GB2458265A

GB2458265A - Low field, cryogenic, termination reservoir

Info

Publication number: GB2458265A
Application number: GB0804322A
Authority: GB
Inventors: Michael John Disney Mallet
Original assignee: Siemens Magnet Technology Ltd; Siemens PLC
Current assignee: Siemens PLC
Priority date: 2008-03-10
Filing date: 2008-03-10
Publication date: 2009-09-16
Anticipated expiration: 2028-03-10
Also published as: GB0804322D0; GB2458265B

Abstract

A cryostat for containing a cooled superconducting magnet has an outer vacuum chamber (14 fig 1) in association with an auxiliary vessel 42. The auxiliary vessel is arranged to receive liquid cryogen, and during operation, is at least partially filled with liquid. At least one article 46 is located within the auxiliary vessel, and during operation the article is in contact with the liquid cryogen. The article can be a switch, a joint, or a switch/diode pack, and the auxiliary vessel can be a header tank. The header tank may be arranged to receive recondensed cryogen 44 from a surface 40 cooled by a refrigerator 17. The header tank may be connected to the cryogen vessel via a port 50, and once the header is filled with liquid cryogen, additional liquid may flow through the port and into the cryogen vessel. The superconducting magnet can be cooled by a thermal conductor linking the cryogen to the magnet, or by a cooling loop arrangement. The header tank may be located at a position such that during operation it experiences a magnetic flux density of no greater than 0.8 T.

Description

LOW FiELD, CRYOGENiC, TERMINATION RESERVOIIR

Fig. I schematically illustrates a radial cross-section of a cryostat containing a solenoidal superconducting magnet, such as is currently used in magnetic resonance imaging (MRI) equipment. 11g. 2 schematically illustrates a corresponding axial cross-section.

A superconducting magnet lOis made up of a number of primary coils 22, and may also include shielding coils 24 and other coils such as shim coils and bucking coils (not shown in this example). Shielding coils 24, as shown in Fig. 2, are effective in reducing the strength of magnetic field outside of the magnet, the so-called stray field. The magnet 10 is placed within a cryogen vessel 12, which is partially filled with a liquid cryogen 13. This liquid cryogen may be helium, but may be another cryogen such as nitrogen, hydrogen, neon, depending on the temperature required for operation of the magnet. An outer vacuum container (OVC) 14 encloses the cryogen vessel, and defines a vacuum region surrounding the cryogen vessel and thermally insulating it from the ambient temperature of the OVC. Typically, at least one thermal shield 16 is positioned between the OVC and the cryogen vessel 12. These shields serve to reduce thermal radiation from the OVC reaching the cryogen vessel. Such shields are usually cooled to a temperature intermediate the temperatures of the cryogen vessel and the OVC. In a helium cooled arrangement, typical temperatures are 300K for the OVC 14, 80K for the shield 16 and 4K for the cryogen vessel 12.

An access turret 19 houses a vent tube, also known as an access neck, 20, which provides access to the cryogen vessel for refilling, and also provides an escape path for expelled cryogen in case of quench. Current leads 21, 21a provide electrical connection to the magnet. A typical arrangement sees positive current lead 21 passing through the vent tube 20, while negative current lead 21a connects to the bottom of the vent tube 20, or to the interior surface of the cryogen vessel 12, such that the external negative connection is made through the body of the cryostat. Current leads 21, 21a typically join the magnet 10 near the bottom, preferably under the level of liqLtid cryogen 13. Such connection is considered advantageous, and the presence of liquid cryogen keeps the associated electrical joints at the cryogen temperature. The electrical joints may be somewhat resistive, and during ramping up or down, that is, the introduction of current to, or the removal of current from, the magnet, some ohmic heating of the joints may occur.

A refrigerator turret houses a refrigerator 17 in a refrigerator sock 15. The refrigerator provides cooling to the cryogen vessel and preferably also to the shield 16. Preferably, the refrigerator is a recondensing refrigerator, and serves to recondense any boiled-off cryogen back into its liquid state, to reduce cryogen consumption. The refrigerator sock 15 may be open to the cryogen vessel, or the refrigerator may be thermally linked to a recondensing surface through a wall of the cryogen vessel, to simplify refrigerator removal and replacement, and to allow the refrigerator sock to be evacuated, or at atmospheric pressure.

The apparatus is substantially symmetrical about axis A-A, other than in the placement of refrigerator turret 18 and access turret 19.

Fig. 3 schematically illustrates electrical connections of a typical superconducting magnet for an MRI system, made up of a number of separate coils of superconducting wire. To effect a superconducting electrical circuit the separate coils must be connected together. In order to obtain a complete superconducting circuit, the joints must be superconducting. The superconducting joints are schematically shown at 30, and will be described in more detail below. A superconducting switch 32 is also provided to complete a circuit through the coils 22, 24 and to allow current to be applied to and removed from the coils. A superconducting switch typically comprises a length of superconductive wire 33 and an electric heater 34. To allow current to be applied to and removed from the coils, the heater 34 is enerqised to raise the temperature of the superconductive wire 33 such that is becomes resistive. The swiich is "open". By de-energising the heater 34, the superconducting wire 33 cools such that ii. becomes superconductive. The switch is "closed". As for superconducting joints 30, the superconducting switches 32 are preferably immersed in the liquid cryogen 13, to provide effective cooling, particularly while current is applied to or removed from the coils, a process commonly known as "ramping".

During ramping, heat is typically introduced into the cryogen vessel by resistive heating of non-superconducting current leads. This raises boil-off of liquid cryogen, and may lead to pockets of relatively warm cryogen gas within the cryostat. Such pockets of relatively warm gas may increase the likelihood of magnet quench.

The superconducting joints 30 are typically provided by exposing superconductive filaments from each end of the superconducting wires to be joined, each over a length of about Im. The filaments from the wires are wound together and potted in Woods metal, a superconducting alloy of 500/0 w/w Bi, 26.7% w/w Pb, 13.3°/a w/w Sn, 10% w/w Cd.

The superconducting joints 30 illustrated show conneclions between coils of superconducling wire. It may he necessary to join two or more pieces of superconducting wire together to form a coil. Such joins may be potted in the same way as joints 30, or may be made by soldering the wires together over a long overlap, such as 1 metre. Such joins are not superconductive, but have very low resistance.

Such conventional potted superconducting joints 30 and superconducting switches 32 will often tolerate only a relatively low background magnetic flux density in which to operate. lypical present designs attempt to locate such switches 32 and joints 30 in regions of low flux density which occur within the magnet. I'ig. 4 schematically illustrates an example of contours of magnetic flux density of a solenoidal magnet, and highlights regions of relatively low magnetic flux density which may be chosen for the placement of superconducting joints 30 and switches 32. Points of zero overall field strength, but small volume, are indicated at 50. Present superconducting magnets for MRI systems have maximum magnetic flux density of up to 3T. Such high field may cause problems with the operation of superconducting joints and switches. Similar problems may also be encountered with relatively short magnets with reduced, internal low field volumes, since insufficient low-field volumes are present for placement of superconducting joints and switches.

A diode pack 36, comprising diodes connected in inverse parallel, is connected across the terminals of the superconducting switch 32 to limit the voltage across the switch and prevent damage to the switch due to excess voltage. Is superconducting switch 32 is open, it will become resistive. There is a risk that, once heater 34 is turned off, resistive heating through the wire 33 would keep the switch "open". The diode pack reduces this risk by limiting the voltage across the superconducting switch to a level at which such self-healing is not sufficient to keep the switch "open". The diode pack is usually located within the cryogen vessel, towards the top where it can be cooled by cryogen gas.

The magnetic flux density of an actively shielded superconducting magnet, such as that illustrated in Figs. 1-4 drops off rapidly with distance from the magnet's isocentre X. The current technology used for coil jointing and switch design requires a maximum background magnetic flux density of O.8T. A relatively low field region must be carefully chosen for placement of the superconducting joints 30 and superconducting switches 32 to allow them to function in the presence of a high field magnet. Typical solutions use either increased radial distance, careful volume selection in magnetic null' volumes 50 between coils 22 or extra shielding coils 24 to produce a

low field region.

The present invention provides an improved cryostat and magnet architecture, which provides superconducting joints 30 and switches 32, which may be conventional in themselves, placed in a relatively low field region, while being effectively cooled, and ensuring that heat derived from the operation of the superconducting switches and superconducting joints does not affect operation of the magnet itself.

In particular, the present invention provides a cryogen "header tank" built into the cryostat, in communication with the cryogen vessel 12, within which termination components, in particular superconducting joints 30 and superconducting switches 32 are immersed in liquid cryogen at a radial distance from the magnet isocentre X which is great enough that the magnetic flux density experienced by the termination components is relatively k)w.

The invention accordingly provides methods and/or apparatus as defined in the appended claims.

The above, and further, objects, athantages and characteristics of the present invention will become more apparent from the following description of certain embodiments thereof, given by way of examples only, in conjunction with the accompanying drawings, wherein: lig. 1 schematically illustrates a radial cross-section of a cryostat containing a solenoidal superconducting magnet; Jig. 2 schematically illustrates a corresponding axial part-cross-section; Fig. 3 schematically illustrates electrical connections of a typical superconducting magnet for an MRI system; Fig. 4 schematically illustrates an example of contours of magnetic flux density of a solenoidal magnet; and Fig. 5 schematically illustrates a superconducting magnet housed within a cryogen vessel, including a cryogen header tank, according to an embodiment of the present invention.

Fig. 5 schematically illustrates a superconducting magnet housed within a cryogen vessel, including a cryogen header tank 42, according to an embodiment of the present invention. In the illustrated embodiment, refrigerator 17 has a recondensing surface 40 exposed to the interior of header tank 42. Alternatively, the recondensirig surface may be located elsewhere, with an outlet from the recondensing stage leading to the header tank 42. The header tank 42 is in communication with the cryogen vessel 12 through a port 50 in the wall of the cryogen vessel. As illustrated, the port 50 may he provided with an entrance tube 52 leading into the cryogen vessel. As in the illustrated embodiment, vent tube 20 preferably joins the header iank 42, rather than any other part of the cryogen vessel. Such arrangements are particularly described in co-pending patent applications GB 0618141.6 and PCT/GB2007/050538.

However, the present invention does not require such combination of the vent tube 20 and header lank 42.

Fig. 6 shows an alternative embodiment of the present invention wherein the vent tube 20 joins the cryogen vessel 12 at thc top, as in the prior art arrangement of lig. 1. As in Fig. 5, a cryogen header tank 42 is provided, according to the present invention. In the illustrated embodimeni, refrigerator 1 7 has a recondensing surface 40 exposed to the interior of a header tank 42. Alternatively, the recondensing surface may be located elsewhere, with an outlet from the recondensing stage leading to the header tank 42. The header tank 42 is in communication with the cryogen vessel 1 2 through a port 50 in the wall of the cryogen vessel. As illustrated, the port 50 may be provided with an entrance tube 52 leading into the cryogen vessel.

Figs. 5 and 6 do not illustrate, for reasons of clarity, OVC 14, thermal shield(s) 16, refrigerator turret 18, refrigerator sock 15, access turret 19, although such features will typically be provided in a practical embodiment such as illustrated in Figs. I and 2. It is known to place the refrigerator turret 18 towards the side of the cryostat, as illustrated in Fig. 1, for improved access to the refrigerator.

The header lank 42 is sealed to the surface of the cryogen tank 12 and effectively forms parl of the volume of the cryogen vessel. The header tank 42 is arranged such that a lower region of the interior of the header tank lies below a lower extremity of the port 50. Recondensed cryogen 44 from the refrigerator 17 collects in the lower region, and spills into the cryogen vessel through port 50 once the level of recondensed cryogen 44 is high enough. In Figs. 5 and 6, the header tank 42 is shown filled with recondensed liquid cryogen 44 up to the level of the port 50. Further liquid cryogen 13 is present within the cryogen vessel 12.

According to an aspect of the present invention, termination components 46, in particular superconducting joints 30 and superconducting switches 32, are located within the lower region of the interior of the header tank 42, such that in operation they are immersed in recondensed liquid cryogen 44. This ensures that the termination components are held at the liquid cryogen temperature during operation.

Conventionally, refrigerator 17 operates continuously. Cryogen gas within the header tank 42 is continuously recondensed, providing continuous replenishment of the recondensed liquid cryogen 44 within the header tank. The termination components 46 are accordingly continuously cooled by liquid cryogen 44.

In operation, some heat may be generated by the termination components 46. This will cause boil-off of some of the liquid cryogen 44 within the header tank 42. Since the recondensing refrigerator 17 is in communication with the interior of the header tank 42, the refrigerator 17 may cool and recondense such boiled-off cryogen back to liquid 44 within the header tank 42, reducing or eliminating the entrance of heated cryogen gas into the cryogen vessel 12 itself, and keeping such heated cryogen gas away from the coils 22, 24. The presence of an entrance tube 52 leading down from the port 50 may serve to reduce the amount of heated cryogen gas entering the cryogen vesse] 12, as stratification of the heated cryogen gas within header tank 42 will oppose diffusion of healed cryogen gas downwards through the tube 52.

Preferably, the header tank 42 is located in a suitably low field region of the magnet. By placing the header tank and termination components 46 radially outside of the cryogen vessel 12 itself, their distance from the magnet's isocentre X is increased as compared to conventional positioning of such components within the cryogen vessel. This typically reduces the magnetic flux density experienced by the components 46. The axial positioning of the header tank 42, and/or the positioning of the termination components 46 within the header tank 42, may be selected such that the termination components 46 lie within a region of low magnetic flux density.

When considering operation during ramping up, that is, introduction of current into the magnet coils 22, 24, embodiments such as that of Fig. 5 may be preferred, as large volumes of heated cryogen gas may be produced by resistive heating of the termination components 46. The produced heated cryogen gas may be more than the recondensing refrigerator 1 7 can recondense. In this case, it is necessary for much of the heated cryogen gas to exit through the vent tube 20. By placing vent tube close to the termination components 46 and in communication directly with the interior of the header tank 42, heated cryogen gas evolved from the termination components 46 can leave the cryostat though the vent tube 20 without passing, and heating, the coils 22, 24. The current leads 21, 21a arc typically not superconducting, and these produce a significant amount of heat during ramping. By arranging the vent tube 20 to connect to the header tank 42 as shown in Fig. 5, the heat from the current leads 21, 21a may be cooled by cryogen gas in the header tank, which leaves through the vent tube 20 without passing, and heating, the coils 22, 24.

Such advantages are not provided by the embodiment of l'ig. 6, in which cryogen gas heated by the current leads 21,21a will be within cryogen vessel 12. Any heated cryogen gas generated within the header tank 42 will have to pass through cryogen vessel 12 to reach vent tube 20.

In the event of servicing or repair operations being required, such that access to the superconducting joints 30 or the superconducting switches 32 is required, the location of the termination components 46 within the header tank 42 according to the present invention simplifies the processes for accessing the termination components 46. The header tank 42, accommodating the termination components 46, is preferably located within the refrigerator turret 18 (shown in Fig. 1). In such arrangements, if access is needed for example for re-jointing or replacing switches, only the refrigerator turret 18 needs to be opened or removed. It would not be necessary to open the cryogen vessel 12 itself in all eventualities.

According to a further aspect of the present invention, the diode pack 36, illustrated in Fig. 3, can also be removed from its conventional position within the cryogen vessel to be located within the header tank 42, so as to reduce thermal load within the cryogen vessel 12 during activation of the diode pack 36. If desired, any other equipment, conventionally located within the cryogen vessel and likely to cause heating by boil-off of cryogen may preferably be located within the header tank. -11 -

While the present invention has been discussed with reference to a limited number of specific embodiments, numerous variations and modifications are possible, within thc scope of the invention, as will be apparent to those skilled in the art. lor example, althouqh the described embodiments have header tanks 42 which receive recondensed liquid cryogen 44 directly from the refrigerator 17, allernative embodiments have the termination components 46 within a separate cryogen reservoir, preferably isobaric with the cryogen vessel 12, and provided with liquid cryogen from the cryogen vessel 12. Any such reservoir may be provided with its own vent tube, in addition to a vent tube provided to the cryogen vessel itself, such that heated cryogen vapour produced by heat from the termination components, may leave the cryostat without passing, and heating, the magnet coils 22, 24. Any such reservoir should preferably be located in a

region or relatively low magnetic field strength.

In some embodiments, no cryogen vessel is provided. Liquid cryogen from the header tank cools the magnet by transfer of heat from the magnet to the liquid cryogen in the header tank. In some embodiments, this is achieved by circulation of liquid cryogen through a cooling loop: a loop of thermally conductive tube in thermal contact with the magnet, and open to the header tank, such that liquid cryogen circulates through the cooling loop by convection. Alternatively, a thermal conductor may be provided to link the magnet to the header tank. The thermal conductor may be any known thermal conductor arrangement, such as a bar, rope, braid or laminate of copper or aluminium. In such arrangements, much less cryogen is needed than the immersion cryogen vessel arrangements of Figs. 5 and 6. For example, if the header tank is sealed with conduction cooling to the magnet, only a relatively small volume of liquid cryogen is required -for example, only 20-30 litres compared to 1000-1500 litres for an immersion cryogen vessel. Cooling may be provided by copper braids, strips etc. and no liquid cryogen need touch the magnet.

Claims

CLAIMS.1. A cryostat for containing a cooled superconducting magnet, the cryostat comprising: -a cryogen vessel (12) for containing the magnet and a liquid cryogen; -an outer vacuum chamber (14) housing the cryogen vessel; and -a header tank (42) arranged to receive liquid cryogen and which, in operation, is at least partially filled with a liquid cryogen, wherein at least one joint (30) joining superconducting wires is located within the header tank such that, in operation, the joint is in contact with the liquid cryogen within the header tank.
2. A cryostat for containing a cooled superconducting magnet, the cryostat comprising: -a cryogen vessel (12) for containing the magnet and a liquid cryogen; -an outer vacuum chamber (14) housing the cryogen vessel; and -a header tank (42) arranged to receive liquid cryogen and which, in operation, is at least partially filled with a liquid cryogen, wherein at least one superconducting switch (32) is located within the header tank such that, in operation, the joint is in contact with the liquid cryogen within the header tank.

:
3. A cryostat for containing a cooled superconducting magnet, the cryostat comprising: -a cryogen vessel (12) for containing the magnet and a liquid cryogen; -an outer vacuum chamber (1 4) housing the cryogen vessel; and -a header tank (42) arranged to receive liquid cryogen and which, in operation, is at least partially filled with a liquid cryogen, wherein at least one diode pack (36) associated with a superconducting switch (32) is -14 -located within the header tank such thai, in operation, the joint is in contact with the liquid cryogen within the header tank.
4. A cryostat according to any preceding claim, wherein the header tank (42) is arranged to receive recondensed cryogen (44) from a recondensing surface (40) cooled by a refrigerator (17).
5. A cryostai. according to claim 4, wherein a recondensing refrigcrator (17) is positioned above the header tank (42) such that cryogen (44) recondensed on a recondensing surface of the refrigerator falls into the header tank (42).
6. A cryoslat according to any preceding claim, wherein the header tank is in communication with the cryogen vessel.
7. A cryostat according to claim 6, wherein the header tank is arranged to receive recondensed cryogen from the cryogen vessel.
8. A cryostat according to claim 6 wherein the cryogen vessel (12) and the header tank (42) are in communication through a port (50) in the cryogen vessel, such that the header tank will fill with liquid cryogen to the level of an edge of the port (50), and any further liquid cryogen supplied to the header tank will flow into the cryogen vessel.
9. A cryostat according to claim 8, further comprising a vent tube (20) for providing access to the cryogen vessel (12) from the exterior, arranged such that such access is obtained through the port (50).
10. A cryosiat according to any preceding claim, wherein the cryostat contains a cylindrical magnet and the header tank (42) is located at a greater radial distance from the isocentre (X) of the magnet than a maximum radius of the magnet in the cryogen vessel.
11. A cryostat according to any preceding claim, wherein the cryoslat contains a magnet and the header tank (42) is located at a position such that, in operation, the header lank experiences a magnetic flux density of no greater than 0.81'.
12. A cryostat containing a cooled superconducting magnet comprising: -an outer vacuum chamber (14) housing the cryogen vessel; and -a sump (42) arranged to receive liquid cryogen and which, in operation, is at least partially filled with a liquid cryogen; and -means for cooling the superconducting magnet by transfer of heat from the cooled superconducting magnet to liquid cryogen within the sump, wherein at least one joint (30) joining superconducting wires is located within the header tank such that, in operation, the joint is in contact with the liquid cryogen within the header tank.
13. A cryostat containing a cooled superconducting magnet comprising: -an outer vacuum chamber (14) housing the cryogen vessel; and -a sump (42) arranged to receive liquid cryogen and which, in operation, is at least partially filled with a liquid cryogen; and -means for cooling the superconducting magnet by transfer of heat from the cooled superconducting magnet to liquid cryogen within the sump, wherein at least one superconducting switch (32) is located within the header tank such that, in operation, the joint is in contact with the liquid cryogen within the header tank.
14. A cryostat containing a cooled superconductinq magnet comprising: -an outer vacuum chamber (14) housing the cryogen vessel; and -a sump (42) arranged to receive liquid cryogen and which, in operation, is at least partially filled with a liquid cryogen; and -means for cooling the superconducting magnet by transfer of heat from the cooled superconducting magnet to liquid cryogen within the sump, wherein at least one diode pack (36) associated with a superconducting switch (32) is located within the header tank such that, in operation, the joint is in contact with the liquid cryogen within the header tank.
15. A cryostat containing a cooled superconducting magnet according to any of claims 12-14 wherein the means for cooling the superconducting magnet by transfer of heat from the cooled superconducting magnet to liquid cryogen within the sump, comprises a cooling loop arrangement.
16. A cryostat containing a cooled superconducting magnet according to any of claims 12-14 wherein the means for cooling the superconducting magnet by transfer of heat from the cooled superconducting magnet to liquid cryogen within the sump, comprises a thermal conductor linking the cooled superconductive magnet to the sump.
17. A cryostat substantially as described, and/or as illustrated in the accompanying drawings. \-1-piiendmet5to the cla%mS have been ted as foOwS 1. A cryo stat for containing a cooled superconducting magnet, the cryostat comprising: -a cryogen vessel (12) for containing the magnet and a liquid cryogen; -an outer vacuum chamber (14) housing the cryogen vessel; and -a header tank (42) in communication with the cryogen vessel and arranged to receive liquid cryogen and which, in operation, is at least partially filled with liquid cryogen, wherein at least one joint (30) joining superconducting wires is located within the header tank such that, in operation, the joint is in contact with liquid cryogen within the header tank.0 2. A cryostat for containing a cooled superconducting magnet, the cryostat comprising: -a cryogen vessel (12) for containing the magnet and a liquid cryogen; -an outer vacuum chamber (14) housing the cryogen vessel; and -a header tank (42) in communication with the cryogen vessel and arranged to receive liquid cryogen and which, in operation, is at least partially filled with liquid cryogen, wherein at least one superconducting swItch (32) is located within the header tank such that, in operation, the superconducting switch is in contact with liquid cryogen within the header tank.3. A cryostat for containing a cooled superconducting magnet, the cryostat comprising: -a cryogen vessel (12) for containing the magnet and a liquid cryogen; -an outer vacuum chamber (14) housing the cryogen vessel; and -a header tank (42) in communication with the cryogen vessel and arranged to receive liquid cryogen and which, in operation, is at least partially filled with liquid cryogen, wherein at least one diode pack (36) associated with a superconducting switch (32) is located within the header tank such that, in operation, the diode pack is in contact with liquid cryogen within the header tank.4. A cryostat according to any preceding claim, wherein the header tank (42) is arranged to receive recondensed cryogen (44) from a recondensing surface (40) cooled by a refrigerator (17).5. A cryostat according to claim 4, wherein a recondensing refrigerator (17) is positioned above the header tank (42) such that cryogen (44) recondensed on a recondensing surface of the refrigerator falls into the header tank (42).6. A cryostat according to any preceding claim, wherein the header tank is arranged to receive recondensed cryogen from the cryogen vessel.7. A cryostat according to any preceding claim wherein the cryogen vessel (12) and the header tank (42) are in communication through a port (50) in the cryogen vessel, such that the header tank will fill with liquid cryogen to the level of an edge of the port (50), and any further liquid cryogen supplied to the header tank will flow into the cryogen vessel.8. A cryostat according to claim 7, further comprising a vent tube (20) for providing access to the cryogen vessel (12) from the exterior, arranged such that such access is obtained through the port (50).9. A cryostat according to any preceding claim, wherein the cryostat contains a cylindrical magnet and the header tank (42) is located at a greater radial distance from the isocentre (X) of the magnet than a maximum radius of the magnet in the cryogen vessel.10. A cryostat according to any preceding claim, wherein the cryostat contains a magnet and the header tank (42) is located at a position such that, in operation, the header tank experiences a magnetic flux density of no greater than 0.8T.11. A cryostat containing a cooled superconducting magnet comprising: -an outer vacuum chamber (14) housing the superconducting magnet; and. . -a header tank (42) arranged to receive liquid cryogen and which, in operation, is at least partially filled with a liquid cryogen; and (.0 -means for cooling the superconducting magnet by transfer of heat from the superconducting magnet to liquid cryogen within the header tank, wherein at least one joint (30) joining superconducting wires is located within the header tank such that, in operation, the joint is in contact with the liquid cryogen within the header tank.12. A cryostat containing a cooled superconducting magnet comprising: -an outer vacuum chamber (14) housing the superconducting magnet; and -a header tank (42) arranged to receive liquid cryogen and which, in operation, is at least partially filled with a liquid cryogen; and -means for cooling the superconducting magnet by transfer of heat from the superconducting magnet to liquid cryogen within the header tank, wherein at least one superconducting switch (32) is located within the header tank such that, in operation, the superconducting switch is in contact with the liquid cryogen Within the header tank.13. A cryostat containing a cooled superconducting magnet comprising: -an outer vacuum chamber (14) housing the superconducting magnet; and -a header tank (42) arranged to receive liquid cryogen and which, in operation, is at least partially filled with a liquid cryogen; and -means for cooling the superconducting magnet by transfer of heat from the cooled superconducting magnet to liquid cryogen within the header tank, wherein at least one diode pack (36) associated with a superconducting switch (32) is located within the header tank such that, in operation, the diode pack is in contact with the liquid cryogen within the CC) header tank. C"14. A cryostat containing a cooled superconducting magnet according to any of claims 12-14 wherein the means for cooling the superconducting magnet by transfer of heat from the cooled superconducting magnet to liquid cryogen within the header tank, comprises a cooling loop arrangement.15. A cryostat containing a cooled superconducting magnet according to any of claims 12-14 wherein the means for cooling the superconducting magnet by transfer of heat from the cooled superconducting magnet to liquid cryogen within the header tank, comprises a thermal conductor linking the cooled superconductive magnet to the header tank.16. A cryostat substantially as descrthed, and/or as illustrated in the accompanying drawings. Co (0 c\J