangement of winding layers such as might be used to produce a homogeneous magnetic field.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The dipole winding of the superconducting magnet coil shown in FIGS. 1 and 2 consist of two component windings, which, in turn, comprise two winding layers 1 and 2 in the case of one component winding and 3 and 4 in the case of the other. The winding layer 1 is on top of the winding layer 2 and the winding layer 3 is on top of the winding layer 4. The winding is positioned on a carrier cylinder 5 which at the same time forms the inner wall of the liquid helium tank of the cryostat in which the coil is located, the outer wall of the cryostat comprising a cylindrical tube 6 of large enough diameter to define a space for liquid helium between it and the outside of the coil.

The winding components are cylindrical segments, being almost semi-cylinders, with the innermost winding layers 2 and 4 fitting the contour of the carrier cylinder 5. All of the winding layers are wound with similar contours, illustrated in the case of the layer 1 by FIG. 1 where it is shown to comprise side portions 8 which extend substantially or approximately parallel to the longitudinal axis 7 of the carrier cylinder 5, and circumferentially extending winding heads 9. The shape of the winding layer 2 which is underneath the winding layer 1 is indicated by dashed lines in FIG. 1; and since it consists of more turns than the winding layer 1, it therefore has wider winding heads than the layer 1. The two component windings are positioned symmetrically with respect to the plane of symmetry of the coil, as defined by the longitudinal axis 7 of the carrier cylinder 5 and the straight line 10 which is normal to the direction of the magnetic field produced by the coil, indicated by the arrow B. In FIG. 1 the coil is positioned with the longitudinal axis 8 horizontal and the plane of symmetry defined by the straight lines 7 and 10, vertical. This has the advantages previously noted.

The winding layers 2 and 4 and the winding layers 1 and 3 jointly form cylinders. Between the inner windings 2-4 and the carrier cylinder 5 a cylindrical coolant duct, or passage system, 11 is positioned, through which the liquid coolant can move in axial, tangential and radial directions with respect to the coil. This is provided, for example, by placing around the carrier cylinder 5 a plastic-coated metal screen or one of several suitably shaped plastic mats. This coolant duct system 11 is followed by the winding layers 2 and 4, each preferably wound from ribbon shaped stabilized conductors composed of high-field superconductive material and electrically normal conductive metal. These ribbons are each wound with the ribbon edges facing the coolant duct or passage system 11 bare and uninsulated so as to have direct contact with the coolant to obtain very effective heat transfer to the coolant. To insulate the individual turns or convolutions of the ribbon-shaped stabilized conductors from each other, a ribbon of insulating material (not shown) may be wound between the interfacing sides of the conductor.

The two winding layers 2-4 are followed in a radial outward direction by a second coolant duct or passage system 12 having the characteristics of the layer 11 and around which is wound a wrapping 13 which may be, for example, epoxy-resinated glass fiber tape of high tensile strength. This wrapping 13 is followed by another coolant duct system 14 on which the winding layers 1-3 are positioned, these being in turn surrounded again by a cylindrical coolant duct system 15 with a further wrapping 16 having the characteristics of the wrapping 13. For the foregoing construction reference should be had to FIG. 2 because in FIG. 1 all of the various wrappings described cannot be illustrated because of the drawing scale of FIG. 1.

Between all of the winding side portions which correspond to 8 in FIG. 1, support fittings 17 are interposed which may, for example, be made of hard fiber material in the form of the bars each having a series of axially interspaced coolant passages or holes 18 formed through them and which extend approximately in the plane of symmetry defined by the straight lines 7 and 10. These fittings or bars 17 are used at both the top and bottom of the coil. The wrappings 13 and 16 and thecoolant duct system layers 12, 14 and 15 formed by the screens or mats, are all provided with coolant passages 19 in registration with the passages 18 of the fittings 17. In this way liquid coolant passages 18-19 are formed which are open to the coolant duct system layer 11 at the latter's surface, and edgewise with respect to the coolant duct system layers 12, 14 and 15, the passages 18-19 extending to and opening from the outer periphery of the coil.

As can be seen from FIG. 1, a core 20, made, for example, of hard fiber material, having radial coolant passages 21 is positioned between the innermost conductor turn of the winding side 8 and winding head 9 to support this innermost turn. This core defines a substantially cylindrical segment to conform to the cylindrical contour of the coil. A similar core is applied to the winding layer 3 and corresponding cores 20a are used for the same purpose in the case of the winding components 2 and 4, the passages 21 being registered radially through to the layer 11 insofar as they extend through both the cores 20 and 20a. At least the wrappings 13 and 16 are provided with holes 22 in registration with the passages 21 and such holes may also be formed through the various coolant ducts.

It can clearly be seen especially from FIG. 1 that liquid helium easily can get from the helium tank of the cryostat from below, via the lower coolant passages 18 and 19 into all of the cylindrical coolant all-directional ducts 11, 12, 14 and 15 and can spread out in these coolant ducts freely in the axial as well as in the tangential and radial directions over the entire coil areas. Helium vapor bubbles generated in these coolant ducts can move upwardly, particularly in the region of the winding heads or ends 9 and can escape through the main discharge openings 18 in the fittings or spacer bars 17 situated on top. The radial coolant passages 21 in the cores 20 offer further paths for a rapid escape of evaporated helium. Through these radial coolant passages 21, fresh liquid helium can furthermore again flow rapidly into the coil winding. Thereby, excellent cooling of the coil is assured with, at the same time, the highest operational safety.

For manufacturing the coil, the individual winding layers of the component coils can advantageously be prefabricated on a winding fixture and be solidified, for instance, by means of a suitable synthetic resin. Around the tubular carrier cylinder 5 are placed, for instance, helium-permeable plastic mats. Subsequently, the prefabricated winding layers 2 and 4 are put on and the cores of the windings are filled by fillers 20. In this connection, security against rotation of the winding must also be provided. The fittings 17 or bars are then inserted in between the long sides of the winding layers. After a further helium-permeable mat is put on, the entire winding shell, including the winding heads or ends, is wrapped under pretension on its entire outer area by means of the wrapping 13. The further build-up proceeds according to the sequence which may be seen in FIGS. 1 and 2. The individual winding layers are then advantageously electrically connected in series. It is advisable to arrange the coil or winding terminals in regions of low mechanical stress and a low magnetic field.

Within the coil windings the electro-magnetic forces are transmitted tensionally connected by the fitted fillers and fittings and the pre-tensioned wrappings. The winding forces resulting toward the outside can be taken up by a relatively strong, inflexible carrier tube 5 and an outer wrapping 16, which is stressed only in tension, as shown in FIGS. 1 and 2. Deviating from this preferred embodiment, although not shown, an inflexible outer wrapping, which consists of annular I-sections joined together in the axial direction can also be used to take up the winding forces resultant toward the outside. This solution, however, is relatively expensive.

As the cryostat for the coil according to FIGS. 1 and 2 is preferably used a cryostat with a horizontally arranged, freely accessible interior space which is at room temperature. Such cryostats, which in addition have a turret-shaped extension for storing a supply of helium, are described, for instance, in the German Patents 1,501,304 and 1,501,319. Of such a cryostat, only the helium tank 5, 6 and additionally in FIG. 1, the inner tube 31 which encloses the freely accessible inner space 30 and is at room temperature, are shown in FIGS. 1 and 2. The space between the tubes 5 and 31 is evacuated for the purpose of thermal insulation. A further vacuum jacket is provided on the outside of the outer tube 6 (not shown) of the helium tank. Within the two vacuum jackets, nitrogen-cooled radiation shields and heat-insulating plastic foil can, for instance, further be arranged in a manner known per se. If the coil shown in FIGS. 1 and 2 is used for a magnetohydrodynamic generator, the plasma channel and the electrodes of the generator must be arranged in the space 30 inside the tube 31.

In FIG. 3 is shown schematically how inside the carrier tube 5 a largerly homogeneous magnetic field can be achieved. The carrier tube 5 is represented in FIG. 3 by a circle 40, and the outer limit of the winding by a second circle 41. Between these two circles, two mutually overlapping circles 42 and 43 are placed in such a manner that they touch the circles 40 and 41. As is well known, one now obtains a homogeneous magnetic field B within the circle 40 if one provides uniform current density within the shaded areas 44 and 45 defined by the circles 42 and 43. This can be accomplished by simulating the areas 44 and 45 by shell-like winding layers 47 to 51, as shown in FIG. 3 to the left of the plane of symmetry 46 and by providing the same current density within the individual winding layers.

With a magnet coil constructed in this manner and having ten winding shells with the design according to the invention, a homogeneous magnetic field of about 5 tesla can, for instance, be obtained in a free inner space of about 77 cm diameter. In such a coil the individual winding layers may, for instance, consist of copper strip 5 mm wide and 2 mm thick, in which a multiplicity of niobium-titanium wires of about 50 um thickness is embedded. Each of the ten winding shells is then about 5 mm thick. The current density in the winding is here about 10.sup.4 A/cm.sup.2. The plastic mats serving as the coolant ducts may, for instance, be 1 mm thick each, and the wrappings, wound with epoxy resin-reinforced fiberglass tape, about 1 mm each. The length of the coil can be about 4 meters.

The superconducting magnet coil construction according to this invention may be modified greatly from the example of the embodiment shown in detail. One can, for instance, provide a different number of winding layers, or provide between two winding layers a common, cylindrical coolant duct which is permeable axially and tangentially. Furthermore, mounting means such as wrappings need not be provided after every winding layer. Tubes which can be pushed over the windings are, moreover, suited as mounting means in lieu of wrappings. The coolant ducts can also be worked into the walls of these tubes in a suitable manner. It is essential, however, that at least one side of each winding layer can be touched by the coolant stream and coolant canals leading radially outward are provided in support and mounting members, respectively.

Instead of circular cross section, the carrier cylinder on which the winding is arranged, may, for instance, also have an elliptic cross section. The individual winding shells must then be fitted to this cross section. There is also the possibility that the carrier cylinder is conically tapered or expanded from one end to the other. The coil winding can thereby be matched, for instance, to the shape of an expanding plasma channel in an MHD generator.

The superconducting magnet coil according to the invention is suited, besides for MHD generators, also for a number of other applications. It can, for instance, be used as a beam deflection magnet in particle accelerators or as a superconducting stator winding of an electric machine. Also, the conductors of the individual winding layers may not have to be bent as sharply as is shown in FIG. 1. They can also run along the cylinder surface in a wider arc. The sharply bent winding heads, however, are suited particularly if the shortest possible structural length of the superconducting magnet coil is important.

Claims

What is claimed is:

1. A superconductive magnet coil having an elongated dipole winding positioned on a carrier tube, the winding comprising two winding components shaped as shells which are symmetrical with respect to a plane of symmetry which goes through the longitudinal axis of the carrier tube and which is perpendicular to the magnetic field of the coil, the winding components each comprising a plurality of superimposed winding layers each having axially extending sides approximately parallel to the longitudinal axis of the carrier cylinder and circumferentially extending winding heads, the mutually corresponding winding layers of these winding components cooperatively forming layers and having support members extending between their mutually adjacent axially extending sides, the coil having liquid coolant ducts through which liquid coolant can pass from the outside of the coil to contact at least one side of the winding layers in axial and circumferential directions; wherein the improvement comprises the formation of radial coolant passages extending through said support members and between said winding layers' mutually adjacent axially extending sides and radially connecting said ducts with the outside periphery of said coil, said passages being free from said winding components.

2. The magnet of claim 1 in which said coolant ducts are formed by one or more layers which are liquid coolant permeable in all directions and conform in shape to said windings' layers and are in contact therewith, said radial coolant passages being open to said permeable layers.

3. The magnet of claim 2 in which said support members are support bars and said radial coolant passages include a series of holes extending transversely through said support bars, the latter extending axially with respect to said coil and said radial coolant passages extending through one or more of said permeable layers, said support bars being at least adjacent to said plane of symmetry.

4. The magnet of claim 2 in which cores are positioned within the innermost turns of said winding layers, said liquid permeable layers being in contact with the radially inner and outer surfaces of said winding layers and the radially inner and outer surfaces of said cores, the latter being formed with holes forming radial liquid coolant passages extending from one of said liquid permeable layers to the other.

5. The magnet of claim 4 in which the outer circumference of at least the outermost one of said liquid permeable layers is encircled by a high-tensile strength cover having holes formed therethrough in registration with the holes formed in said cores.

6. The magnet of claim 1 in which means are interposed between said winding components and said carrier tube for forming longitudinally and circumferentially extending liquid coolant passages open to the radially inside surfaces of said winding components and to said radial passages.

7. The magnet of claim 6 in which said means comprises a layer which is liquid permeable in all directions.

8. The magnet of claim 1 and including a cryostat in which the magnet is positioned substantially horizontally.

9. The magnet of claim 8 in which said radial passages extend substantially vertically.

10. The magnet of claim 5 in which said radial coolant passages which extend through said support members are larger in cross section than are said radial coolant passages formed by the holes through said cores and said wrapping.