GB2514372A

GB2514372A - Quench Protection System for a Superconducting Magnet

Info

Publication number: GB2514372A
Application number: GB1309142.6A
Authority: GB
Inventors: David Warren; Andrew Twin; Ziad Melhem; Paul Noonan; M Hamed Lakrimi
Original assignee: Oxford Instruments Nanotechnology Tools Ltd
Current assignee: Oxford Instruments Nanotechnology Tools Ltd
Priority date: 2013-05-21
Filing date: 2013-05-21
Publication date: 2014-11-26
Anticipated expiration: 2033-05-21
Also published as: GB2514372B; GB201309142D0

Abstract

A quench protection system has a heater 1 positioned adjacent to and thermally isolated from a magnet (101, 102) and comprising an inductively wound heater element (5a, 5b). When the magnet experiences a quench, a current flows in the heater element (5a, 5b) causing a Lorentz force which moves the heater 1 to a position where it is in thermal contact with the magnet, thereby initiating a further quench. The heater element (5a, 5b) may be resistive to act as a heat source and the current flow may be initiated by a quench detection system 60. The heating elements (5a, 5b) may be laminated on a flexible polyimide sheet which deforms by bending to bridge the region between inner coil 102 and outer coil 103. The invention may be retro-fitted to existing superconducting magnets and avoid the need for good thermal bonding between the heaters and the coils.

Description

QUENCH PROTECTION SYSTEM FOR A SUPERCONDUCTING MAGNET

Field of the Invention

The present invention is directed towards a quench protection system for a superconducting magnet. The invention is also directed to a superconducting magnet system comprising a quench protection system.

Background to the Invention

Superconducting magnets typically comprise a coil or coils of superconducting wire, often having cylindrical geometry. The coils are kept below their transition temperature (i.e. in the superconducting regime) by a cooling mechanism or medium, and a power supply is used to deliver the required current to the coils in order to provide the desired magnetic field. The cooling medium may comprise a bath of cryogen, such as liquid helium, or alternatively may be cooled using a mechanical refrigerator such as a pulse tube refrigerator or Gifford McMahon (GM) refrigerator for example. The superconducting nature of the magnets allows large currents to be used, generating magnetic fields of up to about 32T.

For this reason, superconducting magnets are used in many applications such as research magnets, NMR and MRI techniques and particle accelerators, among others.

When a superconducting magnet is energised, a small heat fluctuation (of the order pJ) can load to the superconductor in the magnet coils warming above the transition temperature and causing the coils to become resistive. In such a scenario, the large amount of energy stored in the coils (of the order kJ or even MJ) is dissipated into the now-resistive coils which rapidly heat up, leading to a rapid boiling off of cryogen in a cryogen-cooled system. Such an event is known in the art as a quench.

The magnitude of the temperature rise during a quench and in particular the stresses generated by the differential temperatures within the magnet coils leads to the necessity to extract energy from the magnet coils in a controlled manner.

It is known in the art to use a resistor network connected across the magnet coils such that the excess energy during a quench may be dissipated in the resistors.

However, when a magnet coil quenches and becomes resistive, the neighbouring coils that remain superconducting are inductively coupled to the quenching coil. Therefore, the change (decrease) in current in the quenching coil leads to a corresponding rapid increase in current in the inductively coupled neighbouring coils. This increase in current in the neighbouring coils can lead to very high mechanical and thermal stresses that may lead to irreparable magnet damage. Therefore, magnets with large amounts of stored energy (typically MJ) are not able to rely on a resistor network alone to protect the coils in case of a quench.

Therefore, heaters may be positioned at suitable locations on the magnet and, in the event of a quench on one coil, the heaters are used to quench the remaining coils as quickly as possible in order to minimise the problems outlined above. In order to effect a rapid quench in the coils the heaters are ordinarily wound non-inductively and adhered to the bore of the magnet using epoxy resin. However, this is difficult to achieve and requires large amounts of manufacturing time and materials to effect a good thermal bond without affecting the performance of the coil. Further, due to the high mechanical stresses experienced by some superconducting magnet coils, heaters attached to the coils in such a way may, over time, become loose and thereby become less effective due to the reduced thermal contact between the heater and coil.

There is therefore a requirement to improve quench protection systems for superconducting magnets.

Summary of the Invention

According to a first aspect of the invention there is provided a quench protection system for a superconducting magnet comprising: a first heater positioned when in use in a first configuration adjacent to and thermally isolated from a first part of the magnet, the first heater comprising an inductively wound heater element; an actuator operable in use to cause current to flow through the heater element when a first pad of the magnet experiences a quench, such that the current flow in the heater element causes a resultant force to be exerted on the first heater thereby causing it to move from the first configuration to a second configuration in which it is brought into thermal contact with the magnet.

The quench protection system of the invention overcomes the above problems as the first heater is caused to move from its first configuration to a second configuration when a first pad of the magnet experiences a quench. When in its second configuration, the heater is in thermal communication with the magnet and is thus able to rapidly quench the remaining part of the magnet in order to prevent damage to the magnet. Conversely, when the first heater is in its first configuration it is substantially thermally isolated from any superconducting part of the magnet.

The heating function, which causes the pad of the magnet which is heated to quench, is provided by the first heater when in the second configuration. The first heater may take the form of a thermal link which provides heat to the magnet by bringing it into thermal contact with a heat source. Such a heat source might be a relatively warm additional part of the system for example, with the first heater providing the thermal link rather than the source of heat itself. Indeed with suitable mounting such a heat source may itself be brought into thermal contact rather than using an additional thermal link.

The first heater may provide the actual source of heat and therefore the first heater may have two elements, the inductively wound heater element and a heat source element which are mechanically linked together, the heater element providing the force for the change in configuration and the heat source element providing the heat. For example the heat source element may be ohmically resistive and electrically connected in either series or in parallel with the heater element. Preferably however the heat source element is integrated into the heater element such that the heater element provides a dual function of providing the force for movement and also providing the source of heat.

Advantageously this may be achieved by the heater element being ohmically resistive so as to provide ohmically resistive heating using an inductively wound ohmically resistive circuit.

Typically, the magnet comprises at least one superconducting wire or tape coil" wound in a solenoid geometry. Whore the magnet comprises a single coil, the heater, in its second configuration, comes into contact with the coil so that if a first part of the coil quenches, the heater rapidly quenches the remainder of the coil in order to minimise or prevent further damage to the magnet coil.

In an arrangement where the magnet comprises two or more coils, the coils are not thermally coupled. This means that if one coil experiences a quench, the other coil(s) can continue to operate in their superconducting mode. In the event of such a quench, large transient currents may be induced in the other coil(s) with associated high mechanical and thermal stresses. In the present invention, if one of the coils does experience a quench, in order to prevent damage to the remainder of the magnet due to the high stresses, the first heater is used to rapidly quench the other coil(s). A quench protection system for use with a magnet comprising two or more coils typically comprises a number of heaters corresponding to the number of coils. More than one heater per coil may also be provided, in spaced locations for example.

II a quench occurs in a first part of the magnet, the first heater is brought into contact with a second part of the magnet distal to the first part in order to quench the second part. The first and second parts of the magnet may be located on the same coil, but are preferably on separate coils.

In its first configuration, the first heater is typically suspended adjacent to and spaced from (i.e. thermally isolated from) a first part of the magnet. When a quench occurs in a first part of the magnet, current flows through the heater element generating heat and also a resultant force on the heater such that it is displaced and moves to its second configuration. Although the heater is preferably suspended, the heater may alternatively be moveably mounted (for example pivotally mounted) such that the resultant force causes the heater to be displaced and come into thermal contact with a part of the magnet. Alternatively or in addition, the resultant force may cause the heater to deform (by bending for example) so that at least part of the heater comes into thermal contact with part of the magnet.

In order that the current flow through the heater element causes a resultant force to be exerted on the heater, the heater element is inductively wound. In other words, there is a net "loop" of current flow through the heater element such that, when current flows through the heater element in the presence of the magnetic field generated by the superconducting magnet, the heater element experiences a Lorentz force such that the heater moves from its first configuration to the second configuration in thermal communication with the magnet. This use of an inductively wound heater element is completely different to prior art heaters which are adhered to the coils of the magnet. Such prior art heaters are deliberately non-inductively wound such that they do not experience a Lorentz force which could result in them coming away from their mounting. Conversely, the present invention instead makes use of the Lorentz force to generate the thermal coupling between the heater and coil.

In a simple form, the heater element may comprise two substantially parallel sections wherein, in use, current flows in opposing directions in the two substantially parallel sections. In one such arrangement the heater takes the form of a "U shape" having two ends defined by two substantially parallel tracks connected by a perpendicular track. Current flows into the "U shape" at one end and flows out from the other end. The opposing direction of current flow within the "U shape" causes opposing sides of the heater element to experience opposing Lorentz forces thereby generating a torque on the heater element.

Preferably however, the heater element may take the form of a substantially planar spiral such that, in use, current flows in opposite directions on opposing sides of the spiral. The spiral is most preferably a rectangular spiral with the lengths of parallel tracks decreasing towards the centre of the spiral.

Advantageously, the use of a spiral improves the flexibility of the heater and thus allows improved thermal contact with the magnet.

The quench protection system of the first aspect may be actively or passively actuated. In the case of active actuation, the quench protection system may further comprise a quench detection system operable in use to monitor the magnet to detect when a quench occurs and subsequently control the actuator to cause current to flow through the heater.

Typically, in such an active arrangement, the actuator comprises a capacitor arrangement or an externally triggered power supply arrangement (which may be a pulsed power supply). When a quench is detected by the quench protection system, the quench protection system controls the actuator such that it provides a rapid discharge of current into the heater element when a quench occurs. With an integrated heater element the rapid discharge of current into the heater element not only provides heat by means of Joule heating, but also the generation of a resultant force on the heater as described above.

The quench protection system may alternatively be actuated passively, where typically the actuator comprises a resistor arrangement. Preferably the resistor arrangement is a protection resistor(s) arranged in parallel with the coil(s) of the magnet. Such a resistor arrangement is conventionally used in prior art systems to dissipate energy from a quenched coil. In the present invention, as well as acting as a quench protection mechanism in its own right, the voltage(s) arising across the resistor when a quench occurs are used to drive current through the heater. In an alternative passive arrangement, the first heater itself may act as the actuator (with no resistors present), in which case the first heater would both dissipate the quench energy and provide heating in order to quench the un-quenched coil.

Preferably, the heater further comprises a substantially planar substrate onto which the heater element is mounted (typically by lamination). The substrate not only supports the heater element but also electrically insulates it from the ground and the coil windings in order to prevent electrical shorting. Although the substrate may in some instances be stiff, the substrate (and therefore the heater itself) is preferably flexible in order that the resultant force acting on it brings the heater into thermal contact with the magnet. A flexible substrate is particularly advantageous as it allows the heater to move from the first to the second configurations by deformation and allows good thermal contact between the heater and the magnet by increasing the surface area of the heater in contact with the magnet.

The substrate preferably comprises a polyimide sheet such as Kapton® which is advantageously flexible with a high thermal conductivity and is stable in a wide range of temperatures. However, other substrate materials that exhibit similar mechanical and thermal properties may be used.

An advantage of the quench protection system of the present invention is that it may be retro-fitted to an existing superconducting magnet system. This is because the heater(s) do not need to be mounted onto the coils themselves (unlike prior art systems); instead they make use of the Lorentz force to bring the heaters into thermal contact with the coils. The quench protection system is particularly suited to retro-fitting on superconducting magnet systems already comprising protective resistor arrangements as the heaters may be "passively" actuated by these resistors as explained above. In such a case, the quench protection system is preferably connectable to the protection resistor(s) through electrical leads or the like.

Alternatively, where no resistors are included as part of the magnet, the quench protection system may comprise at least one resistor. In another arrangement where no resistors are included as part of the magnet, the quench protection system may not include any resistors and the heater itself acts as the actuator.

In the case of an "active arrangement", a quench detection system and an actuator (such as a capacitor arrangement or pulsed power supply arrangement) are preferably used as part of the quench protection system.

The quench protection system may further comprise a second heater being positioned when in use in a third configuration adjacent to and thermally isolated from a third part of the magnet. The second heater may take any of the possible forms of the first heater although preferably the second heater comprises an inductively wound, ohmically resistive heater element, such that the current flow in the heater element causes a resultant force to be exerted on the second heater thereby causing it to move from the third configuration to a fourth configuration in which it is brought into thermal contact with the magnet. Such an arrangement is particularly beneficial when the magnet comprises two coils, in which case the first heater is used to quench the first coil, and the second heater is used to quench the second coil.

According to a second aspect of the invention there is provided a superconducting magnet system comprising: first and second superconducting coils, each operable to produce a magnetic field when current flows through the coils, and a quench protection system according to the first aspect, wherein if a quench occurs in one of the coils, the actuator causes current to flow through a heater element such that at least one heater is brought into thermal contact with at least the other of the first and second coils. The coils are typically manufactured from superconducting wire or tape.

In a preferred arrangement, the second coil is coaxial with and mounted within the first coil so as to define an annular region between the first and second coils, wherein the first quench heater is mounted (preferably suspended) within the annular region. In such an arrangement, the first heater is suspended within the annular region such that upon a quench of one of the coils, the first heater is able to move from the first to the second configuration in thermal contact with the non-quenched coil. Preferably in such a coaxial arrangement, the quench protection system comprises a second heater suspended within the bore of the second "inner" coil.

Preferably, when in its second configuration, the first heater is in thermal contact with each of the first and second coils, thereby "bridging the gap" between the coils. For example, if a quench occurs in the second "inner" coil, the first heater comes into thermal contact with each of the first and second coils, assisting in keeping the first heater in thermal contact with the first "outer" coil. Such a second configuration where the heater "bridges the gap" may also be used to simultaneously quench the second "inner" coil (for example if a quench occurs on the first "outer" coil).

In some arrangements, the superconducting magnet system comprises first and second resistor arrangements connected in parallel with the respective first and second wire coils, wherein, when a quench occurs in one of the first and second coils, the respective resistor arrangement acts as the actuator. As described above, the present invention may make use of the voltage generated across the respective "protective" resistor arrangement in order to drive the heater. This is the so-called "passive" actuation.

Brief Description of the Drawings

The invention will now be described with reference to the following drawings, in which: Figure 1 is a perspective view of a typical superconducting magnet as is known in the art; Figure 2 is a schematic view of a quench protection system according to a first embodiment of the invention; Figure 3A illustrates a first example of a heater element according to the first or a second embodiment of the invention; Figure 3B illustrates a second example of a heater element according to the first or a second embodiment of the invention, and; Figure 4 is a schematic view of a quench protection system according to a second embodiment of the invention.

Description of Preferred Example

Figure 1 is a perspective view of a typical superconducting magnet 100. The magnet 100 comprises a first "outer" coil 101 and a second "inner" coil 102 each having cylindrical geometry and wound in the form of a solenoid. The second coil 102 is coaxial with and located within the bore of the first coil 101, thus defining an annular region 104 (see Figures 2 and 4) between the coils. The coils are made of a type II superconductor such as niobium tin or niobium titanium. The winding of superconducting magnet coils is a well understood technique and will not be further discussed herein. As will be appreciated, the present invention is also applicable to other magnet arrangements, such as quadrupole coils having a "racetrack" arrangement.

In practical applications of the invention, the superconducting magnet 100 is located within a cryostat or equivalent apparatus which provides a stable low temperature environment so as to ensure that the superconducting coils remain below their superconducting transition temperature and therefore remain in the superconducting regime during normal use. Typically such cryostats may immerse the coils within a bath of liquid cryogen (typically helium) provided by a so-called "wet" system, although re-condensing or conductively cooled systems comprising a cryocooler such as a pulse tube refrigerator may alternatively be used. The present invention is suitable for all such different cooling techniques.

A DC power supply 110 is used to provide direct current to the coils of the superconducting magnet in order to "ramp" the magnet to the desired magnetic field. Once the desired magnetic field is reached, the magnet is switched to run in steady state or "persistent" mode where the magnetic field is stable, as is understood in the art. It is during this persistent mode that the magnetic field provided by the magnet is used, for example for NMR experiments of MRI imaging.

Figure 2 is a schematic illustration of a quench protection system 50 according to a first embodiment of the invention. Figure 2 shows, in cross section, a superconducting magnet of the type seen in Figure 1 having coaxially wound inner 102 and outer 101 coils. The quench protection system 50 comprises a heater I suspended by means of a mechanical clamp (not shown) within the annular region 104 between the coils 101 and 102, as will be discussed below.

The mechanical clamp maintains the upper end of the heater I in a fixed position. In an alternative embodiment, the heater 1 may be freely suspended (by a wire(s) for example), wherein the heater is not fixed at its upper end.

First 11 and second 12 protection resistors are connected in parallel across the outer 101 and inner 102 coils respectively. In practice, resistor arrangements comprising a series of resistors will be used instead of single resistors, although the following description will refer to single resistors for clarity purposes. The resistances of the protections resistor(s) vary depending on the magnet coils used, with larger resistances being used for high current magnets using large diameter wires (such as those used in High Energy Physics for example).

The following description will refer to a quench occurring on the inner coil 102, although it will be appreciated that the quench protection system 50 may also be used for quenches occurring on the outer coil.

The quench protection system 50 comprises a quench detection system 16, comprising quench detection electronics, connected in parallel across the second resistor 12. If a quench occurs in the inner coil 102, heating the inner coil to above its superconducting critical temperature, this causes current to flow through the second resistor 12 thereby generating a voltage across second resistor 12. If the voltage across the second resistor 12 exceeds a predetermined threshold (due to a quench occurring), this actuates the charged capacitor 20, which rapidly discharges and supplies current to the heater 1. More specifically, current is provided to heater element 5 (illustrated at 5a and Sb in Figure 2), as will be explained in further detail below. The value of the voltage threshold is chosen such that noise fluctuations do not cause unwanted discharges of the capacitor (and thus undesirable quenches). In a similar manner to above, a single capacitor 20 is illustrated and described for clarity purposes; however in practice a capacitor bank comprising a plurality of capacitors having a capacitance of the order IF is typically used.

The heater I comprises a thin, flexible polyimide sheet (such as Kapton®) which acts as a substrate onto which a conductive heater element 5 (see Figures 3A and SB) is laminated using well known techniques. The flexible polyimide sheet is sufficiently thin such that a resultant force acting on the heater element readily changes the geometry of (i.e. "deforms") the sheet. Typically one dimension of the sheet is substantially smaller than its other two dimensions. This advantageously increases the flexibility of the sheet. The non-electrically conducting Kapton® sheet further acts to prevent the electrically conductive heater element shorting to the ground or the coil windings.

In normal use, when no quench has occurred, the top of the Kapton® sheet 3 of the heater I is suspended from its upper end in the annular region 104 between the inner 102 and outer 101 coils of the magnet 100 by means of a mechanical clamp positioned at the upper end of the coils. When suspended within the annular region 104, the heater 1 is not in contact with either the inner or outer coils such that there is a gap between the heater and the magnet, such that the magnet and heater are thermally isolated. This is in contrast to conventional heater systems where any heaters are permanently affixed to the magnet coils, and allows the quench protection system of the present invention to be easily retro-fitted to existing superconducting magnets, as well as avoid the problems with affixed heaters that are set out in the "Background to the Invention" section above.

Figure 3A illustrates a plan view of a first example of a heater element 5. The heater element has a "U shape" in plan form and comprises two substantially parallel sections 5a and Sb connected by perpendicular section Sc. The "U shape" of the heater element 5 comprises two ends 501 and 502, wherein current is fed into the "U shape" through first end 501 and flows around the "U shape" and exits through second end 502. The "U" shape of the heater element 5 creates a current "loop" when current flows through it, as illustrated by the current arrows (labelled "I") depicting opposing current flow in sections Sa and Sb. In other words, the heater element 5 is inductively wound. The heater element 5 is manufactured from ohmically resistive material such as nichrome or stainless steel arranged in wire or track form, and generates heat by Joule heating when current flows through it.

When current flows through the heater element 5, due to the net flow of current around the "loop", sections of the heater element S having current flow perpendicular to the direction of the magnetic field from the magnet 100 experience a Lorentz force, as illustrated in Figure 3A. In the view of Figure 2, the direction of current flow in section 5a is into of the plane of the figure, and the direction of current flow in section Sb is out of the plane of the figure. The direction of the magnetic field produced by the magnet is illustrated by arrow "B".

Therefore, section 5a experiences a force Fa in the direction out of the plane of the figure in the view of Figure 3 (i.e. towards the inner coil 102 in the view of Figure 2), and the section 5b experiences an opposite force Fb into the plane of the figure in the view of Figure 3 (i.e. towards the outer coil 101 in the view of Figure 2).

As a result, when a quench is detected in the inner coil of magnet 1, current flow through the heater element S causes it to pivot about axis X-X' such that at least section Sb of the heater element 5 comes into contact with the outer coil 101. As the Kapton® substrate is flexible, due to the movement of the heater elements 5a and Sb, the heater 1 bridges the region between the inner and outer coils and comes into thermal contact with both coils. Due to the Joule heating provided by the heater element 5, the outer coil is heated to above its superconducting transition temperatures and is rapidly quenched, thus preventing further damage to the magnet. Conventionally the outer surface of the inner coil will comprise an overbinding and/or glass reinforced plastic former and so will not quench when the heater comes into thermal contact with it as the majority of the thermal energy from the heater 1 will be absorbed by the overbinding and/or former rather than the coil itself. However, it is envisaged that in embodiments, the movement of the heater to bridge the region between the inner and outer coils will quench both coils.

Figure 3B illustrates a plan view of a second example of a heater element (here labelled "6"). The heater element 6 is an inductively wound ohmically resistive wire or track (example materials include nichrome and stainless steel) that generates heat by Joule heating when current flows through it. The wire is arranged in a planar rectangular spiral comprising a plurality of sections which decrease in length towards the centre of the spiral. When a quench is detected on the inner coil 102 and current is provided to the heater element 6, the heater element 6 creates a current "loop" which is schematically illustrated by the current arrows labelled u1l. Current flows in opposing directions on opposing sides of the spiral. For example, current flows in opposing directions in parallel sections 6a and 6b, and similarly in parallel sections 6c and 6d, which are perpendicular to sections 6a and 6b.

When current flows through the heater element 6, due to the net flow of current around the loop", sections of the heater element 6 having current flow perpendicular to the direction of the magnetic field B generated by the magnet experience a Lorentz force, as illustrated in Figure 3B. In the view of Figure 2, the direction of current flow in section 6a is into of the plane of the figure, and the direction of current flow in section 6b is out of the plane of the figure. The direction of the magnetic field produced by the magnet is illustrated by arrow "B".

Therefore, section 6a experiences a force Fa in the direction out of the plane of the figure in the view of Figure 3B (i.e. towards the inner coil 102 in the view of Figure 2), and the section 6b experiences an opposite force Fb into the plane of the figure in the view of Figure 3 (i.e. towards the outer coil 101 in the view of Figure 2).

Sections 6a' and 6a" of the heater element 6 which are parallel to section 6a also experience a force in the direction of force Fa, and similarly sections 6b' and 6b" of the heater element parallel with section 6b experience a force in the direction of force Fh. As a result, when a quench is detected in the inner coil of magnet 1, current flow through the heater element 6 causes it to pivot about axis Y-Y' such that the lower portion of the heater element comes into contact with the outer coil 101. As the Kapton® substrate is flexible, due to the movement of the heater element, the heater I therefore bridges the gap between the inner and outer coils and comes into thermal contact with both coils. Further, the spiral nature of the heater element windings increases the flexibility of the heater. Due to the Joule heating in the heater element, the outer coil is heated to above its superconducting transition temperatures and is rapidly quenched, thus preventing further damage to the magnet.

In the view of Figure 2, when a spiral heater element such as that seen in Figure 3B is used, section 5a represents a superposition of each parallel section of the spiral 6a, 6a', 6a" having current flowing in a first direction, and section 5b represents a superposition of each parallel section of the spiral 6b, 6b', 6b" having current flowing in the opposing direction. As will be appreciated, a spiral heater element may have fewer or more sections than those seen in Figure 3B.

Figure 4 is a schematic illustration of a quench protection system 60 according to a second embodiment of the invention. Like reference numerals in Figures 2 and 4 indicate like parts. Further, the portions 5a and 5b in Figure 4 are the same as discussed above in relation to Figures 2, 3A and 3B.

The following description will refer to a quench occurring in the inner coil 102, although it will be appreciated that the quench protection system 60 may also be used for quenches occurring on the outer coil.

The first embodiment relates to an "active" quench protection system comprising a quench detection system 16 and a capacitor discharging mechanism 18 as described above. In other words, the active quench protection system of the first embodiment comprises a dedicated "actuator" (i.e. the capacitor arrangement) in addition to the protection resistors. The second embodiment, as will be described below, relates to a "passive" quench protection system. The quench protection system 60 of the second embodiment comprises first and second protection resistors 11 and 12 connected in parallel with the outer 101 and inner 102 coils respectively. As in the first embodiment, these are in practice resistor arrangements each comprising a plurality of resistors. The heater element is connected across the second protection resistor by actuation circuit 21. Either heater element 5 or 6 may be used in the second embodiment.

As described above, when a quench occurs on the inner coil 102, current flows through the second protection resistor 12 thus generating a voltage across it.

This voltage is used to provide current to the heater element 5. In the same way as described for the first embodiment, in the event of a quench on the inner coil, the flow of current through the heater element means that it experiences Lorentz forces such that the heater 1 comes into contact with both the inner and outer coils, thereby rapidly quenching the outer coil. The actuation circuit 21 is adapted such that the heater element may be actuated accordingly in a number of different quench scenarios.

Both of the first and second embodiments above have been described in the event where the inner coil 102 quenches and the heater I acts to heat (and therefore quench) the outer coil 101 in order to prevent further damage to the magnet. There is also the scenario (although less likely to occur) where the outer coil quenches unexpectedly and it is desired to quench the inner coil in order to prevent further damage to the magnet. Although not shown in Figure 2 for purposes of clarity, the quench protection system also comprises a quench detection system, capacitor discharging mechanism and capacitor connected across first protection resistor 11 in the same manner as that connected across second protection resistor 12 and described above. In the event of a quench occurring on the outer coil 101, current is provided to a second heater (not shown) positioned between the magnet bore tube and the inner surface of the inner coil 102. This second heater is configured in the same way as described above and the heater element of the second heater experiences a Lorentz force such that at least part of the second heater comes into contact with the inner surface of the inner coil, thereby increasing its temperature above its transition temperature and causing a quench on the inner coil.

Similarly, although not shown in Figure 4 for purposes of clarity, the quench protection system of the second embodiment further comprises an actuation circuit connected across first protection resistor 11 operable to provide current to a second heater in the event of a quench on the outer coil. The second heater is located adjacent the inner surface of the inner coil 102 as described above and acts to quench the inner coil.

The inner surfaces of the coils are used for the "protection" quenches as the outer layers of each coil typically have a glass reinforced plastic former and possibly an overbinding that will act to limit the heat pulse from the heater.

Further, the outer wire of a superconducting coil runs far from its full

specification.

A large advantage of quench protection system of the present invention is that it can be retro-fitted to existing superconducting magnets. This is in contrast to existing systems where a heater is permanently attached to a coil. The quench protection system of the present invention is particularly beneficial when used with superconducting magnets having coaxially wound first and second coils, as seen in Figure 1, as a heater may be suspended in the annular space between the two coils. However, the quench protection system may also be used with other superconducting magnet arrangements, for example a single coil or quadrupole coils having a racetrack arrangement. Further, the quench protection system may comprise two or more heaters.

The present invention is also directed to complete systems including a superconducting magnet in combination with a quench protection system according to either the first or second embodiment.

Claims

Claims 1. A quench protection system for a superconducting magnet, comprising: a first heater positioned when in use in a first configuration adjacent to and thermally isolated from a first part of the magnet, the first heater comprising an inductively wound heater element; an actuator operable in use to cause current to flow through the heater element when a first part of the magnet experiences a quench, such that the current flow in the heater element causes a resultant force to be exerted on the first heater thereby causing it to move from the first configuration to a second configuration in which it is brought into thermal contact with the magnet.
2. A quench protection system according to claim 1, wherein the heater element is ohmically resistive so as to act as a heat source.
3. The quench protection system of any of the preceding claims, further comprising a quench detection system operable in use to monitor the magnet to detect when a quench occurs such that the actuator causes current to flow through the heater.
4. The quench protection system of any of the preceding claims, wherein the actuator comprises a capacitor arrangement.
5. The quench protection system of any of claims I to 3, wherein the actuator comprises a power supply.
6. The quench protection system of any of claims 1 to 3, wherein the actuator comprises a resistor arrangement.
7. The quench protection system of claim 6, wherein the resistor arrangement is arranged in parallel with a part of the magnet.
8. The quench protection system of any of the preceding claims, wherein the heater further comprises a substantially planar substrate onto which the heater element is mounted.
9. The quench protection system of claim 8, wherein the heater element is laminated onto the substrate.
10. The quench protection system of claim 8 or claim 9, wherein the substrate is flexible.
11. The quench protection system of any of claims 8 to 10, wherein the substrate comprises a polyimide sheet.
12. The quench protection system of any of the preceding claims, wherein the heater element comprises two substantially parallel sections, wherein, in use, current flows in opposing directions in the two substantially parallel sections.
13. The quench protection system of any of the preceding claims, wherein the heater element is formed as a substantially planar spiral such that, in use, current flows in opposing directions on opposing sides of the spiral.
14. The quench protection system of claim 13, wherein the spiral is a rectangular spiral.
15. A quench protection system according to any of the preceding claims, further comprising a second heater being positioned when in use in a third configuration adjacent to and thermally isolated from a third part of the magnet, the second heater comprising an inductively wound heater element, such that the current flow in the heater element causes a resultant force to be exerted on the second heater thereby causing it to move from the third configuration to a fourth configuration in which it is brought into thermal contact with the magnet.
16. A quench protection system according to claim 15, wherein the heater element of the second heater is ohmically resistive.
17. A superconducting magnet system comprising: first and second superconducting coils, each operable to produce a magnetic field when current flows through the coils, and; a quench protection system according to any of the preceding claims, wherein if a quench occurs in one of the coils, the actuator causes current to flow through a heater element such that at least one heater is brought into thermal contact with at least the other of the first and second coils.
18. The superconducting magnet system of claim 17, wherein the second coil is coaxial with and mounted within the first coil so as to define an annular region between the first and second coils, wherein the first quench heater is mounted within the annular region.
19. The superconducting magnet system of claim 18, wherein the quench protection system comprises a second heater mounted within the bore of the second coil.
20. The superconducting magnet system of claim 18 or claim 19, wherein in its second configuration the first quench heater is in thermal contact with each of the first and second coils.
21. The superconducting magnet system of any of claims 17 to 20, when dependent on claim 7, comprising first and second resistor arrangements connected in parallel with the respective first and second wire coils, wherein; when a quench occurs in one of the first and second coils, the respective resistor arrangement acts as the actuator.