A method and system for maintaining cryogenic temperatures

Abstract

A method and system for maintaining apparatus at cryogenic temperatures and systems configured to maintain the temperatures is described. In one example the method and system comprise maintaining the temperature of a superconductor magnet comprising an internal reservoir, the method and system comprising the steps of: Adding a first portion of cryogenic slurry to the internal reservoir; Determining the first portion has at least partially melted; Replacing the first portion of cryogenic slurry with a second portion of cryogenic slurry.

Description

A METHOD AND SYSTEM FOR MAINTAINING CRYOGENIC TEMPERATURESFIELD OF THE INVENTION

[0001] The present disclosure relates to maintaining apparatus at cryogenic temperatures and systems configured to maintain the temperatures.BACKGROUND TO THE INVENTION

[0002] Superconducting objects, such as superconductor magnets require cooling during use to maintain temperatures suitable for superconduction. If the temperature of the superconductor is allowed to rise the superconductor can exceed its critical temperature and introduce resistance to the circuit.

[0003] In prior art systems, such as the levitated dipole experiment at Columbia University and MIT, a helium cryostat has been used, with a tube heat exchanger for initial nitrogen magnet cooling and daily helium re-cool. In other examples, such as JP2006052921A objects are immersed in a cryogen for cooling to a desired temperature, with the object being placed in a cooling tank. The cryogen(s) would typically be liquid hydrogen or nitrogen, providing a low temperature to cool the system. While immersing the superconductor in a vessel filled with cryogen would provide a cooling effect this is time limited. To improve the length of time available cryogen may be pumped around a system to constantly refresh the cryogen and maintain the required temperature.

[0004] However, these systems do not provide adequate temperature stability in some environments, for example where the superconductor magnets are moveable.SUMMARY OF INVENTION

[0005] It is an object of the present invention to provide a system or method for maintaining cryogenic temperatures of apparatus such as superconducting magnets.

[0006] In one aspect the present invention may be said to comprise a method of maintaining the temperature of a superconductor magnet comprising an internal reservoir, the method comprising the steps of: Adding a first portion of cryogenic slurry to the internal reservoir; Determining the first portion has at least partially melted; Replacing the first portion of cryogenic slurry with a second portion of cryogenic slurry.

[0007] Optionally the cryogenic slurry comprises one or more of nitrogen, neon, hydrogen, argon, oxygen or helium.

[0008] Optionally the cryogenic slurry comprises liquid and solid of the same material. Optionally the cryogenic slurry comprises liquid and solid of different materials. Optionally the cryogenic slurry is a mix slurries of different materials. Optionally the first and second portion of cryogenic slurry are obtained from an external reservoir. Optionally the first and second portion are obtained from the same external reservoir. Optionally the external reservoir comprises a slurry production device. Optionally comprising the step of connecting and / or disconnecting the internal reservoir from the external reservoir. Optionally comprising a valve configured to separate the first portion from the second portion of cryogenic slurry during operation of the superconductor magnet. Optionally the valve allows the separation of the first portion of magnetic slurry from the second portion of magnetic slurry. Optionally the internal reservoir and the external reservoir are disconnected during operation of the superconducting magnet, and reconnecting between operating events of the superconducting magnet. This allows for operation of the superconducting magnet without requiring a connection to the external reservoir.

[0009] Optionally the first portion is added through an inlet of the internal reservoir and removed through an outlet of the internal reservoir. Optionally comprising simultaneously removing the first portion of cryogenic slurry while adding the second portion of cryogenic slurry. Optionally the second portion is used to urge the removal of the first portion. Optionally determining the first portion has at least partially melted comprises determining that the portion of solid has fallen below a preset threshold. Optionally determining the first portion has at least partially melted comprises monitoring one or more of a time and a temperature.

[0010] Optionally determining the first portion has at least partially melted comprises monitoring a portion of the cryogenic slurry that has become liquid. Optionally the first portion is replaced once a threshold portion of the slurry has become liquid. Optionally comprising the step of circulating the portions of cryogenic slurry within the internal reservoir. Optionally comprising the step of cooling thesuperconductor magnet to an operational temperature before adding the first portion of cryogenic slurry. Optionally comprising the step of repeatedly adding and replacing further portions of cryogenic slurry. Optionally comprising the step of operating the superconductor magnet between the steps of adding the cryogenic slurry and replacing the cryogenic slurry to the internal reservoir. Optionally operating the superconductor magnet comprises one or more of: changing the location of the superconductor magnet, and adding current to the superconductor magnet. Optionally changing the location of the superconductor magnet comprises levitating the superconductor magnet.

[0011] Optionally comprising a closed loop process for re-cooling the first portion of cryogenic slurry for use in a later replacement. Optionally the first portion of cryogenic slurry is re-cooled for use in the next replacement.

[0012] Optionally comprising the step of a predictive system determining a time (or time-period) for replacing the first portion of cryogenic slurry. Optionally the predictive system comprises a machine learning system. Optionally a Kalman filter. Optionally the predictive system comprises a thermal system model. Optionally the predictive system is configured to receive a plurality of measurements from one or more sensors associated with the superconductor magnet. Optionally the machine learning method is trained to maximum the uptime of the magnet and / or minimise downtime of the magnet and / or maximise plasma duration. A predictive system may improve maintenance of the temperature of the superconducting magnet.

[0013] Optionally comprising controlling one or more temperature control systems. Optionally the temperature control systems comprise one or more heating or cooling systems. Optionally the temperature control systems are controlled based on the performance of the superconducting magnet. Optionally the temperature control systems are electric. Optionally the temperature control systems are configured to heat or cool a portion of the internal reservoir. Optionally the temperature control systems comprise one or more phase change materials. Optionally the phase change materials are located in the superconducting magnet and / or in the internal reservoir. Optionally the phase change materials are configured to maintain a desired temperature duringreplacement of the cryogenic slurry. Optionally the controller is configured to monitor the temperature of one or more of the phase change materials and initiate a preemptive action based on the monitored temperature. Optionally the one or more temperature control systems are configured to stratify the inner reservoir and / or to ensure the homogeneity of the inner reservoir and / or to prevent stratification of the inner reservoir. Optionally the superconducting magnet comprises one or more pumps to control stratification of the inner reservoir. Optionally the stratification is controlled at the replacement of the first portion with the second portion. Optionally the slurry in one or more containers of slurry is stratified.

[0014] Optionally comprising determining a variable flow rate of the pump to replace the first portion of cryogenic slurry. Optionally comprising the step of monitoring one or temperature sensors and controlling the flow rate based on the one or more temperature sensors. Optionally the temperature sensors are configured to determine the state of the cryogenic slurry. Optionally a delta-pressure is used to determine the state of the cryogenic slurry. Optionally the variable flow rate is increased if a greater percentage of liquid slurry is determined. Optionally the flow rate is dependent on the heat transfer coefficient between the cryogenic slurry and the internal reservoir.

[0015] Optionally wherein the step of replacing the first portion of cryogenic slurry with a second portion of cryogenic slurry comprises replacing one or more containers of cryogenic slurry within the internal reservoirs. Optionally the one or more containers are shaped to match one or more recesses of the internal reservoir. Optionally the one or more containers are substantially identical. Optionally the containers are orientated such that the solid portion of the cryogenic slurry within them is encourage towards a thermal interface of the internal reservoir. Optionally the containers are individually monitored. Optionally the containers have a thermal interface. Optionally the interface is configured to engage a corresponding interface of the interior reservoir. Optionally the containers comprise an inlet to allow the cryogenic slurry to be replaced. Optionally the containers are configured to allow the cryogenic slurry to be re-cooled within thecontainers. The use of containers may reduce contamination of the magnet and / or increase the speed of recharge.

[0016] In one aspect the present invention may be said to comprise a system for maintaining the temperature of a superconductor magnet comprising an internal reservoir, the system comprising: an external reservoir for holding a cryogenic slurry; one or more containers for moving cryogenic slurry into the internal reservoir; A controller configured to: determine the thermal state of the one or more containers of cryogenic slurry in the internal reservoir; and replace the one or more containers containing of cryogenic slurry with one or more further containers of cryogenic slurry based on the determination. The above options may be applied to this aspect.

[0017]

[0018] In one aspect the present invention may be said to comprise a system comprising a controller configured to implement any one or more of the above methods. Optionally the system configured to maintain the temperature of a superconducting magnet.

[0019] In one aspect the present invention may be said to comprise a system for maintaining the temperature of a superconductor magnet comprising an internal reservoir, the system comprising: an external reservoir for holding a cryogenic slurry; a pump for moving a portion of the cryogenic slurry into the internal reservoir; A controller configured to: determine the thermal state of the portion of cryogenic slurry in the internal reservoir; and control the pump to replace the first portion of cryogenic slurry with a second portion of cryogenic slurry based on the determination.

[0020] Optionally the external reservoir is repeatedly connectable to the internal reservoir. Optionally the internal reservoir comprises a path extending between an inlet and an outlet. Optionally the pump is configured to pump the first and second portion through the inlet to the internal reservoir and out of the outlet. Optionally the pump is configured to pump the first and second portion simultaneously. Optionally the inlet and outlet are each repeatedly connectable to the external reservoir. Optionally the external reservoir comprises a cryogenic generator for one or more of producing and cooling the cryogenic slurry. Optionally the superconductor magnet is moveable,optionally wherein the system comprising a dock, to position and / or orientate the superconductor magnet when replacing the cryogenic slurry. Optionally the superconductor magnet comprises a guide configured to guide the position of the superconductor magnet to and from the dock.

[0021] Optionally the superconductor magnet is a levitating magnet. Optionally the superconductor magnet is contained within a pressure vessel and the external reservoir is outside the pressure vessel. Optionally the internal reservoir comprises one or more fins configured to improve the thermal conduction of the cryogenic slurry. Optionally the fins extend into the internal reservoir. Optionally the internal reservoir comprises a pump to move the cryogenic slurry about the internal reservoir. Optionally the internal reservoir comprises a passive thermal system configured to enhance the thermal transfer to the cryogenic slurry. Optionally the passive thermal system is driven by gravity and / or movement of the superconductor magnet. Optionally the passive thermal system allows solid cryogen in the slurry to settle in a first location and a liquid portion of the slurry to settle in a second location. Optionally the external reservoir and / or the connector between the external reservoir and the internal reservoir are insulated.

[0022] In one aspect the present invention may be said to comprise a method of maintaining the temperature of an object comprising an internal reservoir, the method comprising the steps of: Adding a first portion of a coolant slurry to the internal reservoir; Determining the first portion has at least partially melted; Replacing the first portion of coolant slurry with a second portion of coolant slurry. Optionally comprising any one or more of the steps described herein with object replacing magnet or superconducting magnet. The system and method may be used on other objects which require cooling during time periods at which they are detached from a source of coolant.

[0023] In one aspect the present invention may be said to comprise a system for maintaining the temperature of a object comprising an internal reservoir, the system comprising: an external reservoir for holding a cryogenic slurry; a pump for moving a portion of the cryogenic slurry into the internal reservoir; A controller configured to:determine the thermal state of the portion of cryogenic slurry in the internal reservoir; and control the pump to replace the first portion of cryogenic slurry with a second portion of cryogenic slurry based on the determination.

[0024] Optionally the object comprises a medical device. Optionally an MRI system. Optionally the object is portable. Optionally the object is a quantum computer. Optionally the system is configured to maintain the temperature of a portion of a helium cryo-cooler. Optionally the systems provides an operational buffer to a helium cooling system.

[0025] In one aspect the present disclosure may be said to comprise a method of maintaining the temperature of an object comprising one or more internal spaces, the method comprising the steps of: adding a first portion of cryogenic slurry to the one or more internal spaces; determining the first portion has at least partially melted; replacing the first portion of cryogenic slurry with a second portion of cryogenic slurry. Optionally the object is a superconducting magnet. Optionally a levitating superconductor magnet.

[0026] Optionally the internal spaces comprise internal compartments. Optionally the first portion of cryogenic slurry and the second portion of cryogenic slurry are held in one or more containers. Optionally replacing comprises removing one or more of the containers holding the first portion of cryogenic slurry and replacing them with one or more containers holding the second portion. Optionally the one or more containers are shaped to fit in specific recesses or openings of the superconductor magnet.

[0027] In one aspect the present disclosure may be said to comprise a system for maintaining the temperature of a superconductor magnet comprising an internal reservoir, the system comprising: a plurality of containers configured to hold cryogenic slurry, and a superconductor magnet comprised an internal space to receive one or more of the plurality of containers, wherein the controller is configured to determine the thermal state of a first portion of cryogenic slurry in one or more containers within the internal space; and control the replacement the first portion of cryogenic slurry with a second portion of cryogenic slurry in one or more of the containers, based on the determination. Optionally any one or more of the options above may be combined withthis aspect. Optionally the system is a medical device, MRI, quantum computer, or portable device.

[0028] The term "comprising" as used in this specification means "consisting at least in part of". When interpreting each statement in this specification that includes the term "comprising", features other than that or those prefaced by the term may also be present. Related terms such as "comprise" and "comprises" are to be interpreted in the same manner.

[0029] It is intended that reference to a range of numbers disclosed herein (for example, 1 to 10) also incorporates reference to all rational numbers within that range (for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of rational numbers within that range (for example, 2 to 8, 1.5 to 5.5 and 3.1 to 4.7)BRIEF DESCRIPTION OF DRAWINGS

[0030] Examples will be described with reference to the following drawings, of which:

[0031] Figure 1 is a diagram of a cryogen store connected to a magnet with an internal pathway.

[0032] Figure 2 is a diagram of a cryogen store connected to a magnet with an internal reservoir.

[0033] Figure 3 is a flow diagram of a monitoring system for the cryogen monitoring method can be implemented on the respiratory apparatus of Figure 1.

[0034] Figure 4 is a diagram of a system with a plurality of cryogen containers for interchange.DETAILED DESCRIPTION

[0035] While the following description refers to superconductor magnets the principles may be applied to other apparatus, such as magnets, levitating dipoles, electric aircraft, and satellites. While cryogenic slurries are described it is also possible that higher temperature slurries may be used in cases where the temperatures required are higher. In these cases the slurry may be referred to as a coolant slurry.

[0036] In some cases the method are used in systems where the apparatus or device are moveable, so as to not allow a permanent connection to an external cryogen reservoir. In some examples the methods are used where a period of the apparatus or device not moving is sought to be decreased. In both case by transferring a cryogen slurry to the device or apparatus the system allows the temperature to be maintained efficiently.

[0037] A levitated dipole is a type of nuclear fusion reactor where a toroidal magnet (comprising a superconducting ring within) is levitated inside a reactor vessel. The poloidal magnetic field generated by the magnet, in use, is used to contain a plasma surrounding the torus in the reactor vessel. The plasma may then be heated and used to generate power through nuclear fusion reactions. The magnet comprises a pressure vessel, within which sits the superconducting ring or toroid, control electronics, charging electronics and cooling apparatus. A current must be generated within the superconducting ring and maintained to control the plasma. Both the electronics within the magnet, and the plasma surrounding the magnet generate heat. To maintain the superconductor within the magnet below its critical temperature it is therefore necessary to cool, or maintain a cryogenic temperature, of the magnet. This is made more difficult where, in use, the magnet levitates within the pressure vessel without an external connection, or at least without a connection sufficient to transfer a coolant to the magnet.

[0038] It has been found that sufficient cooling can be achieved to maintain a cryogenic temperature of a superconductor by adding a cryogenic slurry to an internal reservoir. The slurry means that the latent heat of fusion (changing from solid to liquid state) can be used, providing additional cooling power compared to a liquid alone. This also helps to ensure that a constant temperature is substantially maintained - because the phase change occurs at a constant temperature. Using a cryogenic slurry allows the coolant to be switched quickly - by pumping and replacing the slurry from the internal reservoir. This means that any downtime of, for example, a magnet is minimised.

[0039] A slurry is a mixture of solids suspended in a liquid. Depending on the relative percentages of solid and liquids the transport parameters of the slurry maychange. Where there is sufficient liquid present the slurry may be pumped or moved by, for example, a centrifugal pump. In some cases, a slurry may be referred to as a slush fluid, where, again, solid particles are mixed with a liquid. A cryogenic slurry is formed from a material at low temperatures. A cryogenic temperature may be less than 120K. To form a slurry a phase transition must occur. Common materials include hydrogen, nitrogen, neon, argon, helium, and carbon dioxide.

[0040] A cryogenic slurry may be formed by a single material in solid and liquid form. Alternatively, a cryogenic slurry may be formed by a mixture of different materials (at least one and optionally all being at cryogenic temperatures). For example, a cryogenic slurry could be formed by solid and liquid neon. In another example the cryogenic slurry could be formed by a first liquid and a different solid, such as solid neon and liquid hydrogen. The materials chosen may be adapted to improve flow and / or temperature characteristics. For example, the (originally) solid material could be a first material selected to retain cooling, while the liquid could be selected for ease of adding and replacing in the internal reservoir. The selection of material for the slurry may improve cooling and / or transport performance. In some cases, the slurry may be referred to as slush or mixture of small solid particles (For example the particle size may be less than 100mm, 50mm, 20mm, 10mm, 5mm, or 2mm approximately, although other sizes may be used in some cases) in liquid. The slurry may act as a solid-liquid two-phase fluid.

[0041] There are known means of producing cryogenic slurry including a repeating freeze / thaw operation, a spray method, and using an auger. The auger is the most common for large scale production. In some cases, the slurry may be provided by an external provider. The external reservoir may comprise, or be connected to, a cryogenic slurry production device. The device may comprise a cryogen source, such as one or more of a cooler and an auger. Using an external reservoir 11 to create and store the cryogenic slurry means that the weight and complexity of the magnet can be reduced. The external reservoir 11 also allows cooling of the currently unused portion(s) of cryogenic slurry to be performed while the magnet is in use. Although it is considered that a single reservoir 11 is used, in practice a separate disposal reservoir may be used,or a loop type arrangement where the slurry is re-cooled between exiting and returning to the magnet.

[0042] The system may use a closed loop process for re-cooling the slurry. For example, the portion of the slurry removed from the magnet 12 may be cooled while the magnet is in operation, and then returned to the magnet 12 in the next stage. The closed loop process reduces potential waste in the system and minimises the amount of slurry required. In some cases the external reservoir 11 can hold a larger amount of slurry to increase the time available to re-cool the removed slurry before it needs to be returned to the magnet 12. The system, except at the connection to the interior reservoir of the magnet 12, may be closed to prevent any contamination or loss of the slurry.

[0043] Figure 1 shows a first system 10 comprising a cryogen store or external reservoir 11 and a magnet 12 with an internal reservoir 17. A conduit 13 between the external reservoir 11 and the internal reservoir 17 allows cryogen to flow between the external reservoir 11, or external reservoir, and the internal reservoir 17. A pump 19 may be arranged on the conduit 13 to assist the movement of the cryogen. A valve 15 may be used to close the conduit 13. The valve 15, or a further or alternative connector 16 may further disconnect the conduit 13 so as the magnet 12 can be moved relative to the cryogen store 11. The connector 16 may be reversibly connectable to allow the magnet to be released, moved and reconnected. The magnet, or other object may have a dock 18 to allow positioning of the magnet relative to the conduit(s) 13 or connector 16. The magnet is shown inside an enclosure such as pressure vessel 40. A controller 41 may be configured to receive inputs from one or more sensors and control the operation of any one or more of the pump 19, valve 15, connector 16 or magnet 12 or other devices.

[0044] In a first example a method of maintaining the temperature of a magnet 12 is explained. The magnet 12 is first cooled down to an operational temperature. The operational temperature may refer to a set temperature or be within a range of temperatures. This cooling may be achieved through an external cooling device, flowing cryogen through the reservoir or any suitable method of cooling the magnet12. The proposed cryogenic slurry method may also be used to cool the magnet 12, however as the magnet 12 does not need to move during the cooling phase it may be easier or more efficient to cool with a different cooling method.

[0045] Once the magnet 12 is cooled to its operational temperature the internal reservoir 17 can have a first portion of cryogenic slurry added. This first portion is used to maintain the temperature of the magnet 12 at the operational temperature. In some cases the method attempts to do this for as long as possible, to increase the time the magnet can be operational. While the first portion of cryogenic slurry is in the internal reservoir 17 it is monitored to determine a melted portion. By monitoring the melted portion of the slurry the solid remaining, or time remaining, where the slurry is able to maintain the temperature is monitored. The method may account for the time required to replace the cryogenic slurry in the calculation so as to begin the replacement earlier and avoid a drop in temperature. This intermittent changing of the slurry allows for sufficient maintenance of the temperature of the magnet to provide long uptimes, while ensuring the slurry temperature remains constant and / or the magnet temperature does not exceed operational requirements.

[0046] Any suitable means for monitoring the amount of melting of the first portion may be used. For example a time period could be used. The time period may be predetermined to allow an estimated portion of the cryogenic slurry to melt. In one example the time period is dependent on a temperature measurement. For example a temperature measurement of the magnet 12 and / or the internal reservoir 17 may be used to adjust the time period. In some cases the temperature may be tracked over the time period, with the historical temperature used to adjust the time period. In a further example a temperature of the cryogenic slurry may be used to determine the amount of the first portion which has melted. During the phase change the temperature should remain substantially constant. However, as the phase change completes the temperature will begin to rise. By monitoring the temperature and determining the rise in temperature the amount of melting of the first portion can be monitored. In further examples the amount of melting may be sensed or determined electrically or mechanically, for example by determining a density of an amount or size of particles inthe cryogenic slurry. In a further example, the method may determine the amount of solid or liquid in the slurry, for example by monitoring a characteristic of the slurry such as a temperature, conductivity or density. A predefined threshold may be used such that, once the time or slurry is past the threshold the replacement of the slurry is initiated. In cases where the replacement is not immediate the threshold may be set so as to provide for the time required to replace the slurry.

[0047] Predictive systems may be used to determine the amount of melting of the first portion, the time to the phase transition, or a time period between interchange of the slurry. For example, machine learning or artificial intelligence systems may be trained. The training may be performed on a test system with additional sensors to determine characteristics of the slurry. The characteristics may be determined under a plurality of conditions. The conditions may represent the operation of the magnet, environmental conditions, plasma operation and / or environmental conditions. The trained machine learning or artificial intelligence system may then be used to predict a state of the slurry, or a time period until a melting of the slurry, based on one or more characteristics of the slurry in use. For example, the machine learning system may use a temperature reading from the magnet, and one or more electrical characteristics of the system to determine an amount of melting. A further example is using the measured power output of a fusion reaction to estimate the heat load deposited into the magnet from neutron production and thereby estimate the amount of melting. The machine learning system may use a neural network, classification, regression, clustering, reinforcement learning, decision tree, Kalman filter or other technique.

[0048] In one example a predictive model (e.g., a machine learning model) would monitor thermal losses. There are several ways in which thermal power may be lost in the system. For example, ohmic losses (iA2*R), which is often dominant. Additional real time measurements may increase the accuracy of the model. Possible measurements include one or more of: magnet current, flux pump duty cycle and re-applied current, support systems current (for example, electronics, batteries, etc...), temperature of the magnet assembly components, temperature and pressure of the cooling slurry, volumetric flow rate of the slurry (if dynamically pumped through the system),temperature of the surface of the cryostat. The model may be trained to maximise the uptime of the magnet (or to minimize magnet downtime) and / or plasma duration. Alternatively, or in conjunction the model may be trained to minimise performance degradation (i.e. to maintain the desired operating temperature).

[0049] In one example a machine learning method is used for monitoring and maintaining the thermal performance of a cryogenic system. The method may comprise receiving a plurality of real-time measurements from one or more sensors associated with the cryogenic system. In some cases, the measurements comprise one or more of: magnet current, flux pump duty cycle, support systems current, magnet assembly component temperatures, cooling slurry temperature and pressure, volumetric flow rate of the cooling slurry, and cryostat surface temperature. The measurements may be processed by a thermal system model. The thermal system model may be configured to represent heat generation and dissipation within the cryogenic system. The thermal system model may account for thermal losses including one or more of: ohmic losses, hysteresis losses, eddy current losses, and thermal conduction through structural components. Based on the thermal system model and / or the measurements, the system may estimate, for example using an Extended Kalman Filter (EKF), the internal state of the cryogenic system. The internal state may comprise one or more of: the phase change status of the cooling slurry, the remaining solid fraction of the slurry, the heat load on the magnet assembly, and the estimated time to phase transition completion.

[0050] The machine learning method, such as the Kalman filter, may predict the evolution of the internal state based on the thermal system model and the received measurements. The machine learning method may further update the predicted internal state based on a comparison between the predicted measurements derived from the thermal system model and the received measurements, thereby refining the estimate of the internal state. The machine learning method may be trained using a plurality of historical operational datasets collected under varying environmental and operational conditions. The training may comprise mapping the relationships between the received measurements, the thermal system model parameters, and thecorresponding internal state variables. The training may further comprise adjusting process noise and measurement noise covariance matrices to improve the accuracy of the machine learning method under real-time operating conditions.

[0051] After training the machine learning method may be deployed to the magnet or system to provide real-time predictions of the magnet's cryogenic system's thermal state. The predictions may be used to control the replacement of the cooling slurry and / or optimize the thermal performance of the cryogenic system. The machine learning method may be deployed in a fixed state, or may continuing training based on outcomes during deployment. In some cases the machine learning method provides control signals to one or more actuators. The control signals may be based on the estimated internal state of the slurry. The control signals may comprise commands to dynamically adjust one or more of: the volumetric flow rate of the slurry, the pump duty cycle, or the timing of slurry replacement. The control signals may be generated to maintain the thermal performance of the magnet / cryogenic system without requiring direct measurements of all variables of the system. In some cases the machine learning method enables continued operation of the cryogenic system during periods of degraded sensor performance or missing data by relying on predictive capabilities derived from the thermal system model and previously trained relationships.

[0052] The system may determine is a threshold portion or percentage of the slurry has become liquid. The determination of a state of the cryogenic slurry may be monitored continuously or periodically. Monitoring the cryogenic slurry allows for a determination of the best time to replace the slurry. This may be a balance between amount of solid remaining in the slurry and the time required to replace the slurry. By changing the slurry before all the solid has disappeared the temperature of the slurry is maintained because of the latent heat during phase change, improving thermal performance. The use of this monitoring of the solid-to-liquid ratio can avoid operation interruptions by ensuring the slurry is replaced before heating of the magnet occurs and / or ensuring the slurry is not replaced before a significant fraction is melted.

[0053] Once a predetermined portion of the first portion of the cryogenic slurry has melted the first portion is replaced with a second, fresh (e.g., re-cooled or with aportion of solid cryogen above a threshold), portion of cryogenic slurry. While the first portion could simply be re-cooled this would take additional time. Therefore it is advantageous to replace the first portion with a second portion. The removed first portion may then be re-cooled while the second portion maintains the temperature of the magnet 12. Alternatively, it may be disposed of and replaced with fresh cryogenic slurry. In some cases, replacing the first portion of the cryogenic slurry comprises removing a part of the first portion of the cryogenic slurry and replacing it. For example a liquid part of the cryogenic slurry may be removed and replaced with further solid cryogenic material, thereby replacing the first portion of the slurry with a second portion and refreshing the cooling effect.

[0054] The replacement of the slurry has been described using the slurry as a pump. However, one or more containers may be used. Where containers are used the internal reservoir may refer to an internal space, such as an internal compartment, configured to receive the containers. The containers may be flowed into the internal reservoir or may be fitted to the internal reservoir. For example, by mechanical latch or securing devices. The containers may form batteries of slurry. The interchange of the batteries allowing rapid replacement of the slurry and / or reducing the potential for contamination, and reducing liquid management. The containers may have substantially the same shape, so as containers can be easily replaced by one another. For example, the containers may be substantially spherical or cuboid. Alternatively, the containers may have a range of shapes. The shapes may be configured to engage with matching recesses in the magnet 12. This may make full use of the available space in the magnet 12 and / or ensure appropriate cooling in particular areas of the magnet 12. The containers may comprise an outer wall surrounding a slurry reservoir. The outer wall may be configured to enhance heat transfer to the slurry. The containers may comprise one or more sensors to monitor the characteristics of the slurry within the container. The system may be configured to receive these characteristics and determine if a replacement of the container (or all of the containers) is required. The containers may have a standardized thermal interface for connecting to the magnet, or for recharging (replacing) the slurry within the container after use. The containers maycomprise an inlet to allow slurry to be removed and replaced, or the slurry may be recooled within the containers. The containers may be relatively small relative to the size of the internal reservoir to allow the containers to be pushed into and / or through the internal reservoir for replacement of the slurry.

[0055] Figure 4 illustrates the use of containers 41. The superconductor magnet 12 is within pressure vessel 40. A dock 18 is shown in connection with magnet 12 and internal reservoir or compartment 12. Two containers 41 are inserted into magnet 12. The containers 41 may be placed in recesses within the magnet 12. The containers 41 may be secured into place, for example by one or more locks or clips, or the shape of the container may allow a wedge or friction fit. The containers 41 may have a thermal interface to engage the internal reservoir 17. The external reservoir 11 is used to re-cool or recharge the containers 41. The containers 41 may have a thermal interface to assist with this process. Containers 41 may have different shapes or size, such as containers 42, 43. This can require and / or ensure containers are placed in a desired location. The different shape may comprise one or more recesses and / or a keying to limit engagement in the internal reservoir 17. The containers may be shaped to fit a particular recess of the internal reservoir.

[0056] Utilising the melting of the cryogenic slurry provides a sufficient time of cooling to the superconductor magnet 12 without necessarily requiring a permanent connection to the, for example, magnet 12. By monitoring and estimating the amount of solid remaining in the slurry (or a corresponding characteristic) the method is able to determine when to replace the first portion of slurry with a fresh portion of slurry to maintain temperature while reducing downtime of the, for example, magnet. The use of a slurry improves the ability to move the cryogen between the external reservoir 11 and internal reservoir 17. Using a slurry also provides a constant temperature. This is advantageous to maintain the temperature of the superconducting magnet instead of having to adjust or control for a changing temperature.

[0057] Figure 1 shows a single connection between the external reservoir 11 and the internal reservoir 17 in conduit 13. A pump 19, or other actuator, may be configured to add and remove the slurry from the internal reservoir 17 or between theexternal reservoir 11 and the internal reservoir 17. The pump 19 may be permanently connected to the external reservoir 17 or conduit 14 (e.g., a pipe or tube).

[0058] Figure 2 shows a second example where the internal reservoir 17 is formed as a pathway 21 which may be a loop or circuit, pathway 21 has an inlet 22 and an outlet 23. Connections may be made to the inlet 22 and outlet 23 to allow the internal reservoir to be filled and emptied concurrently. The connections 16 may be repeatedly connectable to allow connection and disconnection multiple times. Similar to Figure 1 the outlet 23 may be connected to the external reservoir or to a waste outlet. A pump 14 may be used to move the cryogen slurry through the pathway 21. Each of the two conduits 25, 26 may have one or more of a valve 15 and a disconnection point 16, to allow the magnet 14 to be moveable and / or to prevent or allow the flow of cryogen through the pathway 21. The conduits 25, 26 may be insulated for thermal performance. A reserve reservoir 29 may be used to store expelled cryogen before returning it to the external reservoir 11.

[0059] The pathway 21 may be tortuous or configured to provide cooling in specific areas of the superconductor magnet 12. In another example the pathway may be substantially straight or a constant curve to improve addition and removal of slurry. The pathway may follow or be arranged near to the superconductor material (e.g. the ring or coil) within the magnet 12. The inlet 22 and outlet 23 may be at opposite ends of the superconductor magnet 12. In some cases the inlet 22 and outlet 23 are both positioned on a lower or bottom surface of the magnet 12 so as to allow a simple connection when the magnet is lowered to the ground. The weight of the magnet 12 may be used to open or connected the inlet 22 and outlet 23 of the pathway 21 to the respective conduits. The shape and design of the pathway 21, forming the internal reservoir 17 (shown as pathway 21 in Figure 2) may be configured to allow improved cooling of the magnet.

[0060] For example, the pathway may have one or more fins or restrictions. The fins may protrude into or out of the internal reservoir 17, 21. For example fins protruding into the reservoir may improve heat transfer to components of the magnet 12. The fins or protrusion may be formed of a conductor, such as the copper used toelectrically connect the superconductor, or the steel used to support the superconductor shape. Alternatively, or in conjunction fins or paths may extend from the reservoir 17, 21 to reach particular components or features within the magnet 12 to provide targeted cooling. However, in these cases the drainage or removal of cryogen may need to be considered. The shape and location of the fins may position them near to the hottest part of the magnet 12, or where cooling is most required. Alternatively the pathway 21 (forming internal reservoir 17) may be configured to be as wide as possible to allow the cryogen slurry to be added and removed as efficiently or quickly as possible.

[0061] In some cases, the magnet 12 may have one or more additional cooling systems. The additional cooling systems may be configured to improve the performance of the cryogenic slurry and / or to optimise performance depending on operation of the magnet. For example, one or more heaters or coolers may be placed around the internal reservoir 17. These may be used to control the state of the slurry (i.e. increase the length of time before melting or melt more quickly to allow replacement. These heaters / coolers may be electric. In some cases, further phase change materials may be located in the superconducting magnet, such as in the internal reservoir. The phase change materials may be cooled by the slurry. The phase change materials then provide cooling (or maintenance of the desired temperature) during replacement of the slurry. In some cases, the system may control the use of the additional cooling systems for operational performance. For example, the additional cooling systems may only activate if a temperature spike occurs.

[0062] In some cases, the magnet incorporates secondary phase transformation materials. These materials may comprise solid-to-liquid, liquid-to-gas, or solid-to-gas phase change materials (PCMs). The PCMS are configured to provide a thermal buffering capability in the event of transient temperature excursions. For example, they help to maintain the desired temperature when there is an unexpected variation in the slurry or magnet temperature. The PCMs are not intended to serve as the primary cooling mechanism (which is the cryogenic slurry), but rather to act as a temporary heat reservoir to mitigate thermal spikes during specific operational scenarios. One exampleis if the cryogenic slurry has fully transitioned to a liquid state. The PCMs are configured to undergo phase transformation (melting) at a predetermined temperature. The predetermined temperature is higher than that of the slurry in the internal reservoir.

[0063] In some cases, the PCMs may be strategically positioned within the magnet's thermal management system to act as localized heat sinks. Upon reaching the PCM's melting point, latent heat absorption during the phase transformation provides a critical time buffer, allowing the system to transition from active operation to a safe shutdown or to refill the internal reservoir with a new portion of cryogenic slurry. This buffering capability is especially advantageous during high-demand periods or when the system experiences transient heat loads that exceed the cooling capacity of the remaining liquid coolant.

[0064] The use of PCMs may also be integrated with real-time temperature and pressure monitoring to dynamically assess the thermal state of the system. This may be in conjunction with the predictive modeling discussed previously, Sensors positioned near the PCM modules, as well as throughout the magnet assembly, can detect the onset of phase transformation in the PCMs. These measurements may be utilized to alert the system to impending thermal excursions and initiate preemptive actions. Example preemptive actions include engaging refilling operations or temporarily reducing the system's thermal load.

[0065] In some cases, heaters may be employed as an auxiliary system to address stratification issues within the cryogenic slurry. If solid material in the slurry accumulates in localized zones (for example due to gravitational effects or flow dynamics) heaters may be selectively activated to re-melt or redistribute the solid phase. This ensures homogeneity of the slurry and maintaining optimal thermal transfer characteristics. The activation of the heaters may be based on data from temperature and pressure sensors, or other sensors which can detect variations indicative of stratification. In some cases, predictive models, incorporating known thermal and fluid dynamic properties of the slurry, can forecast regions of potential solid buildup and preemptively activate the heaters.

[0066] The integration of either or both PCM-based thermal buffering and stratification control heaters may enhance the system's robustness by minimizing temperature spikes during transitions and ensuring consistent cooling performance.

[0067] In some cases, the second portion of slurry is used to urge, push or encourage the removal of the first portion of slurry. This may allow the slurries to be added and removed at the same time. The second portion may be able to displace the first portion because of the greater solid content (i.e., density) of the fresh slurry. This may improve the speed of replacing the cryogenic slurry, due to the simultaneous addition and removal of the portions. In another example, the system may have two pumps 19, one to remove the first portion and a second to add the second portion. The pumps 19 may be connected to separate conduits. Alternatively, the second portion may be used to propel the first portion out of the superconductor magnet 12.

[0068] The internal reservoir 17 may comprise one or more pumps 27 or devices configured to circulate the slurry within the internal reservoir 17. This allows the step of circulation to improve, or make more constant, the spread of cryogen within the magnet 12. This could occur even when the magnet 12 is moving and / or levitated. In some cases, a passive pump may also or alternatively be used. The pump 27 may circulate cryogen in the internal reservoir, or along a connected pathway in the internal reservoir. The internal reservoir may have one or more valves to allow the connected passageway to be open and closed, for example during cryogen replacement.

[0069] The passive pump or system may by driven by gravity or movement of the magnet to encourage movement of the slurry into particular areas. In some cases, the slurry could be added so as to encourage movement. For example, a liquid portion could be added separately to a solid portion so as the liquid portion would then move into the solid portion over time. Alternatively, the internal reservoir could be configured to allow the solid parts and liquid parts of the slurry to settle in different locations. In a further example, as the slurry becomes more liquid it may be able to pass through a narrower opening into a different section of the internal reservoir.

[0070] Figure 3 shows an example cycling method. The cryogenic slurry is created 31 externally to the object which is to be maintained at a temperature. A portion of theslurry (which may be the entire amount of slurry produced) is transferred 32 to the internal reservoir of the object. The object is operated, and the slurry state is monitored 33 to determine the amount of solid, liquid or time remaining. If, for example, the solid percentage has dropped below a threshold 34, or is estimated to had dropped below a threshold, the cryogenic slurry is removed 35 from the internal reservoir and replaced with a fresh cryogenic slurry. This may require the operation of the object to be paused. For example, the object may return to a docking station 18 (shown in Figure 1), or similar. The cryogenic slurry may be recycled back into the external reservoir 11 upon removal 35 from the internal reservoir 17, or a fresh portion of cryogenic slurry may be created 31.

[0071] In order to maintain a constant temperature, the cryogenic slurry may be regularly cycled. This may mean tens or hundreds of cycles to allow, for example, a magnet 12 to remain operating in superconducting state when in a hot chamber. Because the downtime of the magnet 12 is decreased (due to the quick exchange of the slurry) the magnet 12 can repeatedly be recharged without a large break in operation. In a typical operation the magnet 12 would be cooled to the desired temperature, the cryogenic slurry added, the magnet 12 levitated and operated until the slurry has melted to a threshold amount, then the magnet landed, and the cryogenic slurry replaced. The magnet 12 would then be re-levitated and returned to operation. The cycles of landing, replacing the cryogenic slurry in the magnet and levitating the magnet 12 could then repeat as required. In the case of a levitating dipole operating the magnet 12 comprises levitating the magnet 12 into an operational position and generating a current in the superconductor coils to confine a plasma in the generated dipole field. Where the object is not a magnet 12 the moving of the object may not be levitation. A dock 18 or guide may be used to ensure the magnet, or object, returns to the appropriate positions and / or orientation. For example, the guide may be a protrusion. The protrusion may have one or more tracks or fins which mate or engage with corresponding recesses on the magnet to guide it into position (or vice versa).

[0072] Where a magnet 12 is used, an in other cases, an advantage of the system is that it allows the cryogen store or manufacture to be external to the magnet 12. For example, a levitating dipole magnet is typically a pressure vessel container a superconductor ring or coil. The magnet 12 is then placed within a large pressure vessel 40 to contain a plasma. The described system would allow the external reservoir 11 and pump 19 to be located outside of both pressure vessels, with only the internal reservoir required to be within the magnet. This allows easier processing and maintenance of the cryogen and pump and reduces the weight onboard the magnet 12. Although figure 1 shows the connection on the side of pressure vessel 40 typically it may be on the bottom to allow easier orientation and connection.

[0073] The system may have a controller 41. The controller may be configured to determine the thermal state of the cryogenic slurry (for example by one or more sensors) in the internal reservoir 17 and control the pump 19 to replace the cryogenic slurry (or multiple pumps 19 to remove and replace the cryogenic slurry in the internal reservoir 17. The controller 41 may also control, or send signals to other devices to perform, any of the steps described herein. For example, the controller may monitor or adjust the valve and / or the connector 16.

[0074] The system may use a fixed flow rate for pumping, or a fixed pumping pressure or power. However, the system may control the pump dynamically, having a variable flow rate. Because of the changing state (i.e. liquid / solid portion) of the cryogenic slurry appropriate control may be important. For example, the system may use one or more characteristics of the cryogenic slurry and / or one or more characteristics of the system to determine a suitable flow rate when replacing the slurry. For example, the state of the slurry may be determined. The flow rate could be increased if the slurry has a greater liquid portion, as this would flow more quickly from the inner reservoir. For example, the separation of the slurry may be determined, the flow rate could be controlled to prevent slurry separation (liquid rising above solid). For example, the system may use a characteristic of the magnet or environment to determine the state of the slurry. A temperature of the magnet, or a temperature aboutthe magnet (e.g. a plasma temperature or fusion power) may be used to estimate the slurry state and / or directly be used to determine a suitable flow rate.

[0075] The control of the flow rate may be determined by considering at least the delta-pressure of the system (i.e. of the internal reservoir or internal / external reservoirs. The delta-pressure may be understood as a resistance of the slurry to flow through the system (e.g. the internal reservoir). The higher the viscosity of the slurry, up to being fully solid, the harder it is to pump into the internal reservoir. This results in greater force required to move the slurry and a corresponding higher delta-pressure, especially to move the slurry at a higher flow rate. The higher delta-pressure, and therefore system pressure adds mechanical strain to the system, due to increased load. Therefore, the controller may balance one or more of the delta-pressure, flow rate requirements, system thresholds and the slurry viscosity and / or state.

[0076] The thermal parameters of the flow rate depend on the local convection rates across the boundary of the slurry and where the heat flux is coming from (i.e. the wall of the internal reservoir). The higher the flow rate, the higher the heat transfer coefficient (delta-Temperature) between these features, as the slurry maintains a constant temperature. During exchange of the slurry the heat transfer coefficient should be selected or be controlled (e.g. by the dynamic flow rate) below a threshold. Too little heat transfer and the temperature of the components feasibly increases, or magnet downtime increases. Too much heat transfer and thermal strain issues can lead to mechanical stresses or exceed threshold limits. Because the viscosity of the slurry may change in use (because of a decrease in ice-mix fraction) the determination of flow rates may be altered. For example, the delta pressure limitations may reduce, as will the thermal energy store available. This may lead to an increase in flow rate, with the slurry also being easier to pump. Understanding the state of the slurry (predictive or measured) allows the flow rate to be controlled to balance mechanical thresholds with performance. This may be determined by a parameter such as delta-pressure (other parameters may be measured instead). For example, the controller may ensure delta- pressue remains below a system threshold, while controlling the flow rate to provide adesired heat flux to the magnet. The desired heat flux may be a range, to provide flexibility in flow rate.

[0077] The system may monitor for stratification of the slurry. Preventing stratification ensures the maintenance of the desired temperature across the magnet. The stratification control may be achieved by controlling the pump to spread the slurry evenly through the system. The stratification may be controlled by mixing. For example, by pumping the slurry in and / or out of the external reservoir where flow comes in at the bottom and is pulled out of the top (or vice-versa, depending on density, for example). In some cases, an additional inlet may, or flow path within the internal reservoir, may be used to alter the stratification. In that reservoir there could also be a mixer to stratify the slurry within the internal reservoir.

[0078] In some cases, stratification (i.e., a lack of homogeneity of the slurry) is used advantageously. In a Neon slurry the solid would form beneath the liquid (although this may reverse with alternative slurry materials). In one example a modular slurry container is used. The container may begin as a solid block, which is inserted to the system (removing the need to pump material). This means it does not require an flow path or opening during use. As the solid cryogenic material melts the liquid will rise to the top (or alternatively fall to the bottom, dependent on density). This brings addition solid to the top of the container. If the thermal interface is arranged at the top of the container the solid slurry is kept closer to the thermal interface to enhance cooling.

[0079] In some cases, the outlet 23 may be positioned or shaped so as to encourage liquids to flow out, so as once melted the liquid portion of the cryogenic slurry is easily removed, while any remaining solid is kept, and the replacement second portion added.

[0080] The examples previously described have focused on a levitating dipole magnet 12 which requires cooling to a cryogenic temperature for superconducting. However, the system and method can be applied to other systems where it is advantageous to maintain an operating temperature. There may be particular advantages where the object to be kept at a cryogenic temperature is moveable or requires separation from a cryogen store because the system and method do notrequire continuous pumping of cryogen. The method may also be applied to other applications where limiting downtime of for cooling is important. For example, magnet systems where there is a need to operate at a constant temperature without direct connection to a cooling loop during operation; electric aircraft motors where they need to maintain constant temperature during flight but want a short downtime to minimise time spent waiting on the ground; and satellites who periodically dock with a support station. For example, the satellite could have a limited time period in which to dock with the support station before rejoining its orbit. By quickly replacing the cryogenic slurry on board the downtime can be reduced, while providing sufficient cooling. In these examples the internal reservoir may be within an alternative container (i.e., a magnet is not required). The features specific to magnets or fusion may be replaced by the alternative application.

[0081] For example, the system may be applied to MRI (magnetic resonance imaging) systems to reduce the amount of helium required. MRI machines often use superconductors requiring Liquid Helium for cooling. The present system can allow a reduced amount of superconductor and minimise the complexity of a system, or at least prevent a requirement for continuous operation of the system. The internal reservoir allows the system to be disconnected from the source of the slurry, which may allow the MRI (or other medical device) to be transported while held at the required temperature. In these applications the uptime / downtime of the device (container / magnet) may be much longer than in the described system, because the thermal heating is reduced compared to the fusion reactor example discussed above. The system may be applied to quantum computing and / or supercomputing cooling. In quantum computing, the operational principles rely on superconducting materials and having a near-zero thermal profile to reduce vibrations. Dilution refrigeration (He3 and He4 mixing) is used but there is still a need to carry that heat away. Previously liquid helium cryo-cooler in a still-condensing-type-devices have been used. In some cases the present system could provide an operation buffer to the He3 / He4 system. The described system may improve over liquid helium in terms of managing transients and keeping temperatures consistent. The system may be particularly advantageous forcooling of portable devices, where continuous cooling is not possible so the described intermittent cooling allows operation at low temperatures with reduced downtime for recharging.

[0082] Aspects of the controller and methods described above may be operable or implemented on any type of specific-purpose or special computer, or any machine or computer or server or electronic device with a microprocessor, processor, microcontroller, programmable controller, or the like, or a cloud-based platform or other network of processors and / or servers, whether local or remote, or any combination of such devices.

[0083] The controller described in connection with the examples disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic component, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, circuit, and / or state machine. A processor may also be implemented as a combination of computing components, e.g., a combination of a DSP and a microprocessor, a number of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.

[0084] The methods or algorithms described in connection with the examples disclosed herein may be embodied directly in hardware, in a software module executable by a processor, or in a combination of both, in the form of processing unit, programming instructions, or other directions, and may be contained in a single device or distributed across multiple devices. A software module may reside a storage medium such as RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD- ROM, or any other form of storage medium known in the art. A storage medium may be coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor.

[0085] In its various aspects, embodiments of the disclosure can be embodied in a computer-implemented process, a machine (such as an electronic device, or a general purpose computer or other device that provides a platform on which computer programs can be executed), processes performed by these machines, or an article of manufacture. Such articles can include a computer program product or digital information product in which a computer readable storage medium containing computer program instructions or computer readable data stored thereon, and processes and machines that create and use these articles of manufacture.

Claims

CLAIMS1. A method of maintaining the temperature of a superconductor magnet comprising an internal reservoir, the method comprising the steps of:Adding a first portion of cryogenic slurry to the internal reservoir;Determining the first portion has at least partially melted;Replacing the first portion of cryogenic slurry with a second portion of cryogenic slurry.

2. The method of claim 1 wherein the cryogenic slurry comprises one or more of nitrogen, neon, and hydrogen.

3. The method of any one of claims 1 or 2 wherein the cryogenic slurry comprises liquid and solid of the same material.

4. The method of any one of claims 1 to 3 wherein the first and second portion of cryogenic slurry are obtained from an external reservoir.

5. The method of claim 4 wherein the first and second portion are obtained from the same external reservoir.

6. The method of any one of claims 4 or 5 wherein the external reservoir comprises a slurry production device.

7. The method of any one of claims 4 to 6 comprising the step of connecting and / or disconnecting the internal reservoir from the external reservoir.

8. The method of claim 7 wherein the internal reservoir and the external reservoir are disconnected during operation of the superconducting magnet, and reconnecting between operating events of the superconducting magnet.

9. The method of any one of claims 1 to 8 comprising simultaneously removing the first portion of cryogenic slurry while adding the second portion of cryogenic slurry.

10. The method of any one of claims 1 to 9 wherein the second portion is used to urge the removal of the first portion.

11. The method of any one of claims 1 to 10 wherein determining the first portion has at least partially melted comprises determining that the portion of solid has fallen below a pre-set threshold.

12. The method of any one of claims 1 to 11 wherein determining the first portion has at least partially melted comprises monitoring one or more of a time and a temperature.

13. The method of any one of claims 1 to 12 wherein determining the first portion has at least partially melted comprises monitoring a portion of the cryogenic slurry that has become liquid.

14. The method of any one of claims 1 to 13 wherein the first portion is replaced once a threshold portion of the slurry has become liquid.

15. The method of any one of claims 1 to 14 comprising the step of circulating the portions of cryogenic slurry within the internal reservoir.

16. The method of any one of claims 1 to 15 comprising the step of cooling the superconductor magnet to an operational temperature before adding the first portion of cryogenic slurry.

17. The method of any one of claims 1 to 16 comprising the step of repeatedly adding and replacing further portions of cryogenic slurry.

18. The method of any one of claims 1 to 17 comprising the step of operating the superconductor magnet between the steps of adding the cryogenic slurry and replacing the cryogenic slurry to the internal reservoir.

19. The method of claim 18 wherein operating the superconductor magnet comprises one or more of: changing the location of the superconductor magnet, and adding current to the superconductor magnet.

20. The method of claim 19 wherein changing the location of the superconductor magnet comprises levitating the superconductor magnet.

21. A system for maintaining the temperature of a superconductor magnet comprising an internal reservoir, the system comprising: an external reservoir for holding a cryogenic slurry;a pump for moving portions of the cryogenic slurry into the internal reservoir;A controller configured to: determine the thermal state of a first portion of cryogenic slurry in the internal reservoir; and control the pump to replace the first portion of cryogenic slurry with a second portion of cryogenic slurry based on the determination.

22. The system of claim 21 wherein the external reservoir is repeatedly connectable to the internal reservoir.

23. The system of claim 21 or 22 wherein the internal reservoir comprises a path extending between an inlet and an outlet.

24. The system of claim 23 wherein the pump is configured to pump the first and second portion through the inlet to the internal reservoir and out of the outlet.

25. The system of claim 24 wherein the pump is configured to pump the first and second portion simultaneously.

26. The system of claims 24 or 25 wherein the inlet and outlet are each repeatedly connectable to the external reservoir.

27. The system of any one of claims 21 to 26 wherein the external reservoir comprises a cryogenic generator for one or more of producing and cooling the cryogenic slurry.

28. The system of any one of claims 21 to 27 wherein the superconductor magnet is moveable, optionally wherein the system comprising a dock, to position and / or orientate the superconductor magnet when replacing the cryogenic slurry.

29. The system of claim 28 wherein the superconductor magnet comprises a guide configured to guide the position of the superconductor magnet to and from the dock.

30. The system of any one of claims 21 to 29 wherein the superconductor magnet is a levitating magnet.

31. The system of claim 30 wherein the superconductor magnet is contained within a pressure vessel and the external reservoir is outside the pressure vessel.

32. The system of any one of claims 21 to 31 wherein the internal reservoir comprises one or more fins configured to improve the thermal conduction of the cryogenic slurry.

33. The system of claim 32 wherein the fins extend into the internal reservoir.

34. The system of any one of claims 21 to 33 wherein the internal reservoir comprises a pump to move the cryogenic slurry about the internal reservoir.

35. The system of any one of claims 21 to 34 wherein the internal reservoir comprises a passive thermal system configured to enhance the thermal transfer to the cryogenic slurry.

36. The system of claim 35 wherein the passive thermal system is driven by gravity and / or movement of the superconductor magnet.

37. The system of claim 35 or 36 wherein the passive thermal system allows solid cryogen in the slurry to settle in a first location and a liquid portion of the slurry to settle in a second location.

38. The system of any one of claims 21 to 37 wherein the external reservoir and / or the connector between the external reservoir and the internal reservoir are insulated.

39. A method of maintaining the temperature of an object comprising an internal reservoir, the method comprising the steps of:Adding a first portion of a coolant slurry to the internal reservoir; Determining the first portion has at least partially melted;Replacing the first portion of coolant slurry with a second portion of coolant slurry.

40. The method of claim 39 comprising any one or more of the steps of claims 1 to20.

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