GB2510410A

GB2510410A - Quench pressure reduction for superconducting magnet by reducing heat flux from coil to cryogen

Info

Publication number: GB2510410A
Application number: GB1301909.6A
Authority: GB
Inventors: Eugene Astra; Matthew John Longfield
Original assignee: Siemens PLC
Current assignee: Siemens PLC
Priority date: 2013-02-04
Filing date: 2013-02-04
Publication date: 2014-08-06
Anticipated expiration: 2033-02-04
Also published as: GB2510410B; CN105103247B; JP2016507159A; KR101874039B1; US10365337B2; US20150377991A1; EP3252784B1; EP3252784A1; EP2951844B1; WO2014118390A2; KR20150110785A; GB201301909D0; EP2951844A2; JP6429800B2; CN105103247A; WO2014118390A3

Abstract

A method for reducing gas pressure within a cryogen vessel housing a superconducting magnet during quenching comprises deliberately heating a superconducting coil to cause film boiling of cryogen in contact with a surface of the superconducting coil. The film boiling may reduce the rate of thermal transfer to the liquid cryogen below the rate without heating by reducing the contact surface area between the coil and the liquid cryogen. The heating may be performed by secondary coils of resistive wire inductively coupled and mechanically attached to the superconducting coil. The secondary coils may form closed electrical circuits whereby, in case of a quench, electrical current is induced in the secondary coils and heats the coil surface to cause film boiling. The secondary coil may be provided as an overbinding. Alternatively, the heating may be performed by a quench propagation circuit including at least one quench heater attached to the superconducting coils.

Description

QUENCH PRESSURE REDUCTION FOR SUPERCONDUCTING MAGNET

In a superconducting magnet assembly for MRI applications, a liguid cryogen such as helium is often provided within a cryogen vessel which also contains superconducting coils supported on a mechanical retaining structure such as a former. During a quench event, as is well known, energy stored in the superconducting coils is dissipated as heat causing boiling of the cryogen. The boiling of the cryogen increases the pressure within the cryogen vessel, known as quench pressure, until a pressure-limiting device such as a valve or a burst disc opens to provide a gas egress path at a certain quench pressure.

The standard approach for magnet design is to minimise the increase in coil temperature during a quench, and to design a large supporting former which mechanically supports and retains the coils, and also acts as a heat sink for the magnet coils. During a quench event, heat from the coils is conducted to the former, which is typically of aluminium or stainless steel. This limits the rise in the temperature of the surface in contact with the coils.

Conventional superconducting magnets are cooled down to about 4K using liquid helium to induce a superconducting state. The magnet coils are ramped to a specified electric current, which has an associated stored energy. When a superconducting magnet undergoes a transition from the superconducting state to the normal / resistive state, as in a quench, any stored electrical current is transferred from the superconductor filaments into the copper cladding typically provided around superconducting filaments. An amount of heat is generated by Ohmic heating of the magnet coils. The heat from the magnet coils is then transferred via thermal conduction into the former and the liquid helium, both of which are in thermal and mechanical contact with the coils.

The amount of energy dissipated and the rate at which the stored energy is transferred from the magnet coils into the liquid helium, together with the volume of helium and the geometry of the pressure vessel containing the magnet and helium determine the quench pressure within the helium vessel.

lior example, the design of the helium vessel and the available turret venting path cross-section will influence the fluid impedance experienced by escaping cryogen gas. High quench pressures are undesirable because of the need to increase the pressure vessel wall thickness, and therefore cost and weight, to cope with such pressures and the need to increase the cross-sectional area of the turret to relieve the quench pressure.

Increased turret area will increase its thermal heat load into the helium vessel, which results in the requirement of increasing the cooling power required from an associated cryogenic refrigerator.

Current superconducting magnet designs use the parameters of operating current and number of turns -which determines the energy stored in the magnet coils, guench propagation circuit properties, vent path area, and vessel strength to engineer a solution for managing the quench pressure.

Fig. 2 schematically represents a conventional cylindrical magnet structure with superconducting coils 20 wound onto an aluminium former 22, which acts as a heat sink. A radially outer surface 26 of the coil directly contacts liquid cryogen, and forms the main interface for heat transfer from the coil to the liquid cryogen to cool the coil.

Conventionally, during a quench, radially outer surface 26 is at a temperature T, typically about 80K, while the radially inner surface 28 of the former 22 is at a temperature I, typically about 20K. Heat flows 01, 02 are shown, where 01 represents heat flux from coil 20 to former 22, while Q2 represents heat flux from coil 20 to adjacent cryogen. Some conventional arrangements dispense with the former, thereby providing more effective cooling of the coils due to increased contact surface area between coils and cryogen.

The present invention proposes to provide a superconducting magnet which does not have the conventional former, acting as a heat sink, but to introduce a resistive element into the magnet coil structure that will control the surface temperature of the magnet coil structure, so controlling the rate of heat dissipated into the liquid cryogen. The present invention aims to deliberately increase the temperature of the coil surface and the surface area of the coil in contact with the liquid cryogen, so that the heat transfer from the coil into the liquid helium is reduced. This is achieved by inducing a different boiling phase of the helium at the surface which reduces the quench pressure. This provides reduced quench pressure without the need to increase the vent path cross-sectional area and reduces the need for cryogen vessel strength to resist quench pressures. The present invention provides quench pressures which will be significantly reduced as compared to conventional structures, and therefore the pressure vessel thickness can be reduced and the vent path, or turret, reduced in cross sectional area.

This cross-sectional area may be referred to as "diameter" below, for brevity, but it is to be noted that the vent path cross sectional area may be of any shape, and need not be circular.

US Patent 4,223,723 suggests to remove any film boiling of a liquid cryogen at a surface of a cooled article by providing holes in the cooled article and therefore increase the heat transfer from the article into liquid nitrogen or helium, so as to increase the effectiveness of the cryogen. The present invention runs counter to the teachings of this patent, and provides increased surface temperature, thus promoting the film boiling regime and therefore counter intuitively reducing the heat transfer into liquid cryogen. This reduces the effectiveness of the cryogen cooling during a quench and therefore reduces the quench pressure.

The above, and further, objects, characteristics and advantages of the present invention will become more apparent from the following description of certain embodiments thereof, in conjunction with the accompanying drawings, wherein: Fig. 1A illustrates the relative heat flux over a range of temperature differentials between liquid helium and a bulk coil temperature; Fig. lB illustrates the relative heat flux over a range of temperature differentials between liquid helium and a coil surface temperature; Fig. 2 shows a conventional construction of superconducting coils wound into a former; Fig. 3A illustrates a structure according to an embodiment of the pre5ent invention in which no former i5 provided, but other arrangelnent5 are provided to reali5e the present invention; Fig. 3E illustrates a structure according to an embodiment of the present invention in which a coil structure comprises a resistive secondary coil on a radially outer surface of superconducting coils; and Fig. 4 shows the evolution of quench pressure with time for a conventional arrangement such as illustrated in Fig. 2, and for an arrangement according to the present invention.

The quench pressure is related to the rate at which heat is transferred from the magnet coils into the liquid cryogen. The rate at which heat is transferred from the magnet coils into the liquid cryogen i5 in turn related to the interaction between the liquid cryogen and the coil surface.

The liquid cryogen in contact with a surface of a coil boils when heated. This boiling reduces the surface area of the coil in contact with liquid cryogen and so affects a coefficient of heat transfer from the coil into the cryogen.

The boiling regime, discussed further below, and so also the heat transfer coefficient, changes with temperature of the cooled surface.

The quench pressure can therefore be affected by the coil surface temperature.

Fig. 1A illustrates various boiling regimes of liquid helium in contact with a cooled article over a range of temperatures.

In the present case, it illustrates the variation in wall heat flux as a function of the coil bulk temperature t expressed as a difference in temperature between a coil and a liquid cryogen.

Between about 4K and about 16K, bubble boiling occurs. Heat flux 4 increases as the temperature differential between liquid helium and the coil bulk increases. From about 16K upwards, mixed boiling occurs, where some bubbles are produced, and some film boiling occurs. This results in unstable contact surface area between liquid cryogen and cooled surface. For example, this is observed at T1, where the temperature difference is 20K.

As the temperature difference t rises in this region, the proportion of film boiling increases, and the heat flux 4 decreases, despite the increasing temperature differential t, due to reducing surface area of liquid cryogen in contact with the cooled surface.

This boiling regime of helium is counterintuitive in that, in a certain range of temperature differentials, by increasing the surface temperature, the rate of heat transfer into the helium can be reduced and therefore the quench pressure also reduced. This only works within a certain range of temperature-pressure combinations for a particular structure.

In the illustrated example, this range is between about T1=20K and about I>=80K for the example structure under consideration.

In conventional magnet designs, superconducting coils are wound into a former and the temperature of the coils during a quench is limited by a quench propagation circuit which reacts to the onset of a quench within one coil by inducing a quench in all coils, so that the stored energy is dissipated relatively evenly between the coils. In such magnet designs, the surface temperature of the coils during a quench is determined by the quench propagation circuit and the former structure and material. The former structure acts as a large heat sink into which the energy from the coils is transferred and thus limits the temperature rise. In standard magnet designs, the boiling regime of the liquid cryogen at the surface of the coils and the former structure provides a high enerqy tran3fer rate into the cryogen and therefore a relatively high quench pressure as the liquid cryogen boils rapidly.

According to an aspect of the present invention, a structure is proposed in which the surface temperature of the coils is raised during a quench. The attendant boiling of liquid cryogen remains within the mixed boiling regime, shown between about I-and T2 in Fig. 1A, or may reach the lower-temperature extremity of the film-boiling range. Accordingly, by raising the coil surface temperature within this region, the heat flux -the rate of transfer of thermal energy to the cryogen -actually reduces. Due to this reduction in rate of transfer of thermal energy, the rate of generation of cryogen gas, and so also the quench pressure, may be reduced as compared to its conventional value.

Similarly, the reduced rate of transfer of thermal energy to the cryogen will reduce the required rate of evacuation of boiled-off cryogen gas, in turn reducing the required diameter of quench gas egress path.

According to a feature of the present invention, the rate of heat transfer 4i from the coils to the cryogen can be reduced by changing the cryogen boiling regime. As can be seen from Fig. lA, the minimum rate of heat uransfer from the coils to the liquid cryogen can be achieved when the temperature differential t between the bulk of the coils and the liguid cryogen is about T, being at the upper range of mixed boiling, just as film boiling begins.

Fiq. 13 resembles Fig. lA, but differs in that the temperatures are expressed as temperature differential between liquid helium and surface temperature of a cooled article.

Labelled temperature differentials T, T2, T have the same siqnificance as discussed with reference to Fig. 1A.

Preferably, the coil surface temperatures durinq quench are raised sufficiently to reach the film boiling regime. With film boiling, the temperature difference t between the coil surface and the liquid cryogen is so great that a layer of boiled-off cryogen gas continuously exists between the wall and the liquid. This reduces the thermal conduction between coil and liquid, by a factor of up to fifty. The rate of heat transfer between the coil and the liquid cryogen is least when this insulating layer is in place, but the temperature difference between coil and cryogen is the minimum for film boiling. Tn the example shown in Fig. 1, this is at T3, about 75K.

B

According to the present invention, arrangements are provided to optimise the surface temperature of the magnet coils. This may be achieved by removing the conventional magnet former.

The arrangements for optimising the surface temperature of the coils may include one or more guench propagation circuit(s) and/or one or more inductive circuit(s), arranged to minimise contact between the coils and liquid cryogen during a quench by optimising a surface temperature of the coil assembly.

This may be achieved by limiting a current which is applied to a quench heater.

In an embodiment of the present invention, schematically illustrated in Fig. 3A, no former is provided. Rather, coils 30 are impregnated with resin and bonded together by intermediate mechanical support and retention layers 32, which may be conveniently referred to as "spacers". These spacers may be metal foam or glassfibre structures which are impregnated or adhesively bonded with resin so that a structure of coils and spacers is produced. In some embodiments, a single impregnation step is used to impregnate the coils and the spacers, so that a monolithic structure is formed. The absence of the conventional former removes a large heat sink or thermal shroud which conventionally acts to limit the temperature rise of the coil surfaces in contact with the former. However, the structure of the coils and their mechanical support and retaining structure is not in itself a feature of the present invention, and the present invention may be applied to other coil and support structures.

In embodiments of the present invention wherein a former 22 is provided, holes may be formed in the former to expose coil surface areas to cryogen, or the former may be provided with resistive windings of a quench propagation circuit to heat the former during quench, which will assist with the establishment of film boiling.

In the absence of a former, heat flux Ql illustrated in Fig. 2 cannot occur. As both radially outer 36 and radially inner 38 surfaces of the coils are exposed to cryogen, similar heat flux Q2 flows from each of these surfaces to the cryogen.

liig. 33 shows a magnet structure of the present invention with no conventional former structure, and with resistive, yet relatively conductive, secondary coils 34 wound onto radially outer surfaces of superconducting coils 30, as an overbinding, according to an embodiment of the present invention.

According to a feature of this embodiment of the invention, and as shown in Fig. 33, coils comprise a secondary coil 34 provided on radially outer surfaces of respective superconducting coil 30. These secondary coils are formed of a resistive wire such as high-purity aluminium. As illustrated, the secondary coils 34 may be provided as an overbinding over the superconducting coils 30. The secondary coils 34 are arranged into one or more closed circuits and are electrically isolated from the superconducting coils 30, but are inductively coupled to them. These secondary coils 34 play no part in normal operation of the superconducting magnet.

However, during a quench, they provide a heated radially outer surface 36 of the coils in contact with cryogen. As shown, heat flux Q2 may flow from the radially inner surface 38 as in the arrangement of Fig. 3A. However, a different heat flux Q3 will flow from the radially outer surface 36, due to the effect of ohmic heating by the secondary coil 34. For use as resistive heaters, the turns of the overbinding must be electrically insulated from each other. In similar structures where the overbinding is used for mechanical purposes only, there is no need to provide electrical insulation between the turns. The overbinding is preferably resin-impregnated, in the same manner as the superconducting coils. Such impregnation may be achieved in a common impregnation step with the impregnation of the superconducting coils.

In overbinding provided to form an inductively coupled secondary winding according to the present invention, the turns are electrically insulated from each other to provide a multi-turn resistive coil. In conventional overbinding which is provided for structural strength, the turns are generally not insulated one from other. The overbinding is preferably arranged to have a similar thermal expansion to that of the coil, to minimise thermally induced stresses between coil and overbinding as their temperatures vary. Conventional overbinding for mechanical strength if typically provided by stainless steel wire, and is not electrically connected. The overbinding of the present invention forms one or more closed electrical circuits and is preferably of a wire with higher electrical conductivity, such as aluminium.

In case of a quench, the current flowing in the superconducting coils 30 decreases rapidly, and an opposing electrical current is induced in the secondary coils 34. The resultant ohmic heating of the resistive wire of the secondary coils 34 increases the temperature of the radially outer surface 36 of the coils during the guench.

Such secondary coils 34 may alternatively, or additionally, be provided on the radially inner surface 38 of one or more superconducting coils 30 in other embodiments of the present invention. However, in cylindrical superconducting magnets for MRI systems, such as those illustrated, it is generally reguired to keep the inner diameter of the magnet structure as large as possible, for a given guantity of superconducting wire. It may accordingly be preferred to place the secondary coils 34 only on the radially outer surface of the superconducting coils, as illustrated.

Considering the curves shown in Figs. lA-lB once again, the increase in surface temperature of the coils in conuact with liquid cryogen may be selected by design such that in the case of a quench, the heat flux, representing the rate of transfer of thermal energy from the coils to the liquid cryogen, may be reduced as compared to a similar structure without the secondary coils 34.

The reduced rate of boiloff of liquid cryogen provided by the invention means that a superconducting magnet will retain more of its cryogen after guench, extending the time during which is will remain cold during a quench. The requirement for providing escape paths fro boiled-off cryogen will be reduced.

The reguired guantity of liquid cryogen may accordingly be reduced, saving on material costs.

The reduced heat flow to the cryogen leads to a more even temperature distribution thouch the material of the coil, reducing the likelihood of damage to the coils during guench due to temperature gradients across a cross-section of the coil.

T,chere present, a former of a thermally conducting material such as aluminium may act as an efficient heat sink, drawing heat from a quenching superconducting coil more efficiently than heat transfers from the coil to the cryogen. Heat can then be transferred from the former to the cryogen.

The overbind can be used to introduce another parameter to control the boiling regime. The overbind may be adapted to control a surface temperature of each coil which is in contact with liquid cryogen. The characteristics of the overbind may be used in the optimisation of quench current.

The film boiling phase will occur in a range of temperatures.

The magnet may quench at low current during initial energisation phases, used to "train" the superconducting coils. The temperatures attained at a surface of the overbind need to be optimised to provide acceptable operation during low current quenches.

While a superconducting magnet is at field, in use in the normal persistent mode, it is preferred that a surface boiling regime provides high rates of heat transfer from the coils into the iiguid cryogen and thus rnaximises the cooling efficiency. This is generally obtained within the bubble boiling phase, illustrated in Fig. 1 at a temperature difference of up to about 16K.

According to a feature of the present invention, a phase transition in the surface boiling is caused during quench, so that the rate of heat transfer from the coils into the cryogen is reduced, thus reducing the rate of generation of cryogen gas, the quench pressure inside the cryogen vessel, and the diameter of the necessary guench gas egress path.

Fig. 4 shows the calculated quench pressure for the mixed boiling phase for a standard rragnet design, and a calculated guench pressure the alternative film boiling phase which is achieved by optimising the coil surface temperature and removing a large former heat sink according to an embodiment of the present invention. The lower curve represents the evolution of quench pressure over time for a magnet structure according to an embodiment of the invention during film boiling, while the upper curve represents the evolution of quench pressure over time for mixed boiling in a conventional magnet structure similar to that illustrated in Fig. 2.

It is conventional to employ small heaters in thermal contact with superconducting coils to propagate a quench among several coils of a superconducting magnet structure. Such heaters are typically affixed to the radially outer surface of a coil, and may be employed in a method of the present invention to control a surface temperature of superconductive coils to obtain a desired boiling regime.

In alternative embodiments, solid single turn conductive bands may be provided as single-turn secondary coil to provide resistive heating of coil surfaces.

In embodiments where formers are provided, means may be provided to heat the former is response to a guench, prompting film boiling at interfaces between the former and liquid cryogen, and reducing thermal stress due to temperature differences between coils and former.

In a further embodiment, conventional "dump" resistors, used to dissipate energy outside of a cryostat, may be replaced by resistive wire on the coils, connected into a quench circuit, and will heat the surfaces of the coils to promote film boiling according to the present invention. A resistive overbinding may be electrically connected into a circuit arranged to receive an electric current for the purpose of adjusting a coil surface temperature, rather than simply carrying a current induced in it by the change in current of the superconducting coil due to a quench event. A resistive element may be electrically included within the overbinding if desired to provide an appropriate thermal response to a quench.

Alternatively, other load elements such as diodes may be placed so as to heat a surface of superconducting coils and/or an associated former to provide film boiling according to the invention. Resistive wire or foil may be placed on or within a former, such as a former of composite material, rather than as an overbinding, to provide heating of a surface of the former in contact with liquid cryogen, to induce film boiling at that interface.

Claims

CLAIMS1. A method for reducing a guench pressure within a cryogen vessel housing a superconducting magnet comprising, in case of a quench, a surface of a superconducting coil is heated to a certain temperature to cause film boiling of liguid cryogen in contact with the surface of the superconducting coil.
2. A method according to claim 1, wherein the rate of thermal transfer to the liquid cryogen is reduced below the corresponding rate in the absence of said heating.
3. A method according to claim 1 or claim 2, comprising the step of providing at least one resistive secondary coil (34) inductively coupled and mechanically attached to a superconducting coil (30), said secondary coil(s) forming one or more closed electrical circuit(s) whereby, in case of a quench, electrical current is induced in the secondary coil(s)and heats a coil surface (36) of the secondary coil to cause film boiling of liquid cryogen in contact with the surface of the secondary coil.
4. A method according to claim 1 or claim 2, wherein step is heating is performed by a quench propagation circuit including at least one quench heater attached to respective superconducting coil(s) of the superconducting magnet.
S. A superconducting magnet assembly comprising a number of superconducting coils (30) housed within a cryogen vessel and arranged to be cooled by contact with liguid cryogen, characterised in that at least one of the superconducting coils is provided with a secondary coil (34) of resistive wire inductively coupled to said superconducting coil and mechanically attached to a surface of the superconducting coil, said secondary coil(s) and forming one or more closed electrical circuit(s).
6. A magnet assembly according to claim 5 wherein the or each secondary coil is provided as an overbinding over the associated superconducting coil (s)
7. A magnet assembly according to any of claims 5-6 wherein the superconducting coils are impregnated and bonded together by intermediate mechanical support and retention layers (32)
8. A magnet assembly according to claim 7 wherein the intermediate mechanical support and retention layers are metal foam or glassfibre structures which are impregnated with the same resin used to impregnate the coils.