The present invention relates to mechanical arrangements for providing rotary drive into a vessel containing high-pressure gas from a motive source such as an electric motor which is exposed to atmospheric pressure

In such cases, it is difficult to provide the required high- pressure sealing around a rotary drive shaft linking the motive source into the high-pressure vessel.

The present invention particularly relates to arrangements for providing motive power to a cryogenic refrigerator, commonly and hereafter referred to as a ^coldhead' , including helium gas at a high pressure such as 28bar. It is well known that helium gas is difficult to seal. Such arrangements are currently employed in cooling arrangements for superconducting magnets, for example superconducting magnets as employed in magnetic resonance imaging (MRI) systems.

MRI systems require a strong background magnetic field which is very homogeneous in an imaging region. It is the task of the superconducting magnet to provide this magnetic field, and most other considerations are secondary to that. The coldhead operates to provide cryogenic cooling, typically to around 4K, the boiling point of helium. Such cooling is required to enable the superconducting magnet to operate in its superconducting state. The coldhead is typically operated by an electric motor which provides motive force to the coldhead itself. Electric motors and the magnetic field generated by the superconducting magnet are largely incompatible. Despite significant efforts by designers, a certain amount of stray field escapes from the superconducting magnet, and regions outside of the magnet but in close proximity to it can experience an appreciable magnetic field. This can interfere with the operation of an electric motor in such regions. Conversely, electric motors tend to generate significant electromagnetic interference, which can disrupt the homogeneous magnetic field required for MRI imaging. Accordingly, the location of the coldhead in MRI systems is very carefully considered to ensure that an associated electric motor is positioned in a low magnetic field to reduce interference with operation of the motor. This becomes harder to achieve as the field strength of the magnet increases, especially if the stray field expands.

Fig. 1 illustrates a conventional arrangement for linking a motor 10 to a coldhead 12. A rotary drive shaft 14 transfers motive power from motor 10 to a component 16 of the coldhead. A key-way 17 or similar arrangement such as a splined shaft is provided for mechanical engagement between the rotary drive shaft 14 and the component 16. The coldhead will include a gas such as helium at a relatively high pressure, such as 28 bar. The outside of the coldhead, and the motor, are at atmospheric pressure. It has been considered difficult to directly seal the rotary shaft 14 to prevent escape of gas, as the seal would need to bear against the surface of the rotary shaft and so would be liable to wear. A rotary seal may be used, but is typically not designed so that the conventional arrangement of Fig. 1 allows a certain leakage of gas from the coldhead into a volume 18 located between a casing of the coldhead 12 and a casing of the motor 10. Although it is possible to provide a leak-tight rotary seal between the rotary drive shaft 14 and the coldhead 12, such seals are difficult to achieve and prone to wear. Similarly, as the casing of the motor will not be sealed against the rotary shaft 14, gas is allowed into the casing of the motor, and a gas pressure within the casing of the motor may build up to the same value as the gas pressure within the coldhead. The casing of the motor 10 may accordingly be sealed against gas leaks between the high pressure within the coldhead, such as 28 bar, and atmospheric pressure. A volume 18 between the housing of the coldhead and the housing of the motor is closed by a seal 20. Seal 20 is a static seal, such as a rubber o-ring, and may be compressed sufficiently to withstand the pressure differential between volume 18 and atmospheric pressure. Mechanical means, not illustrated, serve to compress the seal between the housings and seal high pressure gas within the housings and the volume 18 from the atmosphere. As the seal bears only against stationary surfaces, no wear is expected and the required seal is easy to achieve.

The arrangement of Fig. 1 is >nly possible where the motor 10 is directly mounted onto the coldhead 12. Such arrangement does not address the problems of electromagnetic interference from the motor 10 interfering with an imaging process, or the problem of a stray magnetic field from the superconducting magnet interfering with operat on of the motor.

The present invention accordingly aims to provide effective sealing for a rotary drive into a vessel containing high- pressure gas from a motive source which is exposed to atmospheric pressure. Such arrangements are required to permit the motive source, such as an electric motor, to be placed at an arbitrary position, remote from the coldhead and the superconducting magnet. This reduces or eliminates electromagnetic interference in the imaging procedure as generated by a motor, and reduces or eliminates effects of stray magnetic field on the motive source.

The motive source may accordingly be positioned in a low stray field position, even outside of the environment of the MRI system altogether. Furthermore, the present invention permits the motive source to be located in an unusual position, for example as may be required where there is insufficient space for the motive source to be positioned close to the coldhead. The present invention accordingly provides apparatus and methods 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, given by way of non-limiting example only, wherein:

Fig. 1 schematically illustrates a conventional arrangement for sealing against leaks of high-pressure gas from a coldhead at its interface with a source of motive power; and

Fig. 2 schematically illustrates a partially exploded view of an arrangement for sealing against leaks of high-pressure gas from a coldhead at its interface with a source of motive power according to an embodiment of the invention.

The present invention provides an arrangement which enables the source of motive power to a remote position, while still providing effective sealing of the high-pressure gas within the coldhead and effective mechanical linkage between the source of motive power and the coldhead.

The source of motive power is typically an electric motor; however other sources may be used such as a pneumatic or hydraulic motor. However, for brevity and simplicity, a general source of motive power will be referred to below as a "motor" .

A feature of the present invention is that the motor 10 is linked to the coldhead 16 using a flexible drive 30, as illustrated in Fig. 2, where features corresponding to features of Fig. 1 carry corresponding reference numerals.

The flexible drive comprises an inner part 32 which is a flexible rotary shaft of conventional type, such as flexible drives known for use with electric drills. The flexible drive also comprises an outer part 34 which does not rotate, but is affixed to the housing of the coldhead 12, and the housing of the motor 10 in a gas-tight manner to the pressure encountered within the coldhead.

The flexible rotary shaft 32 carries splined ends 36 in the illustrated embodiment. The splined ends are not in themselves flexible, and are preferably provided with a locking device to ensure reliable engagement. Respective splined ends engage with complementary fittings attached to component 16 within the coldhead, and a rotor on motor 10, to provide the required motive power. Either or both splined ends may be replaced by an alternative engagement means, such as a hexagonal protrusion or recess, a screwdriver-type end or similar arrangement, provided that a respectively compatible complementary fitting is provided on component 16 and the rotor of motor 10.

A technical difficulty which the present invention aims to address is the requirement to seal the rotating drive shaft to helium gas at a high pressure such as 28 bar. As with the conventional arrangement of Fig. 1, no attempt is made to form a leak-tight seal directly against the surface of the rotating shaft, but rather provides a high-pressure enclosure comprising the coldhead housing, the motor housing and the outer part 34 of the flexible drive 30. These components do not rotate with respect to one another, and a gas-tight seal to the required quality may be achieved with conventional means . The outer part 34 comprises a high-pressure hose of suitable internal diameter. A braiding of stainless steel or similar may be provided over a flexible hose, which may itself be of convoluted tubing of stainless steel, copper, aluminium or other suitable material. Some embodiments may employ a composite or polymer convoluted hose with an inner or outer layer of aluminium, copper, stainless steel or similar. Such hoses are conventionally available and capable of withstanding gases such as helium at the pressures under consideration. The hose must be chosen such that it has an internal diameter large enough to allow rotation of the flexible drive shaft 32 with a suitable clearance to ensure a minimal contact between the flexible drive shaft and the interior surface of the hose, to minimise wear on both parts. The flexible drive shaft 32 is fed through the flexible hose and engaged with the component 16 of the coldhead 12, and the motor, using suitable mechanical couplings as described above.

The flexible hose is provided with a high-pressure seal arrangement 38 at each end. In the illustrated embodiment, these are shown disassembled, but when in use serve to seal the hose against the housing of the coldhead 12 and against the housing of the motor 10. The high-pressure seal arrangements 38 could for example be Garlock ^® NW40 seals.

Once assembled, and prior to use, the high-pressure enclosure comprising the coldhead housing, the motor housing and the outer part 34 of the flexible drive 30 is evacuated, purged, and charged with helium gas through a suitable vacuum port (not illustrated) . The surface of the flexible drive shaft 32 and/or an inner surface of the house of outer part 34 may be coated with a friction-limiting material, to reduce wear and to reduce a frictional load on the motor. A liquid such as an oil or a grease may be provided between the inner surface of the outer part 34 and the flexible drive shaft to reduce friction; alternatively a solid lubricant such as graphite powder may be used, but care must be taken to ensure that such lubricants do not contaminate the motor or the coldhead. The flexible drive 30 and hose 34 may be or arbitrary length, within reason, and their flexibility allows a great range of relative positions of coldhead 12 and motor 10 to be employed. An alternative embodiment of the invention employs a rigid drive shaft 32 located within a pressurised chamber such as that provided by outer part 34 in the embodiment of Fig. 2. Outer part 34 may be a rigid hose of stainless steel, copper, aluminium or other suitable material . Some embodiments may employ a rigid composite or polymer tube lined with, or enclosed within, a layer of stainless steel, copper, aluminium or similar. This would allow less freedom in positioning of the motor, but would still provide advantages in reducing magnetic interaction between the motor and the superconducting magnet by significantly increasing the distance between motor and magnet. Alternatively, a belt drive may be provided to transmit the rotary drive from the motor to the coldhead. A rather differently-shaped chamber may be required than the outer part 34 shown in Fig. 2 if a belt drive is to be accommodated, but the design and manufacture of such a chamber is within the ability of those skilled in the art. The shape of the pressurised chamber could be designed to suit the type of drive selected. In another embodiment, a flexible drive shaft is provided which comprises two or more rigid shaft parts linked together by flexible joints, such as universal joints or relatively short lengths of flexible drive shaft material . The outer part may include a flexible hose, flexible over its entire length as discussed above with reference to Fig. 2, or may include a number of rigid tubular sections, each corresponding to a rigid drive shaft part, the rigid tubular sections being joined to one another by flexible sections, each corresponding to a flexible joint in the drive shaft. While the present invention has been described with reference to a limited number of exemplary embodiments, those skilled in the art will be able to derive numerous modifications and variants without departing from the scope of the present invention, which is defined by the appended claims.