GB2530537A - Cryostat for actively-shielded superconducting magnet with reduced cryogen requirement - Google Patents

Abstract

A cryostat enclosing an actively-shielded superconducting magnet, itself comprising inner coils 11 and outer coils 10 within a cryogen vessel 12. A receptacle 40 is provided within the cryogen vessel, arranged such that, in use, the receptacle 40 contains liquid cryogen to a first level to partially immerse the inner coils 11 while the cryogen vessel contains liquid cryogen to a second level, lower than the first level, to partially immerse the outer coils 10. The receptacle is arranged such that over filling will cause the cryogen to overflow into the cryogen vessel. The cryostat preferably includes a recondensing refrigerator 17 arranged to condense vapour from the cryogen vessel into liquid cryogen and direct it to the receptacle.

Description

CRYOSTAT FOR ACTIVELY-SHIELDED SUPERCONDUCTING MAGNET

WITH REDUCED CRYOGEN REQUIREMENT

The present invention relates to cryostats for cooling superconducting magnets; in particular, actively-shielded cylindrical superconducting magnets -Fig. 1 shows a convexiblorial arraxigemeriL of a cryosLaL including a cryogen vessel 12. A cooled actively shielded cylindrical superconducting magnet is provided within cryogen vessel 12, and comprises main (inner) coils 11 and shield (outer) coils 10. The cryogen vessel 12 is itself retained within art outer vacuum chamber (CVC) 14. One or more thermal radiation shields 16 are provided in the vacuum space between the cryogen vessel 12 and the outer vacuum chamber 14.

The coils 10, 11 are essentially arranged coaxially about axis A. Herein, references to an waxial direction indicate directions parallel to axis A, including along axis A itself, and references to "radial" directions indicate directions perpendicular to axis A, extending in a plane which contains axis A. A cryogenic recondensing refrigerator 17 is mounted in a refrigerator sock 15 located in a turret 18 provided for the purpose, towards the side of the cryostat. Alternatively, a refrigerator 17 may be located within access turret 19, which retains access neck 20 mounted at the top of the cryostat.

In some known arrangements, the turret 18 and access turret 19 are combined or co-located, at the side of the cryostat or towards the top of the cryostat.

The refrigerator 17 provides active refrigeration to cool cryogen gas within the cryogen vessel 12, and recondense it into a liquid. The refrigerator 17 may also serve to cool the radiation shield 16. As illustrated in Fig. 1, the refrigerator 17 may be a two-stage refrigerator. A first cooling stage is thermally linked to the radiation shield 16, and provides cooling to a first temperature, typically in the region of 80-100K. A second cooling stage provides cooling of the cryogen gas to a much lower temperature, typically in the region of 4-10K.

Actively shielded magnets require a large cryogen vessel 12 because of their size. A significant difference in diameters is required between the inner 11 and outer 10 coils to make the manufacture of such magnets economical. The inner coils 11 produce a statio background magnetic field for MRI imaging and the outer coils 10 provide an opposing static magnetic field to reduce the external stray field. By placing outer coils 11 at a greater diameter, fewer turns would be required to achieve the same shielding effect, so reducing the cost of superconducting wire required, but the overall structure would become larger, and the required cryostat would become significantly more expensive. A compromise is reached based on the conflicting requirements of small system size and large diameter of the shield coils.

The cryogen vessel 12 is partially filled with cryogen liquid which ensures the inner 11 and outer 10 coils are held at a constant temperature, set by the boiling point of the cryogen liquid. Where liquid helium is used, this is at approximately 4.2K at 1 atmosphere pressure.

Conventionally, the cryogen vessel is filled with a significant mass of liquid cryogen. Recent developments have enabled a reduction in cryogen level, yet it is still preferred that each of the inner 11 and outer 10 coils are in physical contact with the liquid cryogen over part of their circumference. This allows effective cooling due to ease of heat transfer where the coil contacts the liquid cryogen, and the very high thermal conductivity of the coils in their winding direction. Typically, the coils 10, 11 are constructed from supercondcuting wire which contains a large proportion of copper with fine superconducting filaments enclosed. The turns of wire are typically embedded in a thermosetting resin. Layers of other materials, such as resin-impregnated glassfibre, glass beads or resistive wire may be provided on radially inner or outer surfaces of the coils as required for other purposes. Where coils are described as being in contact with liquid cryogen, the wire of the coils may not be directly in contact with the liquid cryogen, but may be in contact through a coating of therracsett±ng resin, or a layer of resin-impregnated cloth, beads, wire or similar.

Sensitive components like superconducting switches and superconducting joints used in superconducting magnets are preferably immersed in liquid cryogen to provide the necessary thermal stability.

From consideration of Fig. 1, it is clear that the position of the outer coils 10, extending near the lower extremity of the cryogen vessel 12, means that relatively small quantities of cryogen liqilid are sufficient to ensure that the outer coils 10 are in contact with the liquid cryogen over part of their circumference and therefore thermally stable.

However, the cryogen vessel 12 has to be filled significantly higher: almost to the level of the cryogen vessel 12 bore, to provide physical contact between the liquid cryogen and the inner coils 11. Such filling represents a significant quantity of liquid cryogen.

The present invention seeks to reduce the mass of liquid cryogen required while still providing physical contact between the liquid cryogen and each of the inner 11 and outer 10 coils.

Some attempts have been made to address this requirement, but these reduce the free volume within the cryogen vessel, either by placing displacers within the cryogen vessel, or shaping the cryogen vessel for reduced volume. However, in case of quench, it is advantageous to have a cryogen vessel with a large free volume, to reduce the maximum cryogen vapour pressure reached inside the cryogen vessel during quench. A lower peak pressure will mean that a thinner, lighter, cheaper construction of the cryogen vessel may be

acceptable.

The present invention accordingly provides a cryostat for an actively-shielded superconducting magnet with a reduced cryogen requirement. The present invention provides apparatus as defined in the appended claims.

The above, and further, objects, advantages and characteristics of the present invention will become more

apparent from the following description of certain

embodiments of the present invention, in conjunction with the accompanying drawings, wherein: Fig. 1 schematically illustrates a radial cross-section of a conventional cryostat for cooling a cylindrical, actively-shielded superconducting magnet; Fig. 2 schematically illustrates a radial cross-section of a cryostat for cooling a cylindrical, actively-shielded superconducting magnet according to an embodiment of the present invention; Fig. 3 schematically illustrates an axial cross-section of a cryostat for cooling a cylindrical, actively-shielded superconducting magnet according to an embodiment of the present invention; Fig. 4 schematically illustrates a radial cross-section of a cryostat for cooling a cylindrical, actively-shielded superconducting magnet according to another ertodiment of the present invention; Fig. 5 schematically illustrates an axial cross-section of a cryostat for cooling a cylindrical, actively-shielded superconducting magnet according to another embodiment of the present invention; and Fig. 6 schematically illustrates an axial cross-section of a cryostat for cooling a cylindrical, actively-shielded superconducting magnet according to another embodiment of the present invention.

According to an aspect of the present invention, a receptacle is provided to hold some of the liquid cryogen in contact with the inner coils 11, while another part of the liquid cryogen is in contact with the outer coils 10.

A receptacle is provided within the cryogen vessel 12, specifically for retaining some of the liquid cryogen in contact with the inner coils. The conventional cryogen vessel 12 can be described as the primary reservoir while the rccoptaclo can bc dcscribcd as a sccondary rcsorvoir.

Fig. 2 schematically shows a radial cross-section of an embodiment of the present invention. Features common with Fig. 1 carry common reference numerals, but not all features are shown. Receptacle 40 is provided. In this example, it is in the shape of a shallow trough placed around the lower extremities of the inner coils U, deep enough to ensure that all inner coils 11 are in contact with liquid cryogen over part of their circumference, when the receptacle 40 is filled with liquid cryogen. An arrangement, such as tube 42, is provided to ensure that recondensed liquid cryogen produced by refrigerator 17 is directed into the receptacle 40. The arrangement may direct all recondensed liquid cryogen to this receptacle to ensure that it is kept full, and excess recondensed liquid cryogen may be allowed to over-flow into the bottom of the cryogen vessel 12. Alternatively, two paths may be provided for recondensed liquid cryogen: one directing the liquid cryogen into the receptacle 40; one directing the liquid cryogen into the cryogen vessel 12.

Optionally, an arrangement such as tube 44 may be provided, fed from access neck 20 (Fig. 1) to divert at least some of liquid cryogen supplied during filling of the cryogen vessel to the receptacle 40. The arrangement may direct all liquid cryogen fed from access neck 20 (Fig. 1) to this receptacle to ensure that it is filled, and excess recondensed liquid cryogen may be allowed to over-flow into the bottom of the cryogen vessel 12. Alternatively, two paths may be provided for liquid cryogen added during filling of the cryogen vessel: one directing the liquid cryogen into the receptacle 40; one directing the liquid cryogen into the cryogen vessel 12.

The receptacle 40 may be retained in place by mounting to the cryogen vessel 12 or to the inner coils 11, or mounting to a mechanical arrangement (not shown) provided to retain the coils in their required positions.

The receptaclc 40 is accordingly filled with liquid cryogen when the cryogen vessel 12 is filled, and is kept filled by the recondensing refrigerator. Optionally, a dedicated receptacle filling pipe may be provided, through the refrigerator sock 15 or access neck 20, to allow direct filling of the receptacle 40 when required.

Even if tube 44 is not provided, and receptacle 40 is not filled with liquid cryogen from the cryogen fill, on installation the cryogen vessel would be filled to a relatively low level; well below a lower extremity of the inner coils 11. Within a relatively short time the receptacle 40 would fill with recondensed cryogen from the refrigerator 17. Assuming receptacle 40 has a liquid volume capacity of about 5 litres, this would take about 5 hours to fill just using a current refrigerator 17 to provide recondensed liquid helium cryogen.

As illustrated, it may be found sufficient to use a receptacle 40 whose maximum fill level does not extend vertically above a lower extremity o a bore tube of cryogen vessel 12.

Fig. 3 shows a schematic radial cross-section of the embodiment of Fig. 2. Outer coils 10 are shown mounted in conventional journals lOa, and inner coils 11 are shown mounted on a conventional former ha. Bore tube 12a of cryogen vessel 12 is shown. Receptacle 40 is shown in phantom for clarity.

The present invention may be applied to other arrangements for mounting the coils.

Fig. 4 shows a schematic radial cross-section of another embodimerrt of the present invention. Features corresponding to features of Figs. 1-2 carry corresponding reference nume rals. The embodiment of Fig. 4 differs from the embodiment of Figs. 2-3 in that receptacle 40 is provided with a sump 42 at a lower extremity, the sump 42 housing sensitive components like superconducting switches and superconducting joints used in superconducting magnets.

Operation of this embodiment is as for the embodiment of Fig. 2 and 3, except in that the sump 42 is preferentially filled before the remainder of the receptacle 40. In case of a quench, the sump 42 should be the last part of the cryogen vessel 12 to run dry of cryogen. As such, the sumn fulfils a purpose of ensuring that the sensitive components like superconducting switches and superconducting joints used in superconducting magnets are kept cold as soon and as long as possible.

Fig. 5 shows a radial cross-section of another embodiment of the present invention. This resembles the embodiment of Figs. 2-3, except in that the receptacle 40 extends vertically above a lower extremity of the bore tube 12a of the cryogen vessel 12. Axial ends of the receptacle must be sealed to prevent liquid cryogen overflowing at a lower level than intended. This may be achieved by sealing the receptacle to the bore tube l2a of the cryogen vessel, or to end walls of the cryogen vessel, or to the inner coils 10 structure or a former lOa or other mechanical support which retains the inner coils in place. By sealing the axial ends of the receptacle 40, the receptacle may be filled to its upper extremity. If preferred, an outlet hole or spout may be provided to define a lower maximum fill level.

The receptacle 40 may be held in place by mounting to the bore tube 12a of the cryogen vessel, or to end walls of the cryogen vessel, or to the inner coils 10 structure or a former ba or other mechanical sllpport which retains the inner coils in place.

Fig. 6 illustrates an axial cross-section of another embodiment of the present invention. In this embodiment, inner coils 11 are mounted onto a non-porous support structure lib by their radially outer surfaces. Such an arrangomont may bo roforrcd to as "A2 bonding".

The non-porous support structure llb may be a composite tube, such as a filament-wound resin-impregnated glassfibre tube; or a tube of aluminium or stainless steel or other suitable material. Otherwise-empty spaces 46 between inner coils 11 at the lower radially-inner surface of the non-porous cylindrical support structure lib are used as the receptacle in this instance. The coils 11 are sealed to the support structure, and may be filled with liquid cryogen to a certain depth. lube 42 and optionally also tube 44, described above, is/are provided, as schematically illustrated, to supply liquid cryogen into spaces 46 between coils 11. It is common in such magnet structures for axially outer coils lic to have a somewhat smaller inner diameter than the other inner coils 11. This is useful as it ensures that all spaces 46 will be filled with liquid cryogen 48 before the liquid cryogen overflows 50 into the bottom of the cryogen vessel 12 and into contact with the outer coils 10 and their support structure lOa.

The feed of liquid cryogen to the reservoir is performed in the same manner as above from the recondensing cold head through tube 42 but, in this embodiment, the coils 11 are bonded on their outside diameter to a tubular structure llb.

This provides a liquid tight collection volume which is in contact with the coil, providing excellent cooling.

In embodiments such as represented in Fig. 6, sensitive components like superconducting switches and superconducting joints used in superconducting magnets may be located in the spaces 46 between coils on the lower extremity of the radially inner surface of the non-porous cylindrical support structure llb; or may be located at the lower extremity of the cryogen vessel 12, to ensure that the sensitive components like superconducting switches and superconducting joints used in superconducting magnets are kept cold as soon and as long as possible.

Tho system roquiros enough cryogen to fill the spaces 46 between inner coils 11, and then subsequently partially immerse the outer coils 10.

The present invention accordingly provides a cryostat for an actively-shielded superconducting magnet with a reduced cryogen requirement, in which a receptacle is provided as a secondary reservoir within a cryogen vessel, being a primary reservoir, such that the secondary reservoir is preferentially filled with liquid cryogen to partially immerse inner coils 11. Excess filling of the secondary reservoir allows liquid cryogen to spill into the primary reservoir to partially immerse outer coils 10. Am optional sump ensures cooling for sensitive components.

The cryostat according to the present invention provides reduced requirement of liquid cryogen for cooling inner and outer coils of a cylindrical actively shielded magnet, yet does not require reduction in the free volume of the cryogen vessel 12, which ensures that the peak pressure experienced during quench remains relatively low compared to conventional arrangements.

No change is required to the design of the cryogen vessel 12.

While providing a reduced requirement for liquid cryogen when in operation, the present invention still allows the cryogen vessel 12 to be filled with a relatively large amount of liquid cryogen for cold-shipping purposes. That is, the cryostat containing the superconducting magnet may be filled with liquid cryogen and transported with the refrigerator 17 inoperative, allowing the cryogen to boil off to atmosphere.

By allowing a large fill with liquid cryogen, the magnet may be held at its operating temperature for many days, allowing it to be transported and installed withoilt any need to cool the magnet on site. As the cryostat according to the present invention may be operated with a relatively small mass of liquid cryogen, it may be possible to simply bring the refrigerator 17 into operation when installing the magnet, with no need to fill or top-up the level of liquid cryogen.

It may even be possible to retrieve some liquid cryogen from the installed cryogen for re-use.

Where a particularly onerous operation is planned -for example, a lengthy series of consecutive imaging operations, additional cryogen may be introduced onto the cryogen vessel, for improved cooling of the magnet coils 10, 11 and excess cryogen may be allowed to boil off to recovery or to atmosphere.

If the cryostat and the magnet coils 10, 11 are allowed to warm above their operating temperature, re-cooling may be aided by the presence of the receptacle according to the present invention. Considering the situation where cryogen gas is present within the cryogen vessel 12, but is not yet cold enough to recondense into liquid, the refrigerator 17 will provide cooled cryogen gas which, being denser, will descend along tube 42 and settle in the receptacle, providing for improved gas cooling of the inner magnet coils 11 during a gas cooling phase.