As is well known to those skilled in the art, Magnetic Resonance Imaging (MRI) systems typically include a number of superconducting magnet coils which are maintained at a cryogenic temperature by being enclosed within a cryogen vessel with a cryogen at its boiling point. The type of superconducting wire used will determine the required boiling point, and therefore the required cryogen. Helium has the lowest known boiling point, and so is commonly used as the cryogen. The present invention applies to such magnet coils, but is not limited to helium as a cryogen, or to superconducting magnets used in MRI systems. In recent years, the cost of helium for use in cooling superconducting magnets has increased, and its availability has diminished. Future supply of this finite resource at a reasonable cost is something of a concern to the industry. It is accordingly desirable to be able to run MRI magnets with a low level of liquid cryogen. Although some development has revealed this to be possible, certain arrangements make use of a conventionally sized cryogen vessel to permit circulation of cryogen vapour in thermal contact with the coils. In such arrangements, during initial cryogen fill, or during refill following a service intervention, the cryogen may inadvertently be overfilled. In addition to the unnecessary consumption of cryogen material, an overfilled cryogen vessel may reach a high quench pressure. This possibility in turn requires a thick walled cryogen vessel and large bore turret and quench line, all of which add considerable expense and would not be needed if one could ensure that cryogen overfill will not take place. Overfilling could be addressed by placing displacers in the cryogen vessel to limit the volume available for cryogen fill. Such an arrangement may be difficult and costly to implement. Undesirably high quench pressures may result, even with the correct quantity of cryogen, due to the reduced volume of free space available for gas expansion.

A disadvantage of some known minimum-cryogen solutions is that they do not have the capacity for enough cryogen to achieve a required hold time for transport. They are commonly transported at ambient temperature and require cooling on installation. This raises the problem of cryogen consumption and availability at the installation site.

It would accordingly be advantageous to provide a superconducting magnet cooling arrangement which requires a relatively small volume of liquid cryogen for operation, but which may be filled with a greater volume of liquid cryogen for transport, yet is arranged to prevent overfilling with cryogen after service operations.

The present invention addresses the above issues by providing methods and apparatus as defined in the appended claims.

The above, and further, objects, advantages and characteristics of the present invention will be described with reference to the following description of certain embodiments thereof, in conjunction with the accompanying drawings, wherein:

Fig. 1 shows a schematic cross-section of a superconducting magnet assembly according to an embodiment of the present invention, in an operational orientation;

Fig. 2 shows a schematic cross-section of a superconducting magnet assembly according to an alternative embodiment of the present invention, in an operational orientation; and

Fig. 3 shows a schematic cross-section of a superconducting magnet assembly according to an embodiment of the present invention, in an orientation suitable for filling and shipping . According to the present invention, a cryostat for maintaining a superconducting magnet coil structure is adapted by extending a conventional vent and filling path from an access turret to a position near the bottom of the cryogen vessel.

Fig. 1 schematically illustrates an embodiment of the invention, being a cylindrical magnet coil assembly with a horizontal bore 14a. A superconducting magnet coil structure 10 is provided, supported by conventional means within a cryogen vessel 12. An outer vacuum container (OVC) 14 provides thermal isolation of the cryogen vessel from ambient temperature. Intermediate thermal isolation, such as thermal radiation shields, and metallised polyester layers may be provided as is conventional. An access vent pipe 16, shown mounted towards one side of the cryogen vessel, provides an access path into the cryogen vessel from the exterior, for filling with liquid cryogen, for example; and possibly also for venting boiled-off cryogen. An amount of liquid cryogen 18 is shown in the bottom of the cryogen vessel. The access vent pipe 16 in the illustrated embodiment is shown supported and retained in position by a turret 20. The turret 20 may conventionally also support and accommodate a cryogenic refrigerator (not shown) within a refrigerator sock 22, for cooling the cryogen 18.

According to a feature of the present invention, access vent pipe 16 within the cryogen vessel extends to a location corresponding to a desired maximum fill level for liquid cryogen when the magnet is in its operational position.

Preferably, the access vent pipe 16 extends to a position below a lower extremity of a bore tube 14a of the OVC 14, when the magnet is in its operational position.

When the cryogen vessel 12 is filled with liquid cryogen, no other ports should be open above the lower extremity of the access vent pipe 16. Accordingly, it will be found impossible to fill the cryogen vessel above a level corresponding to an upper level 32 of the lower open end of the access vent pipe 16. Any attempt to fill beyond this level will be prevented by a gas lock: gas pressure of cryogen vapour within the upper part of the cryogen vessel - that is to say, the part of the vessel above level 32 when the magnet is in operation. Another feature limiting the filling of cryogen will be the heat load placed on the cryogen if it is in contact with the access vent pipe. It may be found that the thermal load represented by contact between liquid cryogen and access vent pipe 16 which extends to a temperature of about 300K at its upper end may prevent further filling of liquid cryogen, rather than - or in addition to - the gas lock mechanism described above.

By filling the cryogen vessel in this position, the chance of overfilling the magnet, thereby raising quench pressure and cryogen consumption, is avoided. A relatively large gas space 34 is maintained, which will reduce quench pressure by providing a larger volume for the cryogen 18 to occupy than would be the case with a greater volume of liquid cryogen. If the liquid cryogen 18 has a small contact area with the magnet coil structure 10, transfer of heat energy from the magnet coil structure to the liquid cryogen during quench will be slower than in the case of a larger contact area with the magnet coil structure. This will be especially true in the case illustrated in Fig. 1, where the liquid cryogen does not touch the magnet coil structure 10 at all. This slower transfer will reduce a peak rate of boiling of the liquid cryogen, and so will reduce a peak value of quench pressure. A quench path will be provided in the arrangement of Fig. 1, although not illustrated. During a quench, the pressure within the cryogen vessel will increase sufficiently to open a valve or burst disc closing the quench path. The open quench path allows cryogen to escape, to avoid a dangerously high pressure developing within the cryogen vessel. By providing a relatively low mass of cryogen within the cryogen vessel, and a relatively low peak quench pressure, the required rate of evacuation of cryogen during a quench is reduced, which in turn means that the access vent pipe 16 and quench path (not shown) may be reduced in diameter, saving material and cost.

As mentioned above, while it is generally advantageous to prevent overfilling of the cryogen vessel following a service, it may be preferred to allow a greater mass of cryogen to be introduced into the cryogen vessel for transport. Typically, the cryogenic refrigerator is not operational during transport. The magnet coil structure 10 is kept cool in such arrangements by boiling of liquid cryogen. The resulting cryogen vapour may simply be vented to atmosphere. As cryogen is consumed during transport, the length of time for which the magnet coil structure 10 may be kept cold during transport is reduced by provision of a reduced mass of cryogen in the cryogen vessel.

According to a feature of some embodiments of the present invention, a venting path may be provided from the cryogen vessel to allow escape of cryogen vapour from the cryogen vessel at a position above the level 32. For example, in Fig. 1, a refrigerator outlet pipe 26 is shown, extending from refrigerator sock 22 into the gas space 34. In use, this allows cryogen vapour to pass from gas space 34 into the refrigerator sock 22, there to be cooled and recondensed into liquid by the refrigerator. The resultant liquid cryogen then drips through the outlet pipe 26 into the cryogen vessel 12. In the present invention, a valve 36 or similar may be opened to provide a vent path from the cryogen vessel, through the refrigerator sock. With that vent path open, further liquid cryogen may be introduced into the cryogen vessel, up to the level of an upper extremity of an open end of the outlet pipe in the cryogen vessel. This upper fill level is shown by a dotted line at 24. Precautions should be taken to ensure that the valve 36 is closed again after fill for shipping; and that it is closed during any subsequent refill operations after servicing. Preferably, filling the cryogen vessel with liquid cryogen to upper level 24 provides a thirty day time-to-dry for shipping .

Valve 36 may be an absolute valve typically required to vent the cryogen vessel in case of excessive internal pressure. This valve could be used to open the venting path for cryogen filling to the upper level 24. After arrival on site the valve would be closed. In alternative arrangements, where the valve is not required during operation, it may be removed and the port sealed, avoiding incorrect use in overfilling the cryogen vessel after future servicing operations. A safety interlock may be provided, using an output from a cryogen level sensor to enable a magnet power supply or magnet supervisory system, to ensure that the superconducting magnet can not be brought into operation ( ^Λ ramped' ) while there is too great a volume of liquid cryogen within the cryogen vessel. The criterion of "too great" may be set by a threshold cryogen level.

When ramping the magnet - that is, introducing electric current into the superconducting coils, some heat dissipation is expected. Accordingly, before ramping begins, valve 36 or similar may be opened or removed to provide an egress path for cryogen vapour out of the cryogen vessel 12, through refrigerator outlet pipe 26 and refrigerator sock 22. With this egress path opened, it is possible to fill the cryogen vessel to a higher level: up to the level of an upper extremity of an open end of the outlet pipe 26 in the cryogen vessel. This upper fill level is shown by a dotted line at 40. This higher quantity of cryogen allows for some cryogen boil-off during ramping, without reducing the level of liquid cryogen available for cooling the magnet coil assembly 10 during operation. The pressure produced by a quench during ramping would be self limiting. During the early stages of the ramp when more liquid cryogen is present there will be less energy available to produce the heat required to boil the cryogen. As full field is approached the cryogen level will have diminished to approximately its normal level.

Valve 36 must be closed after ramping, for example using any of the precautions described above with respect to closing valve 36 for transport.

Figs. 2 and 3 illustrate an alternative embodiment which may be employed to achieve similar results to those discussed above with reference to Fig. 1. However, in this embodiment, it is required to rotate the cryostat between a first position for operation and filling after service operations (Fig. 2) and a second position for transport, and for filling for transport (Fig. 3) . In the embodiment of Figs. 2, 3, the access vent pipe 16 is not extended, but rather, the whole turret 20 is positioned lower on the side of the cryostat. In some embodiments, access vent pipe 16 may terminate flush with the wall of the cryogen vessel 12, and not extend into the cryogen vessel 12 at all.

The refrigerator may be tilted away from the vertical, which may not be preferred for certain types of refrigerator. However, such arrangement may provide service and configuration advantages such as simplification of cryogen filling and replacement of the refrigerator. Such procedures can be undertaken by an operator standing on a floor which supports the cryostat, rather than the operator having to be elevated on steps or a platform. The overall height of the cryostat is reduced as compared to conventional cryostats in which the refrigerator and turret are located towards the upper extremity of the cryostat. The reduced overall height facilitates installation and removal of the cryostat. In Figs. 2, 3, features corresponding to features in Fig. 1 carry corresponding reference numerals. When the cryostat is in its first position, shown in Fig. 2, operation corresponds to that discussed above for the structure of Fig. 1 in operation.

Access vent pipe 16, shown mounted towards a lower part of one side of the cryogen vessel, provides an access path into the cryogen vessel from the exterior, for filling with liquid cryogen, for example; and possibly also for venting boiled- off cryogen. An amount of liquid cryogen 18 is shown in the bottom of the cryogen vessel. As in the embodiment of Fig. 1, access vent pipe 16 within the cryogen vessel has a lower extremity which extends to a location corresponding to a desired maximum fill level for liquid cryogen when the magnet is in operation. Preferably, the access vent pipe 16 has a lower extremity which extends to a position below a lower extremity of a bore tube 14a of the OVC 14, when the magnet is in its first, operational, position. When the cryogen vessel 12 is filled with liquid cryogen, no other ports should be open above the lower extremity of the access vent pipe 16. Accordingly, it will be found impossible to fill the cryogen vessel above a level corresponding to an upper level 32 of the lower open end of the access vent pipe 16. Any attempt to fill beyond this level will be prevented by a gas lock: gas pressure of cryogen vapour within the upper part of the cryogen vessel - that is to say, the part of the vessel above level 32 when the magnet is in its operational position. Another feature limiting the filling of cryogen will be the heat load placed on the liquid cryogen if it is in contact with the access vent pipe. It may be found that the thermal load represented by contact between liquid cryogen and access vent pipe 16 which extends to a temperature of about 300K at its upper end may prevent further filling of liquid cryogen, rather than - or in addition to - the gas lock mechanism described above. Filling the cryogen vessel with liquid cryogen to level 40 preferably enables liquid cryogen to come into direct contact with the magnet coil structure 10, to provide more effective cooling during the ramping stage. However, this is unlikely to provide sufficient mass of cryogen to provide cooling during transport.

Fig. 3 illustrates the structure of Fig. 2 rotated into its second position, for transport and for filling for transport. According to this aspect of the present invention, the cryostat is rotated on its axis A such that the lower extremity of the access vent pipe 16 moves towards the top of the cryostat. In this second position, a lower extremity of access vent pipe 16 rises above its former position, thereby enabling the cryogen vessel 12 to be filled with liquid cryogen 18 to a higher level 24, sufficient to provide cooling of the magnet coil structure 10 during transport.

Preferably, filling the cryogen vessel with liquid cryogen to upper level 24 provides a thirty day time-to-dry for shipping.

The cryostat would be transported in its second position, as shown in Fig. 3, to prevent flooding of the turret 20 with liquid cryogen and the consequent additional heat load that would provide. The magnet could be returned to normal orientation as shown in Fig. 2 on installation, for example when lifted from a shipping pallet to be moved to its intended installation location. Although providing the advantage of an extended maximum shipping time, the amount of liquid cryogen introduced into the cryogen vessel may be modulated according to the expected duration of transport. That way, excessive consumption due to over-provision for transport may be reduced. For transport which is not expected to take a significant time, the steps described with respect to Fig. 3 may be avoided, and the cryostat may be transported with the level of liquid cryogen introduced during the steps described with reference to Fig. 2.

After shipping, the cryostat may be rotated to its first position, as illustrated in Fig. 2. If liquid cryogen remains in excess of level 32, some cryogen may be drawn off for re-use, or be allowed to vent to atmosphere.

An enhancement to the above would be to allow an additional volume of cryogen to remain in the vessel to allow for boil off during magnet ramping. By opening valve 36, cryogen gas may egress through outlet pipe 26 to allow cryogen filling to the level of the outlet pipe 26, higher than level 32 of access vent pipe 16.