SUPPORT STRUCTURES FOR HTS MAGNETS

Abstract

Disclosed herein is a support structure for a field coil comprising high temperature superconductor, HTS. The support structure comprises an internal load transfer member configured to attach at one end to the field coil and at another end to an inner surface of a vacuum vessel containing the field coil and configured to support the field coil. At least part of the internal load transfer member is configured to remain at room temperature during operation of the HTS magnet and is not cooled by the cooling system used to cool the field coil.

Claims

1. A support structure for a field coil comprising high temperature superconductor, HTS, the support structure comprising: an internal load transfer member configured to attach at one end to the field coil and at another end to an inner surface of a vacuum vessel containing the field coil and configured to support the field coil against electromagnetic forces acting on the field coil; wherein at least part of the internal load transfer member is configured to remain at room temperature during operation of the field coil.

2. A support structure according to claim 1, and comprising an external support member configured to support the inner support member, wherein the external support member is integrated with the vacuum vessel or attached to an outer surface of the vacuum vessel.

3. A support structure according to claim 1, wherein the internal load transfer member is configured to attach to the upper inner surface of the vacuum vessel, and to an upper portion of the field coil.

4. A support structure according to claim 1, wherein the internal load transfer member comprises a laminated material, with a plane of the laminated material being perpendicular to a load axis of the internal load transfer member.

5. A support structure according to claim 4, wherein the laminated material is a glass fibre epoxy material.

6. A support structure according to claim 1, wherein the field coil is a toroidal field coil for confining plasma in a Tokamak and the internal load transfer member is configured to attach to an upper portion of a central column of the toroidal field coil.

7. A support structure according to claim 1, wherein the field coil is a toroidal field coil for confining plasma in a Tokamak and the internal load transfer member is configured to attach to a return limb of the toroidal field coil.

8. A support structure according to claim 1, wherein the field coil is a poloidal field coil for confining plasma in a Tokamak.

9. A cryostat for a field coil comprising high temperature superconductor, HTS, the cryostat comprising: a support structure according to claim 1; a vacuum vessel enclosing the inner support member and the field coil.

10. A cryostat according to claim 9, further comprising a heat shield located between the vacuum vessel and the field coil, the heat shield being configured for cooling to an intermediate temperature between a temperature of the field coil and a temperature of the vacuum vessel, wherein the internal load transfer member passes through the heat shield.

11. A cryostat according to claim 10, configured to cool the heat shield using liquid nitrogen.

12. A superconducting magnet comprising: a cryostat according to claims 9; a field coil comprising high temperature superconductor, HTS; and a cooling system configured to cool the field coil to a temperature below the critical temperature of the HTS.

13. A superconducting magnet according to claim 12, wherein the internal load transfer member is not directly cooled by the cooling system.

14. A superconducting magnet according to claim 12, further comprising an external support frame outside the cryostat, and one or more external supports for transferring load from the internal load transfer member to the external support frame.

15. A nuclear fusion reactor comprising: a cryostat according to claim 9; a toroidal field coil comprising high temperature superconductor, HTS, to which the internal load transfer member is attached; two or more poloidal field coils comprising HTS; a spherical tokamak plasma chamber; and a cooling system configured to cool the toroidal and poloidal field coils to a temperature below the critical temperature of the HTS.

16. A nuclear fusion reactor according to claim 15, and comprising a second internal load transfer member attached to the poloidal field coil.

17. A superconducting magnet, comprising: a field coil comprising high temperature superconductor, HTS; a cooling system for cooling the field coil to a temperature below a critical temperature of the HTS; a vacuum vessel containing the field coil; an internal load transfer member configured to attach at one end to the field coil and at another end to an inner surface of a vacuum vessel containing the field coil and configured to support the field coil against electromagnetic forces acting on the field coil; wherein at least part of the internal load transfer member is configured to remain at room temperature during operation of the field coil.

18. A superconducting magnet according to claim 17, wherein the field coil is a toroidal or poloidal field coil for confining a plasma in a Tokamak.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Some preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

[0020] FIG. 1 is a schematic diagram of an exemplary HTS field coil, cryostat, and support structure; and

[0021] FIG. 2 is a schematic diagram of a supported toroidal HTS field coil.

DETAILED DESCRIPTION

[0022] Due to the high forces applied to a toroidal field coil during operation, support structures inside the cool volume may be inadequate for high-field and/or low radius toroidal field coils. Due to the perceived need in the prior art to keep the EM support structures cool, it is effectively not possible to transfer forces from the toroidal field coils to external supports, but instead the strength of the supporting structures themselves is relied on to support the field coils. This is a particular issue because the loads on the toroidal field coils are not axisymmetric (rotationally symmetrical about the central column), which makes it difficult to design support structures which can be contained within the cool volume.

[0023] In contrast to the conventional approach to building superconducting magnets, it is proposed that the supports of an HTS toroidal field coil may be left uncooled without making a significant difference to the heat load of the magnet. This can be done as the cost of removing excess heat at HTS operating temperatures (typically around 30K) is much less than the cost of removing heat at LTS operating temperatures (typically around 4K). The extra heat will add to the power required to keep the magnet cool, but allows for greatly simplified design of the support structures, and reduces the size of the cryostat, vacuum vessel and heat shields required (as they need only enclose the magnet itself, and not the supports).

[0024] The use of room temperature supports is particularly attractive for applications with an already high heat load such as nuclear fusion reactorsthe heat load from such a reactor is much greater than the excess heat load due to the room temperature supports, and so the cooling system can easily cope with the extra heat.

[0025] In addition, most conventional superconducting magnets are axisymmetric. Any loads caused by electromagnetic forces can be contained within the cold volume.

[0026] By contrast, the toroidal field coils used to contain plasma in a tokamak fusion reactor are not axisymmetric and have very challenging stress distributions. In particular, during normal operation of the tokamak the self-field of the toroidal field coil leads to a force distribution acting outwards in the plane of the coil.

[0027] The support structure for the field coil comprises an internal load transfer member which connects to the magnet and the inner surface of the vacuum vessel of the cryostat. The support structure may also comprise an external support member which connects to the outer surface of the vacuum vessel at a location corresponding to the point where the internal load transfer member is attached, and bears the load exerted by the internal load transfer member. The external support member may be integrated with the vacuum chamber, e.g. as extra re-enforcement to the vacuum vessel structure.

[0028] The loads supported by the internal load transfer member may include gravitational loads (i.e. due to the weight of the magnet structure) and/or electromagnetic loads (i.e. due to the electromagnetic forces acting on the magnet structure). It is expected that during operation of a toroidal field magnet, the electromagnetic loads will be significantly higher than the gravitational loads.

[0029] FIG. 1 shows an exemplary HTS field coil, cryostat, and support structure according to an embodiment. The HTS field coil 11 is cooled to 30K by a cooling system (not shown), and is inside a vacuum vessel 12 which is at room temperature (about 300K). Between the HTS field coil and the vacuum vessel is a thermal shield 13 which is cooled, also by a cooling system (not shown). This cooling may be to 77K, e.g. by liquid nitrogen (or by hydrogen or helium).

[0030] The temperatures given are by way of example only. The HTS field coil may be cooled to any temperature below the critical temperature of the magnet (depending on the application), and the thermal shield may be at any temperature between the temperature of the vacuum vessel and the temperature of the HTS field coil. Multiple thermal shields may be provided at decreasing temperatures between the vacuum vessel and the HTS field coil. It will also be appreciated that room temperature may not mean precisely 300K, but is intended to cover any temperature above about 270K.

[0031] The HTS field coil is supported by internal load transfer members 14 and 15. The lower internal load transfer member 14 connects to the base of the magnet and to the base of the vacuum vessel. The upper internal load transfer member 15 connects to the top of the magnet and to the upper inner surface of the vacuum vessel. Both internal load transfer members 14 and 15 pass through the thermal shield, and there will be a temperature gradient in the supports from room temperature where they are joined to vacuum vessel 12 to the HTS operating temperature where they are joined to HTS field coil 11. The internal load transfer members transfer loads resulting from the EM forces on the field coil to the vacuum vessel. The loads from the EM forces will generally be in the plane of the field coil and outward from the field coil (with some toroidal loads from the interaction between the current in the toroidal field coil and the poloidal field).

[0032] An external support 16 is attached to the outer upper surface of the vacuum vessel 12, to bear the load exerted by the upper internal load transfer member 15. The external support 16 and upper internal load transfer member 15 may be attached only to the vacuum vessel 12, or they may be attached to each other by structures which pass through the vacuum vessel 12, provided such structures maintain the seal of the vacuum vessel 12. For example, one or more bolts may attach the internal load transfer member 15 to the external support 16 through holes in the vacuum vessel 12, and a seal may be provided between the internal load transfer member 15 and the vacuum vessel 12 and/or between the external support 16 and the vacuum vessel 12 to avoid leaks through the bolt holes. As a further example, the internal load transfer members and external support members may together comprise a strut which passes through the vacuum vessel (i.e. with the sections inside acting as internal load transfer members, and the parts outside acting as external support members). The external support supports the loads exerted by the inner load transfer members onto the vacuum vessel.

[0033] The external support may be provided as a frame or other structure 16 outside the vacuum vessel as shown in FIG. 1, or it may be integrated with the vacuum vessel, e.g. by using a re-enforced vacuum vessel configured to support the loads transferred by the internal load transfer members. The external support may comprise a combination of re-enforcement to the vacuum vessel and support structures outside the vacuum vessel.

[0034] It will be appreciated that load transferring members passing through a heat shield are generally connected to it thermally. This can be done by flexible links so that the mechanical load is still transferred to room temperature, but some of the conducted heat is removed at higher temperatures where it is more efficient. For example an intermediate thermal link may thermally (but not mechanically) connect the internal load transfer member to a liquid nitrogen temperature shield. This imposes a high heat load, but this does not matter because cooling at 77K is inexpensive. This allows for sections of the internal load transfer member close to the HTS coil to be at a reduced temperature, reducing the heat load at low temperature where cooling is more expensive. The intermediate thermal link may comprise a metal plate between the two thermally insulating blocks that make up the internal load transfer member.

[0035] The internal load transfer members 14 and 15 each act to support the field coil 11. The direction of the force on each internal load transfer member defines an axis of load for that member.

[0036] The internal load transfer members 14 and 15 may be of any suitable load-bearing structure, and may be of any sufficiently strong non-magnetic material. The structure of the internal load transfer members and their attachment to the field coil will depend on the shape of the field coil, but this is well within the scope of normal design work for the skilled person, especially as cooling for the supports does not need to be taken into account (unlike with conventional cooled supports).

[0037] For example, as shown in FIG. 2, where the field coil 21 is a toroidal field coil with a central column and a plurality of return limbs, the internal load transfer members 24 and 25 may be columns fixed to the top and bottom of the central column. The internal load transfer members 24 and 25 may be formed of a laminated material with the laminate sheets being perpendicular to the axis of load. One suitable laminated material is formed from G10 or G11 glass fibre epoxy laminate sheets. Additional internal load transfer members 27 may be attached to the return limbs. These additional load transfer members 27 are particularly beneficial for supporting the field coil against electromagnetic forces. The internal load transfer members pass through the thermal shield 23 to the vacuum vessel 22. An external support frame 26 may also be provided to support the load from the internal load transfer members 27, 25, with the ground acting as external support for the internal load transfer member 24. Again, this external frame 26 is beneficial to provide support against the very considerable electromagnetic forces experienced by a toroidal field coil. It will be appreciated that a similar arrangement may be provided for a poloidal field coil (not shown in FIG. 2).

[0038] As mentioned above, such supports may be used for a fusion reactor such as a spherical tokamak reactor. A spherical tokamak comprises a toroidal plasma chamber, a toroidal field coil as described above, and at least two poloidal field coils which are circular field coils in a plane perpendicular to the central column. With suitable additional support for the plasma chamber and poloidal field coils, the support structure shown in FIG. 2 may be used for such a reactor. For example, the poloidal field coil and plasma chamber may be provided with additional internal load transfer members which connect them to the vacuum chamber, they may be mechanically connected to the toroidal field coil and supported by the same support members which support the toroidal field coil, or some combination of the two approaches may be used. There is relatively less advantage to the use of the support structures on the poloidal field coil, as the forces on the poloidal field coil are generally lower than those on the toroidal field coil, and generally axisymmetric.