GB2538748A

GB2538748A - Thermosyphon cooling of an actively shielded superconducting magnet

Info

Publication number: GB2538748A
Application number: GB1509061.6A
Authority: GB
Inventors: James Calvert Simon; Paul Johnstone Adam; Noys Jonathan; Simpkins Michael; Anthony Robert Teah John
Original assignee: Siemens Healthcare Ltd
Current assignee: Siemens Healthcare Ltd
Priority date: 2015-05-27
Filing date: 2015-05-27
Publication date: 2016-11-30
Also published as: GB201509061D0

Abstract

A cylindrical superconducting magnet with inner 10 and outer 12 shield coils is housed within a cylindrical outer vacuum container 14. A thermosyphon cooling arrangement, with cryogen tank 22 smaller than the coils, is positioned within the vacuum container 14, reducing the height of the superconducting magnet. A heat exchanger 32, optionally in a recondensing chamber 40, is exposed to the interior of tank 22, and cooled by a cryogenic refrigerator 30. A thermosyphon tube 44 extends around, and in thermal contact with, outer coil 12 and has a bore connected to tank 22 above maximum cryogen level 28 and below minimum cryogen level 26. A portion of the outer coil is not in thermal contact with the thermosyphon tube containing liquid cryogen. Thermally conductive material 50 may be applied over this portion. Multiple thermosyphon tubes may be used with outer and inner coils, connected to tank 22 through manifolds.

Description

THERMOSYPHON COOLING OF

AN ACTIVELY SHIELDED SUPERCONDUCTING MAGNET

The present invention is directed to the cooling of an 5 actively-shielded magnet using a thermosyphon.

Conventionally, superconducting magnets have typically been cooled by at least partial immersion in a bath of liquid helium. Due to the expense of this scarce resource, magnet manufacturers are now investigating alternative arrangements for cooling magnets using substantially less liquid helium. The favoured way of implementing this cooling is with a gravity-driven thermosyphon. While helium is a commonly used cryogen in this application, other cryogens are also used.

The present invention may be applied to any cryogen appropriate to the superconducting material of the coils.

A conventional thermosyphon arrangement relies upon positioning all the coils that are to be cooled below a minimum operating level of cryogen in a cryogen tank. Fig. 1 shows an example of such an arrangement applied to an actively shielded superconducting magnet.

The actively shielded superconducting magnet comprises at least one inner coil 10 and at least one outer coil 12 housed within an outer vacuum chamber (OVC) 14. The inner coil(s) 10 and outer coil(s) 12 are annular and are aligned along axis A-A. A thermosyphon arrangement 20 is provided to cool the coils to an operating temperature low enough to allow superconduction in the coils.

The thermosyphon arrangement comprises a cryogen tank 22 which is smaller than the coils, and in use contains liquid cryogen 24, typically helium, to a level no lower than a minimum level 26 and no higher than a maximum level 28. A cryogenic refrigerator 30 acts to cool a heat exchanger 32 exposed to the interior of the cryogen tank 22 above the maximum level 28 of liquid cryogen. In use, the heat exchanger is cooled to a temperature below the boiling point of the cryogen in the cryogen tank. Cryogen gas in the cryogen tank condenses to liquid on the heat exchanger and returns to the liquid cryogen 24 within the cryogen tank. In the illustrated embodiment, heat exchanger 32 is located within a recondensing chamber 40 connected to the cryogen tank 22 by at least one tube 42 which is inclined downwards from the recondensing chamber 40 to the cryogen tank 22. In other embodiments, the recondenser 32 may be positioned within the cryogen tank 22, and recondensing chamber 40 and tube 42 omitted. A thermosyphon tube 44 extends around outer coil 12, in thermal contact with its surface. As preferred, the tube 44 may be attached in contact with the radially outer, radially inner or an axial end surface of the coil.

Choice of the surface will at least partly depend on the design of the actively shielded superconducting magnet. The thermosyphon tube 44 has a bore which is connected to the cryogen tank 22 at its two ends. One end of the bore enters the cryogen tank 22 at an upper location 46, above the maximum liquid cryogen level 28 while the other end of the bore enters the cryogen tank 22 at a lower location 48, below the minimum liquid cryogen level 26. In operation, liquid cryogen 24 enters the bore through the lower location 48, and cools the coil 12 by thermal conduction through the wall of the thermosyphon tube 44. The liquid cryogen warms, and boils, as a result of this heat transfer. The resulting cryogen gas rises and flows through thermosyphon tube 44 into the cryogen tank 22 at the upper location 46. The cryogen gas is then recondensed by heat exchanger 32 into cryogen liquid. Thus heat is removed from coil 12 by cryogenic refrigerator 30. To cool all coils, similar thermosyphon tubes 44, 44' are placed in contact with the other inner coils 10 and outer coils 12. All are connected to the cryogen tank 22 as described above, but may be connected through manifolds, such that only a single tube connecting the cryogen tank to a manifold need be connected to the cryogen tank at each of the upper 46 and lower 48 locations.

Outer coils 12 may also be referred to as shield coils, as they function to reduce the stray magnetic field produced by the superconducting magnet in use.

In order to fill the thermosyphon tubes 44 with liquid cryogen, the lower location 48 is located above the upper extremity of the outer coil 12. This means that the thermosyphon arrangement 20 extends above the generally cylindrical shape of the OVC 14.

The position of the thermosyphon arrangement contributes to the overall height of the superconducting magnet. The total height of the superconducting magnet may restrict accessibility for installation of the magnet, meaning that the magnet cannot be installed in some proposed installation sites. The height of the magnet, increased by the thermosyphon arrangement 20, makes servicing operations and siting difficult. For example, it is difficult for a service engineer to gain access to the cryogen vessel 22 to fill with cryogen when it is raised high above the ground. Often, limited clearance will be available above the magnet due to fixed ceiling heights.

The position of the thermosyphon arrangement of Fig. 1 means 25 that relatively complex manufacturing methods are required.

The present invention aims to address these issues and provide a thermosyphon arrangement for cooling a superconducting magnet which does not significantly increase 30 the height of the OVC.

The present invention accordingly provides arrangements as recited in the appended claims.

The above, and further, objects characteristics and benefits of the present invention will become more apparent from the following description of certain embodiments thereof, given by way of non-limiting examples only, in conjunction with the accompanying drawings, wherein: Fig. 1 schematically represents a conventional arrangement of a thermosyphon for cooling a superconducting magnet; Fig. 2 schematically represents an arrangement of a thermosyphon for cooling a superconducting magnet according to an embodiment of the present invention; and Fig. 3 schematically represents an arrangement of a thermosyphon for cooling a superconducting magnet according 10 to another embodiment of the present invention.

While the conventional arrangement of Fig. 1 provides cooling to essentially the whole circumference of the outer coil 12 by the thermosyphon tube 44, the present invention provides cooling to only a section of the outer coil 12, and positions the cryogen tank 22 within the generally cylindrical shape of OVC 14.

Fig. 2 illustrates a thermosyphon cooling arrangement of the present invention. Cryogen vessel 22 is located at a radial position intermediate between inner coils 10 and outer coil 12. While the exact position of cryogen vessel 22 may vary between embodiments, the cryogen vessel 22 should be located within the generally cylindrical shape of OVC 14.

As with the arrangement of Fig. 1, thermosyphon tube 44 extends around a surface of outer coil 12, in thermal contact with the outer coil. The bore of the thermosyphon tube enters the cryogen tank 44 at a lower location 48 and an upper location 46. The thermosyphon tube fills with liquid cryogen to a level determined by the fill level of liquid cryogen in the cryogen tank, between maximum level 28 and minimum level 26. Due to the positioning of the cryogen tank 22 within the generally cylindrical shape of OVC 14, a significant portion of the circumference of the outer coil 12 is not cooled by thermosyphon tube 44 at a position below the minimum liquid cryogen level 26 where it contains liquid cryogen. This portion may be defined by the angle it subtends at the magnet axis A-A. The subtended angle may vary between a value 0 when the liquid cryogen is at its maximum level and a value 0' when the liquid cryogen is at its minimum level. In preferred embodiments, the subtended angle 0' lies between 400 and 1200, although could lie outside of this range.

As the minimum level 26 of liquid cryogen within the cryogen 10 tank 22 is above the upper extremity of inner coil 10, the thermosyphon tube 44' attached to the inner coil 10 will fill with liquid cryogen in the conventional manner.

Over the extent of the outer coil 12 which is not in thermal contact with thermosyphon tube at a location where it is filled with liquid cryogen, the outer coil 12 is cooled by conduction around its circumference to locations which are in thermal contact with thermosyphon tube at a location where it is filled with liquid cryogen. The inventors have found that the structure of the outer coil enables efficient heat transfer in a circumferential direction around the outer coil to a locaton where the heat can be removed by thermosyphon tube 44 at a location where it is filled with liquid cryogen 24.

Outer coils 12 typically have a large cross-sectional area, due to the number of turns required. The outer coils 12 may also be produced from larger Cross section conductor, to withstand the larger hoop stresses that the outer coils are subjected to. These characteristics mean that the outer coil 12 is highly thermally conductive in the circumferential direction.

Gradient coils used in MRI systems are positioned within the 35 bore of the superconducting magnet and produce rapidly oscillating magnetic fields. These oscillating magnetic fields induce currents in inner and outer coils 10, 12.

However, the outer coils 12 are positioned far enough away from the gradient coils that the influence of the gradient coils on the outer coils 12 is much less than the influence of the gradient coils on the inner coils 10.

According to the invention, the cryogen tank 22 of the thermosyphon is lowered to within the generally cylindrical shape of OVC 14. The gravity-driven nature of the thermosyphon means that a portion of the circumference of outer coil 12 is not cooled by a thermosyphon tube at a location where it contains liquid cryogen. Instead, thermal conduction takes place around the circumference of the outer coil so that heat is transferred from the portion of the circumference to a part of the outer coil which is cooled by a thermosyphon tube at a location where it contains liquid cryogen.

Fig. 3 schematically represents an arrangement of a thermosyphon for cooling a superconducting magnet according to another embodiment of the present invention.

In this embodiment, the bore of the thermosyphon tube 44 is in communication with the cryogen tank at the upper location 46 through a wall of the recondensing chamber. In this embodiment, boiled-off cryogen gas re-enters the recondensing 25 chamber direct from the thermosyphon tube 44, without the need to flow up through the tube(s) 42 against a descending flow of liquid cryogen. This may permit a more efficient recondensing arrangement.

The solution of the present invention is found to be effective on outer coils 12, but it is believed that it would be difficult to apply the same solution to inner coils 10, because the outer coils 12 typically have a lower circumferential thermal resistance due to their larger cross sectional area and outer coils 12 receive less heat input generated by gradient fields interacting with them. There may also be difficulties with the cryogen vessel and associated tubes which may conflict with the requirement to keep the open bore of the OVC 14 free.

In some embodiments of the present invention, a layer of 5 thermally conductive material 50 may be applied to one or more surfaces of the outer coil 12, typically extending at least over the portion of the coil which is not c000lcd by a part of the thermosyphon tube 44 containing liquid cryogen. Such thermally conductive material 50 will assist in 10 transferring heat from the portion of the coil which is not c000led by a part of the thermosyphon tube 44 containing liquid cryogen. Such thermally conductive material may be applied to any suitable surface of the coil: radially inner, radially outer or either axial end surface, as appropriate.

By providing a thermosyphon arrangement according to the present invention, the overall height of the superconducting magnet may be reduced, facilitating installation and service, and indeed allowing installation in locations that were previously unable to accommodate a similar magnet. Service operations are facilitated by increasing headroom above the magnet. The arrangement of the invention also requires fewer complex assemblies, facilitating manufacturing of the magnet.

While the present invention has been described with reference to a limited number of specific embodiments, it will be apparent to those skilled in the art that numerous modifications and variations may be made within the scope of the appended claims.

Claims

CLAIMS: 1. A cylindrical superconducting magnet comprising an inner coil (10) and an outer coil (12), aligned along an axis (A-A) 5 and housed within a generally cylindrical outer vacuum container (OVC) (14), further comprising a thermosyphon cooling arrangement (20), the thermosyphon cooling arrangement comprising: -a cryogen tank (22) which is smaller than the inner coil (10) and the outer coil (12); and - a cryogenic refrigerator (30) arranged to cool a heat exchanger (32) exposed to an interior of the cryogen tank (22); - a thermosyphon tube (44) extending around the outer coil (12) in thermal contact with its surface, the thermosyphon tube comprising a bore connected to the cryogen tank (22) at its two ends, one end of the bore being in communication with the cryogen tank at an upper location (46), above a maximum liquid cryogen level (28), the other end of the bore entering the cryogen tank at a lower location (48) below a minimum liquid cryogen level (26), characterised in that the cryogen tank 22 is positioned within the generally cylindrical shape of the OVC (14), and a portion of the outer coil (12) is not in thermal contact with the thermosyphon tube below the minimum liquid cryogen level over a length of the portion.
2. A cylindrical superconducting magnet according to claim 1 wherein the portion subtends an angle (0') at the axis 30 wherein the angle (0') lies in the range of 40°-120°.
3. A cylindrical superconducting magnet according to claim 1 or claim 2 wherein the heat exchanger (32) is located within a recondensing chamber (40) connected to the cryogen tank (22) by at least one tube (42) inclined downwards from the recondensing chamber (40) to the cryogen tank (22).
4. A cylindrical superconducting magnet according to claim 3 wherein the bore of the thermosyphon tube (44) is in communication with the cryogen tank at an upper location (46) through a wall of the recondensing chamber.
S. A cylindrical superconducting magnet according to any preceding claim, further comprising a further thermosyphon tube (44') extending around the inner coil (12) in thermal contact with its surface, the thermosyphon tube comprising a bore connected to the cryogen tank (22) at its two ends, one end of the bore entering the cryogen tank at an upper location (46), above a maximum liquid cryogen level (28), the other end of the bore entering the cryogen tank at a lower location (48) below a minimum liquid cryogen level (26).
6. A cylindrical superconducting magnet according to any preceding claim, comprising a plurality of inner coils and a plurality of outer coils, each provided with a thermosyphon tube (44; 44') each extending around the respective coil (10; 12) in thermal contact with its surface, wherein the thermosyphon tubes are connected to the cryogen tank (22) through manifolds, a single tube extending from each manifold to the cryogen tank at respective upper (46) and lower (48) locations.
7. A cylindrical superconducting magnet according to any preceding claim, wherein a layer of thermally conductive material (50) is applied to one or more surfaces of the outer coil, extending at least over the portion of the outer coil (12) is not in thermal contact with the thermosyphon tube below the minimum liquid cryogen level over a length of the portion.
8. A cylindrical superconducting magnet substantially as 35 described and/or as illustrated in the accompanying drawings.Amendment to Claims have been filed as follows CLAIMS: 1. A cylindrical superconducting magnet comprising an inner coil (10) and an outer coil (12), aligned along an axis (A-A) and housed within a generally cylindrical outer vacuum container (OVC) (14), further comprising a thermosyphon cooling arrangement (20), the thermosyphon cooling arrangement comprising: -a cryogen tank (22) which is smaller than the inner coil (10) and the outer coil (12); and - a cryogenic refrigerator (30) arranged to cool a heat exchanger (32) exposed to an interior of the cryogen tank (22); - a thermosyphon tube (44) extending around the outer coil (12) in thermal contact with its surface, the thermosyphon tube comprising a bore connected to the cryogen tank (22) at cr) its two ends, one end of the bore being in communication with the cryogen tank at an upper location (46), above a maximum -- liquid cryogen level (28), the other end of the bore entering CD 20 the cryogen tank at a lower location (48) below a minimum liquid cryogen level (26), C\J characterised in that the cryogen tank 22 is positioned within the generally cylindrical shape of the OVC (14), and a portion of the outer coil (12) is not in thermal contact with the thermos yphon tube, and the portion of the outer coil is located above the minimum liquid cryogen 2. A cylindrical superconducting magnet according to claim 1 wherein the portion subtends an angle (0') at the axis 30 wherein the angle (0') lies in the range of 40°-120°.3. A cylindrical superconducting magnet according to claim 1 or claim 2 wherein the heat exchanger (32) is located within a recondensing chamber (40) connected to the cryogen tank (22) by at least one tube (42) inclined downwards from the recondensing chamber (40) to the cryogen tank (22).4. A cylindrical superconducting magnet according to claim 3 wherein the bore of the thermosyphon tube (44) is in communication with the cryogen tank at an upper location (46) through a wall of the recondensing chamber.S. A cylindrical superconducting magnet according to any preceding claim, further comprising a further thermosyphon tube (44') extending around the inner coil (12) in thermal contact with its surface, the thermosyphon tube comprising a bore connected to the cryogen tank (22) at its two ends, one end of the bore entering the cryogen tank at an upper location (46), above a maximum liquid cryogen level (28), the other end of the bore entering the cryogen tank at a lower location (48) below a minimum liquid cryogen level (26).plurality of outer coils, each provided with a thermosyphon 1-- tube (44; 44') each extending around the respective coil (10; (:) 20 12) in thermal contact with its surface, 1-- wherein the thermosyphon tubes are connected to the cryogen C\J tank (22) through manifolds, a single tube extending from each manifold to the cryogen tank at respective upper (46) and lower (48) locations.7. A cylindrical superconducting magnet according to any preceding claim, wherein a layer of thermally conductive material (50) is applied to one or more surfaces of the outer coil, extending at least over the portion of the outer coil (12) which is located above the minimum liquid cryogen level.6. A cylindrical superconducting magnet according to any cr) preceding claim, comprising a plurality of inner coils and a 8. A cylindrical superconducting magnet substantially as described and/or as illustrated in Figs. 2 and 3 of the accompanying drawings.