WO2025071418A1

WO2025071418A1 - Passive temperature regulation in superconducting circuits

Info

Publication number: WO2025071418A1
Application number: PCT/NZ2024/050111
Authority: WO
Inventors: Max Ryan Goddard-Winchester; Benjamin Patrick Pennington Mallett; Jamal Rimkeit Olatunji
Original assignee: Victoria Link Limited
Priority date: 2023-09-28
Filing date: 2024-09-30
Publication date: 2025-04-03

Abstract

There is provided an assembly for regulating temperature in a system for supplying current to a superconducting load. The assembly may comprise a thermal switch positioned between a cryocooler and a length of superconducting material comprised in a loop of superconducting material in which a voltage is generated by a flux pump. The cryocooler may additionally be thermally connected to the superconducting load. The thermal switch may be configured such that the cryocooler cools the superconducting load to a lower temperature than the length of superconducting material. The temperature to which the length of superconducting material is cooled may be substantially close to a critical temperature of the length of superconducting material. Forms of the technology also relate to the thermal switch and the flux pump.

Description

PASSIVE TEMPERATURE REGULATION IN SUPERCONDUCTING CIRCUITS

1. FIELD OF INVENTION

The present technology relates to methods, systems and devices for temperature regulation in superconducting circuits. The present technology may find particular application in devices and systems for supplying energy to a load using a flux pump.

2. BACKGROUND TO THE INVENTION

Superconducting circuits have a wide range of applications. Examples of applications for systems including superconducting circuits include (and are not limited to): superconducting magnets; flux pumps; fault current limiters; and magnetic energy storage systems. These may be applied in systems used for, amongst others: space propulsion; nuclear fusion; nuclear magnetic resonance (NMR); magnetic resonance imaging (MRI); and levitation.

Many applications including superconducting circuits require low-voltage, high-current power supplies, for example to supply power to high temperature superconducting (HTS) electromagnets, which may be high-field, low-weight and consequently suitable for storing large amounts of energy. To meet these requirements, traditional power supplies require a large amount of space resulting in a large infrastructure challenge. Also, connecting a normal conducting circuit to a superconducting circuit housed in a cryostat introduces a large thermal load through the physical contacts into the cryostat, creating a cooling challenge. This requires sophisticated thermal design and imposes a considerable heat penalty on the cryostat and cooling system. It also incurs a significant voltage drop across the normal conducting circuit components, necessitating a significantly higher-power supply than required solely to energise the superconducting coil.

Superconducting power supplies in the form of flux pumps help address these issues. The higher current densities that are achievable in superconductors allow the power supply to be more compact. In addition, flux pumps allow energy to be supplied to a load without electrical contact between the power supply and the load. The lack of electrical contact may remove a path of thermal conductivity and reduces the cooling requirements. When a flux pump is used to supply power to a superconducting load, it may be advantageous for the superconducting load (e.g. a HTS coil) to be maintained at temperatures as low as possible as this permits higher magnetic fields to be generated. It may be simultaneously advantageous for some superconducting components of some types of flux pump to be maintained at elevated temperatures, for example close to their critical temperature. This is conventionally achieved using complex, expensive, or inefficient heat regulation systems.

3. OBJECT OF THE INVENTION

It is an object of the technology to provide improved methods, systems and devices for temperature regulation in systems for supplying current to a superconducting load. Alternatively, it is an object of the technology to provide an improved assembly for regulating temperature in a system for supplying current to a superconducting load. Alternatively, it is an object of the technology to provide an improved thermal switch configured for use in regulating the temperature of one or more components of a superconducting circuit. Alternatively, it is an object of the technology to provide an improved flux pump for providing current to superconducting load. Alternatively, it is an object of the technology to at least provide the public with a useful choice.

4. SUMMARY OF THE INVENTION

Aspects of the technology relate to methods, systems and devices for temperature regulation in systems for supplying current to a superconducting load. The present technology may find particular application in methods, devices and systems for supplying current to a superconducting load using a flux pump, for example flux pumps in which there is no electrical contact between the power supply and the superconducting load.

In one aspect of the technology, there is provided an assembly for regulating temperature in a system for supplying current to a superconducting load. The assembly may comprise a thermal switch positioned between a cryocooler and a length of superconducting material. The length of superconducting material may be comprised in a loop of superconducting material in which a voltage is generated by a flux pump. The cryocooler may additionally be thermally connected to the superconducting load. The thermal switch may be configured such that the cryocooler cools the superconducting load to a lower temperature than the length of superconducting material. In some forms, the temperature to which the length of superconducting material is cooled may be substantially close to a critical temperature of the length of superconducting material.

In one aspect of the technology, there is provided a thermal switch configured for use in regulating the temperature of one or more components of a superconducting circuit. In one aspect, there is provided a thermal switch for use in a system for supplying current to a superconducting load. The thermal switch may be configured to switch between a first configuration and a second configuration. In the first configuration, the thermal switch may provide a first thermal pathway between two contacts, which may be thermally connected to a cryocooler and a length of superconducting material in use. In the second configuration, the thermal switch may provide a second thermal pathway between the two contacts. The first thermal pathway may have a greater thermal conductance than the second thermal pathway. The thermal switch may be configured to enable the cryocooler to cool the superconducting load to a lower temperature than the length of superconducting material.

In another aspect of the technology, there is provided a flux pump for providing current to superconducting load. The flux pump may comprise an assembly for generating a voltage in a loop of superconducting material. The flux pump may further comprise an assembly for regulating temperature of the superconducting load and of a length of superconducting material comprised as part of the loop of superconducting material according to another aspect of the technology. The temperature of the flux pump may be regulated to be substantially higher than the temperature of the superconducting load.

According to one aspect of the technology, there is provided an assembly for regulating temperature in a system for supplying current to a superconducting load, the assembly comprising: a loop of superconducting material in which a voltage is generated during use, wherein the loop of superconducting material comprises a length of superconducting material; first and second terminals for connecting a superconducting load in parallel across the length of superconducting material for supply of current to the superconducting load; a cryocooler; and a thermal switch, wherein the cryocooler is configured to be thermally connected to the superconducting load to cool the superconducting load to a first temperature, and wherein the cryocooler is thermally connected to the length of the superconducting material through the thermal switch to cool the length of the superconducting material to a second temperature, wherein the second temperature is higher than the first temperature.

In examples, the thermal switch may be configured such that the second temperature is substantially close to a critical temperature of the length of superconducting material.

In examples, the thermal switch may be configured to switch between a first configuration and a second configuration, wherein, in the first configuration, the thermal switch provides a first thermal pathway between the cryocooler and the length of the superconducting material, and, in the second configuration, the thermal switch provides a second thermal pathway between the cryocooler and the length of the superconducting material, wherein the first thermal pathway has a greater thermal conductance than the second thermal pathway.

In examples, the thermal switch may be configured to switch between the first configuration and the second configuration as a result of changes in temperature of the thermal switch.

In examples, the thermal switch may be passive or otherwise able to switch between the first configuration and the second configuration without requiring a power source.

In examples, the thermal switch may switch between the first configuration and the second configuration via mechanical movement. For example, the thermal switch may include at least one member which expands or contracts as it is heated or cooled in order to provide mechanical movement within the thermal switch. In other examples the mechanical movement may be provided by a spring, differential thermal expansion, or a bi-metallic member.

In other examples, the thermal switch may be a gas-actuated or vapor-tension switch. For example, the thermal switch may transition between the first configuration and the second configuration using the thermal expansion of a gas or vapour. In some examples this thermal expansion may create a corresponding pressure on a diaphragm or piston which in turn may transition the switch between the first configuration and the second configuration. In other examples, the thermal switch may be a phase change thermal switch. For example, the phase change thermal switch may comprise one or more conductive fins with a thin layer of gas between. In these examples, when the temperature drops to a particular level, the gas may condense on the fins to create a vacuum which transitions the thermal switch to the second configuration as the thermal conductance of the switch decreases due to the presence of the vacuum.

In examples, the thermal switch may comprise a first contact thermally connected to the cryocooler and a second contact thermally connected to the length of superconducting material. The first contact and/or second contact may be made of a thermally conductive material such as copper.

In examples, the thermal switch may be configured so that the first contact and second contact move relative to each between the first configuration and the second configuration. In examples, the first contact may contact the second contact in the second configuration, and the first contact may be physically separated from the second contact in the first configuration.

In examples, the thermal switch may comprise a first member constructed from a first material with a first coefficient of thermal expansion, and a second member constructed from a second material having a second coefficient of thermal expansion which is less than the first coefficient of thermal expansion.

In some examples, the first member may be arranged in parallel with the second member. The relative coefficients of thermal expansion of the first material and the second material may be selected in order to control the movement of the first and second members in use. For example, heating or cooling of the thermal switch may cause the first material to expand or contract by a greater amount than the second material, to join or separate the first contact from/to the second contact.

In examples the thermal switch may be configured with the first and second coefficients of thermal expansion selected so that the thermal switch transitions between the first configuration and the second configuration at a thermal switching temperature of between approximately 70K, and approximately 90K, such as between approximately 75K and approximately 80K, such as approximately 78K.

In examples, the cryocooler may be configured to connect to the superconducting load via a thermally conductive contact made from a thermally conductive material such as copper. In examples, the thermally conductive contact may comprise a base plate, configured to in use attach to the cryocooler.

In examples, the thermally conductive contact may comprise a plurality of supports which in use are interposed between one or more layers of the superconducting load.

In examples, the length of superconducting material is comprised as part of a flux pump for providing current to the superconducting load. In some forms, the flux pump may be one of the following types of flux pump: switch rectifier; dynamo; self-rectifier and diode.

In examples, the length of superconducting material may be formed from a high temperature superconductor.

According to one aspect of the technology, there is provided a thermal switch for use in a system for supplying current to a superconducting load, the thermal switch comprising: a first contact configured in use to thermally connect to a cryocooler; and a second contact configured in use to thermally connect to a length of superconducting material, wherein the thermal switch is configured to switch between a first configuration and a second configuration, wherein, in the first configuration, the thermal switch provides a first thermal pathway between the first contact and the second contact, and, in the second configuration, the thermal switch provides a second thermal pathway between the first contact and the second contact, wherein the first thermal pathway has a greater thermal conductance than the second thermal pathway.

In examples, the thermal switch may be configured to switch between the first configuration and the second configuration via mechanical movement. For example, the thermal switch may include at least one member which expands or contracts as it is heated or cooled in order to provide mechanical movement within the thermal switch. In other examples the mechanical movement may be provided by a spring, differential thermal expansion, or a bi-metallic member.

In other examples, the thermal switch may be a gas-actuated or vapor-tension switch. For example, the thermal switch may transition between the first configuration and the second configuration using the thermal expansion of a gas or vapour. In some examples this thermal expansion may create a corresponding pressure on a diaphragm or piston which in turn may transition the switch between the first configuration and the second configuration.

In other examples, the thermal switch may be a phase change thermal switch. For example, the phase change thermal switch may comprise one or more conductive fins with a thin layer of gas between. In these examples, when the temperature drops to a particular level, the gas may condense on the fins to create a vacuum which transitions the thermal switch to the second configuration as the thermal conductance of the switch decreases due to the presence of the vacuum.

In examples, the first contact and/or second contact may be made of a thermally conductive material such as copper.

In examples, the thermal switch may be configured so that the first contact and the second contact move relative to each between the first configuration and the second configuration. In examples, the first contact may contact the second contact in the second configuration, and the first contact may be physically separated from the second contact in the first configuration.

In some examples, the first member may be arranged in parallel with the second member. The relative coefficients of thermal expansion of the first material and the second material may be selected in order to control the movement of the first and second members in use. For example, heating or cooling of the thermal switch may cause the first material to expand or contract by a greater amount than the second material, to join or separate the first contact from/to the second contact. In examples the thermal switch may be configured with the first and second coefficients of thermal expansion selected so that the thermal switch transitions between the first configuration and the second configuration at a thermal switching temperature of between approximately 70K, and approximately 90K, such as between approximately 75K and approximately 80K, such as approximately 78K.

According to one aspect of the technology, there is provided a flux pump for providing current to superconducting load, the flux pump comprising: a loop of superconducting material comprising a length of superconducting material; a voltage generation assembly for generating a voltage in the loop of superconducting material; first and second terminals for connecting a superconducting load in parallel across the length of superconducting material for supply of current to the superconducting load; a cryocooler; and a thermal switch, wherein the cryocooler is configured to be thermally connected to the superconducting load to cool the superconducting load to a first temperature, and wherein the cryocooler is thermally connected to the length of the superconducting material through the thermal switch to cool the length of the superconducting material to a second temperature, wherein the second temperature is higher than the first temperature.

In some examples, the flux pump may be one of the following types of flux pump: switch rectifier; dynamo; self-rectifier and diode.

In some examples, the voltage generation assembly may comprise a transformer comprising a primary coil and a secondary coil. The secondary coil may be comprised as part of the loop of superconducting material and may be electrically connected in series to the length of superconducting material. The secondary coil may be maintained at a superconducting temperature by the cryocooler.

In examples in which the flux pump is a switch rectifier flux pump, the flux pump may further comprise a magnetic field generator configured to apply a magnetic field to the length of superconducting material. The magnetic field generator may be configured to be selectively controlled to switch the length of superconducting material between a low-resistance state when a magnitude of the magnetic field is relatively low and a higher-resistance state when a magnitude of the magnetic field is relatively high. In the low-resistance state the transport current may be substantially less than a critical current of the length of superconducting material, and in the higher-resistance state the transport current may approach the critical current, be substantially equal to the critical current or be greater than the critical current.

In examples in which the flux pump is a self-rectifier flux pump, an alternating current induced in the secondary coil of the transformer may flow through the length of superconducting material, and the flux pump may be configured so that, for a part of a cycle of the alternating current when the current flows in a first direction, the alternating current approaches the critical current of the length of superconducting material, is substantially equal to the critical current or is greater than the critical current, and, when the alternating current flows in a second direction, the second direction being opposite to the first direction, a peak current is substantially less than the critical current. In this way, the length of superconducting material may remain in the low-resistance state when the current flows in the second direction, and may be in the higher-resistance state for the part of the cycle when the current flows in the first direction.

In examples in which the flux pump is a diode flux pump, the flux pump may be configured such that a critical current of the length of superconducting material when current travels through the length of superconducting material in one direction is different to a critical current of the length of superconducting material when current travels through the length of superconducting material in an opposite direction.

In examples in which the flux pump is a dynamo flux pump, the voltage generation assembly may comprise a rotor. The rotor may comprise at least one magnetic field generator configured to rotate with the rotor, the at least one magnetic field generator generating a magnetic field. The rotor may be positioned relative to the length of superconducting material such that rotation of the rotor causes a voltage to be generated in the length of superconducting material.

In examples, the flux pump may comprise a power source configured to supply an AC or switched DC current supply. In some examples, the current supply is provided to the primary coil of the transformer.

In examples, the cryocooler may comprise a cryostat, wherein one or more of the superconducting load and the length of superconducting material may be positioned within the cryostat. Further aspects of the technology, which should be considered in all its novel aspects, will become apparent to those skilled in the art upon reading of the following description which provides at least one example of a practical application of the technology.

5. BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the technology will be described below by way of example only, and without intending to be limiting, with reference to the following drawings, in which:

FIG. 1 shows an exemplary electric-field versus current graph for a high-temperature superconductor;

FIG. 2 is an illustration of graphs of electric field against current for a superconducting material when three external magnetic fields of different magnitude are applied;

FIG. 3 is a schematic illustration of a flux pump for supplying current to a superconducting load according to one form of the technology;

FIGS. 4A-4D are schematic illustrations of flux pumps for supplying current to a superconducting load according to other forms of the technology;

FIG. 5 is a circuit diagram of the flux pump for supplying current to a superconducting load illustrated in FIG. 4B;

FIG. 6A is a perspective view of a magnetic field generator used to transition a length of superconducting material from a low resistance state to a higher resistance state according to one form of the technology;

FIG. 6B is a perspective view of a magnetic field generator used to transition a length of superconducting material from a low resistance state to a higher resistance state according to another form of the technology;

FIG. 7A is a cross-sectioned side view of a thermal switch in a first configuration in accordance with the present technology;

FIG. 7B is a perspective cross-sectioned view of the thermal switch of FIG. 7A;

FIG. 7C is a cross-sectioned side view of the thermal switch of FIG. 7A in a second configuration in accordance with the present technology;

FIG. 7D is a perspective cross-sectioned view of the thermal switch of FIG. 7A in the second configuration; FIG. 8A is a schematic view of a differential thermal expansion type thermal switch in a first configuration in accordance with one example of the present technology;

FIG. 8B is a schematic view of the differential thermal expansion type thermal switch of FIG. 8A in a second configuration;

FIG. 9A is a schematic view of a thermal switch in accordance with one example of the present technology;

FIG. 9B is a schematic view of another thermal switch in accordance with one example of the present technology;

FIG. 10 is a schematic view of a gas gap thermal switch in accordance with one example of the present technology;

FIG. 11 is a schematic view of a sprung passive switch in accordance with one example of the present technology;

FIG. 12A is a perspective view of a flux pump comprising a thermal switch in accordance with one example of the present technology;

FIG. 12B is a close-up cross-sectional view of the thermal switch of FIG. 12A;

FIGS. 13A-D are graphs showing a performance comparison of the flux pump of FIG. 12A, with and without the thermal switch;

FIG. 14A is a perspective view of another flux pump comprising a thermal switch in accordance with one example of the present technology; and

FIG. 14B is a perspective view of another flux pump comprising a thermal switch in accordance with one example of the present technology.

6. DETAILED DESCRIPTION OF EXEMPLARY FORMS OF THE TECHNOLOGY

6.1. Superconductivity

A superconductor is a material that exhibits zero electrical resistance below a certain temperature known as the critical temperature, T_c. A superconductor exhibits a phenomenon known as the Meissner Effect, which is the complete expulsion of any magnetic field from the superconductor. Superconductors are perfect diamagnetic materials up until a certain magnetic field strength known as the critical field, B_c. At this point the superconductor cannot keep the magnetic field out, and thus, the superconducting phenomena is destroyed. This critical field also implies that there is a limit to the current that the superconductor can carry, known as the critical current, l_c. There are two types of superconductors, named type I and type II. Type I superconductors are typically pure metals and behave as described above. Type II superconductors behave differently. Type II superconductors allow some magnetic field to penetrate at a critical field H_ci < H_c without transitioning out of the superconducting state. Because of this, type II superconductors can carry much more current than type I superconductors, making them more useful for practical applications.

The critical temperature for a superconductor is conventionally defined as the temperature below which the resistivity of the superconductor drops to zero or near zero. In other words, a superconductor is said to be in its superconducting state when the temperature of the superconductor is below the critical temperature and in a non-superconducting state when the temperature is above the critical temperature. Many superconductors have a critical temperature which is near absolute zero; for example, mercury is known to have a critical temperature of 4. IK. It is however also known that some materials can have critical temperatures which are much higher such as 30K to 125K; for example, magnesium diboride has a critical temperature of approximately 39K, while yttrium barium copper oxide (YBCO) has a critical temperature of approximately 92K. These superconductors are often generally referred to as high-temperature superconductors (HTS).

6.1.1. Critical Current

The critical current for a high-temperature superconductor wire or tape is conventionally defined as the current flowing in a superconductor wire/tape which results in an electric field drop along the wire of 100 pV/m (= 1 pV/cm). The critical current is a function of both the superconducting material used, and the physical arrangement of the superconducting material. For example, a wider tape/wire may have a higher critical current than a thinner tape/wire constructed of the same material. Nevertheless, throughout the specification, reference to the critical current of the superconductor / superconducting material is made to simplify the discussion.

In a superconductor / superconducting material, if the current / is approximately equal to the critical current l_c, the resistance of the superconductor is non-zero, but small. However, if / is larger than the critical current l_c, the resistance of the superconductor may become sufficiently large to cause heat dissipation which can heat the superconductor to a temperature above its critical temperature, which in turn causes it to no longer be superconducting. This condition is sometimes referred to as a "quench" and can be damaging to the superconductor itself.

Figure 1 shows an exemplary plot depicting the internal electric-field versus current curve for a high- temperature superconductor. The electric field shown in this plot is related to resistance via the following equation: where:

• E is the electric field;

• / is the current through the superconductor;

• R is the resistance of the wire; and

• L is the length of the wire.

Accordingly, the plot of Figure 1 is related to the resistance per-unit length for the superconductor and, because the curve depicted is non-linear, the resulting resistance for the superconductor is non-linear with current.

In Figure 1 it can be seen that the electric field strength in the superconductor is substantially zero below the critical current l_c for the superconductor. As the current in the superconductor approaches the critical current, the electric field in the superconductor starts to increase. At the critical current, the electric field in the superconductor is 100 pV/m. Further increasing the current in the superconductor above the critical current results in rapid increases in the electric-field strength in the conductor.

The transition from the superconducting to the normal state in HTS materials, such as is shown in Figure 1, can be described by an empirical law known as the E-J power law: where E is the electric field in the conductor, J is the current density, and n is an experimentally defined unitless parameter which governs the steepness of the transition. In most superconductors, n has a value between 25-30. The critical current density J_c is defined by some arbitrarily chosen threshold field Eo, which may be 100 pV/m (= 1 pV/cm) as explained above. In this specification reference may be made to the relative resistances of a superconducting material and components comprising a superconducting material. More particularly, the specification refers to a superconducting material being in a low-resistance or higher-resistance state. It will be appreciated that, when in a superconducting state, superconducting materials can have a resistance which is zero or substantially zero, and as such these resistances are often expressed in terms of the electric field present across the superconducting material for a given current. Nevertheless, throughout the present specification, reference is made to relative resistances, for example low-resistance and higher-resistance states of the superconducting material, in order to simplify the discussion.

The term 'low-resistance state' may refer to when the superconducting material has a resistance that is close to or substantially zero in the superconducting state, or when the material has a low resistance in a partially superconducting state. The term 'higher-resistance' state refers to a state in which the superconducting material has a resistance that is substantially greater than the resistance in the low resistance state, for example a substantially non-zero resistance or a resistance that is close to zero but substantially greater than the resistance in the low-resistance state. For the avoidance of doubt, a higher-resistance state as referred to in this specification may, unless the context clearly indicates otherwise, include a superconducting state.

Similarly, where in this specification reference is made to a superconductor being in a higher-resistance state as a result of a current carried by the superconductor exceeding the critical current, it should be understood that, unless the context clearly indicates otherwise, the higher-resistance state may also be achieved if the current carried by the superconductor approaches or is substantially equal to the critical current.

In describing the technology in this specification, material and components comprising the material are referred to as "superconducting". This term is commonly used in the art for such materials and should not be taken to mean that the relevant material is always in a superconducting state. Under certain conditions the material and components comprising the material may not be in a superconducting state. That is, the material may be described as being superconductive but not superconducting. 6.1.2. Superconducting Materials

Certain forms of the present technology may comprise a variety of types of superconducting material. For example, forms of the technology may comprise high-temperature superconducting (HTS) materials. Exemplary HTS materials suitable for use in the forms of technology described include copper-oxide superconductors, for example a rare-earth barium copper oxide (ReBCO) such as yttrium barium copper oxide, gadolinium barium copper oxide or bismuth strontium calcium copper oxide (BSCCO) superconductors, and iron-based superconductors. BSCCO superconductors typically have a strong interdependence between critical current and an applied magnetic field, which may make them particularly suitable for some forms of the present technology. Other types of superconductors may be used in other forms of the technology.

While forms of the technology will be described in relation to high-temperature superconductors, it should be understood that other forms of the technology may use other types of superconductors, for example low-temperature superconductors, in their place.

6.1.2.1. Superconducting Tape

In certain forms of the technology, the superconducting material may take the form of a tape, i.e. a length of material having a length that is significantly larger than its width and its depth, and a width that is significantly larger than its depth. The tape may have two substantially parallel opposed faces, where the faces are separated by the depth of the tape.

6.1.3. Effect of Magnetic Field on Superconductors

The critical current in a superconductor is dependent on the external magnetic field applied to the superconductor. More particularly, the critical current decreases as a higher external magnetic field is applied to the superconductor, up to the value of the critical field, above which the superconductor is no longer in the superconducting (low resistance) state. This relationship is shown in Figure 2, which is an illustration of graphs of electric field against current for a superconducting material when three external magnetic fields of different magnitude are applied. The highest magnitude of external magnetic field, Bappi, results in the lowest critical current, l_ci. In some forms, the external magnetic field to achieve this effect may be applied perpendicular to a surface of the length of superconductor in which the critical current is reduced or suppressed. The applied magnetic field may be in one direction only, which may be referred to as a DC field, as compared to a time-varying magnetic field whose direction cycles, for example sinusoidally, which may be referred to as an AC field.

For all superconductors, the critical current drops off sharply with only a small applied magnetic field. This means that a small change in the applied magnetic field can result in a large change in the critical current. This relationship is dependent on the superconducting material and the way the length of superconducting material that carries current was manufactured.

It should be appreciated that this mechanism to reduce or suppress the critical current by applying an external magnetic field, e.g., a DC field, is different from the phenomenon of dynamic resistance. This occurs when a superconductor is exposed to a time-varying magnetic field while carrying a DC transport current. This creates a DC electrical resistance in the superconductor, which may be sufficiently large that the superconductor switches into a higher-resistance state.

6.2. Flux Pump

Forms of the technology provide flux pumps 600. A flux pump 600 may be understood to be an assembly or system that supplies power to a load 308, and in particular a load formed of superconducting material. For example, the load 308 may comprise an electromagnet comprising a coil of superconducting material. The flux pump 600 may be configured to supply current into the load 308 and to increase the level of current in the load.

Forms of the technology may provide different forms of flux pump 600. Schematics of exemplary forms of flux pump 600 according to certain forms of the technology are illustrated in FIGS. 3 and 4A. In this form, the flux pump 600 may comprise a loop of superconducting material 310 which itself comprises a length of superconducting material 305 as a sub-part of the loop. The flux pump also comprises a voltage generation assembly 610 configured to generate a voltage in the loop of superconducting material 310. The loop of superconducting material 310 may be considered to be a "charging loop" as it may function to charge the load 308. The voltage generation assembly 610 may comprise a power supply 301 configured to supply an alternating current (AC). The voltage generation assembly 610 may further comprise a secondary coil 305B of a transformer 304 (an example of which is shown in FIG. 5) in which a current and voltage is induced by the power supply 301. The secondary coil 305B may be comprised as part of the loop of superconducting material 310 and may be electrically connected in series to the length of superconducting material 305 by the loop 310.

The flux pump 600 further comprises first and second terminals 306A and 306B to electrically connect the load 308 in parallel across the length of superconducting material 305. In this way, the flux pump 600 may be configured to rectify the supply of AC in order to provide a direct current (DC) to the load 308. Different forms of the technology may use different forms of flux pump 600 in order to achieve this, such as those described later.

The flux pump 600 may further comprise a cryocooler 602 for maintaining the length of superconducting material 305 and the load 308 (in forms in which the load comprises superconducting material) at a suitable temperature (as explained in more detail later). The cryocooler 602 may be thermally connected to the load 308 and to the length of superconducting material 305, for example through thermal bus 606. In some forms, the cryocooler may comprise a cryostat 312 within which the superconducting elements of the flux pump 600, for example the load 308 and the length of superconducting material 305, are housed in order to maintain them at the desired temperature. In such forms, the voltage generation assembly 610 may be configured such that there is no electrical contact between the power supply 301 and either the length of superconducting material 305 or the load 308. Exemplary such forms of flux pump 600 are described later.

As explained earlier, it may be advantageous for the superconducting load 308 to be maintained at temperatures as low as possible as this permits higher magnetic fields to be generated. In some forms, it may also be advantageous for the length of superconducting material 305 to be maintained at a higher temperature than the temperature of the load 308, for example at a temperature that is close to the critical temperature of the length of superconducting material 305. In the form of FIG. 3, this may be achieved using thermal switch 604 which is thermally connected between the cryocooler 602 and the length of superconducting material 305. The operation of the thermal switch 604 will be explained further below.

Examples of particular forms of flux pump 600 which may be used in certain forms of the technology will now be described. 6.2.1. Switch Rectifier Flux Pump

FIG. 4B is a schematic illustration of an exemplary flux pump 600, which may be referred to as a switch rectifier flux pump. An exemplary circuit diagram that illustrates the flux pump 600 of FIG. 4B is shown in FIG. 5.

In these example forms, a power supply 301 is provided which comprises an alternating current supply 302 (not shown in FIG. 3) that supplies AC to a primary coil 304A. Consequently, an alternating magnetic field is generated by the primary coil 304A. The primary coil 304A is comprised as part of a transformer 304, that also comprises a secondary coil 304B. Accordingly, the magnetic field generated by the current flow ii in the primary coil 304A induces a second current flow /₂ in the secondary coil 304B of the transformer 304, the ratio of the primary side current being proportional to the ratio of the primary side windings to the secondary side windings, in accordance with Faraday's law. As the secondary coil 304B is comprised as part of a loop of superconducting material 310, a voltage is generated in the loop 310.

In the illustrated examples, the secondary coil 304B is electrically connected in series with the length of superconducting material 305 through further lengths of superconductor as part of the loop 310. A pair of terminals 306A, 306B, are provided in parallel across the length of superconducting material 305. In use a superconducting load 308 is connected between the first terminal 306A and second terminal 306B such that a current flows through the superconducting load 308, in parallel with the superconducting switch 305. In some examples of the technology, the superconducting load 308 may be formed as part of the flux pump 600, while in other examples the flux pump 600 may be configured to have a superconducting load 308 connected to it.

In examples of the technology, the length of superconducting material 305 comprises a length of high temperature superconducting (HTS) material, for example any of the types of HTS material described above. The HTS material has a critical current l_c and a critical temperature T_c. The HTS material may be positioned inside a cryostat 312, or otherwise be thermally connected to a cryocooler 602, configured to maintain the HTS material at a temperature that is less than the critical temperature T_c.

The flux pump 600 in FIGS. 4B and 5 further comprises a magnetic field generator 401 for applying a magnetic field B_app to the length of superconducting material 305. When an external magnetic field B_app is applied to the length of superconducting material 305, its critical current reduces, as shown in Figure 2. The application of the magnetic field B_app can therefore be selectively controlled to cause the length of superconducting material 305 to act as a switch. If the length of superconducting material 305 carries a switch current (i.e. a current flowing through the switch 305) that is less than the critical current when the magnitude of the magnetic field B_app has a certain value then the length of superconducting material 305 will be in a low-resistance state. If the magnitude of the magnetic field B_app is increased from that value to a relatively high magnitude that is sufficiently high that the critical current reduces to a value that is closer to, or below, the magnitude of the current carried by the length of superconducting material 305, then the length of superconducting material 305 will be in a higher-resistance state.

The low-resistance state of the length of superconducting material 305 can be considered equivalent to the closed state of switch 305 while the higher-resistance state is similar to an open state of switch 305. It should be appreciated, however, that the higher-resistance state is not an electrical open-circuit as would be common for a mechanical switch, but rather represents a higher-resistance conductive state.

In this higher-resistance conductive state, the length of superconducting material 305 may remain in the superconducting state but with a higher level of resistance, or it may be in a non-superconducting state.

The magnitude of the magnetic field B_app applied may be varied to provide a plurality of resistance states, including a resistance of continuously variable magnitude and including varying the resistance between two magnitudes. However for sake of simplicity in describing the operation of the present technology, reference herein will be made primarily to at least two states: a low-resistance state, and a higher resistance state. Note also that, in the low-resistance state the magnitude of the magnetic field B_app may be zero or non-zero.

In use the length of superconducting material 305 may be switched between low and higher resistance states in phase with the AC current waveform induced in the secondary coil 304B, either on the positive or negative half of the AC waveform. In the higher resistance state, a greater proportion of the current flow i2 is transferred into the load 308, than when the length of superconducting material 305 is in the lower resistance state. Accordingly, by activating the switch in phase with the positive or negative half of the AC current waveform, it is possible to provide an amplitude current flow in the positive or negative direction. Accordingly, this results in a net-DC current flow within the superconducting load, rectifying the supply of current to the load. In certain forms, the power supply 301 may comprise normally conducting components (i.e. non- superconducting components) while the secondary side of the transformer 304 may comprise superconducting components. For example, the power source 302 can include any suitable source of electrical power including an AC power supply, a switched DC power supply, a current or voltage source, and the primary side windings 304A may comprise normally conducting conductors such as copper conductors.

As the current flow in the primary side of the transformer 304 generates heat due to resistive losses in the normally conducting materials, it may be beneficial to thermally isolate the primary side 304A from the secondary side 304B, using a cooled environment 312, such as a cryostat, or other form of cryocooler 602. A benefit of a transformer 304 is that it may wirelessly transfer power into the cooled environment.

Examples of magnetic field generators 401 include magnets (for example permanent magnets and electromagnets) and conductors carrying currents (for example a length of superconducting material carrying a current).

In the form of technology shown in FIG. 4B, the magnetic field generator 401 comprises a magnetic core 406 formed from a material having a high magnetic permeability, for example iron or ferrite. In some examples, a permanent magnet may be positioned in close proximity to the magnetic core 406, for example sandwiched between two portions of the magnetic core 406. In another form, an electromagnet may be formed by winding a conductor around a portion of the magnetic core 406 and passing current through the conductor in order to generate a magnetic field through the magnetic core 406. In certain forms, the magnetic core 406 may comprise a gap 412, as illustrated in FIG. 4B and the length of superconducting material 305 may be positioned in the gap 412 so as to apply the magnetic field generated by the magnetic field generator to the length of superconducting material 305.

The magnetic core 406 may comprise any suitable shape or form to concentrate the magnetic flux onto the length of superconducting material 305. For example, the magnetic core 406 may comprise one or more tapered ends adjacent to the length of superconducting material. In other forms, the magnetic core 406 may comprise a plurality of teeth adjacent to the length of superconducting material. Further details of exemplary switch rectifier flux pumps are described in PCT Publication Nos. WO 2022/164329 and WO 2022/164330, the contents of which are herein incorporated by reference.

6.2.1. Self-Rectifier Flux Pump

Another exemplary form of flux pump 600 may be referred to as a self-rectifier flux pump. This will now be described with reference to the schematic illustration of FIG. 4A, the components of which have been described earlier.

In the self-rectifier flux pump 600, an AC is induced in the secondary coil 304B of the transformer 304, and this may flow through the loop of superconducting material 310 and through the length of superconducting material 305. The power supply 301 may be configured to induce a suitable AC in the secondary coil 304B so that, for a part of a cycle of the AC in the loop 310 when the current flows in a first direction, the AC approaches the critical current of the length of superconducting material 305, is substantially equal to the critical current or is greater than the critical current. In this part of the AC cycle, the length of superconducting material 305 may be in the higher-resistance state. In addition, the induced AC in the secondary coil 304B may be such that, when the AC flows in the opposite direction, a peak current of the AC waveform in this direction is substantially less than the critical current of the length of superconducting material 305. In this way, the length of superconducting material 305 may remain in the low-resistance state when the current flows in one direction, and may be in the higher- resistance state for the part of the cycle when the current flows in the opposite direction. Consequently, a DC may be supplied to the superconducting load 308 that is connected in parallel across the length of superconducting material 305.

Further details of exemplary self-rectifier flux pumps are described in PCT Publication No. WO 2022/164330, the contents of which are herein incorporated by reference.

6.2.2. Other Examples of Superconducting Switches

In the previously described examples of flux pumps, the length of superconducting material 305 effectively acts as a switch, transitioning between low-resistance and higher resistance states, which enables a DC to be supplied to the superconducting load 308, enabling it to be charged. In other forms, a superconducting switch may be formed in other ways. Examples of these and other forms of superconducting switches, and their applications in flux pumps can be found in PCT Publication No. WO 2021/080443 published on 29 April 2021, PCT Publication No. WO 2022/164329, published on 4 August 2022, and PCT Publication No. WO 2022/164330 published on 4 August 2022, the entire contents of which are herein incorporated by reference in their entirety. Some of these forms will be described briefly in the following paragraphs. In some forms, combinations of the described switching mechanisms may be used.

FIG. 6A illustrates another example of the type of superconducting switch described earlier in relation to the switch rectifier flux pump of FIGS. 4A and 5. In this form, a length of superconducting material 305 may be configured to act as a switch 305 by positioning the length of superconducting material 305 in the presence of a magnetic field generator 401. In the illustrated example, the magnetic field generator 401 comprises a power source 302 configured to generate a time-varying current flow in a length of conductor 404, which may be a normally conducting (non-superconducting) conductor. The time varying current flow induces a time-varying magnetic field in a magnetic core 406 (such as an iron core) which in turn applies the time-varying magnetic field across the surface 408 of the superconducting material, for example in a direction which is substantially perpendicular to or has a component which is substantially perpendicular to the surface 408 of the length of superconducting material 305.

The applied magnetic field reduces the critical current of the superconductor to provide a lower, field- supressed critical current as illustrated in FIG. 2. Accordingly, when the switch of FIG. 6A is used in conjunction with a flowing current in the length of superconducting material 305, the flowing current may equal or exceed the field-suppressed critical current of the length of superconducting material 305, and as a result the superconducting material 310 may act as a switch 305 transitioning from the low- resistance state to the higher resistance state.

The applied magnetic field from the magnetic field generator 401 may be synchronised with the induced current waveform so as to allow for the reduction in critical current of the length of superconducting material 305 to correspond with the positive or negative cycle of the induced current in the secondary coil 304B. For example, the magnetic field generator 401 may be operatively connected to the power source 301, or transformer 304 of FIGS. 4B and 5, so as to synchronise the magnetic field applied by the magnetic field generator. In other examples the power source 301 in the example of FIG. 3 may be synchronised with the power source 302 of the magnetic field generator 401 using any method known to those skilled in the art, such as an external trigger, clock or synchronisation signal. In other examples the magnetic core 406 of the transformer 304 in the example of FIGS. 4B and 5 may be the same magnetic core 406 used in the example of FIG. 6A, thereby synchronising the magnetic field generation used in the switch with the induced current i2 in the secondary coil 304B.

In another example of the technology illustrated in FIG. 6B a length of superconducting material 305 may be configured to act as a switch by providing a loop 408 in the length of superconducting material 310, and passing a time-varying magnetic field through the loop 408 in a direction which is substantially perpendicular to the axis of the loop 408, or has a component which is substantially perpendicular to the axis of the loop 408. Doing so may cause a screening current to flow within the length of superconducting material 310, this screening current, adds to the current already flowing in the length of superconducting material (the transport current), and if the sum of the transport current and the screening current exceed the critical current of the length of superconducting material, the superconducting material may act as a switch, and transition from a low-resistance state to a higher resistance state. Accordingly, like the example of FIG. 6A, the magnetic field generator 401 may be synchronised with the induced current in the superconducting load 308 in order to provide a regulated current flow, with a net-DC component.

A further switching mechanism which may be used is to apply a heat source to the length of superconducting material 305 configured to act as a switch in order to increase the temperature of the superconductor, and reduce the critical current of the superconductor. Accordingly, in some examples of the technology, the flux pump may comprise a heater 410 configured to heat the length of superconducting material 305 configured to act as a switch. An example of a heater 410 is shown in the example form of FIG. 6B. In this form, the heater 410 may be used in combination with the application of a magnetic field by magnetic field generator 401, while in other forms a heater 410 may be configured to heat the length of superconducting material 305 on its own, i.e. without the use of a magnetic field generator 401 in addition and in forms in which there is no loop in the length of superconducting material 310. In some examples of the technology, the heater 410 may be configured to be activated in a synchronised manner with the current induced in the load coil, so as to transition the switch from the low resistance state to the higher resistance state. For example, by using a power source 302 for the heater which is synchronised with the power source 302 for the flux pump. In other examples the heater may be configured to maintain the temperature of the switch at a temperature which is higher than the temperature of one or more other components of the flux pump 600, such as the superconducting load 310, or secondary coil 304B. For example, a temperature controller may be used such as a thermostat, or bimetallic strip. However, using energy to heat the length of superconducting material 305 within a cryocooled environment may be inefficient.

6.2.1. Diode Flux Pump

Another exemplary form of flux pump 600 may be referred to as a diode flux pump. This will now be described with reference to the schematic illustration of FIG. 4C, many of the components of which have been described earlier.

In the diode flux pump 600, an AC is induced in the secondary coil 304B of the transformer 304, and this may flow through the loop of superconducting material 310 and through the length of superconducting material 305. The flux pump 600 may be configured such that a critical current of the length of superconducting material 305 when current travels through the length of superconducting material 305 in one direction is different to its critical current when current travels through it in an opposite direction. This may be achieved by subjecting the length of superconducting material 305 to a plurality of magnetic fields. The flux pump 600 may be configured such that the effect of the plurality of magnetic fields on the length of superconducting material 305 is different when a current flows through it in one direction compared to when the current flows through it in the opposite direction. In some forms, the plurality of magnetic fields may comprise a self-magnetic field generated by the length of superconducting material 305 when current flows through it, and an applied magnetic field generated by a magnetic field generator 401. In other forms, the plurality of magnetic fields comprises: a first applied magnetic field generated by a first magnetic field generator; and a second applied magnetic field generated by a second magnetic field generator. The magnetic field generator 401 may be configured and arranged such that the applied magnetic field is similar to the self-magnetic field when current flows through the length of superconducting material 305 in one direction. In some forms, the magnetic field generator 401 may comprise one or more permanent magnets.

Further details of exemplary diode flux pumps are described in PCT Application No. PCT/IB2023/057123, the contents of which are herein incorporated by reference. 6.2.2. Dynamo Flux Pump

Another exemplary form of flux pump 600 may be referred to as a dynamo flux pump. This will now be described with reference to the schematic illustration of FIG. 4D, many of the components of which have been described earlier.

In the dynamo flux pump 600, the voltage generation assembly 610 may comprise a rotor 810. The rotor 810 may comprise at least one magnetic field generator 401 configured to rotate with the rotor 810, for example mounted on an external rim of a circular or cylindrical part of the rotor 810. The magnetic field generator 401 (which may comprise a plurality of permanent magnets or an electromagnet, for example) generates a magnetic field and the rotor may be positioned relative to the length of superconducting material 305 such that rotation of the rotor causes a voltage to be generated in the length of superconducting material 305 through induction. The length of superconducting material 305 is comprised as part of a loop of superconducting material 310 when a superconducting load 308 is connected to terminals at either end of the length of superconducting material 305 so that the generated voltage causes a current to be supplied to the load 308.

Further details of exemplary diode flux pumps are described in PCT Publication No. WO 2016/024214, the contents of which are herein incorporated by reference.

6.3. Thermal Regulation

In high-temperature superconductors, the critical current increases at a higher temperature (up to a maximum critical current or a maximum temperature, above which the superconductor is no longer superconducting). Many of the forms of flux pump 600 described in the previous sections use in their operation the change in resistance of a length of superconducting material 305 between a low- resistance and a higher-resistance state to act as a switch. Therefore, in some examples of the technology, it may be advantageous for the length of superconducting material 305 to be held at a temperature higher than the superconducting load 308, and furthermore it may be advantageous for the superconducting load 308 to be held at a temperature which is as low as practically possible, in order to maximise the current which can flow within the load. For example, the cryocooler 602 may be configured to operate with a maximum input power. In other words, the cryocooler 602 may be configured to reduce the temperature of the superconducting load 308 as low as is practical or possible. For example, it may be advantageous for the cryocooler 602 to act in a continuous manner to reduce the temperature of the superconducting load 308 as low as possible, rather than being configured to operate in a discontinuous or reduced power mode to regulate the temperature of the superconducting load 308 to a specific temperature, or within a specific temperature range. Doing so may, for example, increase the maximum current which can be carried in the superconducting load 308.

Accordingly, by providing some thermal separation between the superconducting load 308 and the flux pump 600, for example the length of superconducting material 305 that in some forms acts as a switch, it may be possible to operate the superconducting load 308 efficiently without negatively impacting the voltage generation of the flux pump 600. For example, by maintaining the superconducting load 308 and the flux pump 600 at different temperatures, it may be possible to:

• Reduce the delay between switching the length of superconducting material 305 into and/or out of the higher resistance state, as the increased temperature may allow for the current within the length of superconducting material 305 to be closer to its critical current without needing the current flowing to be increased;

• Allow for switching between the low-resistance state and the higher resistance state with a lower current flowing through the length of superconducting material 305;

• Increase an output voltage from the flux pump 600;

• Reduce the magnitude of the magnetic field which otherwise needs to be generated by the magnetic field generator 401 to transition the length of superconducting material 305 from the lower resistance state to the higher resistance state in some forms; and/or

• Simultaneously operate the superconducting load 308 at as cold a temperature as possible, hence increasing the critical current of the superconducting load 308 and increasing the magnetic field able to be generated by the superconducting load 308 (in the example of the superconducting load 308 being an electromagnet) for practical purposes.

It should be appreciated that, in examples of the technology comprising a heater 410, the addition of heat to the cooled environment/cryostat 312 may result in additional cooling requirements in order to maintain the cooled environment 312 at a desired operating temperature. In conventional systems in 1 which a heater 410 is used within a cryogenic environment, measurements indicate that, for every watt of power that is used to heat a component with that environment, approximate 20-30 W are needed to remove that heat. This is energy inefficient, and is particularly problematic in applications in which power is scarce, for example spacecraft propulsion systems (where the total power available may only be in the region of hundreds of watts). In addition, a control mechanism is needed to control the heater 410.

6.3.1. Thermal Switch

In certain forms of the technology, there is provided a thermal switch for use in a system for supplying current to a superconducting load 308, for example in a flux pump 600 such as described in this specification.

In the example of the form shown in FIG. 3 it has been explained that the superconducting load 308 is thermally connected to a cryocooler 602 such that the cryocooler maintains the superconducting load 308 below its critical temperature, and in a superconducting state. As previously mentioned, it may be advantageous to maintain a part of the flux pump 600, for example the length of superconducting material 305, at a higher temperature than the superconducting load 308, such as a temperature closer to the critical temperature of the length of superconducting material 305. This may be advantageous for the reasons explained above.

Accordingly, examples of the present technology provide a thermal switch 604 that is thermally connected between the cryocooler 602 and the length of superconducting material 305. The thermal switch 604 may be configured to transition between a first configuration, in which the thermal switch provides a thermal pathway between the cryocooler 602 and the length of superconducting material 305 having a first thermal conductance, and a second configuration in which the thermal switch provides a thermal pathway between the cryocooler 602 and the length of superconducting material 305 having a second thermal conductance, where the first thermal conductance is higher than the second thermal conductance such that any heat within the length of superconducting material 305 takes longer to conduct through to the cryocooler 602 in the second configuration compared to the first configuration. It should be understood that the thermal conductance of two pathways in a thermal switch according to certain forms of the technology may differ due to differences in the materials used (the intrinsic ability of a material to conduct heat is sometimes referred to as specific thermal conductivity/resistance) as well as in relation to the physical configuration of the materials used. For example, when considering a thermal pathway modelled as a plate having opposing faces with an area of A, and a thickness separating the faces L, the thermal conductance may be defined as: kA Thermal conductance = — L

Reference in this specification to thermal pathways with relative thermal conductances should be understood to mean thermal pathways having a greater or lesser ability to transfer conduct a heat difference quickly, with a higher thermal conductance representing a greater or faster heat flux through the pathway.

In the context of an operational flux pump 600 according to forms of the technology, the heat being transferred through the thermal pathways described herein may be generated from resistive losses in the superconducting components and/or through the use of a heater 410 as described earlier. In some applications, such as applications of the technology to devices for use in space (e.g. space propulsion systems), further sources of heating may include radiated heat, for example from the sun, or conducted heat from any housing which contains the superconducting components.

FIGS. 7A to 7D show one example of a thermal switch 604 in accordance with the present technology. In this example, the switch comprises a first contact 702 which in use, is configured to be thermally connected to one of a cryocooler 602 and a length of superconducting material 305, and a second contact 703 which in use is configured to be thermally connected to the other of the cryocooler 602 and length of superconducting material 305. Since in some forms it may not be important which of the first contact 702 and second contact 703 are thermally attached to the respective cryocooler and length of superconducting material, for the sake of the foregoing discussion we have arbitrarily described the first contact as being configured to thermally connect to the cryocooler, and the second contact as being configured to thermally connect to the superconducting switch 305, but, in other forms, the connections may be the opposite way to that described here. The thermal switch 604 of FIGS. 7A to 7D is configured to adopt two configurations, a first configuration shown in FIGS. 7A and 7B wherein the first contact 702 and second contact 703 are positioned in contact each other (which may be referred to as the contacts or the thermal switch 604 being closed), and a second configuration in which the first and second contacts are physically separated from each other (which may be referred to as the contacts or the thermal switch being open), as shown in FIGS. 7C and 7D. In the first configuration, the thermal switch 604 provides a first thermally conductive pathway 704 between the first contact 702 and the second contact 703 and in the second state the thermal switch provides a second, different thermally conductive pathway between the first contact and second contact via at least the first member 710, and in some examples the second 712 and third members 714. It may be desirable for the first thermally conductive pathway to have a relatively high thermal conductance, or low thermal resistance when compared to the thermal conductance of the second thermally conductive pathway.

For example, the first contact and second contact may be structured to have a large contact area in order to maximise the heat transfer between the length of superconducting material and the cryocooler, and/or the first and second contacts may be constructed of a material having a high specific thermal conductivity, or low specific thermal resistance. For example, the first and/or second contacts may comprise a material with a relatively high specific thermal conductivity such as copper, silver, or diamond.

In the illustrated example, the first contact 702 has a plate or disc-like construction which in the illustrated example is substantially circular in plan view. However, it should be appreciated that the first contact may have any suitable shape in plan view, including rectangular or square. The second contact 703 has a substantially ring-shaped profile in plan view, which allows a second thermal pathway 708 between the first contact 702 and the second contact 703 to extend through the centre of the ring. In the configuration in which the first and second contacts are in physical contact, facing side surfaces of the sections may abut.

In use, either of the first contact 702 or second contact 703 is thermally connected to cryocooler 602, and the part of the flux pump 600 for which temperature regulation is desired is thermally connected to the other of the first contact 702 and second contact 703. In the examples described herein the component which the thermal switch 604 is acting to regulate the temperature of is the length of superconducting material 305, however this should not be seen as limiting. For example, in some forms of the technology it may be advantageous to control the temperature of other superconducting components forming part of flux pump 600.

The second thermal pathway 716 between the first contact 702 and the second contact 703 comprises a first member 710 formed of a material having a first co-efficient of thermal expansion. Accordingly, as the temperature of the first member 710 changes, the relative positioning of the first contact 702 relative to the second contact 703 may be changed, due to the expansion or contraction of the first member 710. This may affect the thermal conductance of the thermal switch 604, i.e. the thermal switch may transition between a first configuration having a first thermal conductance (for example when the first contact 702 and second contact 703 are touching), and a second configuration having a second thermal conductance (for example when the first contact 702 and second contact 703 are separated).

In the illustrated example, the thermal switch 604 further comprises a second member 712 formed of a second material having a second co-efficient of thermal expansion. By selecting materials having different co-efficients of thermal expansion, it may be possible to tune the relative expansions and contractions of the first 710 and second 712 members, for example in order to adjust the response of the thermal switch with changes in temperature.

In some examples it may be beneficial to provide a third member 714 (and optionally further members) to further allow for adjustments of the response of the thermal switch 604 (described in more detail below). However, it should be appreciated that this is not essential to the working of the operating principle of the thermal switch 604, and any number of members may be used, including a single member.

As thermal expansion of materials is generally represented as a percentage change in the size of a material over temperature, a thermal switch 604 may be formed in some examples using a single member with a set co-efficient of thermal expansion, and tuning the inner and outer dimensions of the switch accordingly. In other words, the outer portion of the thermal switch, which in the examples of FIGS. 7A to 7D is represented by the second member 712 (and optionally third member 714), may have a length which is shorter than the internal central connection (shown as first member 710), such that during heating or cooling the total expansion or contraction of the central portion is greater than the total expansion or contraction of the outer portion. In some forms, the first thermal pathway 704, which is provided when the first contact 702 abuts the second contact 703, may have a higher thermal conductance due to the shorter pathway between the contacts. Additionally, the materials of the first 710 and optionally second and third materials 712, 714, may be selected to have a lower thermal conductivity, or otherwise structured to provide a lower thermal conductance than the first thermal pathway, resulting in heat within the switch 305 to be dissipated by the cryocooler 602 more slowly. Accordingly, the product of the thermal conductivity A, and cross-sectional area of these materials may be selected so that the second thermal pathway supports a reduced heat flow per degree of temperature difference (W/K) between the length of superconducting material 305 and cryocooler 602.

In the illustrated example, the first member 710 is arranged in parallel with the second member 712 and optionally the third member 714, such that the ratio of the lengths and thermal expansion characteristics of the first, second and third members 710, 712, 714 can be used to set the response of the thermal switch over temperature. Furthermore, in the illustrated examples, the first contact 702 comprises a substantially cylindrical plate which is attached to a first member 710 which includes a substantially solid cylindrical core that extends away from a top surface of the first contact in a substantially perpendicular direction. At a distal end of the solid cylindrical core, the first member 710 comprises a top plate which extends substantially parallel to the first contact, and perpendicular to the cylindrical core of the first member, the top plate having a larger radius than the cylindrical core. Attached to the top plate is at least one second member 714, and in some examples a third member arranged in parallel with the second member. In each case the second/third members are coaxial with the cylindrical core of the first member, and are dimensioned as substantially hollow cylinders. The second/third members extend from the top plate of the first member down to the second contact 703 which has a substantially annular shape. It should be appreciated however that this form is exemplary, and reference to cylindrical shapes could be replaced with any other suitable geometric shape, for example cuboid or other prism shape. Additionally, other examples of thermal switches 604 may be used, such as the alternative constructions described later.

In a first mode of operation of the thermal switch 604 shown in FIGS. 7A to 7D, the first contact 702 and second contact 703 are in contact with each other which provides a thermal pathway that allows for heat dissipation from the length superconducting switch 305 at a first rate. As the temperature of the thermal switch 604 reduces, the second 710, third materials 712 and optionally fourth 714 materials begin to contract according to their respective co-efficient of thermal expansion (CTE). It may be desirable for materials three 712 and four 714 to have a greater CTE than the second material 710 such that, as the thermal switch 604 is cooled, these materials contract at a greater rate than the second material 710 so that the third and fourth materials pull the second contact 709B apart from the first contact, breaking the first thermal pathway, and forcing heat to be conducted via the second thermal pathway.

Any suitable materials may be used for the first, second and third members. For example, the first member 710 may be stainless steel, and the second material 712 may be Teflon. In another example the first material 710 may be Invar and the second material 712 may be aluminium. In other examples, and one or more of the first 710, second 712, and third 714 materials may comprise a mixture of materials having different CTE values. For example, the mixture may comprise an epoxy mixture which contains a diamond powder to create a material having a customised CTE value.

As a result, at the moment of separation of the first contact 702 from the second contact 703, there is a step-change reduction in the cooling power of the cryocooler 602 in relation to the length of superconducting material 305. Accordingly, the first, second, and optionally third materials may be selected based on their specific CTE while the proportions and thermal conductivities of these materials can be used to tune at what temperature the thermal switch 604 transitions between first thermal pathway and the second thermal pathway.

Accordingly, exemplary thermal switches 604 according to the present technology switch between a first configuration which provides a first thermal pathway having a first thermal conductance, and a second configuration which provides a second thermal pathway having a second thermal conductance. In some examples, the thermal switch 604 may oscillate between these two configurations to effectively regulate the temperature of the components attached to the first and or second contacts.

The use of a second and third members allows the response of the thermal switch to be further adjusted as each material can have a different CTE, and therefore rate of expansion/contraction. Therefore, nonlinear movement of the second contact 703 relative to the first contact may be achieved as the second and third members co-operate to separate and/or connect the first and second contacts. In the illustrated example, the second and third members are provided in parallel with one another, and in parallel with the first member. Accordingly, by using different materials, with different CTE values, including non-linear CTE responses, the third material can either encourage or discourage connection or separation of the contacts based on temperature. For example, the second material may be selected such that at 80K it starts to exhibit expansion which encourages separation of the first contact from the second contact. The third material may be selected such that it has negligible or even a negative thermal expansion at 80K so as to encourage the first contact 702 to remain in contact with the second contact 703. As the temperature continues to reduce to, say, 70K the third material may start to expand which, together with the expansion of the second material, may encourage separation of the first contact from the second contact. As the contacts become separated, the thermal conductance of the pathway decreases, resulting in a temperature increase again (for example to 80K where the process repeats). In certain forms, the thermal switch 604 may transition between multiple configurations.

One application of the technology is in regulating the temperature of a length of superconducting material 305 in a flux pump 600. In this example, as the length of superconducting material 305 transitions from the low resistance state to the higher resistance state described herein, it starts to experience heating due to resistive losses in the superconducting material. Accordingly, the temperature of the length of superconducting material 305 may tend to increase over time. If the temperature of the length of superconducting material 305 exceeds a transition temperature of the thermal switch 604, the first, second, and optionally third members may expand or contract, causing the thermal switch 604 to close, bringing the first contact 702 into contact with the second contact 703 and providing the first thermal pathway 704, improving the thermal connection to the cryocooler and allowing for an increased rate of temperature transfer between the length of superconducting material and cryocooler, lowering the temperature again. This in turn will cool the length of superconducting material 305 to below the transition temperature, causing the thermal switch 604 to open again. With the correct selection of the first, second, and third materials, the thermal switch 604 can therefore open and close around a specific, pre-set temperature to maintain the length of superconducting material 305 at a desired temperature set point. This design may maintain the desired temperature of flux pump components with different operating heat loads and may simultaneously allow a superconducting load 308 to be cooled to much lower temperatures than the superconducting switch, thereby maximising the magnetic field that can be generated and the energy that can be stored in the load. 6.3.2. Other Types of Thermal Switch

While the foregoing example presents one form of a thermal switch in accordance with the present technology, this should not be seen as limiting and any type of thermal switch may be used including:

• A bimetallic or snap action switch which work on the principle on the thermal expansion of dissimilar metals in a similar manner as described in relation to FIG. 7A to FIG. 7D.

• A thermal reed switch which uses a pair of contacts on ferrous metal reeds which are actuated by a magnetic field. For example, the contacts may be normally open or normally closed and change state when a ferromagnetic substance reaches its curie point and alters the magnetic field surrounding the reed switch.

• A gas-actuated, or vapor-tension thermal switch. These use the thermal expansion of a gas or vapor in a sensing bulb to create a proportional pressure on a diaphragm or piston assembly that actuates the electrical switching element.

• A phase change switch. These have highly conductive fins with a thin layer of gas between. When the temperature drops to a particular level the gas condenses on the fins to create a vacuum which limits the thermal conductivity of the switch.

By way of example, FIG. 8A and 8B show an example of a differential thermal expansion switch 604 comprising a first contact 702, and a second contact 703 separated by a first member 710 and a second member 712. In this example the first member and second member(s) are arranged in parallel between the first contact 702 and the second contact 703, and, as in previous examples, the first member and second members may have a combination of different thermal expansions, as well as different thermal conductivities, or otherwise arranged with different thermal conductances in order to provide two thermal pathways having different thermal conductances.

In the illustrated example, in a first configuration illustrated in FIG. 8A, a first thermal pathway is provided from the first contact 702 and 703 through both the first material 710 and second material(s) 712 through to the second contact. As the temperature of the thermal switch 604 changes, for example due to cooling from the cryocooler, or heating from resistive losses in the flux pump 600, relative expansion or contraction of the first member relative to the second member can cause separation of at least one of the first material 710 or second material(s) 712 from either the first 702 or second 703 contacts thereby providing a second configuration illustrated in FIG. 8B where due to separation of at least one connection between the first contact and second contact, results in a thermal pathway with a reduced thermal conductance.

FIG. 9A shows a further example of a passive thermal switch 604 which operates based on the principles of thermal expansion. In this example, a first contact 702 and second contact 703 are provided which are connected by an expansion member 710 which expands or contracts based on its CTE properties. As in the previous examples of the technology, when the thermal switch 604 is used in a flux pump, the first contact 702 may be thermally connected to the cryocooler 602, and the second contact may be thermally connected to the superconducting switch 305 to provide thermal regulation of the superconducting switch within the flux pump.

The expansion member 710 may be attached at one end to the first contact 702. The other end of the expansion member 710 extends towards the second contact 703 and is in thermal contact with the second contact 703. For example a side of the expansion member 710 may abut against a surface of the second contact 703. The plane of the abutment may be parallel to a direction in which the expansion member 701 expands with changes in temperature so that the side of the expansion member 710 slides across the surface of the second contact 703 as the temperature changes. As the first member 710 expands, the amount of surface area of the expansion member 710 in contact with the second contact 703 increases, increasing the overall thermal conductance, while contraction of the expansion member 710 reduces the amount of the expansion member 710 in contact with the second contact, reducing the thermal conductance of the thermal switch 604.

In this example, it should be appreciated that the thermal switch 604 may provide a continuous range of thermal conductances ranging from a first configuration which provides a minimal thermal conductance when the first member is fully contracted, to a second configuration which provides a maximum thermal conductance when the first member is fully extended.

In the example of FIG. 9B, the thermal switch 604 acts and is structured in a similar manner to FIG. 9A with the exception that the second contact 703 comprises two distinct contacts 703A, 703B, each of which may be connected to different components of a superconducting circuit and/or may be provided with different materials to provide different thermal pathways each with different thermal conductances. In this example the expansion member 710 may be positioned so that it is spatially separated from the two second contacts 703A and 703B and the thermal switch 604 may comprise a third contact 709 mounted to the expansion member 710, for example to an end of the expansion member 710 distal to the end that is mounted to first contact 702. A side surface of the third contact 709 may be in thermal contact with a surface of the contacts 703A and/or 703B. In this form, as the temperature changes and the expansion member 710 expands and contracts, the third contact 709 slides along the surfaces of the second contacts 703A, 703B, which alters the thermal pathway between the first 702 and second contacts 703A, 703B.

FIG. 10 shows an example of a gas-actuated, or vapor-tension thermal switch 604 according to one form of the technology, in which the first contact 702, and the second contact 703 are separated by a medium 712. In a first configuration, the medium 712 may be a gas or vapour which is in thermal contact with both the first and second contacts and provides a thermal pathway between them. As the thermal switch 604 is cooled, the gas or vapour condenses against surfaces inside the thermal switch 604 and this transitions the switch into a second configuration, in which a vacuum (or part vacuum) is provided in the switch which reduces the ability for heat to be transferred between the first contact 702 and the second contact 703, therefore reducing the thermal conductance. Each of the first and second contacts 702 and 703 may comprise a plurality of protrusions, such as fins, that extend outwardly into the volume that contains the medium 712. The plurality of fins may interleave as shown in FIG. 10. The provision of such protrusions may increase the surface area on which the gas / vapour may condense, which may help the switch to transition between the first and second configurations.

FIG. 11 shows an example of a spring-based thermal switch 604 according to one form of the technology. In this example, the first and second contacts 702 and 703 are arranged spaced apart, for example with surfaces parallel to each other. A spring member 714 is positioned between the contacts and has respective ends connected to the first contact 702 and to the second contact 703. The spring member 714 may have a layered or coiled structure, as shown in FIG. 11. The spring constant of the spring member 714 may vary with temperature. For example, in a first configuration, adjacent layers of the spring member 714 may contact one another to reduce the path length between the first contact and the second contact, while in a second configuration, the spring member 714 may be extended and the layers of the adjacent layers of the spring may separate. In this second configuration there may be an increased physical path length between the first and second contacts 702 and 703, and therefore a reduced thermal conductance. Accordingly, forms the present technology allow for passive temperature regulation of one or more components of a flux pump 600, i.e. the thermal switch 604 may be able to switch between the first configuration and the second configuration without requiring a power source and without a control system.

In some examples of the technology, such as the thermal switch 604 as described in relation to FIG. 9A, the thermal switch may be able to rest in a steady-state configuration where the thermal conductance of the switch holds the flux pump 600 (or components thereof) at a desired temperature, without alternating between the first configuration and the second configuration.

6.4. Further Examples of the Technology

FIG. 12A shows one example of a flux pump 600 which comprises a thermal switch 604 in accordance with the present technology, while FIG. 12B shows a close-up partial cross-sectioned view of the thermal switch 604 of FIG. 12A.

In this example the components of the flux pump 600 are mounted to a thermal bus 606 in the form of a heat conducting member 802 formed of a thermally conductive material, i.e., a material having a relatively high thermal conductivity, such as copper. In the illustrated example, the heat conducting member 802 thermally connects to a cryocooler 602 (not shown), via a base plate 804. When connected to the base plate 804, the cryocooler 602 is connected with a direct thermal pathway through the heat conducting member 802 to the superconducting load 308. In the illustrated example, the superconducting load 308 is mounted to the heat conducting member 802 via a plurality of connecting arms 806, which thereby act as heatsinks. In some examples of the technology, these connecting arms are interposed between layers of the superconducting load 802, such as between loops of the coil of superconducting material form the load 802, to provide an improved thermal contact between the superconducting load 802, and the cryocooler 602.

Also connected directly to the heat conducting member 802 is a thermal switch 604. This thermal switch is connected thermally in series between the cryocooler, and the length of superconducting material 305 such that two thermal pathways are interchangeably to the length of superconducting material 305 by the thermal switch 604 dependent on temperature, as described earlier. In use, current is supplied to the flux pump 600, using any one or more of the methods described earlier. In the illustrated example, a transformer 304 is used which includes a primary coil 304A which is connected to a power source 302 (not shown), and a secondary coil 304B, which is connected to both the length of superconducting material 305 (via a superconducting loop 310), and to the superconducting load 308, which is connected in parallel across the length of superconducting material 305.

A magnetic core 406 is used to couple the magnetic field from the primary coil 304A to the secondary coil 304B. The primary coil is not subject to the cryocooled environment, for example the cryocooler is not thermally connected to the primary coil 304A. In some examples of the technology, a wall of a cryostat 312 may be positioned between the primary coil 304A and the secondary coil 304B to provide a thermal barrier between the primary coil 304A and power supply 302, and the secondary coil 304B, with the secondary coil 304B and other components of the flux pump 600, and load 308 positioned inside the cryostat 312.

In use, a current flow is induced in the secondary coil of the transformer 304B, which causes a current to flow in the length of superconducting material 305 and superconducting load 308. The length of superconducting material 305 periodically switches between a low resistance state and a higher resistance state, for example in response to an applied magnetic field (not shown). This may cause the temperature of the length of superconducting material 305 to rise. When the temperature of the thermal switch 604 exceeds a transition temperature, such as 80K, the thermal switch 604 is configured to transition from a first configuration in which there is a first thermal conductivity between the length of superconducting material 305 and the cryocooler 602, and a second configuration in which there is a second thermal conductivity between the length of superconducting material 306 and the cryocooler 602. The first configuration has a lower thermal conductivity that the second configuration.

When the switch is in the second configuration, the increased thermal conductivity allows an increased amount of heat within the length of superconducting material 305 to be dissipated in the cryocooler 602. This reduces the temperature of the thermal switch 604, and as a result the switch drops below a transition temperature, such as 80K, causing the thermal switch to transition from the second configuration, back to the first configuration, increasing the reducing the thermal conductivity between the length of superconducting material 305, and the cryocooler 602, which may result in the temperature of the length of superconducting material 305 rising over time once more. FIGS. 14A and 14B show further examples of flux pumps 600 in accordance with the present technology. As with the previous examples, a cryocooler 602 is thermally connected to a superconducting load 308 via a thermal bus 606 in the form of a heat conducting member 802. An induction coil 304B allows for current flow to be induced into a length of superconducting material 305. Connected in parallel across the length of superconducting material 305 is the superconducting load 308. In the example of FIG. 14A, a thermal switch 604 is connected between the heat conducting member 802 and the length of superconducting material 305. In the example of FIG. 14B, the thermal switch 604 is positioned between the length of superconducting material 305 and the superconducting load 308 in order to thermally connect them together. This means that the length of superconducting material 305 is thermally connected to the cryocooler 602 via the superconducting load 308, rather than connecting directly to the heat conducting member.

6.4.1. Thermal Switch Performance

In validating the performance of the flux pump of Fig. 12A, the inventors modelled the thermal properties of the exemplary illustrated flux pump 600. The model assumed copper was used for the thermally conductive material 802, G10 for the material supporting the superconducting material 310 of the superconducting load 308, stainless steel for the magnetic core 406, aluminium for the structure supporting the length of superconducting material 305, and an HTS composite, the thermal properties of which were experimentally derived.

For the purposes of the modelling, the inventors modelled a cryocooler 602 sold under the brand "Sunpower CryoTel MT". From a thermal perspective, the ends of the G10 mechanical suspension Thot were modelled as being held at an ambient temperature of 20°C, and the entire model was considered to be in a radiative thermal equilibrium with an ambient environment again having a modelled temperature of between approximately 20°C . The assembly was modelled as being wrapped in a multilayered insulation (MLI) having an effective emissivity of 0.012 for all surfaces, a value that was derived from laboratory thermal testing.

The length of superconducting material 305 was modelled as dissipating a quantity of heat, Qf_p, while the flux pump 600 is operating. For example, the typical dissipative heating on the superconducting switch may range from 250 to 550 mW. For modelling purposes, Qf_P was modelled as a volume heating source in the length of superconducting material 305, so that it was possible to investigate the system impact of different Qf_p values with and without a thermal switch 604.

The modelling was conducted looking at the transient heat transfer, starting from ambient temperatures. As the flux pump 600 is a superconducting device, Qf_P was modelled as being zero when the bridge temperature was above 90 K.

In a first example, a temperature range was modelled for the length of superconducting material 305 of between 77K to 90K. The limits 90K and 77K were chosen such that the length of superconducting material 305 is in a low-resistance, superconducting state, but not below 77K so that the critical current of the material 305 is not higher than that of the superconducting load 308. As such, the inventors configured a control algorithm for the cryocooler 602 into the model, throttling the input power into the cryocooler to maintain a temperature of the length of superconducting material 305 of approximately 77 K. This way, an equilibrium input power for the cryocooler to maintain a 77 K operating temperature could be obtained automatically for different Qf_P values.

This model was used to demonstrate the benefits of this invention. First, the model was run for a case that did not use a thermal switch 604, the results of which are shown in the graphs of FIGS. 13A and 13B. In this example, the thermal switch 604 was replaced with a copper-to-copper connection to the thermally conductive material 802, thermally connecting the flux pump 600 to the cryocooler 602. Results are shown for different levels of Qf_p.

Each separate instance of Qf_P resulted in the cryocooler throttling itself to maintain a temperature for the length of superconducting material 305 of approximately 77K, and hence the system did not utilise all the cooling power from the cryocooler that it had available. This resulted in elevated superconducting load temperatures, the consequence of which was a lower maximum magnetic field that can be generated.

Taking one example of Qf_p = 250 mW, the cryocooler throttled itself to 39 W of input power to maintain a bridge temperature of 77 K, resulting in a superconducting load temperature of only 70 K, as a result, the maximum magnetic field this magnet could generate was 1 Tesla. This was because the critical current of the superconducting magnet was supressed by being at an elevated temperature. In contrast, the modelling was repeated using a thermal switch 604 such as illustrated in FIGS. 12A and 12B which is configured to have a transition temperature of between approximately 70 K and 75 K with a switching ratio of approximately 1:10. In other words the thermal conductance of the thermal switch in its second configuration was modelled as being approximately 9 times higher than the thermal conductance of the thermal switch in the first configuration. The result of this testing is illustrated in the graphs of FIGS. 13C and 13D.

The thermal switch in this example is able to regulate the length of superconducting material 305 to the desired temperature, which means that the cryocooler is able to be fully utilised to reduce the temperature of the superconducting load, allowing the superconducting load to get as cold as possible. Comparing the result at the same Qf_p = 250 mW, the system using the thermal switch is able to passively maintain a temperature for the length of superconducting material 305 of 78 K while the superconducting load temperature reached 47 K, 23 K colder than in the case without a switch. This results in a substantial increase in the maximum field that can be generated within the superconducting load, of approximately 3 T, or a 300 % increase In the critical current.

6.5. Other Remarks

Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like, are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense, that is to say, in the sense of "including, but not limited to".

The entire disclosures of all applications, patents and publications cited above and below, if any, are herein incorporated by reference.

Reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that that prior art forms part of the common general knowledge in the field of endeavour in any country in the world.

The technology may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, in any or all combinations of two or more of said parts, elements or features.

Claims

Where in the foregoing description reference has been made to integers or components having known equivalents thereof, those integers are herein incorporated as if individually set forth. It should be noted that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the spirit and scope of the technology and without diminishing its attendant advantages. It is therefore intended that such changes and modifications be included within the present technology. 7. CLAIMS

1. An assembly for regulating temperature in a system for supplying current to a superconducting load, the assembly comprising: a loop of superconducting material in which a voltage is generated during use, wherein the loop of superconducting material comprises a length of superconducting material; first and second terminals for connecting a superconducting load in parallel across the length of superconducting material for supply of current to the superconducting load; a cryocooler; and a thermal switch, wherein the cryocooler is configured to be thermally connected to the superconducting load to cool the superconducting load to a first temperature, and wherein the cryocooler is thermally connected to the length of the superconducting material through the thermal switch to cool the length of the superconducting material to a second temperature, wherein the second temperature is higher than the first temperature.

2. An assembly as claimed in claim 1, wherein the thermal switch is configured such that the second temperature is substantially close to a critical temperature of the length of superconducting material.

3. An assembly as claimed in any one of claims 1-2, wherein the thermal switch is configured to switch between a first configuration and a second configuration, wherein, in the first configuration, the thermal switch provides a first thermal pathway between the cryocooler and the length of the superconducting material, and, in the second configuration, the thermal switch provides a second thermal pathway between the cryocooler and the length of the superconducting material, wherein the first thermal pathway has a greater thermal conductance than the second thermal pathway.

4. An assembly as claimed in claim 3, wherein the thermal switch is configured to switch between the first configuration and the second configuration as a result of changes in temperature of the thermal switch.

5. An assembly as claimed in any one of claims 3-4, wherein the thermal switch is passive or otherwise able to switch between the first configuration and the second configuration without requiring a power source.

6. An assembly as claimed in any one of claims 3-5, wherein the thermal switch is configured to switch between the first configuration and the second configuration via mechanical movement.

7. An assembly as claimed in claim 6, wherein the thermal switch may comprise a first contact thermally connected to the cryocooler and a second contact thermally connected to the length of superconducting material, wherein the thermal switch is configured so that the first contact and second contact move relative to each between the first configuration and the second configuration.

8. An assembly as claimed in claim 7, wherein the first contact contacts the second contact in the second configuration, and the first contact is physically separated from the second contact in the first configuration.

9. An assembly as claimed in any one of claims 6-8, wherein the thermal switch comprises a first member constructed from a first material with a first coefficient of thermal expansion, and a second member constructed from a second material having a second coefficient of thermal expansion which is less than the first coefficient of thermal expansion.

10. An assembly as claimed in claim 9, wherein the first member is arranged in parallel with the second member.

11. An assembly as claimed in any one of claims 9-10, wherein the thermal switch is configured with the first and second coefficients of thermal expansion selected so that the thermal switch transitions between the first configuration and the second configuration at a thermal switching temperature of between approximately 70K, and approximately 90K, such as between approximately 75K and approximately 80K, such as approximately 78K.

12. An assembly as claimed in any one of claims 1-5, wherein the thermal switch is a gas-actuated or vapor-tension switch.

13. An assembly as claimed in any one of claims 1-5, wherein the thermal switch is a phase change thermal switch.

14. An assembly as claimed in any one of claims 1-13, wherein the length of superconducting material is comprised as part of a flux pump for providing current to the superconducting load.

15. An assembly as claimed in claim 14, wherein the flux pump is one of the following types of flux pump: switch rectifier; dynamo; self-rectifier and diode.

16. An assembly as claimed in any one of claims 1-15, wherein the length of superconducting material is formed from a high temperature superconductor.

17. A thermal switch for use in a system for supplying current to a superconducting load, the thermal switch comprising: a first contact configured in use to thermally connect to a cryocooler; and a second contact configured in use to thermally connect to a length of superconducting material, wherein the thermal switch is configured to switch between a first configuration and a second configuration, wherein, in the first configuration, the thermal switch provides a first thermal pathway between the first contact and the second contact, and, in the second configuration, the thermal switch provides a second thermal pathway between the first contact and the second contact, wherein the first thermal pathway has a greater thermal conductance than the second thermal pathway.

18. A thermal switch as claimed in claim 17, wherein the thermal switch is configured to switch between the first configuration and the second configuration as a result of changes in temperature of the thermal switch.

19. A thermal switch as claimed in any one of claims 17-18, wherein the thermal switch is passive or otherwise able to switch between the first configuration and the second configuration without requiring a power source.

20. A thermal switch as claimed in any one of claims 17-19, wherein the thermal switch is configured to switch between the first configuration and the second configuration via mechanical movement.

21. A thermal switch as claimed in claim 20, wherein the thermal switch is configured so that the first contact and the second contact move relative to each between the first configuration and the second configuration.

22. A thermal switch as claimed in claim 21, wherein the first contact contacts the second contact in the second configuration, and the first contact is physically separated from the second contact in the first configuration.

23. A thermal switch as claimed in any one of claims 20-22, wherein the thermal switch comprises a first member constructed from a first material with a first coefficient of thermal expansion, and a second member constructed from a second material having a second coefficient of thermal expansion which is less than the first coefficient of thermal expansion.

24. A thermal switch as claimed in claim 23, wherein the first member is arranged in parallel with the second member.

25. A thermal switch as claimed in any one of claims 23-24, wherein the thermal switch is configured with the first and second coefficients of thermal expansion selected so that the thermal switch transitions between the first configuration and the second configuration at a thermal switching temperature of between approximately 70K, and approximately 90K, such as between approximately 75K and approximately 80K, such as approximately 78K.

26. A thermal switch as claimed in any one of claims 17-19, wherein the thermal switch is a gas- actuated or vapor-tension switch.

27. A thermal switch as claimed in any one of claims 17-19, wherein the thermal switch is a phase change thermal switch.

28. A flux pump for providing current to superconducting load, the flux pump comprising: a loop of superconducting material comprising a length of superconducting material; a voltage generation assembly for generating a voltage in the loop of superconducting material; first and second terminals for connecting a superconducting load in parallel across the length of superconducting material for supply of current to the superconducting load; a cryocooler; and a thermal switch, wherein the cryocooler is configured to be thermally connected to the superconducting load to cool the superconducting load to a first temperature, and wherein the cryocooler is thermally connected to the length of the superconducting material through the thermal switch to cool the length of the superconducting material to a second temperature, wherein the second temperature is higher than the first temperature.

29. A flux pump as claimed in claim 28, wherein the flux pump is one of the following types of flux pump: switch rectifier; dynamo; self-rectifier and diode.

30. A flux pump as claimed in any one of claims 28-29, wherein the voltage generation assembly comprises a transformer comprising a primary coil and a secondary coil.

31. A flux pump as claimed in claim 30, wherein the secondary coil is comprised as part of the loop of superconducting material and is electrically connected in series to the length of superconducting material.

32. A flux pump as claimed in any one of claims 30-31, wherein the secondary coil is maintained at a superconducting temperature by the cryocooler.

33. A flux pump as claimed in any one of claims 28-32, wherein the flux pump further comprises a magnetic field generator configured to apply a magnetic field to the length of superconducting material, wherein the magnetic field generator is configured to be selectively controlled to switch the length of superconducting material between a low-resistance state when a magnitude of the magnetic field is relatively low and a higher-resistance state when a magnitude of the magnetic field is relatively high, wherein in the low-resistance state the transport current is substantially less than a critical current of the length of superconducting material, and in the higher-resistance state the transport current approaches the critical current, is substantially equal to the critical current or is greater than the critical current.

34. A flux pump as claimed in any one of claims 30-32, wherein an alternating current induced in the secondary coil of the transformer flows through the length of superconducting material, and the flux pump is configured so that, for a part of a cycle of the alternating current when the current flows in a first direction, the alternating current approaches the critical current of the length of superconducting material, is substantially equal to the critical current or is greater than the critical current, and, when the alternating current flows in a second direction, the second direction being opposite to the first direction, a peak current is substantially less than the critical current.

35. A flux pump as claimed in any one of claims 28-32, wherein the flux pump is configured such that a critical current of the length of superconducting material when current travels through the length of superconducting material in one direction is different to a critical current of the length of superconducting material when current travels through the length of superconducting material in an opposite direction.

36. A flux pump as claimed in any one of claims 28-32, wherein the voltage generation assembly comprises a rotor, wherein the rotor comprises at least one magnetic field generator configured to rotate with the rotor, the at least one magnetic field generator generating a magnetic field, and wherein the rotor is positioned relative to the length of superconducting material such that rotation of the rotor causes a voltage to be generated in the length of superconducting material.

37. A flux pump as claimed in any one of claims 28-36, wherein the flux pump comprises a power source configured to supply an AC or switched DC current supply.

38. A flux pump as claimed in any one of claims 28-37, wherein the cryocooler comprises a cryostat, wherein one or more of the superconducting load and the length of superconducting material is positioned within the cryostat.