GB2463659A

GB2463659A - Method and Apparatus for Improved Cooling of a Cryostat Thermal Shield

Info

Publication number: GB2463659A
Application number: GB0817126A
Authority: GB
Inventors: Michael John Disney Mallet; Stephen Paul Trowell
Original assignee: Siemens Magnet Technology Ltd; Siemens PLC
Current assignee: Siemens PLC
Priority date: 2008-09-19
Filing date: 2008-09-19
Publication date: 2010-03-24
Anticipated expiration: 2028-09-19
Also published as: GB0817126D0; GB2463659B

Abstract

A cryostat includes a cryogen vessel 3, a thermal shield 6 and a bellows 20 filled with a gas. The bellows has a first end 22 in contact with a surface of the thermal shield and a second end 23 thermally coupled to the cryogen vessel. The gas may be nitrogen, argon, neon, helium or hydrogen, and the cryostat may house the windings of a superconducting magnet for an M.R.I imaging system. The thermal coupling may be via a high thermal conductivity section 25 which is pivotally mounted between the thermal shield and the cryogen vessel. The bellows may be of stainless steel, copper or aluminium material and it may be connected to the shield via a mount 34. The surface of the vessel may be provided with a high thermal conductivity layer, such as copper or aluminium, and the cryostat may also include a refrigerator (2 fig 1) to provide additional cooling to the thermal shield. The arrangement allows a reduction in the time taken to cool the thermal shield.

Description

CRYOSTAT

This invention relates to a cryostat in particular for use in a magnetic resonance imaging (MRI) system and a method of cooling a thermal shield of a cryostat.

Superconducting magnet systems are used for medical diagnosis, for example in magnetic resonance imaging systems. A requirement of an MRI magnet is that it produces a stable, homogeneous, magnetic field. In order to achieve the required stability, it is common to use a superconducting magnet system which operates at very low temperature. The temperature is typically maintained by cooling the superconductor by immersion in a low temperature cryogenic fluid, also known as a cryogen, such as liquid helium.

The superconducting magnet system typically comprises a set of superconductor windings for producing a magnetic field, the windings being immersed in a cryogenic fluid to keep the windings at a superconducting temperature, the superconductor windings and the cryogen being contained within a cryogen vessel. The cryogen vessel is typically surrounded by one or more thermal shields, and a vacuum jacket completely enclosing the shield(s) and the cryogen vessel.

An access neck typically passes through the vacuum jacket from the exterior, into the cryogen vessel. Such access neck is used for filling the cryogen vessel with cryogenic fluids and for passing services into the cryogen vessel to ensure correct operation of the magnet system.

Cryogenic fluids, and particularly helium, are expensive and it is desirable that the magnet system should be designed and operated in a manner to reduce to a minimum the amount of cryogen consumed. Leakage of heat into the cryogen vessel evaporates the cryogen which might then be lost from the magnet system as boil-off The vacuum jacket reduces the amount of heat leaking to the cryogen vessel by conduction and convection. The thermal shields reduce the amount of heat leaking to the cryogen vessel by radiation, and by conduction if, as is the usual practice, the cryogen vessel supports and the access neck are thermally linked to the shield, so as to intercept heat being conducted along them. In order to further reduce the heat leaking to the cryogen vessel, and thus the loss of liquid -it is common practice to use a refrigerator to cool the thermal shields to a low temperature. It is also known to use such a refrigerator to directly refrigerate the cryogen vessel, thereby reducing or eliminating the cryogen consumption. It is also known to use a two-stage refrigerator, in which a first stage is used to cool the thermal shield(s), and the second stage is used to cool the cryogen vessel.

It is desirable that such superconducting magnet systems should be traiisported from the manufacturing site to the operational site containing the cryogen, so that they can be made operational as quickly as possible. During transportation of an already assembled system, the refrigerator cooling the one or more shields and/or the cryogen vessel is inactive, and is incapable of diverting the heat load from the cryogen vessel.

Indeed, the refrigerator itself provides a relatively low thermal resistance path for ambient heat to reach the cryogen vessel and shield(s). This in turn means a relatively high level of heat input during transportation, leading to loss of cryogen liquid by boil-off to the atmosphere. It is desirable to reduce the loss of cryogen to the minimum possible, both since ciyogens are costly and in order to prolong the time available for delivery, also known as the hold time, the time during which the system can remain with the refrigerator inoperable, but still contain sonic cryogen.

The processes required to achieve a thermal equilibrium include the necessity of cooling the thermal shield to a level of typically 30-50K. Under normal operating conditions the only source of cooling for the radiation shield is the first stage of the refrigerator. The refrigerator has a limited cooling capability and there can be long delays before the radiation shield is cold enough for the superconducting magnet to be energised. The problem during the cold transit of a superconducting magnet, is that no power is available to the shipping container, so the only form of cooling of such a system is enthalpy of the liquid Helium. The thermal shield is typically poorly coupled to this source of cooling and so the temperature of the radiation shield increases during the magnet transportation, increasing the therma' load on the Helium vessel due to radiation.

As is well known in the art, a difficulty arises when first cooling such a cryostat from ambient temperature. One option is to simply add working cryogen to the cryogen vessel until the cryogen vessel and the magnet settle at the temperature of the working cryogen. While this may be acceptable when using an inexpensive, non-polluting, essentially inexhaustible cryogen such as liquid nitrogen, it is not considered acceptable to use this approach for a working cryogen such as helium, which is relatively costly to produce, or to re-liquefy, and is a finite resource.

When cooling cryostats from ambient temperature to helium temperature, it is known to pre-cool the cryostat to a first cryogenic temperature by other means, before finally cooling the cryostat to operating temperature by the addition of liquid helium.

One conventional method for pre-cooling the cryogen vessel to a first cryogenic temperature involves first adding an inexpensive sacrificial cryogen, typically liquid nitrogen, into the cryogen vessel. The cryostat is then left for some time for temperatures to settle. This may be known as soaking'. The temperature of the cryogen vessel is then allowed to rise above the boiling point of the sacrificial cryogen, to ensure that it is completely removed from the cryogen vessel, before working cryogen is added. Although the material of the cryogen vessel itself quickly cools on addition of a cryogen, an issue arises with the cooling of the thermal radiation shield(s). In use, these thermal radiation shields must be cooled, typically to about 50K in the case of a single thermal radiation shield in a helium-cooled system. They must be thermally isolated from both the cryogen vessel and the OVC, to reduce the thermal influx from the room-temperature OVC to the cryogen vessel when in operating condition. When pre-cooling the cryostat, the thermal isolation of the thermal radiation shield(s) prevents the shield(s) from cooling rapidly on introduction of cryogen into the cryogen vessel. Known methods of pre-cooling a thermal radiation shield include: operating the refrigerator to cool the thermal radiation shields, or softening' the vacuum between the OVC and the cryogen vessel by the operation of an amount of gas, so allowing the thermal radiation shields to be cooled by convection heat transfer to the cryogen vesseL Each of these will now be discussed.

1) Operating the refrigerator to cool the thermal radiation shields has the disadvantage that any sacrificial cryogen within the cryogen vessel would need to be removed beforehand, since otherwise the sacrificial cryogen will be liquefied or frozen in the cryogen vessel. In known methods, the cryogen vessel is pre-cooled with nitrogen, allowed to warm up to a temperature in excess of the boiling point of nitrogen to ensure that no liquid nitrogen remains, and then is flushed with gaseous helium and then evacuated to ensure no contamination remains, before turning on the refrigerator.

The refrigerator then cools the thermal radiation shield at a rate of about lKlhr.

2) Softening' the vacuum between the OVC and the cryogen vessel will allow some thermal conductivity by convection, allowing heat to be transferred from the thermal radiation shield to the cryogen vessel, where it is removed by boiling of the sacrificial cryogen. Further cooling of the thermal radiation shield may occur by radiation once the working cryogen has been added into the cryogen vessel. Vacuum softening has been found to cool the thermal radiation shield rapidly to about 150 K when the cryogen vessel is filled with liquid nitrogen. Typically, the thermal radiation shield warms to 200 K during the phase when the cryogen vessel is allowed to warm to K to ensure all liquid nitrogen is removed prior to filling with a liquid helium working cryogen. The refrigerator is then used to cool the thermal radiation shield from K to 50 K. This process takes approximately 6 days, during which time approximately 200 litres of liquid helium are typically lost in boil off, at a significant cost.

While the financial cost of the lost helium is significant, the length of time required for cooling is also troublesome. Conventionally, the recondensing operation of the refrigerator is tested before the cryostat is shipped to a customer. This requires cooling of the thermal radiation shield to about 50K, since higher thermal radiation shield temperatures will radiate more heat to the cryogen vessel than the recondensing refrigerator can remove. However, more recently, the time taken to cool the thermal radiation shield has become the dominant factor in the time taken for magnet tests as a whole. This is particularly so in arrangements with a particularly low quench rate, which is otherwise most desirable. The pressure to ship completed cryostats and magnet systems to customers as soon has possible has led to the refrigerator recondensing test being omitted from some testing protocols. This, in tum, can lead to difficulties later. For example, if any of these cryostats or magnet systems exhibit boil-off issues on, or after, installation, rapid problem diagnosis and correction will be hindered as their baseline cryogenic performance is unknown.

The present invention provides apparatus and a method as defined in the appended claims.

An example of a clyostat and a method of a cooling a thermal shield of a cryostat according to the present invention will now be described with reference to the accompanying drawings in which: Figure 1 shows an example of a conventional arrangement of a refrigerator on a superconducting magnet system; Figure 2 illustrates schematically, the present invention applied to the cryostat of Fig. 1; Figure 3 illustrates operation of part of Fig. 2 in more detail; Figure 4 illustrates improvements which can be applied to Figs. 2 and 3; Figure S illustrates one embodiment of the example of Fig. 2 in more detail; and, S Figure 6 illustrates another embodiment of the example of Fig. 2 in more detail.

Figure 1 shows a cross-section of a conventional superconducting magnet system 1 for use in an MRI system. A two-stage cryogenic refrigerator 2 is removably connected to a magnet system 1 by an interface sock 4 (also known as an interface sleeve, or refrigerator interface), such that the first stage S of the refrigerator is thermally linked to a thermal shield 6 to cool the thermal shield 6 and the second stage 7 cools a cryogen vessel 3. The refrigerator is preferably, but not necessarily, arranged as a recondensing refrigerator. A heat exchanger, cooled by the second stage 7 of the refrigerator 2, is exposed to the interior of the cryogen vessel 3, for example by a tube 9. The refrigerator is, in operation, thereby enabled to reduce the consumption of cryogen by recondensation of evaporated cryogen back into its liquid state.

Superconductive magnet coils (not shown) are provided in the cryogen vessel.

The coils are immersed in a liquid cryogen 10. The thermal shield 6 completely surrounds the cryogen vessel 3. A vacuum jacket 11 completely encloses the cryogen vessel and the shield in a vacuum. A central bore 12 is provided, to accommodate a patient for examination. An access neck 13 is provided to allow access to the cryogen vessel 3. The access neck 13 is thermally linked to the thermal shield 6.

The present invention addresses the problem of efficient cooling, with or without a functioning refrigerator by using the enthalpy of the liquid cryogen, typically Helium, to provide cooling of the thermal shield to a predetermined level and then arranging for the shield and cryogen vessel to cease to be thermally linked.

Conventionally, Helium enthalpy has been used to cool the thermal shield by passing some of the escaping Helium gas through pipes around the thermal shield. This has the advantage of being simple to implement, but its major disadvantage is that the cryogen vessel 3 has a permanent thermal contact with the pipes and thermal shield 6.

The present invention enables the thermal linkage between the shield and the cryogen vessel to be removed when the thermal shield has cooled sufficiently, so reducing the heat load on the cryogen vessel. Fig. 2 illustrates the invention schematically, showing the cryogen vessel 3 and thermal shield 6, with a gap in the thermal shield to allow a lever mechanism 25 to move in and out of contact with the cryogen vessel. A bellows unit 20 is fixed at one end 22 to a thermally conducting mount 34 on the thermal shield and the free end 23 of the bellows unit 20 pushes onto a S surface 38 of one end 32 of the lever mechanism 25 which has a pivot point 30, such that rotation of the lever around the pivot provides a thermal connection between the thermal shield and the cryogen vessel by pressing the other end 33 of the lever onto the surface of the cryogen vessel to provide thermal contact there. This enables cooling of the thermal shield from the cryogen vessel, for example through the thermal connection via the lever and the pivot to the thermal shield, or through other coupling of the lever and the thermal shield. In this example, the bellows is positioned on the outer surface of the thermal shield, with one end fixed and expands and contracts along the surface of the thermal shield. The lever is typically a section of high thermal conductivity material in the thermal shield.

Figs 3a and 3b, illustrate the operation in more detail. As shown in Fig. 3a, one or more sealed bellows units 20, preferably of stainless steel are fixed at one end to the thermal radiation shield 6, typically via the thermally conducting mount 34, although in suitable orientations, such as the example of Fig.5, the bellows may be directly connected. The sealed bellows unit 20 is filled with a suitable gas 21, typically, but not exclusively, nitrogen, which is at an internal pressure adjusted according to the mechanical advantage offered by the specific lever geometry, such that good thermal contact is made with the helium vessel at room temperature, but allows adequate retraction of the lever during cooldown. Other possible gases include helium, argon, neon and hydrogen. The bellows 20 has one end 22 fixed to the thermal shield 6, with the other end 23 free to move.

An increase in the pressure of the gas 21 inside the bellows unit 20, related to an increase in temperature of the thermal shield, causes the bellows to expand, such that the free end 23 of the bellows unit moves towards and touches a surface 38 of the lever 25, when the shield 6 is relatively hot. The size of the bellows unit is designed such that at high internal pressures the length of the bellows unit is sufficient for the free end 23 of the bellows 20 to be in hard thermal contact with the end of the lever. The lever then provides a good thermal link between the shield 6 and the cryogen vessel. In this example, the first, fixed, end 22 is in contact with a surface of the thermal shield 6 and the second, free end 23 moves into contact with the surface 38 of the lever 25, During precool, the enthalpy of the cryogen in the helium vessel causes the thermal shield to cool down further and the gas pressure inside the bellows unit 20 reduces. The bellows 20 tend to shrink as the gas pressure reduces and at the boiling point of the gas 21 the internal pressure drops to a very low value. Once the internal pressure of the bellows unit has dropped sufficiently, it shrinks to such an extent that a separation 24 exists and the bellows unit is no longer in contact with the surface 38 of the lever 25 and the other end 33 of the lever moves out of contact with the cryogen vessel 3 when the shield is relatively cold, as illustrated by Fig. 3b. The bellows unit 20 now provides no heat load into the cryogen vessel, and where a refrigerator is provided, the temperature of the shield may be maintained at a constant level by the attached refrigerator 2.

In the present invention, the thermal shield 6 cools down due to the enthalpy of the cryogen vessel. As the bellows unit 20 cools down its internal pressure reduces. At the boiling point of the gas 21 the pressure reduces significantly to cause the free end 23 of the bellows 20 to disengage from the lever 25 and the lever from the cryogen vessel 3. This reduces the thermal load on the cryogen vessel and ensures that cryogen boil off after pre-cooling of the magnet is kept to a minimum.

If the thermal shield 6 starts to warm up again, the gas 21 inside the bellows unit 20 expands and reconnects the free end 23 of the bellows to one end 32 of the lever and the far end 33 of the lever to the cryogen vessel 3, repeating the shield cool down process. During normal operation of the magnet the temperature of the thermal shield 6 is sufficiently low (of the order of 50K) to ensure that the bellows is fully retracted and the Helium vessel experiences a minimum heat load.

An additional feature is to provide an extra cooling path by thermal contact through the bellows, as well as by direct contact between the lever and the thermal shield. As illustrated in Fig. 4a, the free end of the bellows may have a high conductivity plate 26, such as copper, or aluminium, which may connect with a similar high conductivity plate 27 on the surface of the lever 25 to improve the thermal linkage and help to cool the thermal shield. Further improvements in thermal contact may be made, as shown in Fig. 4b, by use of a high conductivity material braid 28, 29, such as copper or aluminium, extending between the ends of the bellows. Such a braid could be configured inside 29, or outside 28 the bellows unit 20. Alternatively, the bellows unit could be made from a high conductivity material. Any, or all, of the braid, plate and bellows material may be combined in one embodiment.

Figs. 5 and 6 illustrate more specific embodiments of the present invention. In Fig. 5, a lever is pivoted 31, such that it is out of contact with the cryogen vessel when the bellows 20 are collapsed and when the bellows expand, they force the lever up at the end 32 closest to the thermal shield, such that the other end 33 is moved into contact with the cryogen vessel. When the temperature of the thermal shield drops again, the bellows contract and the weight of the lever causes it to pivot out of contact with the cryogen vessel. The end 33 of the lever which comes into contact with the cryogen vessel may be adapted in shape to increase the area over which the lever 25 and cryogen vessel 3 are in thermal contact.

An alternative arrangement is illustrated in Fig. 6, where expansion of the bellows (Fig.6a) pushes an end piece 36, pivotally (and slideably) mounted 35 on the end 32 of the lever remote from the cryogen vessel, in order to pivot 30 the lever 25 into contact with the cryogen vessel. The lever is typically provided with an elbow 37 at the remote end 32 for better transfer of the forces from the expanding bellows.

When the bellows contract as the temperature of the thermal shield drops (Fig. 6b), the bellows draw back the end piece 36 and with it the remote end 32 of the lever 25, so that the lever pivots 30 and moves out of contact with the cryogen vessel 3.

Another application of the mechanism of the present invention is to extend the shipping time of a cold magnet. During shipping the thermal shield 6 has no cooling power from the refrigerator 2 and as the thermal shield 6 increases in temperature the radiation load on the cryogen vessel increases. By providing a thermal short circuit to the cryogen vessel 3, automatically triggered by the shield temperature, the temperature of the shield can be maintained for a longer period of time during shipping.

Thermal calculations show that a copper link of cross section 400 mm2, thermal length 10 cm, in good thermal contact between a shield at 300 K and the helium vessel held at 77 K will conduct 368 W. This drops to 200 W when the shield cools to 200 K. During the precool process liquid helium is introduced to the helium vessel. For shields at 200 K and helium vessel at 4 K, the conducted power increase to 480 W. For an aluminium shield of mass 180 kg, the change in enthalpy between 300 K and 80 K is 2.89 10 J. Hence, assuming an average cooling power of 200 W, and neglecting suspension heat loads, the shield cools to 80 K, typically sufficient to allow recondensing of Helium to 4K, in 40 hours. This is a significant improvement over the current time of approximately 120 hours.

The improvement in shipping time has been modelled, indicating that for a shield temperature reduction from 100 K to 70 K, a factor 2 increase in hold time results.

hi summary, the present invention provides a sealed bellows unit, containing a suitable gas, which operates as a cryogenic thermostat. The bellows unit is connected at one end to the thermal shield and at the other end to a high conductivity connector that either directly, or indirectly, connects the thermal shield to the cryogen vessel when the thermal shield is above a set point temperature. Cooling of the gas inside the bellows unit causes the bellows to contract and the thermal link to the cryogen vessel from the shield is broken.

This arrangement provides a means of rapidly cooling the radiation shield during the pre-cool process, whilst still retaining good thermal isolation of the shield and cryogen vessel once the shield temperature has dropped below a predefined limit.

During cold shipment of the magnet the mechanism provides a method to use the enthalpy of the cryogen to ensure that the radiation shield temperature does not exceed a predefined limit, so reducing radiation loads and thereby extending the shipping time of the magnet.

Claims

CLAIMS1. A cryostat comprising a cryogen vessel and a thermal shield, the cryostat further comprising a bellows having a first end in contact with a surface of the thermal shield and a second end removably thermally couplable to the cryogen vessel; wherein the bellows are filled with a gas.
2. A cryostat according to claim 1, wherein the second end is removably thermally couplable to the cryogen vessel by thermal conductions through a high thermal conductivity section, pivotally mounted between the thermal shield and the cryogen vessel.
3. A cryostat according to claim 2, wherein, when the second end of the bellows contacts one end of the section, the other end of the section contacts the cryogen vessel.
4. A cryostat according to any preceding claim, wherein the gas is at a pressure greater than atmospheric pressure at room temperature.
5. A cryostat according to any preceding claim, wherein the first and second ends of the bellows are further coupled together by a flexible high thermal conductivity coupling.
6. A cryostat according to claim 5, wherein the coupling comprises one of copper or aluminium braid.
7. A cryostat according to claim 5 or claim 6, wherein the coupling is mounted within the bellows.
8. A cryostat according to aiy preceding claim, wherein the gas is one of nitrogen, argon, neon, helium or hydrogen
9. A cryostat according to any preceding claim, wherein the second end is provided with a high thermal conductivity layer.
10. A cryostat according to any preceding claim, wherein a surface of the cryogen vessel facing the thermal shield is provided with a high thermal conductivity layer.
11. A cryostat according to claim 7 or claim 8, wherein the high thermal conductivity layer comprises a copper or aluminium plate.
12. A cryostat according to any preceding claim, wherein the bellows comprise one of stainless steel, copper or aluminium.
13. A cryostat according to any preceding claim, wherein the clyostat further comprises a refrigerator to provide additional cooling to the thermal shield.
14. A method of cooling a thermal shield of a cryostat, the method comprising providing on a surface of the thermal shield, a bellows filled with a gas, such that the bellows are expanded to provide thermal coupling between the thermal shield and a cryogen vessel when the thermal shield is at a temperature above a first predetermined temperature; and cooling the thermal shield via the thermal coupling until the temperature of the thermal shield drops below a second predetermined temperature; wherein the bellows retract in response to a sufficient reduction in the temperature of the gas, such that the thermal coupling is disconnected when the temperature of the thermal shield drops below the predetermined temperature.
15. A method according to claim 14, wherein the pressure is greater than atmospheric pressure at room temperature.
16. A method according to claim 14 or 15, wherein the bellows are expanded into thermal contact with the cryogen vessel at room temperature and retracted out of thermal contact with the cryogen vessel at crvostat operating temperatures.
17. A method according to any of claims 14 to 16, wherein cooling due to the thermal coupling is in addition to cooling from a refrigerator.
18. A method according to any of claims 14 to 17, wherein providing the thermal coupling between the thermal shield and the cryogen vessel comprises causing a high thermal conductivity section, pivotally mounted between the thermal shield and the cryogen vessel, to be moved in response to expansion of the bellows, such that a heat path between the thermal shield and the cryogen vessel is formed.
19. A magnetic resonance imaging system comprising a superconducting magnet winding housed within a cryogen vessel of a cryostat according to any of claims 1 to 13.
20. A cryostat or magnetic resonance imaging system as hereinbefore described with reference to the accompanying drawings.amended claims have been filed as follows:-CLAIMS1. A cryostat comprising a cryogen vessel and a thermal shield, the cryostat further comprising a bellows having a first end in contact with a surface of the thermal shield and a second end removably thermally couplable to the cryogen vessel; wherein the bellows is filled with a gas.2. A cryostat according to claim 1, wherein the second end is removably thermally couplable to the cryogen vessel by thermal conductions through a thermally conducting section, pivotally mounted between the thermal shield and the cryogen vessel.3. A cryostat according to claim 2, wherein, when the second end of the bellows contacts one end of the section, the other end of the section contacts the cryogen vessel.0) 15 4. A cryostat according to any preceding claim, wherein the gas is at a pressure greater than atmospheric pressure at room temperature.5. A cryostat according to any preceding claim, wherein the first and second ends of the bellows are further coupled together by a flexible thermally conducting coupling.6. A cryostat according to claim 5, wherein the coupling comprises one of copper or aluminium braid.7. A cryostat according to claim 5 or claim 6, wherein the coupling is mounted within the bellows.8. A cryostat according to any preceding claim, wherein the gas is one of nitrogen, argon, neon, helium or hydrogen 9. A cryostat according to any preceding claim, wherein the second end is provided with a thermally conducting layer.10. A cryostat according to any preceding claim, wherein a surface of the cryogen vessel facing the thermal shield is provided with a thermally conducting layer.11. A cryostat according to claim 7 or claim 8, wherein the thermally conducting layer comprises a copper or aluminium plate.12. A cryostat according to any preceding claim, wherein the bellows comprise one of stainless steel, copper or aluminium.13. A cryostat according to any preceding claim, wherein the cryostat further comprises a refrigerator to provide additional cooling to the thermal shield.14. A method of cooling a thermal shield of a cryostat, the method comprising providing on a surface of the thermal shield, a bellows filled with a gas, such that the 0) 15 bellows are expanded to provide thermal coupling between the thermal shield and a cryogen vessel when the thermal shield is at a temperature above a first predetermined temperature; and cooling the thermal shield via the thermal coupling until the temperature of the thermal shield drops below a second predetermined temperature; wherein the bellows retract in response to a sufficient reduction in the temperature of the gas, such that the thermal coupling is disconnected when the temperature of the thermal shield drops below the predetermined temperature.15. A method according to claim 14, wherein the pressure is greater than atmospheric pressure at room temperature.16. A method according to claim 14 or 15, wherein the bellows are expanded into thermal contact with the cryogen vessel at room temperature and retracted out of thermal contact with the cryogen vessel at cryostat operating temperatures.17. A method according to any of claims 14 to 16, wherein cooling due to the thermal coupling is in addition to cooling from a refrigerator.18. A method according to any of claims 14 to 17, wherein providing the thermal coupling between the thermal shield and the cryogen vessel comprises causing a thermally conducting section, pivotally mounted between the thermal shield and the cryogen vessel, to be moved in response to expansion of the bellows, such that a heat path between the thermal shield and the cryogen vessel is formed.19. A magnetic resonance imaging system comprising a superconducting magnet winding housed within a cryogen vessel of a cryostat according to any of claims 1 to 13.20. A cryostat or magnetic resonance imaging system as hereinbefore described with reference to the accompanying drawings. 0) 15 Co (\J