Superconducting magnet arrangement

Abstract

A high temperature superconducting, HTS, magnet. The HTS magnet comprises a coil pack, a heat spreader, a yoke, and a cooling system. The coil pack comprises a plurality of HTS coils, each HTS coil co

Description

[0001] Superconducting magnet arrangement

[0002] Field of the Invention

[0003] The present invention relates to superconducting magnets, in particular to high temperature superconducting magnets.

[0004] Background

[0005] Superconducting materials are typically divided into “high temperature superconductors” (HTS) and “low temperature superconductors” (LTS). LTS materials, such as Nb and NbTi, are metals or metal alloys whose superconductivity can be described by BCS theory. All low temperature superconductors have a self-field critical temperature (the temperature above which the material cannot be superconducting even in zero external magnetic field) below about 30K. The behaviour of HTS material is not described by BCS theory, and such materials may have self-field critical temperatures above about 30K (though it should be noted that it is the physical differences in composition and superconducting operation, rather than the self-field critical temperature, which define HTS and LTS material). The most commonly used HTS are “cuprate superconductors” - ceramics based on cuprates (compounds containing a copper oxide group), such as BSCCO, or ReBCO (where Re is any rare earth element, commonly Y or Gd). Other HTS materials include iron pnictides (e.g. FeAs and FeSe) and magnesium diborate (MgB2).

[0006] Applications of superconducting materials include superconducting magnets. As the magnet coils have zero resistance, superconducting magnets can carry high currents with very low losses (zero or near-zero loss within the superconductor, but there will be some losses from non-superconducting components), and can therefore reach high fields with much lower losses than conventional electromagnets.

[0007] For superconducting magnets having a high magnetic field, a “yoke” is often provided as shown in Figure 1. The yoke 101 (also known as an “iron return path”) is a structure of ferromagnetic material which provides a return path for magnetic flux from a coil pack 102 comprising a plurality of field coils, e.g. in order to limit the intensity of the magnetic field outside of the field coils. The presence of a yoke constrains stray fields, boosts the field achieved in the centre of the magnet, and damps vibrations. Due to the very high fields achievable with superconducting magnets, the yoke commonly substantially surrounds the superconducting field coil, to maximise the reduction in stray field. In Figure 1 , a wedge is taken out of the yoke 101 to show the coil pack 102 for illustration only, and the only gaps in the yoke would be the bore 103 and access for current leads. The coil pack 102 may also include an overbind 104 for each field coil (comprising a stainless-steel hoop, for example) on their radially outer surfaces to provide mechanical integrity.

[0008] The yoke may be “cold”, i.e. within the same vacuum vessel as the field coil and cooled to cryogenic temperatures or to some intermediate temperature, or “warm”, i.e. allowed to remain at ambient temperature.

[0009] A superconducting magnet is formed by arranging HTS cables (or individual HTS tapes, which for the purpose of this description can be treated as a single-tape cable) into coils, either by winding the HTS cables or by providing sections of the coil made from HTS cables and joining them together. HTS coils come in three broad classes:

[0010] • Insulated, having electrically insulating material between and separating the turns. In this arrangement, current can flow only around the turns of the coil (i.e., in a spiral path along the HTS cables).

[0011] • Non-insulated (Nl), where the turns are electrically connected radially, as well as along the cables. This can be achieved, for example, by forming the coil such that a copper stabilizer layer (or other metal cladding) on HTS tapes connects the turns, and / or by potting the coil with a conductive solder.

[0012] • Partially insulated (PI), where the turns are connected radially with a controlled resistance. This can be achieved by separating the turns with a material having a high resistance compared to copper (e.g., a co-wound stainless steel tape or any layer with a desired resistance separating adjacent turns), and / or by providing intermittent insulation between the turns, and / or by providing resistive material (which may comprise components such as resistors) along the side of a coil and connecting at least some of the turns. Different forms of partial insulation are described in W02019150123, W02020079412 and WO2021224279 as just some examples.

[0013] Non-insulated coils can be considered as the low-resistance case of partially insulated coils, in that both are coils in which current may be shared radially between turns via normally conductive (i.e. not superconducting, resistive) material. This alternative current path is in thermal contact with the coil such that resistive heating of the normally conductive material heats the coil.

[0014] Figure 2A illustrates an Nl or PI coil, having a plurality of turns 201 of HTS cable, which are connected by normally conductive material such that current may be shared radially between turns.

[0015] Current flow in non-insulated or partially insulated HTS magnet is more complex than in a fully insulated coil as the current can take two paths, either around the spiral path 202 via the coil of superconducting material, or through the radial path 203 via the normally conductive material connecting the turns. Figure 2B shows an equivalent circuit of the coil of Figure 2A. The spiral path 202 has negligible resistance and high inductance when the coil is fully superconducting, whilst the radial path 203 is resistive and has negligible inductance. In equilibrium, current will primarily flow in the spiral path. However, if current is prevented from flowing fully in the spiral path, e.g. due to HTS of the spiral path becoming normal, current will divert via the radial path. Current will also flow in the radial path if it is required to complete the circuit of the spiral path - e.g. if the power supply 204 is disconnected, and there is still current in the spiral path. Due to the resistance of the radial path, current travelling radially will cause heating of the magnet.

[0016] Figure 3 shows one example of a diversion of current in a non-insulated or partially- insulated coil. The coil has a spiral path 301 formed from HTS material, with conductive material connecting the turns radially. In the event of a part of the HTS material 302 becoming normally conducting (e.g. due to defects or cooling failure), current will divert from the spiral path 301 to avoid the normally conducting HTS, as it is significantly more resistive than the radial path 303. This will cause additional heating of the coil, potentially causing the magnet to quench. The overall stability of the magnet in the event of current diverting due to defects or thermal disruptions is improved by having a low radial resistance.

[0017] When a major quench occurs in a large PI or Nl coil, for example, caused by failure of the cryocooler, this causes a large shift in the magnetic field produced by the magnet, as current is quickly forced from the spiral path to the radial path. This will produce high mechanical loads in the magnet. This could cause damage to the magnet structure, and even if the magnet as a whole survives there will likely be degradation of the HTS material in the magnet due to the high stresses. This degradation will cause changes to the local current paths within the magnet during operation, and therefore will affect the homogeneity of the magnetic field, requiring the magnet to be re-shimmed.

[0018] In Nl or PI coils, if the magnet is disconnected from its power supply, current is driven through the radial path by the magnet’s inductance. This heats the magnet and rapidly reduces the transport current due to resistive losses. Such a scenario may be triggered in various situations, for example due to a fault in the PSU, a fault in the current leads connecting the magnet to the PSU (effectively disconnecting the power supply), or due to deliberate shutdown of the PSU e.g. in response to cryocooler failure or other situation requiring rapid action. Another scenario is a loss of system power, which will shut down both the cryocooler and the PSU.

[0019] Figure 4A shows the flow of current in the coil of Figure 2 in the event of a system power failure, with Figure 4B being the equivalent circuit. Current continues to flow in the spiral path 401 due to its high inductance and this current discharges over time through the radial path 403 and via a bypass diode 404 in or across the power supply. The bypass diode 404, is a “warm” diode that is not being cryogenically cooled. Nevertheless, a significant amount of the energy in the magnet can potentially be dissipated through the bypass diode 404 rather than through the radial path 403, reducing the chance of quench. In insulated magnets or partially insulated magnets with a sufficiently high radial resistance, the voltage across the coil will be greater than the knee voltage of the bypass diode 404, fully opening the diode such that significant current can flow through it. In non-insulated magnets, or partially insulated magnets with a low radial resistance, however, the low voltage generated across the coil will not allow significant current to flow via the bypass diode 404 of the power supply. Instead, as illustrated in Figures 4A and 4B, the majority of the current discharging from the spiral path 401 will complete the circuit via the radial path 403, flowing outward through the entire coil. The bypass diode 404 will also be ineffective if a current lead is damaged or disconnected. Either situation effectively forms a closed loop with the inductance of the spiral path 401 and the resistance of the radial path 403, which causes the coil current to discharge into the radial path 403. Figure 5 shows graphs of simulated magnet properties following a loss of system power resulting in failure of the PSU and cryocooler. At t=0 the power supply fails (i.e. transitioning from the scenario illustrated in Figure 1 to that illustrated in Figure 4A and B). Over the next few hundred seconds, the energy of the magnet is dumped into the HTS field coil by resistive heating of the radial path, causing a rise in coil temperature 501 and complex variations in “max gamma” 502. Max gamma is the maximum value of the ratio of the coil current to the critical current (or the current carrying capacity) of the HTS material of the coil. The critical current of the HTS material will vary throughout the coil depending upon the local temperature and magnetic field. The rapid increase in coil temperature 501 , however, causes max gamma 502 to rise overall despite the fall in coil current and magnetic field as the coil discharges. The energy is typically spread around the entire coil (as the radial heating will be substantially evenly distributed), but for large coils the temperature rise can still be more than enough to cause the HTS field coil to quench. This happens after approximately 120 seconds in this simulation, when at least some of the HTS material of the coil is unable to carry the full current flowing through it, as indicated by max gamma reaching one. This accelerates the dumping of the remaining energy in the coil, and rapidly reduces the field to zero over a few seconds or less, which can cause permanent damage to the coil. Throughout this process, the current through a bypass diode 503 remains low - on the order of a few amps compared to a typical operating current through the spiral path of over 1000A - such that the bypass diode cannot be used to dissipate significant energy from the magnet.

[0020] Max gamma is not a precise measure and a value of max gamma of one is not a hard line. The magnet will not necessarily quench if max gamma reaches one, and values of max gamma approaching one may cause the magnet to decline rapidly into a quench, depending upon the available cooling power and other factors. Nevertheless, maintaining max gamma substantially below one is a reasonable target when modelling and designing a magnet that will be resistant to quenching under different scenarios such as power failure.

[0021] The possibility of a quench is increased by a system power failure since both the power supply for the magnet coil and the cooling for the magnet may be affected or disconnection of the power supply may be triggered in response to a cooling failure, preventing the temperature rise from being offset by increased power to the cryocooler. A power failure may also prevent any active quench protection circuits from functioning (i.e. those relying on monitoring the magnet and routing power to an external energy dump in response to said monitoring). Any degradation of HTS performance can have significant knock-on effects, particularly in applications requiring tight control of the magnetic field such as magnetic resonance imaging.

[0022] As such, there is a need for solutions to prevent quenching of a non-insulated or partially insulated HTS magnet following the disconnection or failure of the power supply or in the event of a total system power failure.

[0023] Summary

[0024] According to a first aspect, there is provided a high temperature superconducting, HTS, magnet. The HTS magnet comprises a coil pack, a heat spreader, a yoke, and a cooling system. The coil pack comprises a plurality of HTS coils, each HTS coil comprising a plurality of turns of HTS material, wherein current can flow radially between turns of each HTS coil via resistive material. The heat spreader extends between first and second HTS coils of the plurality of HTS coils and extends beyond an outer diameter of the first and second HTS coils. The heat spreader is in conductive thermal contact with the first and second HTS coils, and the heat spreader is formed from a first metal. The yoke is formed from a second metal and is a cold yoke in thermal contact with the coil pack. In other words, the yoke is in thermal contact with the coil pack in order to be at substantially the same temperature as the coil pack during operation of the magnet. The cooling system is configured to cool the coil pack. The second metal is ferromagnetic, and the first metal has a greater thermal conductivity than the second metal.

[0025] In another aspect, the present invention provides a high temperature superconducting, HTS, magnet comprising a coil pack comprising a plurality of non-insulated and / or partially insulated HTS coils electrically connected in series. The magnet further comprises a metal heat spreader comprising: a plate extending between first and second HTS coils of the plurality of HTS coils and in conductive thermal contact with the first and second HTS coils; and a thermal battery outside an outer diameter of the coil pack and in conductive thermal contact with the metal plate. The plate further provides the series electrical connection between the first and second HTS coils. During run down of the magnet, such as in the event of disconnection or failure of the power supply, heat generated within the coils is transferred from the first and second coils to the plate and into the thermal battery. The plate and thermal battery may be made from copper or another metal with a high thermal conductivity to ensure that heat is transferred away from between the coils sufficiently quickly to prevent quench. The heat spreader may be formed as a single piece or from two or more pieces connected together in good conductive thermal contact.

[0026] During operation of the magnet of this aspect of the invention, the heat spreader will be charged, with the metal plate and thermal battery at substantially the same electrical potential. The thermal battery is therefore configured to avoid short circuits with other coils in the pack and, optionally, a yoke substantially surrounding the coil pack and the thermal battery. The yoke may be electrically connected to each end (or terminal) of the coil pack and will therefore be a split yoke having first and second parts such that, during operation of the magnet, the first part of the yoke is at a first electric potential, the second part of the yoke is at a second electric potential, and the heat spreader is at an intermediate third electric potential.

[0027] Brief Description of the Drawings

[0028] Figure 1 is a schematic illustration of a superconducting magnet having a yoke according to the prior art;

[0029] Figure 2A illustrates a non-insulated or partially-insulated coil;

[0030] Figure 2B is an equivalent circuit to the coil of Figure 2A;

[0031] Figure 3 shows an example of diversion of current in a non-insulated or partially insulated coil;

[0032] Figure 4A shows the flow of current in the coil of Figure 2 in the event of a system power failure;

[0033] Figure 4B is the equivalent circuit to the coil of Figure 4A;

[0034] Figure 5 shows graphs of simulated magnet properties following a loss of system power Figure 6 is a schematic illustration of an exemplary magnet; and

[0035] Figure 7 shows graphs of simulated properties of the magnet of Figure 6 following disconnection or failure of the PSU and cryocooler. Detailed

[0036] The heat generated within a coil pack comprising a plurality of Nl or PI HTS field coils following disconnection or failure of the power supply is essentially fixed by the energy of the transport current in the coil during operation. Tweaks to the coil parameters (e.g. to the radial resistance between turns) could affect the rate of heating of the coil, but these will also have further effects e.g. for ramp-up time of the coil. As such, the primary means of preventing quench during such a scenario is to improve the rate at which heat is extracted from the HTS coil, so that the rate of heating of the coil as it discharges, and therefore the peak in the ratio of the coil current to the critical current of the HTS material (gamma) is reduced.

[0037] The first part of this solution is to increase the volume or heat capacity of the cold mass in thermal contact with the coil pack, so that the energy released during discharge can all be absorbed. The “cold mass” is the material in thermal contact with the field coil that is cooled to the same or similar temperature as the operating temperature of the magnet, and can generally be considered to also include the field coil itself. In principle, increasing the heat capacity of the cold mass could be done through the addition of any material to the magnet - though metals are generally better due to their relatively high thermal conductivity and heat capacity. In HTS magnets which have a yoke surrounding a coil pack comprising a plurality of Nl or PI field coils, this may be achieved by arranging the yoke as a cold yoke in good thermal contact, e.g. conductive thermal contact, with the coil pack to act as a large thermal reservoir in the event of cryogenic failure. During operation of the magnet, the temperature of a cold yoke (or temperatures of all parts of the cold yoke) is therefore typically below and / or at substantially the same temperature as the coil pack. For example a cold yoke will typically be within approximately 10 K of the temperature of the coil pack (or a temperature of a portion of the coil pack) or, for a magnet operating at approximately 20 K and cooled by a cryocooler operating at approximately 10 K, the cold yoke will be between those two temperatures during normal operation.

[0038] The yoke will typically be made of iron or an alloy thereof, and for a high field magnet will already be required to be relatively large to avoid magnetic saturation. Additionally, the close contact between the yoke and the coil pack will help to align the magnetic field of the magnet with the HTS tape plane, reducing the impact of the magnetic field on the critical current of the HTS.

[0039] However, iron has a relatively low thermal conductivity compared to other metals, and so the heat generated by discharge of the HTS field coils may not be able to spread through the yoke quickly enough to avoid quenching the HTS field coils. HTS field coils and the interfaces between adjacent coils in a pack also have a relatively low thermal conductivity. As a second part of the solution, to ensure that the HTS field coils can dissipate heat effectively, a heat spreader of a more conductive metal (e.g. copper or a copper alloy) is added, extending between HTS field coils of the coil pack, and in conductive thermal contact with the HTS field coils. This may be direct thermal contact, e.g. where the heat spreader is in direct contact with the HTS field coils or attached via solder joints, or may be indirect e.g. via an electrically insulating layer on the surface of the HTS field coils or a terminal plate providing electrical connection between the HTS field coils and the heat spreader.

[0040] An example magnet is shown in Figure 6. The magnet comprises a coil pack 601 containing a stack of HTS field coils, ten in this example, connected in series. The magnet further comprises a heat spreader 602, a yoke 603, terminal plates 604, and a cryocooler 605.

[0041] Each HTS field coil is a Nl or PI coil, i.e. a coil where current can be shared radially between turns via conductive paths. Each coil is electrically connected to the next in series by any suitable means such as an interface plate as described in W02020079412. One or more overbinds 610 may be provided around the coil pack 601 , or each coil is provided with its own overbind 610.

[0042] The heat spreader 602 is a volume of a first metal which is in direct conductive thermal contact with the coil pack 601 , i.e. thermal contact such that conduction is the primary means of heat transfer. The heat spreader 602 is located between first and second coils of the coil pack 601 , e.g. at the mid-plane of the coil pack 601 , and is in direct thermal contact with the first and second coils. The heat spreader 602 may also provide an electrical connection between the first and second coils. The yoke 603 is formed from a second metal and is in conductive thermal contact with the coil pack 601. In this example, thermal conduction from the coil pack 601 to the yoke 603 is via the terminal plates 604. The first metal (forming the heat spreader) has a greater thermal conductivity than the second metal (forming the yoke).

[0043] The terminal plates 604 connect the coil pack 601 to the cryocooler via the outer coils of the coil pack 601. The terminal plates 604 also connect the coil pack 601 to the power supply (not shown) - in other examples these may be any suitable means of heat transfer e.g. heat pipes, fluid coolant systems, and separate components may be provided for electrical connection to the power supply and thermal connection to the cryocooler. The cryocooler provides cooling to the terminal plates and thereby to the heat spreader, iron yoke, and HTS field coil. In general, the cryocooler and terminal plates may be replaced with any suitable arrangement for cooling the HTS field coil, heat spreader, and iron yoke to cryogenic temperatures (i.e. the operating temperature of the HTS field coil).

[0044] In the example of Figure 6, the terminal plates 604 act as both electrical connection to the PSU and thermal connection to the cryocooler 605. As such, care must be taken in the design to avoid short circuits between the terminal plates 604 via the yoke 603 and / or the heat spreader 602. In the example of Figure 6, the yoke is in thermal contact with the terminal plates, and this can be achieved by also having the yoke in electrical contact with the terminal plates, with an electrically insulating gap 607 between upper and lower halves of a split yoke, the two halves being at different electric potentials during operation of the magnet. This electrically insulating gap will not significantly affect the magnetic performance of the yoke. The electrically insulating gap may contain an insulator, or may be a vacuum or air gap to reduce or avoid mechanical stresses caused by different thermal expansion properties of different parts of the magnet. The voltage across the coil in normal operation will be low (on the order of 1 volt), so any conventional electrical insulation or a small gap will generally be sufficient.

[0045] Similarly, the heat spreader 602 will be at a different electric potential than either of the two halves of the yoke and is electrically insulated from the terminal plates 604, the yoke 603 and the radially outermost surfaces of the coil pack 601 (including any overbind 610) by insulating gaps 608 or suitable electrically insulating material. The only electrical connection is via the HTS coils. The heat spreader in Figure 6 is therefore not in good conductive thermal contact with the yoke or terminal plates despite the illustrated physical proximity and is only cooled by the cryocooler via the HTS coils. In operation, the heat spreader therefore cannot readily dissipate heat generated by the coils to the surrounding environment and acts as a thermal battery to absorb and spread heat out from between the HTS coils.

[0046] Upon a sudden power failure, the HTS coils will generate heat as discussed previously and dissipate that heat into the terminal plates, iron yoke, and heat spreader. The outer coils of the pack have a direct thermal connection to the terminal plates and will quickly dissipate heat. Heat cannot escape from the inner coils of the pack through the outer coils to the terminal plates as effectively. If the heat spreader were not present, then the inner coils of the coil pack and, in particular, the coils at or near the mid-plane of the coil pack would likely overheat and quench, causing a sudden and deleterious collapse of the magnetic field. By providing the heat spreader in good thermal contact with the inner coils or conveniently at the mid-plane of the coil pack, the rate of heating of the inner coils can be substantially reduced.

[0047] Figure 7 shows graphs of simulated properties of the magnet of Figure 6 following disconnection or failure of the PSU and cryocooler, equivalent to the corresponding graphs of Figure 5. As can be seen, the additional heat capacity (or thermal mass) of the heat spreader allows a quench to be avoided entirely, as the coil temperature 701 rises less rapidly, and so the critical current is greater and max gamma 702 is lower in the simulation illustrated in Figure 7 than at equivalent times in the simulation illustrated in Figure 5. The current through the bypass diode 703 remains low, slowly dropping to zero over roughly two hours, indicating that the coil has completely and safely discharged in this simulation.

[0048] The magnet structure shown in Figure 6 may be used in magnetic resonance imaging, having a cylindrical bore 606 through the yoke, heat spreader, and coil pack, so that samples may be placed within the bore to expose them to the strong magnetic field. It will be appreciated that the teachings of this document are not restricted to this particular structure - i.e. in any magnet structure, and advantageously a magnet structure having a yoke in thermal contact with the coil pack (a “cold yoke”) to provide additional cold mass. Simulation techniques as well known in the art may be used to determine the required volume and shape of the heat spreader to keep the temperature of the HTS field coil below a threshold during e.g. PSU disconnection, and similar simulations may be used to determine any additional volume or changes of shape required for the yoke to compensate for the reduced volume of iron.

[0049] In the example of Figure 6, the heat spreader has a structure which is the union of a disc between adjacent HTS field coils of the coil pack (with a hole for the magnet bore) and an annular cylinder radially outward of the coil pack. The disc and the annular cylinder may be formed as a single piece or as two or more separate pieces connected together in direct conductive thermal contact to ensure that heat spreads out from between the coils to the annular cylinder. In the illustrated configuration, the heat spreader 602 extends beyond an outer diameter the coil pack 601 and further extends along a radially outer surface of the coil pack 601 to provide additional volume into which to spread and absorb heat. However, the heat spreader 602 may be any suitable shape, with the primary design considerations being a large or good thermal contact area with the coil pack (e.g., between first and second coils of the coil pack and in direct thermal contact with the first and second coils), relatively high thermal conductivity compared to the yoke and / or the coil pack, and a sufficiently large heat capacity to mitigate the heating of the HTS coil during power failure. In particular, the configuration of the heat spreader is such that, if both the PSU and cryocooler are disconnected or fail when the magnet is at its operating current and at its operating temperature, the temperature of the HTS material in the coil pack, particularly in the inner coils in the pack adjacent to the heat spreader, does not rise above a threshold temperature where its current carrying capacity is less than the remaining current flowing through it as the coils discharge. In other words, the coils are prevented from quenching or, alternatively, max gamma in the coils remains substantially below one until the coils have completely discharged.

[0050] The heat spreader may alternatively be designed to safely handle scenarios other than complete system power failure. As one example, it may be determined that the risk of complete power failure is low enough that enabling safe discharge from an equilibrium operating state in the event of power supply failure, such as a disconnected power lead, but with the cryocooler still operating, is sufficient.

[0051] Suitable sizes, shapes and materials of the yoke and the heat spreader may be determined by modelling using techniques as known in the art. For example, the thermal and electrical behaviour of the magnet during run-down following a power failure may be modelled via a thermal-electric network model, and the optimisation of the magnetic field (i.e. the yoke) may be modelled using finite element analysis.

[0052] The heat spreader may comprise a plurality of fins which extend into the yoke, to allow increased dissipation of heat from the heat spreader into the yoke’s cold mass. Where electrical isolation of the heat spreader and the yoke is required, thermally conductive electrical insulation may be provided between the heat spreader and the yoke, or a vacuum / air gap with heat transfer between the yoke and heat spreader by radiation. Radiative heat transfer will be slow such that the outer annular cylinder (or other structure) of the heat spreader will require a sufficient heat capacity to store the heat, acting as a thermal battery as the magnet discharges.

[0053] In addition to the “warm” bypass diode 404 connected across or in the power supply, a ’’cold” bypass diode may be provided across the coil pack in thermal contact with the cold mass (e.g. connected across the terminal plates). This ensures that a bypass diode across the coil is present even if there is a fault in the current leads, or a loss of superconductivity in superconducting parts of the current leads due to cooling failure. In a non-insulated or partially insulated magnet with low radial resistance, the voltage across the coil may be less than the knee voltage of the cold diode, but at least some current will be driven through the cold diode, as illustrated by the diode current line 703 in Figure 7, reducing the current on the radial path and reducing the rate of temperature increase of the coil pack.

[0054] Further passive control of the temperature rise may be achieved by incorporating a reservoir of a “phase change material”, i.e. a material which undergoes at least one phase change between the operating temperature of the magnet and the self-field critical temperature of the HTS (e.g., between around 20 K and around 80 K). The use of a phase change material causes “buffering” of temperature rises in the magnet due to the heat needed to overcome the latent heat of the phase change (i.e. the latent heat of vaporisation and / or melting, as appropriate). The reservoir of phase change material may be placed in direct thermal contact with the heat spreader, the HTS coil, or both.

[0055] The phase change may be a solid to liquid transition (i.e. melting), a liquid to gas translation (i.e. boiling) or a solid to gas transition (i.e. sublimation). Where the result after the phase change is a gas, it may be allowed to vent from the magnet. Where the result is a liquid, it may be held within the reservoir.

[0056] One suitable candidate for the phase change material is neon, which at atmospheric pressure melts at 24K and boils at 27K, with latent heats of melting and vaporisation of 16.4 kJ / kg and 85.9 kJ / kg respectively. In addition, neon is a noble gas and essentially inert, meaning that it does not introduce any hazards or possibility for corrosion. Another suitable phase change material would be nitrogen, which at atmospheric pressure melts at 63K and boils at 77K.

[0057] The phase change material may be introduced to the reservoir by providing it as a gas (or liquid, as appropriate) at a temperature above the operating temperature range, and cooling the magnet with the phase change material in-situ such that it condenses and / or freezes within the central region and / or coolant annulus. During controlled heating up of the magnet (e.g. for maintenance), the phase change material may be removed by pumping it out while liquid or gaseous to prevent unnecessary wastage.

[0058] Using a phase change material which transitions from solid to liquid to gas within the operating temperature will generally provide the greatest total latent heat. However, the heat must be distributed evenly through the phase change material (e.g. by cooling fins and / or portions of the heat spreader extending through the material) to prevent pockets of gas forming within the solid material. Selecting the phase change material, operating temperature range, and pressure such that only the transition from liquid to gas occurs within the operating temperature range will provide somewhat less latent heat (and therefore a greater temperature rise for a given amount of energy), but will allow for a simpler design to avoid this issue. The melting and boiling point of the phase change material may be varied by changing the pressure of the chamber housing the material - though this will generally have a greater effect on the boiling point than on the melting point.

[0059] Where only the liquid-gas phase change is desired, the lower bound of the operating temperature range should be above the melting point of the phase change material. For example, for neon, the lower bound of the operating temperature range should be above the melting point of 24.6K, e.g. the lower bound may be 25K.

Claims

CLAIMS:

1. A high temperature superconducting, HTS, magnet comprising: a coil pack comprising a plurality of HTS coils, each HTS coil comprising a plurality of turns of HTS material, wherein current can flow radially between turns of each HTS coil via resistive material; a heat spreader extending between first and second HTS coils of the plurality of HTS coils and extending beyond an outer diameter of the first and second HTS coils, wherein the heat spreader is in conductive thermal contact with the first and second HTS coils, the heat spreader being formed from a first metal; a cold yoke in thermal contact with the coil pack, the yoke being formed from a second metal; a cooling system configured to cool the coil pack; wherein the second metal is ferromagnetic, and the first metal has a greater thermal conductivity than the second metal.

2. A HTS magnet according to claim 1 , wherein the first metal is copper or an alloy thereof.

3. An HTS magnet according to claim 1 or 2, wherein the second metal is iron or an alloy thereof.

4. An HTS magnet according to any preceding claim, and comprising first and second electrical terminal plates, each electrical terminal plate being electrically connected to the coil pack such that current can flow from the first electrical terminal plate through each of the plurality of HTS coils of the coil pack in series to the second electrical terminal plate.

5. An HTS magnet according to claim 4, wherein: the yoke is formed from two sections, each section being electrically connected to a respective one of the electrical terminal plates, and each section being separated from the other section by a vacuum or air gap or electrical insulator.

6. An HTS magnet according to claim 4 or 5, wherein the heat spreader is separated from the yoke and the electrical terminal plates by a vacuum or air gap or electrical insulator.

7. An HTS magnet according to any of claims 4 to 6, wherein the cooling system is configured to cool the coil pack by cooling the electrical terminal plates.

8. An HTS magnet according to claim 7 wherein the yoke is in conductive thermal contact with the electrical terminal plates.

9. An HTS magnet according to any preceding claim, wherein the heat spreader electrically connects the first and second HTS coils in series.

10. An HTS magnet according to any preceding claim and comprising a diode connected across the electrical terminal plates and arranged to be cooled by the cooling system.

11. An HTS magnet according to any preceding claim, wherein the cooling system is further configured to cool the yoke.

12. An HTS magnet according to any preceding claim, and comprising a reservoir in direct thermal contact with the heat spreader and / or the HTS coil, the reservoir containing a phase change material, wherein the phase change material is a material which undergoes at least one phase change at a temperature between a normal operating temperature of the HTS field coil and a critical temperature of the HTS material.

13. An HTS magnet according to claim 12, wherein the phase change material is neon.

14. An HTS magnet according to any preceding claim, wherein the cooling system comprises a cryocooler in conductive thermal contact with the coil pack.

15. An HTS magnet according to any preceding claim, wherein the first metal has a coefficient of thermal conductivity which is at least double a coefficient of thermal conductivity of the second metal.

16. An HTS magnet according to any preceding claim wherein the heat spreader is configured such that if a supply of current to the magnet is interrupted when the magnet is in operation and a current in the magnet discharges through the resistive material, generating heat within the plurality of HTS coils, the temperature of the first and second HTS coils during the discharge does not rise above a threshold temperature at which the first and second HTS coils quench.

17. An HTS magnet according to any claim 16 wherein the heat spreader is configured to have a heat capacity sufficient to absorb and store the generated heat without the temperature of the first and second HTS coils rising above the threshold temperature.

18. A high temperature superconducting, HTS, magnet comprising: a coil pack comprising a plurality of non-insulated and / or partially insulated HTS coils electrically connected in series; and a metal heat spreader comprising: a plate extending between and providing the series electrical connection between first and second HTS coils of the plurality of HTS coils and in conductive thermal contact with each of the first and second HTS coils; and a thermal battery outside an outer diameter of the coil pack and in conductive thermal contact with the plate.

19. An HTS magnet according to claim 18 further comprising a yoke substantially surrounding the coil pack and the thermal battery and in conductive thermal contact with the coil pack.

20. An HTS magnet according to claim 19 wherein the yoke comprises first and second parts and wherein, during operation of the magnet, the first part of the yoke is at a first electric potential, the second part of the yoke is at a second electric potential, and the heat spreader is at a third electric potential.

21. An HTS magnet according to any of claims 18 to 20 further comprising a pair of electrical terminal plates respectively connected to third and fourth HTS coils of the coilpack, the third and fourth HTS coils being outer coils of the coil pack and the first and second HTS coils being inner coils of the coil pack.

22. An HTS magnet according to claim 21 further comprising a cooling system configured to cool the third and fourth HTS coils via the terminal plates and to cool the first and second HTS coils via the third and fourth HTS coils.

23. An HTS magnet according to any of claims 18 to 22 wherein the thermal battery comprises an annular cylinder radially outward of the coil pack and extending along a radially outer surface of the coil pack.

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