GB2545735A

GB2545735A - Cryostats for superconducting magnets

Info

Publication number: GB2545735A
Application number: GB1522865.3A
Authority: GB
Inventors: James Bickell William; James Calvert Simon; Simpkins Michael
Original assignee: Siemens Healthcare Ltd
Current assignee: Siemens Healthcare Ltd
Priority date: 2015-12-24
Filing date: 2015-12-24
Publication date: 2017-06-28
Also published as: GB201522865D0

Abstract

A cylindrical superconducting magnet structure 10, with a cryogen tank 20 comprising a cylindrical bore tube 22, cylindrical outer tube 24, and end pieces 26, forming a hollow cylindrical chamber. A magnet assembly 12, with axially aligned superconducting coils 14 and a coil support arrangement 16, is mechanically joined to, or in contact with, the bore tube 22 of the cryogen vessel, improving its stiffness, which may prevent buckling. Attachment may be by bolts 30. Annular ribs may also be provided on the radially outer surface of the bore tube 22, to locate and mount magnet assembly pieces. In a second aspect, the magnet assembly comprises coils mounted externally to the cryogen vessel 20, bonded to a radially inner surface of the bore tube 22. Shield coils may be bonded to a radially outer surface of the cylindrical outer tube 24, or mounted in journals which may be integrally formed.

Description

CRYOSTATS FOR SUPERCONDUCTING MAGNETS

The present invention relates to cryostats for superconducting magnets. In particular, it relates to a cylindrical superconducting magnet structure comprising a magnet assembly, itself comprising a plurality of axially-aligned coils of superconducting wire and a coil support arrangement, housed within a cryogen vessel which comprises a cylindrical bore tube, a cylindrical outer shell and end pieces joining the cylindrical bore tube to the outer shell to form a hollow cylindrical vessel.

Cryogen vessels for cooling superconducting magnets are partially filled with a liquid cryogen. The cryogen is held at its boiling point. A superconducting magnet structure within the cryogen vessel is kept cool by boiling of the liquid cryogen, while the boiled-off cryogen is recondensed to liquid by a cryogenic recondensing refrigerator. In normal operating conditions, this process stabilises, and the superconducting magnet structure is held at a stable temperature and the cryogen gas pressure within the cryogen vessel reaches a stable value. Typical of current superconducting magnet arrangements, the cryogen is helium and the cryogen gas pressure is approximately atmospheric pressure. The cryogen vessel is typically suspended within a vacuum vessel for thermal insulation. There is, therefore, typically a pressure differential of about atmospheric pressure across the wall of the cryogen vessel.

In certain circumstances, intentional or unintentional, the magnet may quench. In a quench, the superconducting coils become resistive and lose their stored energy to the liquid cryogen. The gas pressure within the cryogen vessel rises significantly. Typically, a valve or burst disc opens to allow cryogen gas and liquid to escape along a quench path. Significant quantities of costly cryogen are lost in this way.

The present invention seeks to reduce this loss of cryogen material in case of a quench. In particular, the present invention provides structures of cryogen vessels which are capable of withstanding an increased cryogen pressure for a given wall thickness of cryogen vessel, so that the cryogen vessel may withstand a higher pressure which arises during a quench, so that less cryogen is vented in case of a quench and more cryogen material is retained within the cryogen vessel. A conventional approach to this problem is to make the cryogen vessel of thicker or stronger material to withstand the peak quench pressure. This will significantly increase the weight and cost of the cryogen vessel. Additional thickness of the cryogen vessel may mean that the diameter of the coils of superconducting wire needs to increase, leading to an increase in wire consumption and an increase in the cost and weight of the coils of superconducting wire.

The present invention aims to provide a cryogen vessel which is capable of withstanding increased pressure, but which does not require a change of material or an increase in vessel wall thickness. Alternatively, or in addition, the present invention provides structures of cryogen vessels which are of thinner material than conventional, for a given pressure-retaining capability.

Accordingly, the present invention provides cylindrical superconducting magnet structures as defined in the appended claims .

The above, and further, objects, characteristics and advantages of the present invention will become more apparent from the following description of certain embodiments thereof, taken in conjunction with the appended drawings, wherein:

Fig. 1 shows an axial part-cross-section of a first embodiment of the present invention;

Fig. 2 shows an axial part-cross-section of a second embodiment of the present invention;

Fig. 3 shows an axial part-cross-section of a third embodiment of the present invention; and

Fig. 4 shows an axial part-cross-section of a fourth embodiment of the present invention.

In Fig. 1, a cylindrical superconducting magnet structure 10 is shown. The structure is essentially symmetrical about axis A-A. The structure comprises a magnet assembly 12, which itself comprises a plurality of axially-aligned coils of superconducting wire 14 and a coil support arrangement 16, housed within a cryogen vessel 20. The cryogen vessel 20 comprises a cylindrical bore tube 22, a cylindrical outer shell 24 and end pieces 26 joining the cylindrical bore tube 22 to the cylindrical outer shell 24 to form a hollow cylindrical cryogen vessel 20.

According to a feature of this embodiment of the present invention, attachments 30 such as bolts or similar are provided, mechanically joining the coil support arrangement 16 of the magnet assembly 12 to inner surface 34 of the bore tube 22 of the cryogen vessel 20.

Pressure P within the cryogen vessel tends to displace the cylindrical bore tube 22 radially inwardly towards axis A-A. By providing attachment 30 as shown in Fig. 1, the structure of the magnet assembly 12 retains the bore tube in position, preventing its deformation. A given pressure P may be withstood by a thinner wall of the cylindrical bore tube 22 when attached to the magnet assembly 12 than would be possible in the conventional case where the cylindrical bore tube is not retained by, or in contact with, the magnet assembly 12. The mechanical strength of the magnet assembly 12 is used to strengthen and retain the cylindrical bore tube 22.

Fig. 2 shows a second embodiment of the present invention. The bore tube 22 is in contact with a radially inner surface of the magnet assembly 12. This may be achieved by various methods such as bonding, welding or bolting. Cryogen gas does not enter between magnet assembly 12 and bore tube 22. The pressure P within the cryogen vessel does not act on the bore tube 22 where it is in contact with the magnet assembly 12, and the pressure is borne by the magnet structure 12. There is thus no tendency for that part of the bore tube to deform. For those parts of the bore tube which are not in contact with the magnet assembly 12, their proximity to a part of the bore tube which is in contact with magnet assembly 12 means that those parts are unlikely to deform. In this embodiment, a thinner bore tube 22 may be used, although the remaining parts of the cryogen vessel: end pieces 26 and cylindrical outer shell 24 will need to be of sufficient strength to withstand the pressure P.

In the illustrated embodiment, the magnet assembly 12 is of the so-called "serially bonded" type, where resin-impregnated coils 14 are bonded by their axial surfaces to resin-impregnated annular spacers 16. The serially bonded magnet structure bears the pressure P of the cryogen gas. The invention may also be applied to more conventional magnet structures where coils are mounted on a former, in which case the bore tube 22 will be in contact with the former, and the pressure P will essentially be borne by the former. The mechanical strength of the magnet assembly 12 is used to strengthen and retain the cylindrical bore tube 22.

In another embodiment, not separately illustrated but corresponding in appearance to Fig. 2, the bore tube 22 may not be bonded or attached to the magnet assembly 12, but the proximity of the magnet assembly 12 to the bore tube 22 means that there is no scope for the bore tube to buckle, which is a common failure mode, meaning that the bore tube 22 is capable of withstanding a significantly higher pressure when in close proximity to the magnet assembly 12 than when it is spaced away from the magnet assembly by a distance sufficient to allow buckling. The mechanical strength of the magnet assembly 12 is used to strengthen and retain the cylindrical bore tube 22.

Fig. 3 illustrates another embodiment of the present invention. In this embodiment, annular ribs 36 are provided on the radially outer surface 34 of the bore tube 22. They may be attached to the bore tube by bonding, welding, bolting or simply by an interference fit. These ribs strengthen the bore tube and reduce its tendency to buckle. The bore tube 22 may accordingly withstand a greater cryogen vessel pressure than would be possible with the same bore tube 22 wall thickness and no annular ribs 36. Preferably, these ribs 36 are employed in the locating and mounting of the magnet assembly 12 and indeed may be employed in the construction of the magnet assembly. The magnet assembly 12 may be assembled from a number of pieces which are located by joining to the stiffener ribs, which should be accurately positioned. This reduces manufacturing risk in that, should a fault be detected in one of the coils, only the corresponding piece need be replaced. Conventionally, where the magnet assembly 12 is constructed as a single piece, any fault in a coil would mean replacing the whole magnet assembly 12. Once assembled, the magnet assembly provides mechanical retention of the ribs 36 and stiffness to the bore tube 22. The mechanical strength of the magnet assembly 12 is used to strengthen and retain the cylindrical bore tube 22.

The ribs 36 may be inserted into the bore tube with the magnet assembly 12, for example in an interference fit by sliding a combination of magnet assembly 12 and ribs 36 onto the bore tube 22. The ribs may then be welded, bolted or otherwise affixed in position.

By employing the illustrated ribs 36, only relatively short lengths of bore tube 22 are free, reducing the risk that those parts of the bore tube will buckle. In the illustrated embodiment, the ribs 36 have a T-shaped cross section, providing a relatively wide surface for connection to the bore tube 22, while providing a narrow interface to the magnet assembly 12, so that it can be incorporated into the support structure 16 of the magnet assembly without difficulty. Of course, other shapes of ribs may be employed, such as flat annular strips, square or triangular cross-sections .

Fig. 4 shows another embodiment of the present invention. In this embodiment, coils 14 are mounted externally to the cryogen vessel, bonded to a radially inner surface 35 of the bore tube 22. This arrangement may be referred to as "A2 bonding". In the illustrated embodiment, shield coils 16 are also provided. These are bonded to a radially outer surface of the cylindrical outer shell 24 of the cryogen vessel 20. This arrangement may be referred to as "A1 bonding". Alternatively, they may be mounted in journals 38 affixed to the outer surface of the cylindrical outer shell 24. In another alternative, journals 38 may be integrally formed with the cylindrical outer shell 24. The shield coils provide strengthening to the cylindrical outer shell 24. This is of particular significance when the system is warm, in which case the cryogen pressure P within the cryogen vessel is particularly elevated. Similarly, the coils 12 provide strengthening to the cylindrical bore tube 22.

The bore tube 22 may be strengthened by provision of ribs 36 mounted within the cryogen vessel, on a radially outer surface 34 of the bore tube. Other structures may be provided within the cryogen vessel to further mechanically strengthen it. Since the coils are not located within the cryogen vessel, there is a large freedom to place strengthening structures as required. The bore tube 22 is strengthened against buckling by the presence of the coils 14 bonded to the radially inner surface 35, and further strengthened against buckling by ribs 36 and any other structures placed within the cryogen vessel 20 to strengthen it. The cylindrical outer shell 24 is similarly strengthened by the presence of shield coils 16.

In the arrangement of Fig. 4, one may regard an increase in thickness of the wall of bore tube 22 of the cryogen vessel as acceptable, as this will not increase the diameter of the superconducting coils. Indeed, since the cryogen vessel bore tube 22 is located radially outside of the coils 14, it may be possible to reduce the diameter of the coils 14 into the space conventionally occupied by the cryogen vessel bore tube, saving on wire cost.

As the coils are cooled by conduction through the walls of the cryogen vessel, the cryogen vessel should be constructed of a material of suitable thermal conductivity, for example, stainless steel, aluminium or copper.

With coils 14 and shield coils 16, since the coils are positioned externally to the cryogen vessel, wall thickness of the cryogen vessel and internal structures may be optimised without limitation due to the location of the coils 14. The cryogen vessel 20 is reduced in radial dimensions as compared to a similar magnet in which the coils 14, 16 are located within the cryogen vessel. This will reduce the material and thickness requirements of the walls of the cryogen vessel 20. This arrangement also enables the use of shield coils 16 with increased axial dimension, if required.

In another embodiment, the individual coils 14 shown in Fig. 4 may be replaced by a "serially-bonded" magnet structure, comprising magnet coils and spacers as illustrated in Figs. 1-2, for example. The mechanical strength of the magnet structure, comprising coils 14 and optionally also spacers 16, is used to strengthen the cylindrical bore tube 22.

The present invention accordingly provides alternative embodiments of cryogen vessels for superconducting magnets wherein the structure of the superconducting magnets is employed to stiffen and strengthen the bore tube of the cryogen vessel.

Claims

1. A cylindrical superconducting magnet structure comprising a magnet assembly (12), itself comprising a plurality of axially-aligned coils (14) of superconducting wire and a coil support arrangement (16), housed within a cryogen vessel (20) which comprises a cylindrical bore tube (22), a cylindrical outer shell (24) and end pieces (26) joining the cylindrical bore tube to the outer shell to form a hollow cylindrical vessel, characterised in that the bore tube (22) is stiffened by mechanical interaction with the magnet assembly (12).

2. A cylindrical superconducting magnet structure according to claim 1, wherein attachments (30) such as bolts or similar are provided, mechanically joining the coil support arrangement (16) of the magnet assembly (12) to inner surface (34) of the bore tube (22) of the cryogen vessel (20).

3. A cylindrical superconducting magnet structure according to claim 1, wherein the bore tube (22) is in contact with a radially inner surface of the magnet assembly (12).

4. A cylindrical superconducting magnet structure according to claim 1, wherein the bore tube (22) is in such proximity to the magnet assembly (12) that there is no scope for the bore tube to buckle.

5. A cylindrical superconducting magnet structure according to claim 1, wherein annular ribs (36) are provided on the radially outer surface (34) of the bore tube (22), the ribs (36) being employed in the locating and mounting of the magnet assembly (12).

6. A cylindrical superconducting magnet structure according to claim 5, wherein the ribs (36) are employed in locating and mounting of the magnet assembly (12).

7. A cylindrical superconducting magnet structure according to claim 6, wherein the magnet assembly (12) is assembled from a number of pieces which are located by joining to accurately positioned stiffener ribs.

8. A cylindrical superconducting magnet structure comprising a cryogen vessel (20) which itself comprises a cylindrical bore tube (22), a cylindrical outer shell (24) and end pieces (26) joining the cylindrical bore tube to the outer shell to form a hollow cylindrical vessel, and a magnet assembly (12), itself comprising a plurality of axially-aligned coils (14) of superconducting wire, wherein the coils (14) are mounted externally to the cryogen vessel, bonded to a radially inner surface (35) of the bore tube (22).

9. A cylindrical superconducting magnet structure according to claim 8, further comprising shield coils (16) bonded to a radially outer surface of the cylindrical outer shell (24) of the cryogen vessel (20).

10. A cylindrical superconducting magnet structure according to claim 8, further comprising shield coils (16) mounted in journals (38) affixed to the outer surface of the cylindrical outer shell (24) of the cryogen vessel (20).

11. A cylindrical superconducting magnet structure according to claim 8, further comprising shield coils (16) mounted in journals integrally formed with the cylindrical outer shell (24) .

12. A cylindrical superconducting magnet structure according to any of claims 8-11, wherein the bore tube (22) is strengthened by provision of ribs (36) mounted within the cryogen vessel, on a radially outer surface (34) of the bore tube.

13. A cylindrical superconducting magnet structure according to any of claims 8-12, wherein the coils (14) are provided as part of a "serially-bonded" structure, comprising coils axially bonded to annular spacers (16).