GB2537888A

GB2537888A - Cooling arrangement for superconducting magnet coils

Info

Publication number: GB2537888A
Application number: GB1507359.6A
Authority: GB
Inventors: Charles Tigwell Neil
Original assignee: Siemens Healthcare Ltd
Current assignee: Siemens Healthcare Ltd
Priority date: 2015-04-30
Filing date: 2015-04-30
Publication date: 2016-11-02
Also published as: GB201507359D0

Abstract

A cooling arrangement for a superconducting magnet coil, or an assembly of multiple coils, has a cooling channel 26 with a bore, in thermal contact with the coil 10. A cryogen tank 24 provides a reservoir, for example liquid hydrogen, which passes through the bore, cooling the coil by thermal conduction. A refrigerator 18 and heat exchanger 22 may be provided in an isolatable sock 20, to cool and condense cryogen gas entering the tank. The cooling channel may extend between 10 to 90 percent around the circumference of an annular coil. The cooling channel may be constructed from aluminium or copper, have a rectangular or D shaped cross section, and a flexible joint 28. The tank may have a sump 35 with temperature sensors 36. A pipe 32 may be provided extending from the cooling channel to a sealable end 34 outside a vacuum container 14 enclosing the coil.

Description

COOLING ARRANGEMENT FOR SUPERCONDUCTING MAGNET COILS

The present invention relates to cooling arrangement for superconducting magnet coils, particularly cooling arrangements for cylindrical superconducting magnet coil assemblies, and yet more particularly to cooling arrangements for cylindrical superconducting magnets for MRI systems.

Conventionally, superconducting magnet coils assemblies such as cylindrical superconducting magnet coil assemblies have been cooled by at least partial immersion in a bath of liquid cryogen at its boiling point. This cryogen is frequently helium due to its very low-boiling point: about 4K.

Recently, issues of cost and scarcity have prompted some low- cryogen alternatives to the cryogen immersion baths. How-ever, these have brought other drawbacks. For example, a thermal bus in contact with a small quantity of helium may heat the helium to such an extent that film boiling occurs, interrupting thermal transfer to the liquid cryogen. Alternatively, so-called thermosiphons have been proposed. These involve a thermally conducting tube encircling a superconducting coil, with a cryogen recondensing refrigerator mounted at or near the top of the superconducting magnet coil structure. This has the disadvantage of requiring extra headroom for installation and maintenance of the refrigerator, and the tube may become blocked with ice in case of unintentional ingress of air or water vapour.

The present invention provides an alternative arrangement for cooling of superconducting coils with a reduced volume of cryogen, which does not suffer from the above-mentioned problems.

Accordingly, the present invention provides apparatus as set out 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 of the present invention, given by way of examples only, in conjunction with the appended drawings, wherein: Fig. 1 shows a radial section view of a cylindrical superconducting magnet coil assembly according to an embodiment of the present invention; and Fig. 2 shows an axial section view of a cylindrical supercon10 ducting magnet coil assembly according to an embodiment of the present invention.

Fig. 1 schematically illustrates a radial section through a cylindrical superconducting magnet for an MRI system, accord-15 ing to an embodiment of the present invention.

A number of annular coils 10 are aligned along axis A-A. Herein, the term "axial" indicates a direction parallel to axis A-A, while the term "radial" indicates a direction per-20 pendicular to axis A-A, in a plane which includes axis A-A.

As is conventional with cylindrical superconducting magnet coil assemblies for MRI systems, inner coils 10 are provided, and outer, or shield, coils 12 are provided, with greater di-ameter than the inner coils 10. However, Fig. 1 shows only inner coil 10 within an outer vacuum container (OVC) 14 which provides thermal insulation of the superconducting magnet coil structure from ambient temperature.

As is conventional in itself, a turret is attached to the OVC 14, and the interior of the turret is open to the interior of the O/C. In effect, the turret forms part of the O/C. A cryogenic recondensing refrigerator 18 is housed within the turret. It is preferably housed within a sock 20, and acts to cool a heat exchanger 22 which is exposed to a cryogen environment. Commonly, helium is used as the cryogen although other cryogens may be employed, as will be apparent to those skilled in the art. Any further references to helium should be understood to include other cryogens as preferred, unless the context clearly requires helium.

According to a feature of the illustrated embodiment of the present invention, a relatively small liquid helium tank 24 is provided, generally below the heat exchanger 22 and in communication with the interior of the sock 20, so that liquid helium condensed on the heat exchanger 22 will flow into the liquid helium tank 24. As will be apparent to those skilled in the art, means may be provided for isolating the interior of the sock 20 from the interior of the helium tank 24 for removing and replacing the refrigerator 18 during service operations.

Attached to, and in communication with, the helium tank 24, is a cooling channel 26. The cooling channel 26 comprises a relatively large-bore tube, in thermal contact with the superconducting coil 10 over a portion of the length of the channel. As illustrated, the cooling channel 26 is attached to the radially outer surface of coil 10. In other embodi-ments, the cooling channel 26 may be attached to a radially inner surface or an axially outer surface, as preferred and as space permits. It may be found advantageous to provide cooling channel 26 with a rectangular or 1)-shaped cross sec-25 tion, to maximise its contact surface area with the coil. The cooling channel may be formed as an extrusion.

The cooling channel 26 is attached in thermal contact with the coil 10. This may be achieved by means known to those skilled in the art, such as a thin layer of resin-impregnated glassfibre cloth, use of thermally conductive resin in that arrangement?, or the cooling channel 26 may simply be clamped on to the coil 10. Preferably, the cooling channel 26 is formed of a material having a similar coefficient of thermal expansion to that of the material of the coil. For example, superconducting wire is typically made up of a large proportion of copper, or aluminium. It may accordingly be benefi-cial to construct the cooling channel of copper, or aluminium respectively.

The interior of the cooling channel 26 is in communication with the interior of helium tank 24. Preferably, flexible joint is provided between the cooling channel 26 and the helium tank 24. For example, a stainless steel bellows 28 may be brazed between the cooling channel 26 and the helium tank 24 to provide a certain degree of flexibility, sufficient to adapt to differential thermal contraction of the various components involved. Alternatively, a longer stainless steel convoluted pipe may be used, welded or brazed at respective ends to the helium tank 24 and the cooling channel 26. Such flexible joint, such as a bellows or convoluted pipe, may fa-cilitate installation, as rigid alignment between helium tank 24 and cooling channel 26 would not be necessary.

Cooling channel 26 preferably extends downwards from a point at or near a lower extremity of the helium tank 24. In use, liquid helium condenses on the heat exchanger 22 and flows into helium tank 24 and thence into the cooling channel 26. The cooling channel 26 extends around the coil 10 towards its lowest point 30. The cooling channel may extend somewhat beyond the lowest point, but should not extend so far that gas locking in the cooling channel reduces its effectiveness in use.

The cooling channel 26 is in contact with the coil over a significant portion of its circumference. This may be ex-pressed as a value of 0, the angle subtended at the magnet axis A by the extent of thermal contact between the cooling channel 26 and the coil 10. The cooling channel 26 should be in contact with the coil 10 over an extent corresponding to a subtended angle 0 of between 10°-90°. If the cooling channel 26 is in contact with the coil 10 over an extent corresponding to a subtended angle 0 of significantly more than 90°, then heating of the cryogen in the cooling channel 26 by heat transfer from coil 10 may lead to the creation of gas locks in the cooling channel distant from the helium tank, limiting thermal contact with the coil 10 but requiring liquid helium to fill that part of the cooling channel 26.

Thermal conductivity of the coil 10, typically being predominantly of copper or aluminium, is typically sufficient to ensure effective cooling of the whole coil, oven though the cooling channel 26 is in contact with the coil only over a limited extent of the coil circumference. The cooling chan-nels have a sufficiently large bore to avoid gas locking and resultant overheating of the coils.

Preferably, the mass of helium within the arrangement is at least sufficient to fill the cooling channel 26 to the level of the lower extremity of helium tank 24. Heat from the coil 10 is conducted through the wall of the cooling channel 26 and causes boiling of the helium in the cooling channel 26. The resultant helium gas rises and flows back into helium tank 24 and thence into refrigerator sock 20 to be recon-densed on recondenser 22. Refrigerator 18 removes the heat from the system, recondensing the helium gas to liquid which returns to the helium tank 24 to complete the cycle.

An optional pipe 32 is shown, extending from an extremity of the cooling channel 26, distal from the helium tank 24. This pipe 32 extends to a valve or other sealable closure 34 accessible from outside the OVC. The closure 34 may be covered by a service cover, hatch or seal to prevent its use by a system user. On installation, or re-installation, the coil 10 may be at room temperature or at least a temperature significantly above the boiling point of helium. To simply cool the coil to operating temperature by adding liquid helium would be wasteful and expensive, as boiling helium may be al-lowed to escape and be lost. Rather, closure 34 may be opened, and an external pump and refrigerator used to drive cooled helium around a circuit including the sock 20, helium tank 24, cooling channel 26 and additional pipe 32. Helium gas may be circulated and externally cooled until the coil 10 reaches a temperature near its operating temperature, whereupon final cooling may be achieved by adding liquid helium to the cooling channel.

Also shown in Fig. 1 is an optional sump 35, at or near a lower extremity of the helium tank 24. Temperature-sensitive devices 36 such as superconducting terminations, switches or joints may be placed inside this sump to ensure that they remain covered by liquid helium provided by the hea:, exchanger 22. In addition, or alternatively, temperature-sensitive devices 38 may be placed on the exterior of the helium tank, and may be cooled by conduction through the material of the helium tank 24, at a position which should remain cooled by liquid helium at all times that the system is in operation.

The helium tank may be constructed of stainless steel or aluminium, as examples. In a current full-body MRI system, the helium tank may have a capacity of about 200 litres.

Fig. 2 shows a radial view of a cylindrical superconducting magnet according to the present invention. Features corre- sponding to features shown in Fig. 1 carry corresponding reference numerals. Outer vacuum container 14 is not illustrated in Fig. 2, although it would in fact be provided as illus-trated in Fig. 1.

In the embodiment of Fig. 2, a number of inner coils 10 are shown, aligned along axis A-A. Two outer, or "shield" coils 12 are also provided. These are also axially aligned along axis A-A. In use, they reduce the strength of a stray magnetic field emanating from the cylindrical superconducting magnet, as will be familiar to those skilled in the art. The outer coils 12 preferably have an inner diameter which is larger than the outer diameter of the inner coils 10. A cool-ing channel 26 is provided for each coil, thermally joined to the coil and extending from the helium tank 24 towards respective lower extremities 30 of the coils. In this embodiment, the helium tank 24 also performs the function of a manifold, connecting several cooling channels 26 to a single source of liquid helium 22. In other embodiments, a separate manifold may be provided.

It is a common design requirement for cylindrical superconducting magnet assemblies to keep the free bore of the inner coils 10 as large as possible, while keeping the outer dimensions of the cylindrical superconducting magnet assembly as small as possible. For this reason at least, in the illus- trated embodiment, cooling channels 26 are placed on radially outer surfaces of inner coils 10 and on one of the axial end-surfaces of the outer coils 12. The cooling channels 26 could be placed on the radially inner surfaces of the outer coils 12, although that surface by be used for a mechanical mounting arrangement. Inner coils 10 are retained by a retaining structure 40 of any suitable conventional type, which does not form part of the present invention.

While the present invention has been described by reference to a limited number of example embodiments, those skilled in the art will appreciate that numerous modifications and amendments are possible within the scope of the invention defined by the appended claims.

Claims

CLAIMS1. A superconducting magnet coil (10) in conjunction with a cooling arrangement, the cooling arrangement comprising: -a cooling channel (26) in thermal contact with a surface of the superconducting magnet coil, the cooling channel having a bore; and - a cryogen tank (24) in communication with the bore of the cooling channel, whereby, in use, liquid cryogen is provided from The cryogen tank to the bore of the cooling channel to cool the coil by thermal conduction.
2. A superconducting magnet coil (10) in conjunction with a 15 cooling arrangement, according to claim 1, the cooling arrangement further comprising: - a cryogenic refrigerator (18); and - a heat exchanger (22) exposed to the interior of the cryogen tank (24) and arranged to be cooled by the cryogenic re-20 frigerator (18), such that, in use, cryogen gas entering the cryogen tank is cooled by the cryogenic refrigerator and condenses to liquid.
3. A superconducting magnet coil (10) in conjunction with a cooling arrangement, according to claim 1 or claim 2, wherein the superconducting magnet coil is annular about an axis (AA), and the cooling channel (26) is in thermal contact with the surface of the superconducting magnet coil over a portion of its circumference which subtends an angle (0) at the axis, wherein the angle (0) is between 10°-90°.
4. A superconducting magnet coil (10) in conjunction with a cooling arrangement, according to claim 2, wherein the cryogenic refrigerator (18) and heat exchanger (22) are housed within a sock (20), the interior of the sock being in communication with the cryogen tank (24).
5. A superconducting magnet coil (10) in conjunction with a cooling arrangement, according to claim 4, wherein means are provided for isolating the interior of the sock (20) from the interior of the cryogen tank;24).
6. A superconducting magnet coil (10) in conjunction with a cooling arrangement, according to any preceding claim, wherein the cooling channel has a rectangular cross-section.
7. A superconducting magnet coil (10) in conjunction with a cooling arrangement, according to any of claims 1-5, wherein the cooling channel has a fl-shaped cross-section.
8. A superconducting magnet coil (10) in conjunction with a 15 cooling arrangement, according to any preceding claim, wherein the cryogen tank is joined to the cooling channel by a flexible joint.
9. A superconducting magnet coil (10) in conjunction with a cooling arrangement, according to any preceding claim, wherein a pipe (32) is provided, extending from an extremity of the cooling channel (26) distal from the cryogen tank to a sealable closure (34) accessible from outside of an outer vacuum container (14) which encloses the superconducting mag-net coil.
10. A superconducting magnet coils assembly comprising a plurality of annular coils aligned along an axis (A-A), each superconducting magnet coil being provided in conjunction 30 with a cooling arrangement, according to any preceding claim.
11. A superconducting magnet coils assembly according to claim 10 comprising a number of inner coils (10) and a number of outer coils (12), the outer coils having an inner diameter 35 which is larger than the outer diameter of the inner coils.
12. A superconducting magnet coils assembly according to claim 10 or claim 11 wherein a single cryogen tank (24) is provided, in communication with the bore of corresponding cooling channels of a plurality of coils.
13. A superconducting magnet coil (10) in conjunction with a cooling arrangement according to any of claims 1-9 or a superconducting magnet coils assembly according to claim 12, wherein the cryogen tank is provided with a sump (35), containing temperature-sensitive devices (36).
14. A superconducting magnet coil (10) in conjunction with a cooling arrangement according to any of claims 1-9 or a superconducting magnet coils assembly according to any of claims 10-13, wherein the cooling channel comprises an aluminium or copper extrusion.
15. A superconducting magnet coil (10) in conjunction with a cooling arrangement, substantially as described and/or as illustrated in the accompanying drawings.
16. A superconducting magnet coils assembly substantially as described and/or as illustrated in the accompanying drawings.