GB2596781A

GB2596781A - Method and apparatus for cooling a superconducting magnet

Info

Publication number: GB2596781A
Application number: GB2008890.2A
Authority: GB
Inventors: Gouveia Clint; Shaw Nick; Shepheard Adam
Original assignee: Scient Magnetics Ltd
Current assignee: Scient Magnetics Ltd
Priority date: 2020-06-11
Filing date: 2020-06-11
Publication date: 2022-01-12
Also published as: GB202008890D0

Abstract

A magnet system 1 comprises a chamber 9 for containing liquid helium 16 and a superconducting magnet 2 within the chamber. The magnet comprises a plurality of coils of superconducting wire, each comprising a termination 10. The terminations are mounted on a termination plate 12 at an upper region of the magnet. A high conductivity thermal bus 22 is connected between the terminations and a lower region of the chamber. The thermal bus may be made of high purity aluminium and may have a thermal conductivity of at least 500 W/m K. The magnet system may be able to operate at with a minimum liquid helium level of 20 litres in the chamber, or a minimum liquid helium level of 20% of the capacity of the chamber. The magnet system may further comprise a helium recirculation system comprising a re-condenser 18.

Description

Method and Apparatus for Cooling a Superconducting Magnet

BACKGROUND OF THE INVENTION

This invention relates to cryogenically cooled superconducting magnets.

Superconducting magnets are known generally in the art. A superconducting magnet is an electromagnet made from coils of superconducting wire. When the superconducting wire is cooled cryogenically to a sufficiently low temperature (typically around 4.2K) the wire transitions to being in a superconducting state in which it has no electrical resistance, allowing it to carry much larger electrical currents than a standard wire, and thus to create very high strength magnetic fields. Such superconducting magnets have particular usefulness both in MRI machines e.g. in hospitals, and in scientific equipment such as NMR spectrometers.

It is known to cool the superconducting coil using cryogenic liquids, for example liquid helium.

The present invention seeks to provide an improved apparatus for cooling a superconducting magnetic coil.

SUMMARY OF THE INVENTION

From a first aspect, the invention provides a magnet system, comprising: a chamber, suitable for containing liquid helium; a superconducting magnet, contained within the chamber, wherein the superconducting magnet comprises a plurality of coils of superconducting wire, each comprising a termination, said terminations being mounted on a termination plate, positioned at an upper region of the superconducting magnet; and a thermal bus, made of a material having a high thermal conductivity, connected between a lower region of the chamber and the terminations.

Thus it will be seen that, in accordance with the invention, by positioning one end of the thermal bus in the lower region of the chamber, in use, a part of the thermal bus may still be submerged in liquid helium even if the level of liquid helium within the low the termination plate. Due to the high thermal conductivity of the thermal bus, the thermal bus is able to cool the terminations of the superconducting magnet, thereby cooling the coils of the superconducting magnet, even when only the lower part of the chamber contains liquid helium. This allows the magnet to be adequately cooled with a smaller amount of liquid helium than is possible in prior art systems, advantageously helping the magnet system to operate safely for longer between re-fills of liquid helium.

The thermal bus may be connected to the terminations via any suitable thermal pathway, provided that the thermal bus is able to cool the plurality of coils of the superconducting magnet due to at least the other end thereof being submerged in the bath of liquid helium. In a set of embodiments, the thermal bus is connected directly to one or more of the terminations of the coils of superconducting wire, such that they are cooled directly. Alternatively, it is envisaged that the thermal bus could be connected between a lower region of the chamber and the termination plate. Thus the terminations would be cooled by their connection to the termination plate which would in this case be highly thermally conductive.

The thermal bus is typically made of a material having high thermal conductivity. It will be understood by the skilled person that a "high" thermal conductivity, in the context of the present invention, is a thermal conductivity which is sufficient to allow the thermal bus to cool a contact point of one end of the thermal bus (e.g. connected to the superconducting magnet) to approximately the same temperature as the other end, which is in contact with a cooling medium (e.g. liquid helium). The two regions are considered to be "approximately" the same temperature, if they are sufficiently close in temperature that they function as if they were at the same temperature. Thus there is no measurable temperature gradient across the superconducting magnet, or any temperature gradient across the superconducting magnetic coil is sufficiently small that its effect is not observable.

In some embodiments, the thermal conductivity of the thermal bus is at least 200 W/m K, preferably at least 500 W/m K, further preferably at least 1000 W/m K. In some embodiments, the thermal bus is made of aluminium, preferably high purity aluminium e.g. 5N pure aluminium. 3 -

The terminations of the coils of superconducting wire are mounted to the termination plate. In some embodiments the termination plate is made of glass fibre laminate or hard anodised aluminium. These terminations may be joined together e.g. by twisting or soldering, and put into joint cups, wherein the joint cups are mounted on the termination plate. The joint cups may be made of a material having high thermal conductivity. In some embodiments, the joint cups are made of aluminium. In some embodiments, the thermal bus is connected to one or more of the joint cups. The termination plate may also comprise a superconducting switch. In some embodiments, the superconducting switch includes a wire formed from superconducting filaments in a matrix of resistive materials and a small heater, both surrounded by an insulating resin. Such switches, known per se in the art, are typically used for starting up the superconducting coils.

The thermal bus is connected between a lower region of the chamber and a termination plate of the superconducting magnet. The termination plate is positioned at an upper region of the superconducting magnet. It will be understood by the skilled person that the terms "upper" and "lower" refer to regions of the magnet system, when it is in the standard use configuration. In this configuration, the "lower region" of the chamber will be lower i.e. gravitationally, than the upper region of the chamber. Thus, when the magnet system is in use and the chamber contains liquid helium, the lower region of the chamber will be the region which the liquid helium drains towards i.e. it will be the region which is last to empty of liquid helium as helium is lost from the system. The superconducting magnet likewise has a lower and upper region, corresponding to the same "lower" and "upper" directions as the chamber. Thus the "upper" region of the superconducting magnet is the first part of the magnet which will cease to be submerged in helium as helium is lost from the system. In some embodiments, the thermal bus is connected between a lower region of the superconducting magnet and the termination plate of the superconducting magnet.

In both cases it may not be necessary to define a strict division between the upper and lower regions since they can simply be considered relative to each other. However in some circumstances the upper region can be considered to be that part extending from the top of the chamber down to and including the termination plate and the lower region is the region of the chamber below the termination plate to the bottom. 4 -

In some embodiments, the chamber is arranged along a vertical axis which is approximately parallel to the direction of action of gravitational forces on the system. The lower region of the chamber may be the lowest part of the chamber relative to this vertical axis. Similarly in some embodiments, the superconducting magnet is arranged along a vertical axis which is approximately parallel to the direction of action of gravitational forces on the system. The upper region of the superconducting magnet may be the highest part of the superconducting magnet relative to this vertical axis. In some embodiments the chamber and the superconducting magnet are arranged coaxially along the same vertical axis.

The thermal bus and the superconducting magnet are contained within a chamber. The chamber may be made of stainless steel. The chamber may further be surrounded by an outer chamber, creating a cavity between the outer chamber and the chamber.

The cavity may be evacuated to form a vacuum, which helps to insulate the superconducting magnet and thermal bus, and prevent heating of the contents of the chamber. In some embodiments, the magnet system further comprises a radiation shield, located in the cavity.

The superconducting magnet may be made of Niobium-Tin or Niobium-Titanium.

In use, the magnet system in accordance with the present invention is cooled using a cryogenic liquid. Liquid helium is particularly well suited to cooling superconducting magnetic systems. Thus, in some embodiments, the magnet system further comprises liquid helium, contained within the chamber i.e. forming a liquid helium bath. In some embodiments, the liquid helium cools the thermal bus to a temperature of approximately 4.2K. In some embodiments, the magnet system is able to operate with a minimum quantity of (approximately) 20 litres of liquid helium present in the system. In some embodiments, the magnet system is able to operate with a minimum quantity of liquid helium which is (approximately) 20% of the total liquid helium capacity of the chamber. In some embodiments, the magnet system further comprises a liquid helium circulation system, arranged to circulate liquid helium into and out of the chamber. In some embodiments, the liquid helium circulation system comprises a re-condenser, to re-condense evaporated liquid helium. Optionally, the re-condenser is a cryocooler. -5 -

pect or embodiment described herein may, wherever appropriate, be applied to any other aspect or embodiment described herein. Where reference is made to different embodiments or sets of embodiments, it should be understood that these are not necessarily distinct but may overlap.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic drawing showing a prior art superconducting magnetic coil, cooled using liquid helium.

Figure 2 is a schematic drawing showing a superconducting magnetic coil, cooled using liquid helium, in accordance with the present invention.

DETAILED DESCRIPTION

Figure 1 shows a schematic drawing of a superconducting magnetic coil, arranged to be cooled by liquid helium, in a manner which is known in the art.

The superconducting magnetic system 1 contains a superconducting magnetic coil 2. The superconducting magnetic coil 2 may for example be made of Niobium-Tin (Nb3Sn) or Niobium-Titanium (NbTi). The superconducting magnetic coil 2 is contained within a liquid helium vessel 4. The liquid helium vessel 4 is surrounded by a radiation shield 6, which is then contained within an outer vacuum vessel 8 (often abbreviated to OVC). A vacuum is created between the liquid helium vessel 4 and the outer vacuum vessel 8, so as to insulate the chamber 9 created inside the liquid helium vessel and maintain the cold temperature.

As is known in the art, different pieces of superconducting wire, used to make the magnetic coil 2, are joined together, for example by twisting or soldering, and are then put into small joint cups 10, which are mounted on a termination plate 12. Typical termination plates have superconducting lead runs & terminations (i.e. magnet coil circuit wiring in them will generally be non-electrically conductive). The termination plate 12 is typically made of glass fibre laminate or hard anodised aluminium e.g. 10G/40 (often called G10). The termination plate 12 may also be made of non-hard anodised aluminium plates, to which 10G/40 insulative channels are added. Joint cups 10 are typically made of aluminium. Also mounted to the termination plate 12 is a -6 -witch 14. The superconducting switch 14 includes a wire formed from superconducting filaments in a matrix of resistive materials, a small heater is fitted, and the whole assembly is then surrounded by an insulating resin. In operation, the switch is 'opened' using the heater, which heats the superconducting filaments above their transition temperature so that they are not superconducting (but the insulation prevents much heat from passing into the cooling medium e.g. the helium bath) so that the resistance of the switch is approximately 10-50 D. Then the magnet can be energized using a power supply. The heater can then be turned off, so that the switch becomes superconducting, and then the current will continue circulating and the magnetic field will remain essentially constant. This is referred to as "persistent mode".

In order to cool the superconducting magnetic coil 2, the chamber 9 contains liquid helium, which surrounds the superconducting magnetic coil 2, forming a helium bath 16 around the magnetic coil 2.

Over time the temperature of the liquid helium bath 16 will rise, and some of the liquid helium will transition to a gas. In order to keep the liquid helium bath 16 sufficiently cool to keep the magnet from "quenching" (transifioning out of the superconducting state due to reaching the transition temperature) helium gas generated from the helium bath 16 evaporates from the liquid helium vessel 4 and passes through the cooling device 18, which re-condenses the helium. The re-condensed liquid helium is then returned to the helium path 16, as demonstrated by the arrows shown in Figure 1. The cooling device 18 may, for example, be a cryocooler, which is a stand-alone cooling device.

Despite this, over time some helium gas is lost from the system, causing the level of helium in the helium bath 16 to drop slowly. In the system as shown in Figure 1, this can have undesirable consequences. In particular, as the liquid helium level in the bath drops, the top part of the magnetic coil 2 is no longer surrounded by liquid helium, and so begins to heat up. This results in a thermal gradient being set up along the coils of the magnet. This is undesirable because as any part of the coil 2 is heated up, the risk of the coil 2 quenching increases. This poses a serious risk as, if the magnetic coil 2 switches to being non-superconducting, it will heat up very rapidly, causing the liquid helium to transition to helium gas, thus expanding rapidly and posing a serious asphyxiation risk as it is forced out of the cooling system. 7 -

In order to avoid this risk, the level of liquid helium in the helium bath 16 is generally prevented from going below the superconducting switch 16, as indicated by the dashed line showing the minimum helium level 20. As can be seen in Figure 1, only a small amount of helium needs to be lost from the system before the level of the helium bath 16 will be at or below the minimum 20, at which point the system will need to be topped up with additional liquid helium, which is a time consuming and costly process but which therefore has to be carried out at frequent intervals. A typical refill frequency in a known prior art system might be anything from 2 to 12 months.

As shown in Figure 2, a system is provided which addresses some of these shortcomings. The superconducting magnetic system 1' embodying the present invention has the same basic structure as the prior art system shown in Figure 1. Like components have been labelled with the same reference numeral as is shown in Figure 1, with an additional prime marking indicating that these components belong to a system according to an embodiment of the present invention. Thus a superconducting magnetic coil 2' is located in a chamber 9' formed by a liquid helium vessel 4' and an outer vacuum vessel 8' in a bath 16' of liquid helium.

In this embodiment of the invention however the superconducting magnetic system 1' shown in Figure 2 further comprises a highly thermally conductive bus 22'. In this example the thermally conductive bus 22' is made of pure aluminium, for example, aluminium of purity "5N". The thermal bus 22' connects from a lower region of the liquid helium vessel 4', specifically in this case a lower region of the magnetic coil 2', to the termination cups 10', which are on the termination plate 12' which forms an upper part of the magnetic coil 2' in an upper part of the chamber 9'.

Thus, the thermal bus 22', due to its high thermal conductivity, keeps the termination cups 10' cooled which in turn cools the magnetic coil 2', provided that at least some part of the thermal bus 22' is submerged in liquid helium. This allows the minimum level of liquid helium 20' to be greatly reduced, compared to the system as known in the prior art, and as described with reference to Figure 1. As shown in Figure 2, the level of liquid helium is able to drop very low within the liquid helium vessel 4', for example to approximately 20% of a total liquid helium volume at which a prior art system is considered "full". It has now been appreciated by the Applicant that even a -8 -quid helium provides sufficient thermal capacity to cool the entire coil. As a result of the introduction of the thermal bus 22', the superconducting magnetic system 1' according to the present invention can last for much longer without requiring any liquid helium to be added. Such systems can operate safely until there is almost no liquid helium left in the helium bath 18'. In some examples the refill frequency of the claimed system is halved compared to such a prior art system, through the use of a thermal bus, because the additional heat-load to the magnet provided by the bus allows the level of liquid helium to drop much lower than would previously have been acceptable e.g. 4-24 months, rather than 2-12 months.

It will be appreciated by those skilled in the art that the invention has been illustrated by describing one or more specific embodiments thereof, but is not limited to these embodiments; many variations and modifications are possible, within the scope of the accompanying claims. -9 -

Claims

Claims 1. A magnet system, comprising: a chamber, suitable for containing liquid helium; a superconducting magnet, contained within the chamber, wherein the superconducting magnet comprises a plurality of coils of superconducting wire, each comprising a termination, said terminations being mounted on a termination plate, positioned at an upper region of the superconducting magnet; and a thermal bus, made of a material having a high thermal conductivity, connected between a lower region of the chamber and the terminations.
2. The magnet system of claim 1, wherein the thermal bus is connected directly to one or more of said terminations.
3. The magnet system of claim 1 or 2, wherein the thermal bus is made of high purity aluminium.
4. The magnet system of any of claims 1 to 3, wherein the thermal conductivity of the thermal bus is at least 500 W/m K.
5. The magnet system of any preceding claim, wherein the thermal bus is connected between a lower region of the superconducting magnet and the termination plate
6. The magnet system of any preceding claim, wherein the superconducting magnet is made of Niobium Tin or Niobium Titanium.
7. The magnet system of any preceding claim, wherein the termination plate is made of glass fibre laminate or hard anodised aluminium.
8. The magnet system of any preceding claim, wherein the termination plate further comprises a superconducting switch.
9. The magnet system of any preceding claim, wherein the chamber is made of stainless steel.-10 -
10. The magnet system of any preceding claim, wherein the chamber is further surrounded by an outer chamber, creating a cavity between the outer chamber and the chamber and wherein the cavity is evacuated.
11. The magnet system of claim 10, further comprising a radiation shield located in the cavity.
12. The magnet system of any preceding claim, further comprising a liquid helium circulation system.
13. The magnet system of claim 12, wherein the liquid helium circulation system comprises a re-condenser, arranged to re-condense evaporated liquid helium.
14. The magnet system of any preceding claim, further comprising liquid helium contained within the chamber.
15. The magnet system of claim 14, wherein the magnet system is able to operate with a minimum quantity of 20 litres.
16. The magnet system of claim 14 or 15, wherein the magnet system is able to operate with a minimum quantity of liquid helium which is 20% of the total liquid helium capacity of the chamber.