Superconducting magnet

Abstract

A superconducting magnet for producing part of a substantially toroidal field in a device is described. The magnet comprises: a set of conductors comprising one or more first conductors (31f) and one or more second conductors (32f), and a set of joints (33). Each of the joints (33) connects a region of a first conductor (31f) with a region of a second conductor (32f) to form a series of alternating first and second conductors corresponding to at least part of a winding of the magnet. Each of the joints (33) is positioned away from a midplane of the toroidal field. The joints (33) are positioned on alternating sides of the midplane. Each first conductor (3 If) passes through the midplane at a smaller distance from an axis of rotation of the toroidal field than does each second conductor (32f). Each of the regions is elongate and extends in a direction at least partly away from the midplane.

Claims

1. A superconducting magnet for producing part of a substantially toroidal field in a device, the toroidal field having an axis of rotation and a perpendicular midplane, and the magnet comprising: a set of conductors comprising: a first inner superconducting conductor, the first inner superconducting conductor comprising an elongated region that extends substantially parallel to the axis of rotation and that is proximate an end of the first inner superconducting conductor at a furthest extent of the first inner superconducting conductor from the midplane in a first direction, a first outer superconducting conductor, the first outer superconducting conductor comprising a first elongated region that extends substantially parallel to the axis of rotation and that is proximate a first end of the first outer superconducting conductor at a furthest extent of the first outer superconducting conductor from the midplane in the first direction and a second elongated region that extends substantially parallel to the axis of rotation and that is proximate a second end of the first outer superconducting conductor at a furthest extent of the first outer superconducting conductor from the midplane in a second direction that is opposite the first direction, and a second inner superconducting conductor, the second inner superconducting conductor comprising a first elongated region that extends substantially parallel to the axis of rotation and that is proximate a first end of the second inner superconducting conductor at a furthest extent of the second inner superconducting conductor from the midplane in the first direction and a second elongated region that extends substantially parallel to the axis of rotation and that is proximate a second end of the second inner superconducting conductor at a furthest extent of the second inner superconducting conductor from the midplane in the second direction; and a set of joints comprising a first joint and a second joint; wherein: the first joint connects the elongate region of the first inner superconducting conductor with the first elongate region of the first outer superconducting conductor; the second joint connects the second elongate region of the second inner superconducting conductor with the second elongate region of the first outer superconducting conductor; and the second joint is positioned at a different distance from the rotation axis and at a different distance from the midplane with respect to the first joint.

2. A magnet according to claim 1, having a “D” shape.

3. A magnet according to claim 1, wherein the joint comprises solder having an upper critical field that is higher than a maximum operating field at the position of the joints in use.

4. A magnet according to claim 1, wherein a portion of the first outer superconducting conductor comprises a second region that extends in a direction that is not parallel to the axis of rotation, wherein the second region is distinct from the first elongated region of the first outer superconducting conductor.

5. A magnet according to claim 1, wherein one of the first inner superconducting conductor or the first outer superconducting conductor behaves as a superconductor at a first temperature, and another of the first inner superconducting conductors or the first outer superconducting conductor behaves as a superconductor at a second temperature, wherein the first temperature is higher than the second temperature.

6. A magnet according to claim 1, wherein, under a set of operating conditions: said first inner superconducting conductor has a first critical current at a first field; and said first outer superconducting conductor has a second critical current at a second, lower field; wherein the first and second fields correspond to maximum operating fields in regions of the device to be occupied by the first inner and first outer superconducting conductors, respectively; and wherein the first and second critical currents exceed the operating current by first and second margins, respectively.

7. A magnet according to claim 1, wherein said first inner superconducting conductor comprises a superconductor having a first upper critical field corresponding to a first magnetic flux density; and said first outer superconducting conductor comprises a superconductor having a second upper critical field corresponding to a second magnetic flux density, wherein the first upper critical field magnetic flux density is higher than the second upper critical field magnetic flux density.

8. A magnet according to claim 1, wherein the set of conductors comprises three inner superconducting conductors and three outer superconducting conductor.

9. A magnet according to claim 1 wherein: the set of conductors further comprises a second outer superconducting conductor, the second outer superconducting conductor comprising a first elongated region that extends substantially parallel to the axis of rotation and that is proximate a first end of the second outer superconducting conductor at a furthest extent of the second outer superconducting conductor from the midplane in the first direction; the set of joints further comprises a third joint; and the third joint connects the first elongate region of the second inner superconducting conductor with the first elongate region of the second outer superconducting conductor.

10. A magnet according to claim 9, wherein the third joint is positioned on a line parallel with the rotation axis and at a different distance from the midplane with respect to the first joint and the second joint.

11. A superconducting magnet for producing part of a substantially toroidal field in a device, the toroidal field having an axis of rotation and a perpendicular midplane, and the magnet comprising: a set of conductors comprising: a first inner superconducting conductor, the first inner superconducting conductor comprising an elongated region that extends substantially parallel to the axis of rotation and that is proximate an end of the first inner superconducting conductor at a furthest extent of the first inner superconducting conductor from the midplane in a first direction, a first outer superconducting conductor, the first outer superconducting conductor comprising a first elongated region that extends substantially parallel to the axis of rotation and that is proximate a first end of the first outer superconducting conductor at a furthest extent of the first outer superconducting conductor from the midplane in the first direction and a second elongated region that extends substantially parallel to the axis of rotation and that is proximate a second end of the first outer superconducting conductor at a furthest extent of the first outer superconducting conductor from the midplane in a second direction that is opposite the first direction, and a second inner superconducting conductor, the second inner superconducting conductor comprising a first elongated region that extends substantially parallel to the axis of rotation and that is proximate a first end of the second inner superconducting conductor at a furthest extent of the second inner superconducting conductor from the midplane in the first direction and a second elongated region that extends substantially parallel to the axis of rotation and that is proximate a second end of the second inner superconducting conductor at a furthest extent of the second inner superconducting conductor from the midplane in the second direction; and a set of joints comprising a first joint and a second joint; wherein: the first joint connects the elongate region of the first inner superconducting conductor with the first elongate region of the first outer superconducting conductor; the second joint connects the second elongate region of the second inner superconducting conductor with the second elongate region of the first outer superconducting conductor; and the second joint is positioned on a line parallel with the rotation axis and at a different distance from the midplane with respect to the first joint.

12. A magnet according to claim 11 wherein: the set of conductors further comprises a second outer superconducting conductor, the second outer superconducting conductor comprising a first elongated region that extends substantially parallel to the axis of rotation and that is proximate a first end of the second outer superconducting conductor at a furthest extent of the second outer superconducting conductor from the midplane in the first direction; the set of joints further comprises a third joint; and the third joint connects the first elongate region of the second inner superconducting conductor with the first elongate region of the second outer superconducting conductor.

13. A magnet according to claim 12, wherein the third joint is positioned at a different distance from the rotation axis and at a different distance from the midplane with respect to the first joint and the second joint.

14. A magnet according to claim 11, having a “D” shape.

15. A magnet according to claim 11, wherein the joint comprises solder having an upper critical field that is higher than a maximum operating field at the position of the joints in use.

16. A magnet according to claim 11, wherein a portion of the first outer superconducting conductor comprises a second region that extends in a direction that is not parallel to the axis of rotation, wherein the second region is distinct from the first elongated region of the first outer superconducting conductor.

17. A magnet according to claim 11, wherein one of the first inner superconducting conductor or the first outer superconducting conductor behaves as a superconductor at a first temperature, and another of the first inner superconducting conductors or the first outer superconducting conductor behaves as a superconductor at a second temperature, wherein the first-temperature is higher than the second temperature.

18. A magnet according to claim 11, wherein, under a set of operating conditions: said first inner superconducting conductor has a first critical current at a first field; and said first outer superconducting conductor has a second critical current at a second, lower field; wherein the first and second fields correspond to maximum operating fields in regions of the device to be occupied by the first inner and first outer superconducting conductors, respectively; and wherein the first and second critical currents exceed the operating current by first and second margins, respectively.

19. A magnet according to claim 11, wherein said first inner superconducting conductor comprises a superconductor having a first upper critical field corresponding to a first magnetic flux density; and said first outer superconducting conductor comprises a superconductor having a second upper critical field corresponding to a second magnetic flux density, wherein the first upper critical field magnetic flux density is higher than the second upper critical field magnetic flux density.

20. A magnet according to claim 11, wherein the set of conductors comprises three inner superconducting conductors and three outer superconducting conductor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

(2) FIG. 1 illustrates, by way of a cutaway view, a known design of a tokamak;

(3) FIG. 2A illustrates a vacuum vessel and one of the toroidal field coils of the tokamak illustrated in FIG. 1;

(4) FIG. 2B illustrates, by way of a radial sectional view, the vacuum vessel and the toroidal field coil illustrated in FIG. 2A;

(5) FIG. 3A illustrates a winding pack and joints of the toroidal field coil of the tokamak illustrated in FIG. 1;

(6) FIG. 3B illustrates a cross-section of the winding pack illustrated in FIG. 3A;

(7) FIG. 3C illustrates a double pancake of the winding pack illustrated in FIG. 3A;

(8) FIG. 4 shows a cross-section of a cable-in-conduit conductor that may be used in the toroidal field coil of the tokamak illustrated in FIG. 1;

(9) FIG. 5 illustrates, by way of a horizontal cross-sectional view, an inter-pancake joint of the winding pack illustrated in FIG. 3;

(10) FIG. 6A illustrates a vacuum vessel and one of the magnets of a further tokamak;

(11) FIG. 6B illustrates, by way of a radial sectional view, the vacuum vessel and the magnet illustrated in FIG. 6B;

(12) FIG. 6C illustrates the vacuum vessel and the magnet illustrated in FIG. 6A during demounting;

(13) FIG. 7 illustrates a pancake of the magnet illustrated in FIG. 6A;

(14) FIGS. 8A-B illustrate, by way of flow diagrams, methods of (A) constructing and (B) operating the tokamak partly illustrated in FIG. 6A;

(15) FIG. 9 illustrates a further pancake which may be used in the magnet illustrated in FIG. 6A;

(16) FIG. 10 illustrates some features of a magnet which may be used in the tokamak partly illustrated in FIG. 6A;

(17) FIGS. 11A-D illustrate various magnets which may be used in the tokamak partly illustrated in FIG. 6A;

(18) FIGS. 12A-B illustrate (A) a joint which may be part of the magnet illustrated in FIG. 11D and (B) fields in the region of the joint;

(19) FIG. 13 illustrates a part of a magnet with two operating temperatures which may be used in the tokamak partly illustrated in FIG. 6A; and

(20) FIG. 14 shows the critical current density versus field at a temperature of generally 4.2 kelvin for various superconductors which may be used, for example, in the magnet illustrated in FIG. 6A.

DETAILED DESCRIPTION OF THE CERTAIN EMBODIMENTS

(21) Superconducting Magnet

(22) Referring to FIGS. 6-7, a (superconducting) magnet 30 will now be described. The magnet 30 corresponds to a TF coil 30 of a “first” tokamak 10.

(23) Except for the magnets 30 and some related components, the first tokamak 10 may have the same components as the abovedescribed “known” tokamak 1. In particular, the first tokamak 10 may include a vacuum vessel 20, PF coils (not shown) and a central solenoid (not shown) which are the same as the vacuum vessel 2, PF coils 4 and central solenoid 5 of the known tokamak 1, respectively.

(24) The first tokamak 10 includes a plurality of magnets, each of which may be substantially the same as the magnet 30 illustrated in FIGS. 6A-C and described below.

(25) The magnet 30 has a “half-Φ” shape, which corresponds to a “D” shape with an extended upright of the D. The region enclosed by the half-Φ shape is broadly D-shaped. The magnet 30 encircles a section of the toroidal vacuum vessel 20. During operation, the magnet 30 produces part of a toroidal field 60. The toroidal field 60 has a toroidal shape with a cross-section that approximately corresponds to the region enclosed by the magnet 30. The toroidal field 60 has a (substantially vertical) main axis 70 and a (substantially horizontal) midplane 80, which are defined in the same way as in the known tokamak 1. The toroidal field 60 has substantially circular field lines circulating around the main axis 70.

(26) The magnet 30 comprises two parts 31, 32 (hereinafter referred to as inner and outer legs). Referring in particular to FIG. 6C, the parts 31 are separable, making the magnet 30 demountable, as will be explained below. The inner leg 31 is elongate and extends in a substantially straight line in a direction parallel with the main axis 70. The inner leg 31 passes through the hole in the toroidal vacuum vessel 20. The outer leg 32 is elongate and extends along a curved path. An uppermost part of the outer leg 32 is substantially parallel with, and positioned side-by-side with, an uppermost part of the inner leg 31. A lowermost part of the outer leg 32 is substantially parallel with, and positioned side-by-side with, a lowermost part of the inner leg 31. The remaining part of the outer leg 32 is approximately D-shaped, except that it curves smoothly towards each of the uppermost and lowermost parts. The outer leg 32 passes around the outside of the vacuum vessel 20.

(27) Accordingly, the outer leg 32 is generally further from the main axis 70 and/or from the midplane 80 (i.e. less central) than the inner leg 32. The toroidal field 60 generally decreases with increasing distance from the main axis 70 and with increasing distance from the midplane 80. Accordingly, the outer leg 32 is generally subject to lower (operating) fields than the inner leg 31. Other fields in the tokamak (in particular the field produced by the central solenoid, which is the highest of the other fields) may vary in an equivalent way and so emphasise this difference.

(28) Referring in particular to FIG. 6A, the magnet 30 includes a shell 30a, which may be divided into “inner” and “outer” parts 31a, 32a associated with the inner and outer legs 31, 32, respectively.

(29) Referring in particular to FIG. 6C (in which the shell 30a has been removed), the inner leg 31 includes an “inner” part 31b of a winding pack 30b of the magnet 30, and the outer leg 32 includes an “outer” part of the winding pack 30b. In particular, the inner leg 31 includes a number of one or more “inner” pancake parts 31c, and the outer leg 32 includes the same number of “outer” pancake parts 32c. Each inner pancake part 31c pairs with a corresponding outer pancake part 32c to form a pancake 30c.

(30) Referring in particular to FIG. 7, one of the pancakes 30c will now be described.

(31) The inner pancake part 31c includes an “inner” plate 31c, and the outer pancake part 32c includes an “outer” plate 32d. The inner plate 31d has a similar shape to the inner leg 31, and the outer plate 32d has a similar shape to the outer leg 32, except that the uppermost and lowermost parts of the inner and outer plates 31d, 32d have a more complex shape, which will be described below.

(32) The inner and outer plates 31d, 32d may abut each other, and may be releasable connected to each other in any suitable way.

(33) The pancake 30a is a single pancake. Alternatively, the pancake 30a may be a double pancake or another type of pancake. The inner plate 31d has a set of grooves 31e (hereinafter referred to as inner grooves) on one of its major surfaces, and the outer plate 32d has a set of grooves 32e (hereinafter referred to as outer grooves) on the equivalently orientated one of its major surfaces.

(34) A conductor 31f (hereinafter referred to as an inner conductor) is arranged in each of the inner grooves 31e, and a conductor 32f (hereinafter referred to as an outer conductor) is arranged in each of the outer grooves 32e. The inner conductors 31f may be substantially the same as the outer conductors 32f, and each of the inner and outer conductors 31f, 32f may be substantially the same as the conductor 3f shown in FIG. 4. Alternatively, the inner conductors 31f may be different from the outer conductors 32f, as will be explained below.

(35) Only three inner conductors 31f.sub.1, 31f.sub.2, 31f.sub.3 and three outer conductors 32f.sub.1, 32f.sub.2, 32f.sub.3 are shown in the figure. However, there may be any number of inner and outer conductors 31f, 32f.

(36) The inner conductors 31f meet the outer conductors 32f at joints 33 (hereinafter referred to as intra-pancake joints), which will be described below.

(37) The conductors 31f, 32f together form a distorted (half-Φ) spiral. The spiral may be left- or right-handed

(38) Each inner conductor 31f follows a straight path. Each outer conductor 32f follows a path that approximately corresponds to the shape of the outer plate 32d, except that the uppermost and lowermost parts of each of the outer conductors 32f follow a more complex path, which will be described below.

(39) Intra-pancake joint(s) 33 associated with different inner conductors 31f are positioned at different distances from the main axis 70. This enables each inner conductor 31f to follow a straight path parallel to the main axis 70. Furthermore, intra-pancake joint(s) 33 associated with different outer conductors 32f are positioned at different distances from the midplane 80. This enables each outer conductor 32f to follow a smooth path to the relevant intra-pancake joint(s) 33, while avoiding other intra-pancake joints 33. Depending on the size of the intra-pancake joint(s) 33, the outer conductors 32f may follow paths that are splayed towards their ends.

(40) The inner and outer plates 31d, 32d meet in the vicinity of each intra-pancake joint 33, while being shaped to carry the inner and outer conductors 31f, 32f, respectively, to/from the intra-pancake joints 33. Accordingly, as illustrated in the figure, the inner plate 31d narrows in a stepwise way towards its uppermost and lowermost ends, and the outer plate 32d widens in a complementary way.

(41) The centremost conductor in the spiral (in this instance, the third outer conductor 32f.sub.3) extends in a suitable way to a suitable position for an inter-pancake joint 30g or (in this instance) a terminal joint 30h. For example, such a conductor may follow a partially raised or lowered path. In a double-pancake, such a conductor may extend through the inner and/or outer plate 31d, 32d and may correspond to (or be joined to) the centremost conductor of a similar spiral on the other major surfaces of the inner and outer plates 31d, 32d.

(42) Each intra-pancake joint 33 forms an (electrical) connection between an inner conductor 31f and an outer conductor 32f (and is insulated from other intra-pancake joints 33). Accordingly, the conductors 31f, 32f form part of a winding of the magnet 30. Each intra-pancake joint 33 may be formed in any suitable way. For example, each intra-pancake joint 33 may have some or all of the features of the inter-pancake joint 3g of the known tokamak 1 (see in particular FIG. 5). Such an intra-pancake joint 33 may be elongate and may be orientated substantially parallel with the main axis 70. The inner and/or outer plates 31d, 32d may have a cut-out region to at least partly accommodate each intra-pancake joint 33. The inner and/or outer plates 31d, 32d may serve to at least partly clamp each intra-pancake joint 33.

(43) The pancake 30c illustrated in the figure, which may correspond to the first or last pancake 30c in the winding pack 30b, has (when assembled) one inter-pancake joint 30g and one terminal joint 30h.

(44) The inter-pancake joint 30g may be approximately the same as the inter-pancake joint 3g of the known tokamak 1 (see in particular FIG. 5). The inter-pancake joint 30g is for connecting a suitable conductor (in this instance, the first inner conductor 31f1) with a suitable conductor of another pancake 30c.

(45) The terminal joint 30h may be approximately the same as the terminal joint 3h of the known tokamak 1. The terminal joint 30h is for connecting a suitable conductor (in this instance, the third outer conductor 32f.sub.3) with an electrical power system.

(46) Each of the joints in the magnet 30 (i.e. each intra-pancake joint 33, each inter-pancake joint 30g and each terminal joint 30h) may include features to facilitate demounting such as releasable clamps, solder that is readily meltable, etc.

(47) Other pancakes 30c, which may correspond to intermediate pancakes 30c in the winding pack 30b, may have two inter-pancake joints 30g, each of which is for connecting a suitable conductor of the pancake 30c with a suitable conductor of another pancake 30c.

(48) The pancakes 30c may be connected to each other in any suitable way that maintains the same rotation of electrical current.

(49) If the winding pack 30b includes only one pancake 30c, then the pancake 30c may include two terminal joints 30h.

(50) The magnet 30 is demountable, which involves some or all of the joints in the magnet 30 (i.e. some of all of the intra-pancake joints 33, inter-pancake joints 30g and terminal joints 30h) being demountable.

(51) Methods of Constructing and Operating a Tokamak

(52) Referring to FIG. 8A, a method of constructing the first tokamak 10 will be briefly described.

(53) At a first step S1, at least part of the vacuum vessel 20 is provided (e.g. assembled). Several other components of the first tokamak 10 may also be provided at this stage.

(54) At a second step S2, the inner and outer conductors 31f, 32f are arranged around the vacuum vessel 20. For example, pre-assembled inner and outer legs 31, 32 may be put in place. The second step S2 may be carried out even if the toroidal vacuum vessel 20 etc. has already been fully assembled (and without requiring its disassembly).

(55) At a third step S3, some or all of the joints in the magnet 30 are made. This may involve, for example, soldering and/or clamping these joints.

(56) Thereafter, various further steps may be taken to complete assembly of the magnet 30 (e.g. adding the shell 30a) and to complete construction of the first tokamak 10

(57) Referring to FIG. 8B, a method of operating the first tokamak 10 will be briefly described.

(58) At a first step S11, it is determined that at least part of the magnet 30 is to be replaced. For example, it may be determined that one or both winding packs 31b, 32b, one or more pancake parts 31c, 32c, or one or more conductors 31f, 32f are to be replaced. The determination may be made in any suitable way. For example, it may be based on data from one or more sensors associated with the magnet 30. It may be that an operating/service lifetime of the magnet 30 or the part of the magnet 30 has ended.

(59) The inner conductors 31f may be subject to more intense radiation due to their more central position. Accordingly, the inner conductors 31f will generally have shorter lifetimes and require more frequent replacing than the outer conductors 32f. This is particular so in a spherical tokamak, in which the plasma (and hence the inner conductors 31f) are particularly close to the main axis 70.

(60) At a second step S12, relevant joints are demounted. If, for example, only one conductor (e.g. an inner conductor 31f) is to be replaced, then the relevant joints may be the two joints associated with the conductor 31f, or the joints associated with the pancake 30c in which the conductor 31f is included, or the joints associated with two or more or all of the pancakes 30c in the magnet 30. Other components of the magnet 30 (e.g. the shell 30a) and/or other components of the first tokamak 10 may be disassembled prior to demounting the relevant joints.

(61) At a third step S13, the magnet 30 or the part of the magnet 30 is replaced. For example, the inner leg 31 may be set aside and a new inner leg 31 may be put in place, together with the original outer leg 32. The third step S13 may be carried out without requiring disassembly the toroidal vacuum vessel 20.

(62) At a fourth step S14, the relevant joints are made. As described above, this may involve, for example, soldering and/or clamping these joints.

(63) Thereafter, various further steps may be taken to fully re-assemble the magnet 30 (e.g. adding the shell 30a) and/or to fully re-assemble the first tokamak 10.

(64) A Different Pancake

(65) Referring to FIG. 9, a different, “second” pancake 130c will now be described.

(66) The second pancake 130c may be used in place of the first pancake 30c in the abovedescribed magnet 30. The second pancake 130c is substantially the same as the first pancake 30c except for the following: all of the intra-pancake joints 133 of the second pancake 130 are positioned on a line parallel with the main axis 70; the inner and outer plates 131d, 132d meet on this line; and each inner groove 131e and associated conductor 131f follows a path that curves towards one or both of its ends to the relevant intra-pancake joint(s) 133.

(67) The second pancake 130c has a potential advantage of a simpler connection between the inner and outer pancake parts 131c, 132c, and a potential disadvantage of curved inner conductors 131f.

(68) Different Magnets

(69) Referring to FIG. 10, another (“second”) magnet 230 will now be described.

(70) The second magnet 230 may be used in an equivalent tokamak to the first tokamak 10.

(71) The second magnet 230 includes a plurality of straight conductors 231 (which may correspond to inner conductors) and a plurality of curved conductors 232 (which may correspond to outer conductors). In contrast to the abovedescribed “first” magnet 30, all of the straight conductors 231 may be substantially the same as each other, and all of the curved conductors 232 may be substantially the same as each other. Each curved conductor 232 may follow a (three-dimensional) path that approximately corresponds to the shape of the outer conductors 32f of the abovedescribed magnet 30. The second magnet 230 includes a plurality of joints 233. Each joint 233 connects a straight conductor 231 and a curved conductor 232 to form a series of alternating straight and curved conductors 231.sub.1, 232.sub.1, 231.sub.2, 232.sub.2, etc. The conductors 231, 232 together form a distorted (half-Φ) helix.

(72) Accordingly, the conductors 231, 232 of the second magnet 230 are arranged three-dimensionally (in a distorted helix), in contrast to the conductors 31f, 32f of the first magnet 30, which are arranged substantially two-dimensionally (in a distorted spiral).

(73) Other magnets may have an arrangement of conductors with both spiral and helical properties.

(74) The second magnet 230 may have any suitable structure (not shown) to support the conductors 231, 232.

(75) Referring to FIGS. 11A-D, some further magnets will now be described.

(76) Each magnet is a variation of the first magnet 30 and may be used in an equivalent tokamak to the first tokamak 10. In particular, each magnet has differently-shaped inner and outer legs to the first magnet 30. Each magnet generally has substantially the same (although suitably differently-shaped) components as the first magnet 30.

(77) Referring to FIG. 11A, a “third” magnet 330 may include an inner leg 331 that curves slightly such that it partly mirrors the curvature of the outer leg 332. Thus, the third magnet 330 has a potential advantage of having a less curved outer leg 332 and/or joints that are positioned in less central, lower field regions.

(78) Referring to FIG. 11B, a “fourth” magnet 430 may include substantially symmetrical inner and outer legs 431, 432, which is potentially advantageous.

(79) Referring to FIG. 11C, a “fifth” magnet 530 includes an inner leg 531 which curves away from the main axis towards its uppermost and lowermost ends with a substantially constant curvature. The outer leg 532 has matching uppermost and lowermost parts.

(80) Generally, the shape of a magnet and hence the conductors in the magnet may be tailored for better performance, for example by producing favourable operating stresses/strains in the conductors, by producing a favourable orientation of anisotropic superconductors relative to the (operating) field, by enabling straight conductors to be used, etc.

(81) Referring to FIG. 11D, a “sixth” magnet 630 includes an extended inner leg 631 and an outer leg 632 with extended uppermost and lowermost parts.

(82) Referring to FIGS. 12A-B, an intra-pancake joint 633 between an inner conductor 631f and an outer conductors 632f of the sixth magnet 630 will now be described.

(83) As will be explained below, the intra-pancake joint may include solder 633a, and the inner and/or outer conductors 631f, 632f may include solder.

(84) The inner and outer conductors 631f, 632f extend a particularly large distance away from the midplane. The joint 633 is positioned at the furthest extent (e.g. top) of each of the inner and outer conductors 631f, 632f.

(85) Accordingly, the joint 633 is positioned in a particularly low field region 90, where an (operating) field 91 may be below an upper-critical field 92 of solder (e.g. the solder 633a) in the region of the joint 633.

(86) The current flowing from one of the conductors 631f, 632f to the other generally flows via solder (e.g. the solder 633a) and, if the solder is in a superconducting state, then this may advantageously reduce the resistance of the joint 633.

(87) The table below shows some properties of some solders:

(88) TABLE-US-00001 Resistivity at Resistivity at Melting point room temp. 77 kelvin Critical Upper critical (degrees (10.sup.−7 ohm (10.sup.−7 ohm temperature field at absolute Solder alloy Celsius) meters) meters) (kelvin) zero (tesla) Bi.sub.49Pb.sub.18In.sub.21Sn.sub.12 58 9.4 8.1 6.4 3.3 (Cerrolow 136) Pb.sub.20Sn.sub.34Bi.sub.46 ~96 5.5 2.6 8.4 2.3 Pb.sub.20Sn.sub.60Bi.sub.20 ~170 2.6 1.1 8.5 2.2 In.sub.52Sn.sub.48 118 2.6 1.3 6.4 0.34 Pb.sub.38Sn.sub.62 183 1.5 0.48 7.3 0.30 Pb.sub.57Bi.sub.36Sb.sub.7 200-230 6.1 3.7 8.5 2.5

(89) The upper critical field of certain solders, e.g. Cerrolow 136, is relatively high (i.e. of the order of 1 tesla at an operating temperature of ˜4-5 kelvin). Such solders may be used to facilitate the abovedescribed positioning of the joint 633.

(90) In the sixth magnet 630, each joint 633 may be particularly elongated (compared, for example, to the joints 33 illustrated in FIG. 7). Accordingly, each joint 633 may have a larger surface area, which may also reduce its resistance.

(91) Regardless of the length of the joint 633, the (extended) uppermost and lowermost parts of the inner and outer conductors 631f, 632f may be positioned side-by-side to avoid unduly increasing the magnetic field energy.

(92) As illustrated schematically in the figure, each conductor 631f, 632f may include a superconducting part 631fa, 632fa and a parallel normal part 631fb, 632fb. Each superconducting part 631fa, 632fa includes a number of superconducting strands (with circular cross-sections) or tapes (with rectangular cross-sections), and may also include solder. Each normal part 631fb, 632fb includes a high-conductivity metal such as copper, a copper alloy or a silver alloy. The current flowing from one of the conductors 631f, 632f to the other generally flows via the normal parts 631fb, 632fb.

(93) The solder 633a may be omitted from the joint 633 and the joint 633 may be made by merely pressing together the conductors 631f, 632f. In such instances, the relevant surfaces of the conductors 631f, 632f are preferably substantially conformal.

(94) A Magnet with Different Operating Temperatures

(95) Referring to FIG. 13, a part of a “seventh” magnet 730 will now be described.

(96) In the seventh magnet 730, the inner conductors 731f have a different operating temperature to the outer conductors 732f. As will be explained in more detail below, this may lead to a greater choice of superconductors and so may enable an improved magnet 730 to be produced (even though cooling the magnet 730 may be more complex).

(97) Only a single joint 733 between an inner conductor 731f and an outer conductor 732f is illustrated in the figure and described below. The other joints and the other conductors of the seventh magnet 730 may be substantially the same.

(98) Each of the inner and outer conductors 731f, 732f is preferably a cable-in-conduit conductor, i.e. includes a (central) cooling channel 731fc, 732fc. Each cooling channel 731fc, 732fc is closed at each of its ends by the relevant joints (e.g. by an intra-pancake joint 733, as illustrated in the figure) or in any suitable way.

(99) A “first” (cryogenic) fluid F1 is introduced into the cooling channel 731fc of the inner conductor 731f via a first fluid input 734. The connection between the first fluid input 734 and the cooling channel 731fc may be made in any suitable way, for example via the side (as illustrated in the figure) or via the end of the inner conductor 731f. The connection is preferably near an end of the inner conductor 731f. The first fluid F1 passes through the cooling channel 731fc and exits near the other end of the inner conductor 731 via a first fluid output (not shown), which may be similar to the first fluid input.

(100) A “second” (cryogenic) fluid F2 is introduced into the cooling channel 732fc of the outer conductor 732f via a suitable second fluid input 735. The second fluid F2 passes through the cooling channel 732fc and exits via a suitable second fluid output (not shown).

(101) The first fluid inputs and outputs of each of the inner conductors 731f of the seventh magnet 730 may be connected to a “first” cryogenic system. The first cryogenic system controls, for example the temperature and pressure of the first fluid F1. Similarly, the second fluid inputs and outputs of each of the outer conductors 732f of the seventh magnet 730 may be connected to a suitable “second” cryogenic system. Suitable cryogenic fluids include, for example, helium or nitrogen.

(102) Different Inner and Outer Conductors

(103) As explained above, the magnet (e.g. the first magnet 30) may have inner and outer conductors (e.g. the inner and outer conductors 31f, 32f) made from the same conductor and including the same superconductor.

(104) However, additional advantages (beyond e.g. demountability, advantageous positioning of joints, etc.) may be obtained if the magnet has inner and outer conductors made from different conductors and/or including different superconductors. Such a magnet need not be demountable.

(105) As explained above, an outer leg (e.g. the outer leg 32) is generally subject to different conditions, e.g. lower fields, than an inner leg (e.g. the inner leg 31). The inner and outer conductors may be tailored for these different conditions. This may involve taking advantage of differences between properties of different superconductors, for example differences between high-temperature superconductors (HTS) and low-temperature superconductors (LTS).

(106) FIG. 14 shows the critical current density of various candidate superconductors versus field at a temperature of 4.2 kelvin (except for niobium, which is at 5 kelvin). The critical current density corresponds to the critical current per unit area of the superconducting component of the superconductor. For anisotropic superconductors, closed and open symbols represent data with the field parallel and perpendicular to the a-b plane, respectively.

(107) For all of the superconductors, the critical current (i.e. the maximum current that a superconductor can carry with ˜zero resistance) decreases with increasing magnetic field and/or temperature. The critical current goes to zero at an upper critical field (i.e. the highest field in which a superconductor remains superconducting). The upper critical field decreases with increasing temperature.

(108) As can be seen from the figure, HTS yttrium-barium-copper-oxide (YBCO) conductors have a relatively high critical current in high magnetic fields. Moreover, by using suitably aligned, anisotropic YBCO conductors (i.e. with the a-b planes generally parallel to the field), this critical current may be further increased.

(109) Accordingly, YBCO conductors may be advantageously used in the inner leg of the magnet, where the (operating) fields are highest. In principle, this may enable the magnet to produce extremely high fields.

(110) YBCO conductors may also be operated at relatively high temperatures (e.g. ˜30 kelvin). This may lead to significant reductions in cooling costs. As mentioned above, the inner leg is subject to much higher levels of radiation and hence a much higher heat load due to the radiation, compared to the outer leg. Accordingly, maintaining the inner leg at relatively low temperatures (e.g. ˜4-5 kelvin) requires relatively high cooling power.

(111) However, the YBCO conductors themselves may be relatively high cost.

(112) Accordingly, a superconductor with a lower cost may be advantageously used in the outer leg of the magnet.

(113) For example, LTS niobium-titanium (NbTi) conductors may be used in the outer leg. Such conductors are relatively low cost and easy to work with (in particular, they are flexible). NbTi has an upper critical field of ˜10 tesla at 4 kelvin and so its use is limited to relatively low magnetic fields (less than ˜10 tesla) and relatively low temperatures (less than ˜5 kelvin).

(114) In this instance, where the inner leg includes YBCO conductors operated at a relatively high temperature (˜30 kelvin), then the magnet will need two cryogenic systems. These may be configured as described above in relation to the seventh magnet 730.

(115) Alternatively, HTS bismuth strontium calcium copper oxide (Bi-2212) conductors may be used in the outer leg. These are higher cost compared to NbTi conductors, but have advantages including that a single cryogenic system may be used to cool both inner and outer conductors to a similar temperature (˜30 kelvin).

(116) The inner and outer conductors may be optimised for use in the relevant fields. For example, the inner and outer conductors may be selected such that they can just safely (i.e. with appropriate margin(s)) carry the operating current with ˜zero resistance in the relevant field.

(117) The upper critical field of the inner conductor will generally be higher than that of the outer conductor.

(118) The use of different inner and outer conductors may also enable the radiation tolerance of the magnet to be increased without unnecessarily increasing its cost. As explained above, the inner leg is subject to higher levels of radiation than the outer leg. Accordingly, a conductor which is higher cost but less prone to radiation damage may be advantageously used in the inner leg.

(119) As will be appreciated, the flexibility provided by different inner and outer conductors opens up possibilities for magnets for commercial tokamaks with higher magnetic fields, lower costs, greater reliability, etc.

(120) Other Variations

(121) It will be appreciated that there may be many other variations of the abovedescribed embodiments.

(122) For example, instead of the magnet corresponding to a TF coil of the tokamak 10, the magnet may be used in another different type of tokamak. For example, the magnet may be used in a spherical tokamak. A spherical tokamak has an aspect ratio (i.e. a ratio of the major radius to the minor radius of a toroidal region occupied by the plasma) that is relatively small (e.g. less than 2 or less than 1.5). In other words, the plasma and hence the toroidal vacuum vessel have a relatively small central hole. Accordingly, the inner leg of the magnet may be subject to higher magnetic fields and higher levels of radiation and there may be less space available for it (and so higher critical current densities may be required). In these circumstances, dividing the magnet into different inner and outer legs including different superconductors with different properties may be particularly important to meet these requirements at a reasonable cost.

(123) Furthermore, the magnet may be used in a device other than a tokamak, such as a stellarator.

(124) The magnet may include additional intra-pancake joints (and hence additional conductors). For example, there may be a pair of joints between inner and outer conductors.

(125) The joints may have any suitable shape, size and orientation.

(126) Relatively high levels of ohmic heating in a joint may be tolerated in some instances. This may be because of the distance between the joint and the part of the magnet that produces the toroidal field.

(127) The conductors may have any suitable shape. For example, one or more conductors may follow a path that turns and then approaches a joint in a direction substantially towards the midplane.

(128) The inner and outer conductors may include the same superconductor, but may have other differences, e.g. different sizes, shapes, non-superconducting components, different versions of the same superconductor, etc. The conductors may include a combination of two or more different superconductors. The inner conductors may comprise a different such combination from the outer conductors.