Closed-loop precooling of cryogenically cooled equipment

Abstract

Apparatus for pre-cooling cryogenically cooled apparatus housed within a cryogen vessel (26), comprising a first closed-loop cooling circuit (30) containing heat transfer fluid, a circulator (32) for causing the heat transfer fluid to circulate around the closed circuit and a heat extractor (34) arranged to extracts heat from the heat transfer fluid, wherein the circuit carries heat transfer fluid into and from an interior volume of the cryogen vessel (26).

Description

The present invention relates to methods and apparatus for pre-cooling cryogenically cooled-equipment. In particular, it relates to such cooling by a closed-loop refrigeration system. The present invention may be particularly applied to the pre-cooling of superconducting magnets for MRI (magnetic resonance imaging) systems, but may of course be applied to other cryogenically cooled equipment.

In typical current arrangements, apparatus to be cryogenically cooled is housed within a cryogen vessel. The cryogen vessel is contained within an outer vacuum chamber and the volume between the outer vacuum chamber and the cryogen vessel is evacuated, providing effective thermal insulation. Pre-cooling of the apparatus is performed by simply adding liquid cryogen to the cryogen vessel and allowing it to boil off. While effective, this arrangement has certain drawbacks.

If a working cryogen, such as liquid helium, is used for this pre-cool step, the quantity of helium boiled off and vented to atmosphere is costly, and it may be difficult to obtain sufficient supplies in some regions. Also, since liquid helium is a non-renewable resource, its consumption should be minimized where possible.

In certain arrangements, a sacrificial cryogen such as liquid nitrogen is initially used to cool the apparatus to a first temperature, typically higher than the temperature of the working cryogen. Once the equipment has been cooled to the first temperature by the sacrificial cryogen, a quantity of the working cryogen is added to cool the apparatus to the required temperature. The advantages of this arrangement include the fact that an abundant, inexpensive sacrificial cryogen, such as liquid nitrogen, may be used as the sacrificial cryogen; and that the consumption of working cryogen is significantly reduced below the consumption of the alternative arrangement where the working cryogen is used alone. However, difficulties with this method include the likelihood of contamination of the working cryogen with residual quantities of sacrificial cryogen. If an amount of liquid nitrogen remains in a cryogen tank when liquid helium is added, a significant quantity of liquid helium will be required to cool the nitrogen itself down to liquid helium temperature, cancelling some of the benefit of reduced helium consumption.

A complete flow-chart of a method for cooling cryogenically cooled apparatus according to the prior art is shown in FIG. 1.

The following description will be made with particular reference to superconducting magnets for MRI imaging apparatus, but it should be understood that the present invention may be suitably applied to the pre-cooling of any cryogenically cooled apparatus within a cryogen vessel.

In the first step 10, the cryogen vessel is evacuated and then filled with helium gas at atmospheric pressure and ambient temperature. This enables the cryogen vessel to be tested for leaks. Any leakage of the helium gas into the vacuum between the cryogen vessel and the outer vacuum container typically provided around the cryogen vessel for thermal insulation may be easily detected.

In the second stage 12, the helium gas is flushed from the cryogen vessel, and pre-cooling is initiated by the addition of liquid nitrogen. The liquid nitrogen boils off to atmosphere as it cools the magnet structure within the cryogen vessel. Liquid nitrogen has a relatively large thermal heat capacity and so is an effective coolant. It is also inexpensive, and so offers rapid and inexpensive cooling to a first cryogenic temperature.

As shown at stage 14, the addition of liquid nitrogen continues until a predetermined quantity of liquid nitrogen remains in the cryogen vessel.

At step 16, the magnet is allowed to soak in the liquid nitrogen for a certain time, to allow the magnet structure to reach a consistent temperature throughout, equal to the boiling point of nitrogen. Once this is complete, the liquid nitrogen is flushed from the cryogen vessel. This may be by the well known siphon effect, where helium gas at ambient temperature is introduced into the cryogen vessel. Gas pressure in the cryogen vessel is used to force out liquid cryogen. Care must be taken to remove all, or as much as possible, of the nitrogen from the cryogen vessel. The cryogen vessel is then pumped out to vacuum to remove as much nitrogen as possible.

In the next step 18, liquid helium, or another working cryogen as required, is introduced into the cryogen vessel. The working cryogen boils off to cool the magnet to the required operating temperature. Working cryogen is added until a required quantity of working cryogen remains in the cryogen vessel.

Finally, at step 20, the magnet structure is at the required temperature, and contains the required quantity of working cryogen.

While effective, this method still consumes a large quantity of sacrificial and working cryogens. In one known system, with the magnet cooled to 70K by a liquid nitrogen sacrificial cryogen, 1200 litres of liquid helium are consumed in cooling the structure from 70K to 4K. If the nitrogen is not completely removed, there is a significant increase in the amount of helium required, since the remaining liquid nitrogen must be frozen and cooled to the liquid helium temperature. If any cryogen remains within the cryogen vessel, it may act as a ‘poison’, in that it may act to form an ice around the superconducting magnet coils, which in turn may cause the superconducting magnet coils to quench in operation.

Prior art precool arrangements are described, for example, in EP1586833, U.S. Pat. No. 5,187,938, US2005/016187 and GB1324402. In the arrangements disclosed in both US20051016187 and U.S. Pat. No. 5,187,938, a closed loop cooling circuit is used wherein the circulating heat transfer material is cooled by a tank of liquid cryogen, such as nitrogen, but is warmed up to room temperature before passing through a circulator. This warming wastes any cooling power remaining in the heat transfer material and causes significant inefficiencies in the system. In U.S. Pat. No. 5,187,938, the heat transfer material is pressurised slightly in excess of atmospheric pressure to prevent external contaminants from leaking in.

The present invention accordingly aims to alleviate at least some of the disadvantages of the prior art. For example, a desire to reduce the volume of helium required, and to remove risks associated with the introduction of nitrogen into the cryogen vessel. It also aims to simplify the pre-cool procedure. By using only one type of cryogen within the cryogen vessel, the need for repeated evacuations is avoided.

The present invention provides cooling apparatus in which it is not necessary to warm the heat transfer material to room temperature before passing through the circulator. This significantly increases the efficiency of the proposed system. In preferred embodiments, the present invention also provides pressurised heat transfer material, pressurised significantly above atmospheric pressure to improve the effectiveness of heat transport.

Accordingly, the present invention provides methods and 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 consideration of the following description of certain embodiments; given by way of examples only, in conjunction with the accompanying drawing, wherein

FIG. 1 illustrates a flow chart of a conventional pre-cooling method for apparatus cooled to liquid helium temperature;

FIG. 2 shows a schematic diagram of a first embodiment of the present invention; and

FIG. 3 shows a schematic diagram of a second embodiment of the present invention.

According to the present invention, the current open-loop cooling method, based on the boiling of a liquefied cryogen by contact with the cooled apparatus, is replaced by a closed-loop pre-cooling arrangement. A coolant is circulated between the magnet, or other apparatus to be cooled as appropriate, and a cold source. The cold source may be an active refrigerator, or may be a boiling tank of liquid cryogen, a cooled tank of liquid cryogen, or a frozen block of solid cryogen.

FIG. 2 shows a schematic diagram of a first embodiment of the present invention. In FIG. 2, a superconducting magnet structure 20, comprising coils 22 of superconducting wire wound onto a former 24, is shown housed within a cryogen vessel 26, itself housed within an outer vacuum container 28. Such arrangement is entirely conventional, and may be substituted for any other cryogenically cooled apparatus, as the application requires.

In accordance with the present invention, a closed-loop cooling circuit 30 is provided. The cooling loop comprises a closed circuit-containing heat transfer fluid, a circulator 32 such as a compressor or a fan for causing the heat transfer fluid to circulate around the dosed circuit and a heat extractor 34 which extracts heat from the heat transfer fluid. In the illustrated embodiment, a circuit carries gaseous helium into and from the cryogen vessel 26. When inside the cryogen vessel, the helium absorbs heat from the magnet structure and warms up. A compressor acting as circulator 32 compresses the gaseous helium to a certain pressure, typically in the range of 100-300 kPa absolute. Care must be taken not to pressurise the helium in excess of the capability of the cryogen vessel 26. The compressor causes the helium gas to travel around the circuit and increases the density of the helium gas, thereby increasing its capability for heat transfer. The compressed gas flows from the compressor through closed pipes 36 into the cryogen tank 26. The helium absorbs heat from the magnet and is drawn through further pipes to heat extractor 34. The heat extractor may be an active cryogenic refrigerator such as a mechanical refrigerator. An example of a mechanical refrigerator is one operating according to the Stirling cycle. Alternatively, the heat extractor 34 may be a passive refrigerator, such as a tank of liquid cryogen, or a mass of solidified, frozen, cryogen in thermal contact with the pipe 36 carrying the heat transfer fluid.

In a particular embodiment, a passive cooling arrangement employing a tank of liquid cryogen or a mass of solid cryogen may be employed until the magnet has cooled to a first temperature, being no lower than the temperature of the liquid or solid cryogen, with the flow of heat transfer fluid them being switched away from the liquid or solid cryogen to an active refrigerator, to continue with cooling down to a desired pre-cool temperature, lower than that which could be obtained from the passive cooling arrangement alone.

As the magnet structure cools down, the density of the helium heat transfer fluid at a certain pressure will increase, increasing the heat transfer efficiency. If the heat extractor is sufficiently powerful, and parasitic thermal influx is kept to a minimum, the magnet will eventually be cooled to approximately its operating temperature. Alternatively, if the arrangement is not sufficiently powerful or efficient, the temperature of the magnet will stabilise. The gas in the pipes 36 and compressor 32 may even liquefy. At this point, the cryogen vessel may be filled with working cryogen. Since the working cryogen is preferably used for the pre-cool cooling, there is no risk of contamination of the cryogen vessel with remnants of a sacrificial cryogen. Relatively little working cryogen is consumed in this process, since boiling of cryogen is used only to cool from one cryogenic temperature to the operating temperature, not for cooling from ambient temperature.

Cooling is achieved either from electrical energy consumed by an active refrigerator, or by boiling of a liquid cryogen, or by the melting of a frozen cryogen, or the heating or phase transition of any cooled cryogen.

Embodiments employing only electrically powered active cooling refrigerators are the most portable.

While described with reference to helium, other cryogen will of course be used as appropriate for the material of the apparatus being cooled.

In embodiments as described with reference to FIG. 2, it should be possible to cool known MRI system magnets at a rate of about 4K per hour, such that the magnet may be cooled from ambient temperature to a temperature of 4K in 74 hours. The efficiency of the heat transfer system in removing heat from the magnet is limited by the mass flow rate of the heat transfer fluid. There are two alternatives for increasing the mass flow rate. Firstly, the density of the fluid may be increased by increasing the pressure of the gas; or the volume flow rate may be increased. In the described embodiment, the pressure of the heat transfer fluid is applied to the interior of the cryogen vessel. Typically, the cryogen vessel can only withstand pressures of about 300 kPa absolute. This limits the pressure which can be applied to the heat transfer fluid. Therefore, if it is necessary to increase the rate of cooling by increasing the mass flow rate through the cryostat, this must be done by increasing the volume flow rate: the velocity of the heat transfer fluid through the pipes 36. This mass flow rate is determined by the compressor 32. A fan may also be provided to assist with providing the required volume flow rate. In certain embodiments, a fan may be provided in place of the compressor. The fluid will circulate at a lower pressure, but its heat capacity will increase as it cools, leading to an effective cooling arrangement.

FIG. 3 schematically illustrates another embodiment of the present invention. In this invention, dual closed-loop cooling circuits are provided. A first closed loop cooling circuit 50 acts to cool the magnet 20 in a manner similar to that described with reference to FIG. 2, except in that the heat extractor is in a heat exchanger 42. A circulator 52 is provided, to ensure a certain volume flow rate of first heat transfer fluid around the circuit. The first heat transfer fluid flows into and out of the cryogen vessel 26, and so should be chosen to be the same as the working cryogen to be employed within the cryogen vessel. Presently, this is most commonly helium. According to this embodiment of the invention, a second closed loop cooling circuit 40 cools the heat exchanger 42 by circulating a second heat transfer fluid through pipes between the heat exchanger 42 and a heat extractor 44. The heat extractor may comprise an active refrigerator such as an electrically powered cryogenic refrigerator, for example one operating according to the Stirling cycle, or a passive heat extraction means such as a tank of liquid cryogen, or a mass of frozen cryogen. In a particular embodiment, illustrated, a tank of cryogen 46 is provided in parallel with a mechanical refrigerator 44, and operation of this arrangement will be discussed in more detail below. The second heat transfer fluid need not be the same as the heat transfer fluid of the first closed loop cooling circuit 50. More particularly, it need not be the same as the working cryogen to be employed in the cryogen vessel.

A particular advantage of the embodiment of FIG. 3 is that the pressure of the second heat transfer fluid employed in the second closed loop cooling circuit 40 is not limited by the pressure holding capabilities of the cryogen vessel 26. In operations the second closed loop cooling circuit 40 may be brought into operation first, cooling the heat exchanger 42, before the magnet itself is available. In certain preferred arrangements, it is considered advantageous to cool the heat exchanger 42 to a temperature of about 20K before commencing operation of the first cooling loop 50. Since such operation of the second loop 50 is not constrained by the pressure limits of the cryogen vessel, an active refrigerator 44 may be operated at its optimum pressure and efficiency. In such a method, when the first cooling loop 50 begins operation to cool the magnet, the first heat transfer fluid is immediately cooled by the heat exchanger 42. This will increase the initial density of a heat transfer fluid flowing to the magnet, increasing its mass flow rate, and will also increase the temperature difference between the magnet 20 and the heat exchanger 42. Each of these effects will increase the initial efficiency of cooling the magnet, thereby enabling effective cooling of the magnet to be completed in a shorter time. The heat exchanger 42 should be designed to have a significant thermal mass, so that when cooling of the magnet begins, the heat exchanger will only warm slowly, keeping the rate of cooling the magnet relatively high and substantially constant.

In this embodiment, as with the embodiment of FIG. 2, the heat extraction may be performed using an active mechanical refrigerator 44. Alternatively, the second closed loop cooling circuit may be arranged to pass in thermal contact with a low temperature thermal mass. For example, the pipes of the second closed loop cooling circuit may be placed in contact with a bath of liquid nitrogen, to provide cooling to approximately 70K. In another embodiment, the pipes of the second closed loop cooling circuit may be placed in contact with a mass of frozen nitrogen, to provide cooling to significantly below 70K. In a more advanced version of such an embodiment, stainless steel pipes carrying the heat transfer fluid may be embedded in a block of aluminum, and the whole structure immersed in a bath or block of sacrificial cryogen. For efficient cooling of the magnet, cooling may begin by circulation of second heat transfer fluid around the second closed loop cooling circuit 40 through a passive refrigeration means, such as a tank of liquid cryogen 46 or a block of solid cryogen. Once the heat exchanger 42 has been cooled to the temperature of the liquid or solid cryogen, the flow of heat transfer fluid may be switched to flow to an active mechanical refrigerator 44 to allow further cooling of the heat exchanger 42 below the temperature of the tank of liquid cryogen 46 or block of solid cryogen. Should the temperature of the heat exchanger 42 rise above the temperature of the tank of liquid cryogen 46 or block of solid cryogen again, for example due to an influx of heat removed from the magnet 20, second heat transfer fluid may once again be allowed to flow through the tank of liquid cryogen 46 or block of solid cryogen, to cool the heat exchanger again.

Of course, the heat exchanger 42, the refrigerator 44, the tank of liquid cryogen 46 and the pipes connecting these components must be effectively thermally isolated to prevent thermal influx from the surroundings. Similar considerations apply to embodiments such as shown in FIG. 2.

While the present invention has been described with reference to a limited number of particular embodiments, those skilled in the art will recognise that various modifications and amendments may be made, within the scope of the invention as defined by the appended claims.

For example, electrically powered refrigerators operating according to the Stirling cycle have been found to be very efficient and powerful in the context of the present invention. (Such refrigerators have been found to be particularly compact, powerful and transportable). However, other types of cryogenic refrigerator are known, and could be employed in the present invention. The present invention has been described with particular reference to helium as the working cryogen. While this is appropriate for conventional low-temperature superconducting magnets, other working cryogens may be employed within the scope of the present invention, according to the nature of the cryogenically cooled apparatus. For example, so-called high temperature superconductors are known, and these may be cooled to a superconducting state by liquid nitrogen.

The heat exchanger discussed with reference to FIG. 3 may also be considered as a thermal battery: “cold” is stored in the heat exchanger, either by provision of a suitably cooled cryogen material or operation of the second closed loop cooling circuit. The stored “cold” is later 'supplied” to the cooled equipment. The heat exchanger may be constructed of any appropriate material. The material chosen should have a high thermal diffusivity and thermal capacity at the required temperature of operation. Suitable materials need to be chosen for the heat exchanger according to the intended temperature of operation. For operation at a temperature of 20K, frozen nitrogen has been found to be appropriate. For operation at a temperature of 80K, water ice has been found effective. Both of these materials are abundant, inexpensive and non-polluting.

Certain aspects of the present invention provide certain particular advantages. By using a frozen block of cryogen as the secondary cooling source, or the heat exchanger, cooling may be achieved to temperatures below the boiling point of the cryogen used. For example, nitrogen may be economically used as the cooling cryogen. Without further cooling, liquid nitrogen will cool to about 70K, by boiling at that steady temperature. By initially cooling the cryogen, cooling to temperatures such as 20K may be effected, which in turn substantially reduces the amount of working cryogen needed to cool the magnet or other equipment to its operating temperature. For example, using helium as an example working cryogen, the use of boiling nitrogen for cooling the heat transfer fluid will require cooling from about 80K to about 4K by consumption of liquid helium, whereas if the magnet or other equipment can be cooled to 20K, a much reduced quantity of liquid helium will be required to cool from 20K to 4K.

Since the second cooling loop is not exposed to the interior of the cryogen vessel, the pressure of the second heat transfer fluid is not constrained by the maximum pressure which the cryogen vessel can withstand. For example, a typical cryogen vessel may have a maximum pressure capacity of 300 kPa absolute. The second closed-loop cooling circuit may contain a gaseous cryogen pressurised significantly in excess of the pressure of the heat transfer fluid of the first closed-loop cooling circuit. This increase of pressure significantly increases the heat transfer capacity of the fluid, by increasing its density. Accordingly, the heat transfer capacity of the second cooling loop may be improved beyond that of the first closed-loop cooling circuit, increasing the rate of cooling of the heat exchanger 42, and so also the rate of cooling of the magnet or other equipment.

Claims

1. Apparatus for pre-cooling cryogenically cooled apparatus housed within a cryogen vessel, comprising a first closed-loop cooling circuit containing heat transfer fluid, a circulator for causing the heat transfer fluid to circulate around the closed circuit and a heat extractor arranged to extract heat from the heat transfer fluid, wherein the circuit carries heat transfer fluid into and from an interior volume of the cryogen vessel.

2. Apparatus according to claim 11 wherein the circulator comprises a compressor which acts to compresses a gaseous heat transfer fluid to a pressure in the range of 100-300 kPa absolute.

3. Apparatus according to claim 11, wherein the heat extractor is an external mechanical active cryogenic refrigerator.

4. Apparatus according to claim 1, wherein the heat extractor is a passive cryogenic refrigerator comprising a reserve of a cryogen in thermal contact with the closed-loop cooling circuit.

5. Apparatus according to claim 4 wherein the reserve of cryogen comprises a quantity of solid cryogen.

6. Apparatus according to claim 4, wherein the reserve of cryogen provides cooling to a temperature of below 70K.

7. Apparatus according to claim 1, wherein the heat extractor comprises both an active cryogenic refrigerator and a passive cryogenic refrigerators arranged such that passive cooling may be applied to the heat transfer fluid until the cryogenically cooled apparatus has cooled to a first temperature, with further cooling being provided by switching the flow of heat transfer fluid to the active refrigerator, to continue with cooling down to a desired temperature, below that obtainable from the passive refrigerator alone.

8. Apparatus according to claim 1, wherein the circulator comprises a fan.

9. Apparatus according to claim 1, wherein the heat extractor which extracts heat from the heat transfer fluid is a heat exchanger, itself cooled by a second closed-loop cooling circuit containing a second heat transfer fluid, a second circulator for causing the second heat transfer fluid to circulate around the second closed-loop cooling circuit and a second heat extractor arranged to extract heat from the second heat transfer fluid.

10. Apparatus according to claim 9 wherein the first and second heat transfer fluids are both gases, and the second heat transfer fluid in the second closed-loop cooling circuit is at a higher pressure than the pressure of the heat transfer fluid in the first closed-loop cooling circuit.

11. Apparatus according to claim 9, wherein the second heat transfer fluid in the second closed-loop cooling circuit is of a different material than the heat transfer fluid in the first closed-loop cooling circuit.

12. Apparatus according to claim 9, wherein the second heat extractor is an external mechanical active cryogenic refrigerator.

13. Apparatus according to claim 9, wherein the second heat extractor is a passive cryogenic refrigerator comprising a reserve of a cryogen in thermal contact with a pipe carrying the second heat transfer fluid around the second closed-loop cooling circuit.

14. Apparatus according to claim 13 wherein the reserve of cryogen comprises a quantity of solid cryogen.

15. Apparatus according to claim 9, wherein the second heat extractor comprises both an active cryogenic refrigerator and a passive cryogenic refrigerators arranged such that passive cooling may be applied to the second heat transfer fluid until the cryogenically cooled apparatus has cooled to a first temperature, with further cooling being provided by switching the flow of second heat transfer fluid to the active refrigerator, to continue with cooling down to a desired temperature, below the temperature obtainable using the passive refrigerator alone.

16. Apparatus according to claim 13, wherein the reserve of cryogen provides cooling to a temperature of below 70K.

17. Apparatus according to claim 9, wherein the second circulator comprises a fan.

18. Apparatus according to claim 9, wherein the heat exchanger comprises a volume of liquid or solid nitrogen; or water ice.

19. A method for pre-cooling a cryogenically cooled apparatus within a cryogen vessel, comprising circulating a heat transfer fluid through a first closed-loop cooling circuit by operation of a circulator causing the heat transfer fluid to circulate around the first closed-loop cooling circuit and extracting heat from the heat transfer fluid by use of a heat extractor in thermal connection with the first closed-loop cooling circuit, wherein the heat transfer fluid flows into and from an interior volume of the cryogen vessel.

20. A method according to claim 19 wherein the circulator comprises a compressor compressing gaseous heat transfer fluid to a certain pressure in the range of 100-300 kPa absolute.

21. A method according to claim 15, wherein heat extractor comprises an external mechanical active cryogenic refrigerator.

22. A method according to claim 19, wherein heat extractor comprises a passive cryogenic refrigerator, being a reserve of cryogen in thermal contact with the first closed-loop cooling circuit.

23. A method according to claim 19, wherein heat extraction is initially performed by passive cooling employing a reserve of cryogen until the cryogenically cooled equipment has cooled to a first temperature, being no lower than the temperature of the sacrificial cryogen, with further heat extraction then being performed by an active refrigerator, to continue with cooling down to a desired pre-cool temperature.

24. A method according to claim 19, wherein the heat extractor is a heat exchangers and the heat exchanger is itself cooled by a second closed-loop cooling circuit which cools the heat exchanger by circulating a second heat transfer fluid by operation of a circulator causing the second heat transfer fluid to circulate around the second closed-loop cooling circuit and extracting heat from the second heat transfer fluid by use of a second heat extractor in thermal connection with the second closed-loop cooling circuit.

25. A method according to claim 24 wherein the second closed loop cooling circuit is brought into operation to cool the heat exchanger before the first closed loop cooling circuit is brought into operation.

26. A method according to claim 24, wherein the second heat extractor is an external mechanical active cryogenic refrigerator.

27. A method according to claim 24, wherein the second heat extractor comprises a passive cryogenic refrigerator, being a reserve of cryogen in thermal contact with the first closed-loop cooling circuit.

28. A method according to claim 24, wherein heat extraction by the second closed-loop cooling circuit is initially performed by passive cooling employing a reserve of cryogen until the cryogenically cooled equipment has cooled to a first temperature, being no lower than the temperature of the reserve of cryogen, with further heat extraction then being performed by an active refrigerator, to continue with cooling down to a desired pre-cool temperature, below the temperature obtainable by use of the reserve of cryogen alone.

29. A method according to claim 24, wherein the second circulator comprises a fan.

30. A method according to claim 24, wherein the first and second heat transfer fluids are both gases, and the second heat transfer fluid in the second closed-loop cooling circuit is at a higher pressure than the pressure of the heat transfer fluid in the first closed-loop cooling circuit.

31. A method according to claim 24, wherein the second heat transfer fluid in the second closed-loop cooling circuit is of a different material than the heat transfer fluid in the first closed-loop cooling circuit.

32. A method according to claim 24, wherein heat extraction from the second closed-loop cooling circuit is performed by an external mechanical active cryogenic refrigerator.

33. A method according to claim 24, wherein heat extraction from the second closed-loop cooling circuit is performed by a passive cryogenic refrigerator comprising a reserve of cryogen in thermal contact with the second closed-loop cooling circuit.

34. A method according to claim 24, wherein heat extraction from the second closed-loop cooling circuit is performed by both an active cryogenic refrigerator and a passive cryogenic refrigerator, arranged such that passive cooling is applied to the second heat transfer fluid until the cryogenically cooled apparatus has cooled to a first temperature, with further cooling being provided by switching the flow of second heat transfer fluid to an active refrigerator, to continue with cooling down to a desired pre-cool temperature.

35. A method according to claim 24, wherein the second circulator comprises a fan.

36. A method according to claim 24, wherein the heat exchanger is formed of a volume of liquid or solid nitrogen; or water ice.

37. A method according to claim 24, wherein the heat exchanger is cooled to a certain cryogenic temperature before operation of the first closed-loop cooling circuit.

38. Apparatus or a method according to claim 1, wherein in the first closed-loop cooling circuit, the circulator acts on cooled first heat transfer fluid from the heat extractor on its way to the apparatus to be cooled.

39. Apparatus or a method according to claim 1, wherein in the second closed-loop cooling circuit, the circulator acts on cooled first heat transfer fluid from the second heat extractor on its way to the heat exchanger.