GB2528919A

GB2528919A - Superconducting magnet assembly

Info

Publication number: GB2528919A
Application number: GB1413814.3A
Authority: GB
Inventors: Eugene Astra; Adam Paul Johnstone; Anders Mortensen
Original assignee: Siemens PLC
Current assignee: Siemens PLC
Priority date: 2014-08-05
Filing date: 2014-08-05
Publication date: 2016-02-10
Also published as: GB201413814D0

Abstract

A superconducting magnet operating method comprises: providing a cryogen vessel housing 12 at least one superconducting coil 10; providing a cryogenic cooling arrangement for the said coil(s) 10 including providing a cooling medium 14 within the cryogen vessel 12; sealing the cryogen vessel 12 prior to the operation of the magnet; and quenching the superconducting magnet when it is in operation. Alternatively, the cooling medium 14 is heated to a temperature within the range of 0Â°C to 40Â°C instead of the quenching process. Also disclosed is a superconducting magnet assembly comprising: a cryogen vessel 12 containing at least one superconducting coil 10 and a cooling medium 14; a cryogenic cooling arrangement characterised in that the cryogen vessel 12 is provided with a pressure-limiting safety device for venting the cooling medium 14 when the absolute pressure of the medium reaches a value in excess of 40 bar but does not vent when the pressure remains below 30 bar. Alternatively, the pressure-limiting safety device is arranged to vent when the absolute pressure is in excess of 100 bar but does not vent when the pressure remains below 100 bar. The superconducting magnet may be used in an MRI system.

Description

SUPERCONDUCTING MAGNET ASSEMBLY

Superconducting magnets are known in the field of MRS systems, providing a high-strength homogeneous background

magnetic field.

Superconducting magnets must be maintained at a cryogenic temperature below the transition temperature of the superconductive material involved. Such temperatures have conventionally been achieved by provision of a cryogen cooling medium at its boiling point.

The cooling medium should be chosen to suit the superconductor material involved. For example, for NbTi and Nb3Sn superconductor, helium would be used; for Mg52 conductor, helium, hydrogen, or neon could be used; for HTS a number of cryogens are suitable.

The most common existing superconducting magnets for MRI systems are cooled by partial immersion in a bath of liguid helium. A current superconducting magnet assembly for use in an MRI system, is schematically illustrated in Fig 1.

Cylindrical superconducting magnet assembly 5 comprises one or more superconducting magnet coils 10 housed within a cryogen vessel 12. The cryogen vessel 12 typically has a service turret (not shown) near the top, to allow for filling with cryogen, and venting of cryogen vapour as necessary. All outer vacuum chamber (OVC) provides thermal insulation of the cryogen vessel from ambient temperature. Liguid helium 14 is provided, in contact with the superconducting coils 10. A guench pipe and a cryogenic refrigerator are typically provided, but are not shown, for clarity.

When a superconducting magnet assembly such as illustrated is assembled, the cryogen vessel 10 is filled with liquid helium 14 through the service turret to an upper level 16. As the superconducting magnet assembly is transported to a user site, heat influx reaches the cryogen vessel 12 by thermal radiation and conduction. The heat influx causes boiling of helium 14, which keeps the superconducting coils 10 at operating temperature. During transport, it is typical that no cryogenic refrigeration is provided. Boiled-off helium gas 20 is allowed to escape through vent tube 22.

Typically, a pressure-limiting guench valve is provided, which opens when the pressure within the helium vessel reaches a certain limiting value, typically about 1.2 bar absolute ("bara") By allowing boiled-off cryogen 20 to escape, the superconducting coils 10 are kept at operating temperature, but the liguid helium 14 is typically lost to atmosphere.

The cryogen vessel typically contains several hundred litres of liquid cryogen 14 which is vented 22, 20 during transport, but also during fault conditions.

In operation, a cryogenic refrigerator is used, which removes heat from the cooling medium, which in turn cools the superconducting magnet coils. Should the cryogenic refrigerator cease to operate, the heat influx to the cryogen vessel and any heat generated by the superconducting coils and ancillary equipment within the cryogen vessel will cause liguid helium 14 to boil, increasing the pressure within the cryogen vessel 12 until a pressure valve opens and allows helium to vent through the vent tube 22 to atmosphere. Tn operation, should one part of a superconducting coil reach a temperature in excess of its critical temperature, a quench will ensue, meaning that energy stored in the magnetic field of the magnet will be released as heat into the helium 14.

This typically causes egress of a large mass of helium through the vent tube 22 or a guench tube provided for the purpose, but not illustrated in Fig. 1.

Due to the increasing expense and scarcity of helium, it is desired to reduce the quantities of helium reguired for cooling superconducting magnets in MRI systems and the quantities of helium which are released during transport and fault conditions such as quench.

The present inventors have found that superconduceing magnet coils can be effectively cooled when placed above a bath of liquid cryogen, in an atmosphere of cryogen vapour and out of contact with the liquid cryogen. Cooling of the superconducting magnet coils may then be provided by heat transfer into a circulating cryogen vapour, which is in turn cooled by the cryogenic refrigerator.

Certain aspects of cooling superconducting magnet coils by circulating cryogen vapour were discussed U57832216.

Low helium mass cooling arrangements are discussed in E91522867.

A warm gas buffer arrangement is discussed in US5979176.

"Room temperature" is not a clearly defined value, but for present purposes, it will be assumed to be within the range of 0°C to 40°C.

The present invention accordingly aims to provide apparatus and methods which provide effective cooling of superconducting coils with minimal consumption of cryogen.

Accordingly, the present invention provides apparatus and methods as defined 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 thereof, taken in conjunction with the accompanying drawings, wherein: Fig. 1 schematically illustrates a conventional arrangement for cooling of superconducting coils in a superconducting magnet assembly for use in an MRS system; and Fig. 2 schematically illustrates an arrangement for ccoling of superconducting coils in a superconducting magnet assembly for use in an MRI system, according to an aspect of the present invention.

The present invention provides a sealed cryogen vessel enclosing at least one superconducting coil and a mass of a cooling medium, such as helium. A schematic of this is shown in Fig. 2. The mass of cooling medium 14 and the mechanical properties of the cryogen vessel 12 are arranged such that no venting of the cooling medium is reguired in case of quench. In preferred embodiments, no venting of cooling medium is required, even in case of a failure of the cryogenic refrigerator which allows the superconducting magnet coil(s) and the cooling medium to warm up to room temperature. The mass of cooling medium must be sufficient to provide adequate heat transfer between the cryogenic refrigerator and the superconducting coil (s) Preferably, the cryogen vessel 12 is provided with the mass of cooling medium 14 and is sealed at the point of manufacture. The cryogen vessel 12, enclosing cooling medium 14 and one or more superconducting coil 10 may then remain sealed during its lifetime.

In such an arrangement, the cooling medium 14 must be contained by the cryogen vessel 12 during fault conditions.

During fault conditions such as quench, heat is dissipated in the cooling medium which experiences a phase transition and warming.

Such an arrangement must combine characteristics such as: free volume of the cryogen vessel; mechanical strength of the cryogen vessel; mass of cooling medium; thermal properties of the cooling medium. The mechanical strength of the cryogen vessel must be sufficient to withstand pressures expected during a quench; or respectively during a warming to room temperature without venting of cooling medium. Trade-offs must be had between mass of cooling medium and vessel volume, to reduce maximum cryogen vessel pressures to a tolerable level but without increasing the thickness and mass of the cryogen vessel to unacceptable proportions.

A sufficient guantity of cooling medium 14 must be provided that the superconducting coils 10 can be effectively cooled by boiling of liquid cryogen at times that the cryogenic refrigerator is not operational. This may be due, for example, to failures of an electrical supply or supply of cooling water to a compressor supplying the cryogenic refrigerator with compressed helium. Such requirement suggests a relatively large mass of cooling medium should be used. A certain mass of cooling material 14 is also required to ensure adequate cooling of the superconducting coils 10 when subjected to heat loads, for example during energisation and operation of the magnet.

Such heat loads or fault conditions cause expansion of the cooling medium and an increase in pressure within the cryogen vessel. The cryogen vessel must be mechanically robust enough to withstand such pressures. Such requirement suggests that a relatively small mass of cooling medium should be used.

The present invention provides a "sealed" cryogen vessel 12 containing a relatively small mass of a cooling medium 14 inside a cryogen vessel 12 enclosing a superconducting coil structure 10. Tn certain embodiments, as illustrated in Fig. 2, the mass of cooling medium is so small that, in normal operation, the superconducting coils 10 remain above the surface of the cooling medium. The geometry of the cryogen vessel may be adapted to ensure that the superconducting coils do not touch liquid cooling medium, for example by providing a well on the bottom of the cryogen vessel to accommodate liquid cooling medium.

Cooling of the superconducting magnet coils 10 is provided by heat transfer between the superconducting magnet coils and the vapour of the cooling medium 14. The vapour transports heat away from the superconducting magnet coils 10 to a cooling arrangement such as a cryogenic refrigerator or a cooling loop (not shown, for clarity) Preferably, the cooling arrangement is positioned such that a convection current of vapour of cooling medium 14 is set up.

Such convection current will assist transfer of heat from superconducting coils 10 to the vapour of the cooling medium and thence to the cooling arrangement. Tn the case of helium, the thermal coefficient of expansion is relatively large at temperatures of around 4K, as commonly used in cooling of superconducting magnet coils. Simply by placing a cryogenic refrigerator off-centre with respect to the cryogen vessel may be sufficient to set up an effective convection current, when in use. The relatively small mass of cooling medium provided within the cryogen vessel ensures that a maximum pressure within the cryogen vessel remains tolerable, and within the mechanical capability of the cryogen vessel, in case of a quench; or respectively in case of heating to room temperature.

Preparation of the cryogenically cooled superconducting magnet structure of the invention may proceed according to any of the following possible scenarios, among others.

Cooling medium 14 is introduced into the cryogen vessel in gaseous form at room temperature at relatively high pressure.

For example, the charging pressure may lie between 10 bara to lOObara.

The cryogen vessel is then sealed. This may be achieved by introducing the gaseous cryogen through a valve which is held closed once charging is complete. Alternatively, or in addition, a seal may be applied, for example a metal seal may be welded to the cryogen vessel over a charging port used for charging the cryogen.

Alternatively, the cooling medium may be introduced into the cryogen vessel in gaseous form at an intermediate cryogenic temperature. This may be achieved by using an external cooling apparatus to cool tho cooling medium before it is introduced into the cryogen vessel. The charging pressure may then lie between lbara to lOObara, for example.

The cryogen vessel is then sealed. As above, this may be achieved by introducing the gaseous cryogen through a valve which is held closed once charging is complete.

Alternatively, or in addition, a seal may be applied, for example a metal seal may be welded to the cryogen vessel over a charging port used for charging the cryogen.

In another alternative, the cryogen vessel 12 may be charged with liguid cooling medium 14. This may serve to cool the superconducting coil structure 10 and the cryogen vessel 12.

Alternatively, the superconducting coil structure 10 and cryogen vessel 12 may be pre-cooled to a temperature of approximately the boiling point of the cooling medium before the cooling medium is added. This reduces the need for cooling after the cooling medium is added, and means that the cryogen vessel may be charged at a much lower pressure. For example, the cryogen vessel may be charged at a pressure of approximately lbara.

The cryogen, and therefore also the cryogen vessel and the superconducting magnet coils within the cryogen vessel, are then cooled. This may be achieved using a second cooling medium disposed within a separate cooling loop. The second cooling medium may in fact be a separate volume of the same cooling medium, but sealed separately from the cooling medium introduced into the cryogen vessel. The cooling loop may penetrate the cryogen vessel to be supported within the interior volume of the cryogen vessel 12. The cooling loop may contact the superconducting magnet coils, or their support structure. Alternatively, the cooling loop may be thermally linked to an external surface of the cryogen vessel 12 to cool the cryogen vessel, the cooling medium and the superconducting magnet coils through the material of the cryogen vessel. Alternatively, a cold head (cryogenic refrigerator) built in to or on to the cryogen vessel may be used to cool the cooling medium, the cryogen vessel and the superconducting magnet coils. For example, the cold head may act to cool a heat exchanger which is exposed to the interior of the cryogen vessel, or may cool a heat exchanger which is thermally linked to an exterior surface of the cryogen vessel, cooling of the cooling medium taking place through the material of the cryogen vessel.

Preferably, the cooling arrangement comprises a cryogenically cooled heat exchanger exposed to the cooling medium in the cryogen vessel. The heat exchanger is preferably cryogenically cooled by a mechanical refrigerator in thermal and mechanical contact with the heat exchanger.

The cooling medium may then be maintained in liguid form, or at least at its boiling point, by cooling using the cold head or other cooling arrangement, as appropriate.

In operation, a cryogen vessel experiences a range of operating pressures. These may vary between sub-atmospheric, above-atmospheric and high pressures encountered during fault events and failure modes such as guench.

During sub-atmospheric pressure within the cryogen vessel, minimal risk of ice contamination occurs where the cryogen vessel is sealed with a permanent seal. For at least this reason, it is preferred that the cryogen vessel is not sealed only by action of its charging valve. Sub atmospheric operation is achieved by operating the coldhead so that a hcat balance is achieved at temperatures that correspond to a sub atmospheric cryogen vessel pressure. The operating point of the magnet can either be within the saturation curve of the cooling medium or outside of the saturation curve whereby cooling is only provided by gas. Within the saturation curve, the cold head may operate in recondensing mode, cooling gaseous cryogen and producing drops of liquid cryogen on the heat exchanger. In a "gas only" configuration, a heat exchanger mounted on the coldhead with a high surface area may be required to effectively cool the gaseous cryogen.

Certain known failure modes of superconducting magnets must be provided for in a superconducting magnet assembly of the present invention. Tolerance mechanisms for various failure modes in the case of superconducting magnet assemblies according to the present invention are discussed below.

In the presence of a gas-liquid two-phase cooling medium, typical pressure rise before a quench occurs is -1bar.

When the magnet quenches, a typical pressure rise is between bar and 3Obar.

when the magnet coils and cooling medium warms to room temperature, a typical pressure rise is lOObar.

According to certain embodiments of the present invention, the cryogen vessel is sealed against pressures up to at least a maximum pressure that could be expected to be reached during a quench event, for example 40 bara. By preventing egress of cooling medium, energy transferred from the superconducting coils during a quench is contained within the cryogen vessel. Some of the energy is absorbed by heat capacities of the superconducting coils, their associated support structures, and the cryogen vessel itself. This reduces the amount of energy which must be absorbed by the cooling medium, and helps to limit the maximum cryogen vessel pressure on case of a guench. Taking into account the savings in consumption of cooling mcdium, thc superconducting magnet assembly of the present invention may be cheaper and less dependent on supply of a volatile commodity than

equivalent assemblies of the prior art.

Preferably, the cryogen vessel is sealed against pressures up to at least a maximum pressure that could be expected to be reached during warming to room temperature, for example lOObara. The cryogen vessel can be designed to withstand pressures seen during all failure modes. However, the magnet becomes costly and heavy if design pressures significantly in excess of lOObara are provided for.

Should the magnet warm to temperatures significanrly greater than room temperature, for example in the rare event of a fire, the pressure within the cryogen vessel may significantly exceed lOObara. It may be found appropriate, however, not to construct the cryogen vessel to withstand pressures reached during a fire, as the rarity of such events may not warrant to considerable added weight and material costs required to ensure that a cryogen vessel remains sealed even at fire temperatures. The pressure within the cryogen vessel may preferably be limited to a value somewhat above the pressure experienced when the magnet coils and cooling medium warms to room temperature by safety devices such as a limiting valve or burst disc. A burst disc may be preferred, as it is less likely to leak in normal use.

In other embodiments, a limiting valve or burst disc may be provided to allow egress of cooling medium if the pressure within the cryogen vessel significantly exceeds a maximum

II

pressure that could be expected to be reached during a quench event, for example 30 bara.

The present invention is believed to be applicable to superconducting magnet assemblies of a range of sizes and field strengths, at least in the range of 0.1T to 11.7T.

In particular, the present invention provides a cooling method by which a superconducting magnet for MRI is cooled by heat transfer to vapour of a cooling medium, the cooling medium and the superconducting magnet being sealed from atmosphere within a cryogen vessel, that cryogen vessel remaining sealed from atmosphere through a range of pressures, from a sub-atmospheric pressure to a pressure in excess of that which could be expected in the case that the superconducting magnet coils were to quench from their normal operating state. Preferably, the cryogen vessel remains sealed in case the superconducting coil(s) and the cooling medium warm to room temperature.

The main advantages of this invention are that the coils can be effectively cooled whilst consuming reduced quantities of cooling medium, which is sealed within the cryogen vessel.

This brings a number of advantages including the elimination of the requirement for providing cooling medium at the installation and operational site of an MRI system. A reduced quantity will be required per MRI system produced, which will in turn lead to a reduced and more stable price for the end-user.

Preferably, the cryogen vessel is charged with cooling medium, for example according to any of the methods described above, and no more cooling medium need be supplied for the lifetime of the superconducting magnet. If the superconducting magnet assembly undergoes a quench, or is allowed to warm to room temperature, no cooling medium escapes from the cryogen vessel. At the end of the useful life of the superconduoting magnet assembly, the cooling medium may be retrieved for re-use, or re-sale.

Although the superconducting magnet assembly of the present invention may be arranged such that the cooling medium does not escape even in the case that the cryogen vessel is engulfed in fire, the inventors presently regard it as preferable to provide over-pressure venting to allow cooling medium to escape from the cryogen vessel when it reaches a pressure eguivalent to heating to a temperature significantly in excess of room temperature (for example, to 100°C or more) . This pressure may be at least 100 bara. Provision of such emergency venting is regarded as a trade-off between the undesirable loss of cooling medium at such times, against the undesirable increase in cost, material consumption and weight whioh would be involved in sealing the oryogen vessel against egress of cooling medium in such events.

In its liguid state, the cooling medium preferably fills no more than 10%, and preferably no more than 5% of the volume of the cryogen vessel. In a cryogen vessel of 2000 litres free volume, superconducting coil cooling may be aohieved with 150 litres of liquid helium.

The present invention accordingly provides a superconducting magnet assembly for use in an MRI system, comprising at least one superconducting coil 10 nounted within a crycgen vessel 12, a cooling medium 14 provided within the cryogen vessel, and a cryogenic cooling arrangement provided for cooling the cooling medium, the cryogen vessel being constructed to withstand a pressure of cooling medium corresponding to a guench event in the superconducting coil(s), the mass of cooling medium being sufficient to ensure transfer of heat from the superconducting magnet coils to the cryogenic cooling arrangement when the superconducting coils are in use.

Preferably, the cryogen vessel is constructed to withstand a pressure corresponding to superconducting coil temperature of 40°C without egress of cooling medium.

The present invention also provides a method for operating a superconducting magnet assembly for use in an MRT system, comprising: -providing a cryogen vessel housing at least one superconducting coil therein; -providing a cryogenic cooling arrangement for cooling the superconducting coil (s) -providing a mass of cooling medium inside the cryogen vessel, said mass being sufficient to transfer heat from the superconducting coil(s) to the cooling arrangement to maintain the superconducting coil(s) in a superconducting state, in operation; -sealing the cryogen vessel prior to operation, -introducing current into the superconducting coil(s) to an operating condition; -guenching the superconducting coil(s), releasing stored energy to the cooling medium, such that the cryogen vessel remains sealed.

Preferably, the cryogen vessel remains sealed in case the cooling medium reaches a temperature of 40°C.

In current MRI systems of the type considered here, helium is typically used as the cooling medium, with an inventory of about 150 litres.

When heated to room temperature, the helium cooling medium boils to produce a pressure of about lOObara within the cryogen vessel.

During a quench event, the pressure reached depends on the size of the magnet concerned and the amount of energy which had been stored in it. As examples, for a current 1.5T magnet, quench pressure from a low stored energy state may reach 6 bara; from a standard stored energy state, l4bara; and from a high stored energy state, l7bara. In contrast, a 31 magnet which guenches from a standard stored energy state may generate a quench pressure of 3obara.

Warming of the magnet to room temperature Pressure at room temperature typically causes a cryogen vessel pressure of lOObara.

According to the present invention, the cryogen vessel (12) and connecting parts that contain cooling medium, such as the service turret, are designed such that they are mechanically robust enough to withstand the pressure generated in the case of a quench. Such designs incorporate, but are not constrained to, mechanical bracing, thickening of components, and use of high strength materials. The magnitude of these pressures being determined by the stored energy of the magnet and the act of sealing the entire mass of the cooling medium within the cryogen vessel. Quench pressures typically range from Sbara to 3cbara and do not exceed 4obara.

Claims

CLAIMS: 1. A method for operating a superconducting magnet assembly for use in an MRI system, comprising: -providing a cryogen vessel (12) housing at least one superconducting coil (10) -providing a cryogenic cooling arrangement for cooling the superconducting coil(s); -providing a mass of cooling medium (14) inside the cryogen vessel, said mass being sufficient to transfer heat from the superconducting coil(s) to the cooling arrangement to maintain the superconducting coil(s) in a superconducting state, in operation; -sealing the cryogen vessel prior to operation, -introducing current into the superconducting coil(s) to an operating condition; and -quenching the superconducting coil(s), releasing stored energy to the cooling medium, such that the cryogen vessel remains sealed and gaseous cryogen within the cryogen vessel reaches a guench pressure.
2. A method according to claim 1, wherein the cryogen vessel remains sealed when the pressure within the cryogen vessel remains below the quench pressure, and provides venting of the cooling medium when a pressure within the cryogen vessel reaches a value in excess of the guench pressure.
3. A method according to claim 1 or claim 2, wherein the guench pressure is a pressure in excess of 5 bara, but inferior to 40 bara.
4. A method for operating a superconducting magnet assembly for use in an MRI system, comprising: -providing a cryogen vessel (12) housing at least one superconducting coil (10) -providing a cryogenic cooling arrangement for cooling the superconducting coil (s) -providing a mass of cooling medium (14) inside the cryogen vessel, said mass being sufficient to transfer heat from the superconducting coil(s) to the cooling arrangement to maintain the superconducting coil(s) in a superconducting state, in operation; -sealing the cryogen vessel prior to operation, -heating the cooling medium to a temperature within the range of 0°C to 40°C, such that the cryogen vessel remains sealed.
5. A method according to claim 4, wherein the cryogen vessel remains sealed when the pressure within the cryogen vessel remains below 100 bara, and provides venting of the cooling medium when a pressure within the cryogen vessel reaches a value in excess of 100 bara.
6. A method according to any preceding claim, wherein the step of providing a mass of cooling medium (14) inside the cryogen vessel comprises charging the cryogen vessel (12) with liguid cooling medium (14)
7. A method according to any preceding claim, wherein the cooling arrangement comprises a cryogenically cooled heat exchanger exposed to the cooling medium in the cryogen vessel.
8. A method according to claim 7 wherein the heat exchanger is cryogenically cooled by a mechanical refrigerator in thermal and mechanical contact with the heat exchanger.
9. A superconducting magnet assembly for use in an MRI system, comprising: -a cryogen vessel (12), containing: -at least one superconducting coil (10); and -a cooling medium (14); -a cryogenic cooling arrangement for cooling the cooling medium, characterised in that the cryogen vessel is provided with a pressure-limiting safety device for venting of the cooling medium when a pressure within the cryogen vessel reaches a value in excess of 40 bara, and will provide no venting of the cooling medium when the pressure within the cryogen vessel remains below 30 bara.
10. A superconducting magnet assembly for use in an MRI system, comprising: -a cryogen vessel (12), containing: -at least one superconducting coil (10); and -a cooling medium (14); -a cryogenic cooling arrangement for cooling the cooling medium, characterised in that the cryogen vessel is provided with a pressllre-limiting safety device for venting of the cooling medium when a pressure within the cryogen vessel reaches a value in excess of 100 bara, but will provide no venting of the cooling medium when the pressure within the cryogen vessel remains below 100 bara.Amendments to the claims have been made as follows: CLAIMS: 1. A method for operating a superconducting magnet assembly for use in an MRI system, comprising: -providing a cryogen vessel (12) housing at least one superconducting coil (10) -providing a cryogenic cooling arrangement for cooling the superconducting coil (s) -providing a mass of cooling medium (14) inside the cryogen vessel, said mass occupying no more than 10% of the volume of the cryogen vessel in a liquid state, in operation, said cooling medium being operative to transfer heat from the superconducting coil(s) to the cooling arrangement to maintain the superconducting coil(s) in a superconducting U) 15 state; -sealing the cryogen vessel prior to operation, -introducing current into the superconducting coil(s) to an 0 operating condition; and 0 -quenching the superconducting coil(s), releasing stored r 20 energy to the cooling medium, such that the cryogen vessel remains sealed and gaseous cryogen within the cryogen vessel reaches a quench pressure.2. A method according to claim 1 wherein the mass of cooling medium occupies no more than 5% of the volume of the cryogen vessel in a liquid state in operation.3. A method according to claim 1 or claim 2, wherein the cryogen vessel remains sealed when the pressure within the cryogen vessel remains below the quench pressure, and provides venting of the cooling medium when a pressure within the cryogen vessel reaches a value in excess of the quench pressure.4. A method according to any preceding claim, wherein the quench pressure is a pressure in excess of 5 bara, but inferior to 40 bara.5. A method according to any preceding claim, wherein the step of providing a mass of cooling medium (14) inside the cryogen vessel comprises charging the cryogen vessel (12) with liquid cooling medium (14) 6. A method according to any preceding claim, wherein the cooling arrangement comprises a cryogenically cooled heat exchanger exposed to the cooling medium in the cryogen vessel.IC) 15 7. A method according to claim 9 wherein the heat exchanger is cryogenically cooled by a mechanical refrigerator in thermal and mechanical contact with the heat exchanger. r