Superconducting magnet system and method for generating a magnetic field

Abstract

A superconducting magnet system comprising a superconducting magnet having a plurality of field coils connected in series, each field coil having a plurality of turns including a superconducting material. The system also comprises a primary current source connected across the field coils to supply a DC current to the plurality of field coils to generate a magnetic field. The system further comprises a secondary current source connected in parallel with the primary current source across a subset to supply an additional DC current to that or each field coil within the subset of field coils to modify or correct the magnetic field.

Description

Technical Field

[0001] The present invention relates to a system comprising a superconducting magnet, particularly a high-temperature superconductor (HTS) magnet, and a method for generating a magnetic field. In particular, the present invention relates to modifying or correcting a magnetic field generated using a superconducting magnet.

Background Art

[0002] Superconducting materials are typically divided into "high-temperature superconductors" (HTS) and "low-temperature superconductors" (LTS). LTS materials such as Nb and NbTi are metals or metal alloys whose superconductivity can be described by the BCS theory. All low-temperature superconductors have a peak critical temperature below 30 K (the temperature above which the material cannot become superconducting even in zero magnetic field). The behavior of HTS materials is not explained by the BCS theory, and many have critical temperatures far above 30 K. The most commonly used HTS materials are "cuprate superconductors", which are ceramics based on cuprates (compounds containing a copper oxide group), such as BSCCO or ReBCO (where Re is a rare earth element, generally Y or Gd). Other HTS materials include iron pnictides (e.g., FeAs and FeSe) and magnesium diboride (MgB 2 ) is included.

[0003] ReBCO superconducting tapes are typically manufactured as tapes having a width of 2 mm to 12 mm and a thickness of about 100 micrometers. The structure of a typical tape 100 is shown in FIG. 1 and includes a substrate 101 (typically an electropolished nickel-molybdenum alloy, such as Hastelloy (trademark) having a thickness of about 50 micrometers), on which is deposited a series of buffer layers known as buffer stack 102 having an approximate thickness of 0.2 micrometers. The epitaxial ReBCO-HTS layer 103 covers the buffer stack and is typically 1 micrometer thick. A silver layer 104 and a copper stabilizer layer 105 of 1 to 2 micrometers are deposited on the tape and often completely encapsulate the tape. The silver layer 104 and the copper stabilizer layer 105 extend continuously around the tape 100 (not shown in FIG. 1 for clarity) and can thus also be referred to as a "clad". The silver layer 104 forms a low-resistivity electrical interface to the ReBCO layer 103 and an airtight protective seal around the ReBCO layer 103, and the copper layer 105 enables external connections to the tape from both sides (e.g., by soldering) and provides a parallel conduction path for electrical stabilization. "Lift-off" HTS tapes can also be manufactured, which lack the substrate and the buffer stack.

[0004] HTS tapes and other superconducting materials can be characterized by critical surfaces of maximum current, temperature, and magnetic field at which the superconductor transitions from the superconducting state to the normal state. For example, the critical current I c is the current at which the superconductor becomes normal at a given temperature and magnetic field, and the critical temperature T c is the temperature at which the superconductor becomes normal for a given magnetic field and current. The critical temperature is often formally defined for zero magnetic field, but this term is used more generally in this specification for convenience. The critical surfaces of many HTS tapes can also depend strongly on both the magnitude and direction of the magnetic field.

[0005] The HTS cable comprises one or more HTS tapes connected along its length via a conductive material, usually copper. Under this definition, a single HTS tape is an HTS cable. The HTS tapes may be stacked (i.e., arranged such that the HTS layers are parallel), or may have some other tape arrangement that can vary along the length of the cable.

[0006] The superconducting magnet is formed by arranging the HTS cable in a coil including one or more turns. A turn (or winding) of the coil is a section of the HTS cable that surrounds the inside of the coil (i.e., can be modeled as a complete loop). Generally, the structure of a magnetic coil has two types: by winding or by assembling several sections. The wound coil is manufactured by continuously spirally winding the HTS cable 201 around a former 202 as shown in FIG. 2. The former is shaped to provide the required inner circumference of the coil and may be a structural part of the final wound coil or may be removed after winding. The section coil is composed of several sections 301 as schematically shown in FIG. 3, each of which can include several cables or pre-formed busbars 311 and forms an arc of the entire coil (i.e., a continuous section smaller than the entire turn). These sections are connected by joints 302 to form a complete coil. The coil or coil section is often integrated by encapsulating or potting them, and the encapsulating material can fill the space between the turns. Suitable encapsulating materials include both insulating materials such as epoxy and conductive materials such as solder.

[0007] Figure 4 shows a cross-section of a particular type of wound coil known as a "pancake coil", where the HTS cable 401 is wound in a planar spiral such that it forms a flat coil. The pancake coil can be made with an inner circumference of any two-dimensional shape. Often, the pancake coil is provided as a "double pancake coil" as shown in the cross-section of Figure 5. These comprise two pancake coils 501, 502 wound in opposite directions with an insulator 503 sandwiched therebetween. The inner terminals are connected 504. This means that it is generally only necessary to supply a voltage to the outer terminals 521, 522, which are generally more accessible, to drive a current through the turns of the coil to generate a magnetic field. Many other coil configurations are possible, including helical solenoids.

[0008] One use of the HTS field coil is in a tokamak plasma chamber, including spherical tokamaks where a strong magnetic field is required to confine and control the plasma. Another potential use of the HTS field coil is in proton beam therapy (PBT) devices and proton boron capture therapy (PBCT) devices where a proton beam is used for cancer treatment. PBT and PBCT devices require very high magnetic fields to both accelerate and steer the proton beam.

[0009] There are three broad classes of HTS coils. · Have an electrically insulating material that is insulated and located between turns to separate the turns. In this configuration, the current can only flow around the turns of the coil (i.e., in a helical path along the HTS cable). · Are non-insulated and the turns are connected with low resistance, for example by a conductive metal. This can be achieved, for example, by forming the coil such that a copper stabilizer layer (or other metal cladding) connects the turns and / or the coil is potted with a conductive solder. ·Partially insulated, where the turns are connected to the resistive intermediate between the conductor and the insulator. This can be achieved by separating the turns with a material having a higher resistance compared to copper (e.g., co-wound stainless steel tape or any layer with a desired resistance), and / or by providing intermittent insulation between the turns, and / or by providing a resistive material along the side of the coil (which may include components such as resistors) and connecting at least some of the turns. The resistance between turns in a partially insulated coil can be controlled to be 100 times to 10 15 times that of copper to achieve a desired ratio L / R between the inductance L around the coil and the resistance R across the coil. Different forms of partial insulation are described, as just a few examples, in WO 2019 / 150123 and WO 2020 / 079412.

[0010] Non-insulated coils can be considered as the low-resistance case of partially insulated coils. Generally, in both partially insulated pancake coils and non-insulated pancake coils, the turns are connected by a normal (i.e., non-superconducting) conductive material, or equivalently, a resistive (but not insulating) material, such that current can be shared between the turns through the conductive material. For example, in a pancake coil, current can flow not only around the spiral path but also radially. In a solenoid, an additional longitudinal current path is provided.

[0011] A non-insulated or partially-insulated HTS coil can be modeled as having three current paths: two helical paths (one in the HTS and one in the metal stabilizer) following the HTS cable around the turns, and an inter-turn path across the magnet between the coil terminals. In a pancake coil, for example, the inter-turn path is a radial path through the metal stabilizer and any other resistive material connecting the turns. This can be modeled as a single path, but in reality represents the sum of all resistive paths across the magnet. Only the current flowing within the helical paths generates a significant magnetic field. The HTS helical path can be modeled as an inductor with large inductance and zero or negligible resistance when all the tape is superconducting. The stabilizer helical path is parallel to the HTS helical path and (in a simple model) has the same inductance, but also has a significant resistance. Thus, negligible current flows through it unless part of the HTS helical path starts to quench. The inter-turn path across the magnet can be modeled as having negligible inductance and a resistance much larger than the HTS helical path while the HTS material is superconducting. Negligible current flows through this path unless part of the HTS helical path starts to quench or (due to the large inductance of the HTS helical path opposing changes in current) the current in the HTS helical path changes. When the HTS helical path starts to quench, the excess current above the critical current I c is shared between the helical stabilizer path and the inter-turn path according to their relative resistances and L / R time constants.

[0012] HTS field coils are generally designed to operate with all HTS tapes in all turns operating below their local critical current I c which varies around the coil due to fluctuations in the magnetic field and coil temperature. However, due to various fault conditions, the HTS tape current can exceed the critical current.

[0013] * Cooling faults, (local or overall) temperature rise, and thereby Ic decrease. *Transport current I 0 Transient increase, for example, overcurrent fault of the power supply. *Damage to the HTS material (e.g., due to stress cracking, fatigue caused by thermal cycling or energization cycling of the magnet). *Local energy accumulation sufficient to cause thermal runaway.

[0014] If the current in any tape exceeds (or approaches) the local critical current, part of the current is diverted to the metal layer of the tape (mainly the copper stabilizer layer), to any other normal conductive (i.e., non-superconducting) material separating the turns of the non-insulated or partially insulated coil, and to any "reserve" I of the nearby HTS material c capacity. The current flowing through the normal conductive material generates heat, further reducing the local critical current I c and may lead to thermal runaway.

[0015] The region of the HTS tape first affected by the fault condition is known as the "hot spot". Early detection of the hot spot is important so that damage to the HTS magnet can be avoided by "quenching" the magnet and dissipating its energy. Various techniques for detecting hot spots are known, for example, using temperature sensors, strain sensors or voltage taps distributed around the magnet. Large HTS magnets can store a huge amount of magnetic energy that needs to be dissipated safely and quickly in the event of a quench.

[0016] Japanese Patent Laid-Open No. 10-097900 describes a superconducting wiggler having a pair of center coils. The first excitation current is made to flow through the inner part of each center coil where the magnetic field is large. A second excitation current stronger than the first excitation current is made to flow through the outer part of each center coil where the magnetic field is small. To apply different currents to each part of the center coil, the first excitation current flowing through the inner part is added to the second excitation current flowing through the outer part.

Prior Art Document

Patent Document

[0017]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Means for Solving the Problems

[0018] According to a first aspect of the present invention, there is provided a superconducting magnet system including a superconducting magnet having a plurality of field coils connected in series, each field coil having a plurality of turns including a superconducting material. The system also includes a primary current source connected across the field coils to supply a DC current to the plurality of field coils to generate a magnetic field. The system further includes a secondary current source connected in parallel with the primary current source across a subset to supply an additional DC current to that one or each field coil within the subset of field coils to modify or correct the magnetic field.

[0019] The secondary current source can be configured to supply an additional DC current to the field coils within the subset, for example, to make the magnetic field more uniform in a specific region of space. The secondary current source can be adjustable to vary the additional DC current supplied to the field coils within the subset to compensate for changes in screening currents within the superconducting material.

[0020] The field coils that are "connected in series" mean that the coils are connected in sequence such that there is a path through which current flows continuously (i.e., successively) through the coils. Each turn of the field coil refers to a complete rotation of a tape, wire, cable, etc. containing a superconducting (e.g., HTS) material around an axis (however, in some cases, the field coil may be asymmetric such that different turns do not surround a common axis).

[0021] Each field coil may have an alternative current path across it. The alternative current path contains a resistive material and has a lower inductance compared to each respective field coil such that a changing current across the field coil preferentially flows through the alternative current path. The alternative current path is in thermal contact with the field coil such that heating of the resistive material caused by the current flowing through the alternative current path causes heating of the superconducting material of each respective field coil. For example, successive turns within the coil, or at least a portion of the turns, may be connected in series by a conductive material or by a conductive layer separating the turns. Thus, current can pass from one turn to the next, or be shared between turns, by flowing around the turns within the superconducting material (in a "helical" path) and / or by flowing through the conductive material. The conductive material provides an alternative current path that may be referred to as an inter-turn path or a radial path within a planar coil.

[0022] A subset of the field coils can be a continuous subset of the field coils, i.e., a subset in which each field coil within the subset is connected in series to another field coil without any intervening field coils not within the subset.

[0023] DC current can be defined as a current that persists over many times (e.g., exceeding 5, 10, 100, etc.) the time constant of one or more series-connected field coils in the context of a current flowing through one or more series-connected field coils. The time constant can be defined as the ratio (L / R) of the inductance (L) of one or more field coils to the combined inter-turn resistance or radial resistance (R) of one or more field coils.

[0024] An additional DC current supplied to a subset of the field coils enables control of the contribution of the magnetic field generated by the field coils within the subset to the overall magnetic field generated by the superconducting magnet. For example, the additional DC current supplied by a secondary current source may be smaller than the DC current supplied by a primary current source (e.g., 1%, 10%, or 20% thereof), and by controlling the total current flowing through each superconducting material of the field coils within the subset, it becomes possible to correct or "fine-tune" the overall magnetic field. By correcting the magnetic field, the magnetic field actually generated by the magnet can more closely match a predetermined magnetic field, such as what would be intended by the magnet designer. The secondary current source can be configured to pass an additional DC current in the same direction as, or opposite to, the DC current supplied by the primary current source, depending on whether the contribution of the magnetic field generated by the field coils within the subset to the overall magnetic field generated by the magnet should be increased or decreased. In some cases, the contribution of the magnetic field generated by the field coils within the subset can be increased or decreased to achieve a more uniform magnetic field within the target region.

[0025] The additional DC current supplied by the secondary current source may, in some cases, avoid or reduce the need to use shim coils to correct or adjust the magnetic field generated by the superconducting magnet.

[0026] The system may further comprise a control system for adjusting an additional DC current supplied by a secondary current source so as to generate a magnetic field having one or more predetermined target parameters. The one or more predetermined target parameters may include one or more of the magnitude of the magnetic field within a region of space, the magnitude of the component of the magnetic field along a certain direction within the region of space, the direction of the magnetic field within the region of space, and the gradient of the magnetic field within the region of space. The system may further comprise a magnetic field sensor (e.g., a Hall probe) for measuring one or more parameters of the magnetic field generated by a superconducting magnet within or adjacent to the magnet in a region of space. The control system may be configured to adjust the additional DC current supplied by the secondary current source so as to reduce the absolute difference between one or more measured parameters of the magnetic field and the corresponding one of the predetermined target parameters. For example, the control system may comprise a feedback controller such as a proportional integral derivative (PID) controller. In some cases, the control system may also be configured to adjust the additional current to produce a time-varying magnetic field (regardless of the use of feedback control).

[0027] The system can be configured such that when a DC current from a primary current source is supplied to the field coils, the superconducting material of the field coils within the subset has a higher critical current than the superconducting material of the field coils not within the subset. The primary and secondary current sources can be configured such that an additional DC current supplied by the secondary current source is less than the DC current supplied by the primary current source. In this case, the additional DC current supplied to the field coils within the subset can enable the generation of an increased magnetic field by the magnet without the transport current exceeding the critical current of any of the superconducting materials of the coils. Additionally, thus, the maximum magnitude of the screening current in the superconducting material that can be carried during the steady-state operation of the magnet, which depends on the difference in magnitude between the transport current (i.e., DC current) and the critical current, can be reduced (i.e., a lower screening current can occur at a higher current “saturation”). A lower screening current can mean that the magnetic field generated by the HTS magnet more accurately matches its design specifications and / or is more stable. In some applications, the increased stability resulting from a lower screening current can mean that the need for additional shim coils is reduced or eliminated. The increased stability can be particularly advantageous for applications such as nuclear magnetic resonance (NMR) and / or magnetic resonance imaging (MRI).

[0028] The maximum transport current to critical current ratio of the superconducting material can occur, for example, at the innermost radial turn of the field coil. The maximum transport current to critical current ratio of each of the field coils within the subset can be less than or equal to the maximum transport current to critical current ratio of the field coils not within the subset. Alternatively, the maximum transport current to critical current ratio of each of the field coils within the subset can be greater than the maximum transport current to critical current ratio of the field coils not within the subset.

[0029] The field coil can be a planar (i.e., pancake) coil. Each pancake coil has an axis about which the turns are wound, and the turns are nested radially with respect to the axis, one inside the other. The field coils may be arranged facing each other within a stack (e.g., such that the turns of each pancake coil surround a common axis and the field coils are arranged along that axis). A subset of the field coils can include one or more individual adjacent field coils within the stack. A subset of the field coils can exclude one or both of the field coils at opposite ends of the stack. In such a configuration, the critical current of the superconducting material of the field coils at opposite ends of the stack can be lower than the critical current of the superconducting material of the field coils closer to the midpoint of the stack. For example, when the superconducting material is an HTS material, the magnetic field angles at opposite ends of the magnet may not be well aligned with the ab-axis of the HTS material of the coil, which means that the HTS material of the coils towards opposite ends of the stack has a lower critical current than the HTS material of the coils located closer to the center of the stack. Thus, additional DC current can be provided to the coils located closer to the center of the stack to increase the total DC current flowing through the HTS material. In some implementations, the DC current supplied by the primary current source and the additional DC current supplied by the secondary current source can be adjusted (e.g., iteratively) to obtain a desired (e.g., increased or maximum) magnetic field.

[0030] In some implementations, each turn of the field coil is connected by a conductive material and / or separated by a conductive layer so that current can be shared between the turns of the field coil. For example, when the field coil is a pancake coil, the conductive layer enables current to be shared radially between turns (in addition to the helical current path provided by the turns, where current flows almost exclusively within the superconducting material). The conductive material is in thermal contact with the superconducting material. Particularly good thermal contact can be achieved when the conductive material includes a conductive layer that separates the turns. Another configuration is to provide the conductive material along the coil. The secondary current source can be configured to pass an additional AC current between the turns of the field coil within the subset through the conductive material of the field coil, thereby heating the superconducting material of the turns of the field coil within the subset by resistive heating of the conductive material. The resistive heating of the conductive material preferably reduces the critical current of the superconducting material (e.g., HTS material) in the turns of the field coil within the subset such that the maximum transport current to critical current ratio of each of the turns of the field coil within the subset is greater than the maximum transport current to critical current ratio of the field coil not within the subset. In some cases, the transport current to critical current ratio of each superconducting material of the field coils within the superconducting magnet can differ by less than 20%, preferably less than 10%, or more preferably less than 5%. Thus, the combination of the additional DC current and the AC current can act synergistically to reduce the magnitude of the screening current in the subset of field coils.

[0031] In implementations where the field coil is an insulated coil, an additional AC current may be provided to the subset of coils to disrupt or "scramble" the screening current in these field coils and / or the field coils not within the subset. This process can be referred to as demagnetization.

[0032] The system may further comprise a cryostat for housing the magnet, the cryostat being configured to maintain the superconducting material at a temperature below the critical temperature of the superconducting material during operation of the magnet. The primary current source and the secondary current source may be housed within the cryostat. In this case, the cryostat comprises feedthroughs (i.e., electrical connectors extending from a higher temperature outside the cryostat to a lower temperature inside the cryostat) for supplying power to the primary current source and the secondary current source. The primary current source and the secondary current source may be configured to receive power from different feedthroughs. For example, the feedthrough for supplying power to the primary current source may be electrically insulated from the feedthrough for supplying power to the secondary current source. In some cases, the primary current source and the secondary current source may be configured to receive power simultaneously from the same feedthrough. Using different feedthroughs to supply power to the primary and secondary current sources may mean that the current required to pass through each feedthrough is less compared to supplying power to the primary and secondary current sources simultaneously using the same feedthrough. This may enable the use of feedthroughs with a lower cross-sectional area. Alternatively, the primary current source and / or the secondary current source may be provided outside the cryostat, in which case feedthroughs may be provided for supplying current from the primary current source and / or the secondary current source to the field coils.

[0033] The system may also comprise additional secondary current sources connected across additional subsets for supplying additional DC and / or AC current to the field coils within further subsets of the field coils. Thus, the additional secondary current sources may provide further “fine tuning” of the magnetic field and / or compensate for differences in critical current in the superconducting material of the field coils within each subset. The secondary current sources and the additional secondary current sources may be connected in parallel across further subsets of the field coils. In this case, the further subsets of the field coils are subsets of subsets of the field coils. Thus, the field coils within the further subsets can receive additional current from both secondary current sources.

[0034] According to a second aspect of the present invention, there is provided a method of generating a magnetic field using a superconducting magnet comprising a plurality of field coils connected in series. Each field coil has a plurality of turns including a superconducting material. The method includes using a primary current source connected across the field coils to supply a DC current to the plurality of field coils to generate a magnetic field. The method further includes using a secondary current source connected in parallel with the primary current source across a subset of the field coils to supply an additional DC current to the field coils within the subset to modify or correct the magnetic field.

[0035] The method may further include adjusting the additional DC current supplied by the secondary current source to generate a magnetic field having one or more predetermined target parameters (e.g., having a desired magnetic field quality). The one or more predetermined target parameters may include one or more of the magnitude of the magnetic field within a region of space, the magnitude of the component of the magnetic field along a direction within a region of space, the direction of the magnetic field within a region of space, the gradient of the magnetic field within a region of space, the spatial uniformity of the magnetic field, and the stability of the magnetic field over time. The angular dependence of the magnetic field with respect to an axis may be described, for example, by a weighted sum of spherical harmonic functions. The weighting of the spherical harmonic functions may be varied by adjusting the additional DC current supplied by the secondary current source.

[0036] The method may also include obtaining measured values of one or more parameters of the magnetic field generated by the superconducting magnet, and adjusting the additional DC current supplied by the secondary current source includes reducing the absolute difference between one or more measured parameters of the magnetic field and a corresponding one of the predetermined target parameters.

[0037] The additional DC current supplied by the secondary current source may be smaller than the DC current supplied by the primary current source. The additional DC current supplied by the secondary current source can be adjusted such that the transport current to critical current ratio of the superconducting material in each of the field coils differs by less than 20%, preferably less than 10%, or more preferably less than 5%. The transport current to critical current ratio of each of the field coils within the subset can be greater than or equal to the transport current to critical current ratio of the field coils not within the subset.

[0038] The superconducting magnet can have a time constant (L / R) defined by the ratio of the inductance of the magnet (L) to the inter-turn resistance or radial resistance of the magnet (R), and the DC current and the additional DC current are supplied over a period greater than 5, 10, 50, 100, or 1000 time constants (for example). The radial resistance refers to the resistance between the respective ends of the radially innermost and radially outermost turns of the magnet when the superconducting material is in the superconducting state.

[0039] Each turn of the field coil can be connected by a conductive material and / or separated by a conductive layer so that current can be shared between the turns of the field coil. The method can further include using a secondary current source to supply an additional AC current between the turns of the field coils within the subset through the conductive material of the field coil, whereby resistive heating of the conductive material heats the superconducting material of the turns of the field coils within the subset.

[0040] According to a third aspect of the present invention, a superconducting magnet system is provided that includes a superconducting magnet having a plurality of field coils connected in series. Each field coil has a plurality of turns including a superconducting material. The superconducting magnet also includes a primary current source connected across the field coils to supply a DC current to the plurality of field coils to generate a magnetic field. The superconducting magnet further includes a secondary current source connected in parallel with the primary current source across the subset to supply an additional AC current to the field coils within the subset of field coils.

[0041] Each turn of the field coil can be connected by a conductive material and / or separated by a conductive layer so that current can be shared among the turns of the field coil. The secondary current source can be configured to pass an additional AC current through the conductive material of the field coil (between the turns of the field coil within the subset), whereby the resistive heating of the conductive material heats the superconducting material of the turns of the field coil within the subset. As described in connection with the above aspect, by heating the superconducting material, the critical current of the superconducting material can be reduced such that the ratio of the transport current to the critical current in the superconducting material increases, thereby reducing the magnitude of the screening current of the subset of the field coil. The amount of heating can be controlled, for example, by adjusting the amplitude, frequency, and / or waveform of the AC current.

[0042] Alternatively, each of the field coils may be an insulated coil (such that the turns are separated by an electrically insulating material). In this configuration, the secondary current source can be configured to disrupt the screening current in the superconducting material with an additional AC current to generate a more stable magnetic field.

[0043] According to a fourth aspect of the present invention, there is provided a method of generating a magnetic field using a superconducting magnet comprising a plurality of field coils connected in series, each field coil having a plurality of turns including a superconducting material. The method includes using a primary current source connected across the field coils to supply a DC current to the plurality of field coils to generate a magnetic field. The method further includes using a secondary current source connected in parallel with the primary current source across the subset to supply an additional AC current to the field coils within a subset of the field coils. As in the third aspect, the resistive heating of the conductive material connecting the turns can reduce the critical current of the superconducting material in the turns of the field coil within the subset, or the additional AC current applied to the insulated coil can be used to disrupt the screening current and generate a more stable magnetic field.

[0044] The maximum transport current to critical current ratio of each superconducting material of the field coils of the superconducting magnet may differ by less than 20%, preferably less than 10%, or more preferably less than 5%. The maximum transport current to critical current ratio of the superconducting material may occur, for example, in the innermost turn in the radial direction of the field coil. The maximum transport current to critical current ratio of each of the field coils within the subset may be greater than or equal to the maximum transport current to critical current ratio of the field coils not within the subset. Alternatively, the maximum transport current to critical current ratio of each of the field coils within the subset may be smaller than the maximum transport current to critical current ratio of the field coils not within the subset.

[0045] The superconducting magnet may have a time constant defined by the ratio of the inductance of the magnet to the radial resistance of the magnet. The DC current and the additional AC current may be supplied over a period greater than 5 time constants, preferably greater than 10 time constants, more preferably greater than 100 time constants.

[0046] The method may further include receiving a measured value of a parameter related to the magnetic field generated by the superconducting magnet within a region of space and adjusting an additional AC current supplied by a secondary current source to reduce the difference between the measured value and a predetermined target value for the parameter. The additional AC current may, for example, reduce or eliminate a screening current and enhance the magnetic field quality / uniformity in a specific region of space.

[0047] In each of the above aspects, the superconducting material is preferably an HTS material, such as ReBCO, although in some implementations an LTS material may be alternatively used.

[0048] The turns of the field coil may include, for example, an HTS tape as described above with reference to FIG. 1.

Brief Description of the Drawings

[0049]

Figure 1

Figure 2

Figure 3

Figure 4

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DETAILED DESCRIPTION OF THE INVENTION

[0050] The present disclosure provides a system and method for generating a desired magnetic field from a superconducting dipole magnet including a plurality of field coils connected in series. Some such dipole magnets can be arranged to form quadrupole magnets, sextupole magnets, or other magnetic configurations.

[0051] In the following description, it is assumed that the coil is a planar spiral wound coil (i.e., a pancake coil) having partial insulation provided by a conductive layer separating turns. This is merely for ease of explanation. It will be understood that the techniques described below can be applied to many coil structures including those described in the background art introduction, and the following is merely one non-limiting example.

[0052] FIG. 6 shows a circuit diagram of a superconducting magnet system 600 including a superconducting magnet 602 having first, second, and third field coils 604A - 604C connected in series with each other. Each of the field coils 604A - 604C is represented in the figure by resistors 606A - 606C connected in parallel with inductors 608A - 608C. In this example, each of the field coils 604A - 604C is partially insulated by interposing a conductive layer between turns. The conductive layer allows current to flow radially between turns, thereby bypassing the helical path provided by the HTS material in the turns where current circulates around them to generate a magnetic field. However, in some cases, a fully insulated field coil (where radial inter - turn conduction is prevented by an insulating layer between turns) may be used instead or in addition.

[0053] In this example, each of the field coils 604A - 604C is a pancake coil including turns of HTS tape, as described above with reference to FIG. 1 for example. However, in some implementations, tapes including LTS materials may alternatively be used. The field coils 604A - 604C are arranged opposite to each other and overlap one on the other to form a stack. The superconducting magnet 602 is supplied with current from a primary current source 610 connected across a pair of terminals 611A - 611B provided at both ends of the superconducting magnet 602. In use, the primary current source 610 supplies a DC current to the superconducting magnet 602 such that the current passes through the first of the terminals 611A, circulates around each successive turn of the field coils 604A - 604C within the stack, and then exits the superconducting magnet 602 through the second of the terminals 611B.

[0054] The secondary current source 612 is connected across the second (central) field coil 604B of the superconducting magnet 602 using a first terminal 613A located between the first field coil 604A and the second field coil 604B, and a second terminal 613B located between the second field coil 604B and the third field coil 604C. In this example, the secondary current source 612 is configured to supply an additional DC current to the second field coil 604B. During use, the additional DC current flows around the turns of the second field coil 604B in order to exceed and go above the DC current supplied by the primary current source 610 and increase the transport current flowing within the HTS material. The magnetic field generated by the superconducting magnet 602 has a greater curvature towards the ends of the stack of pancake coils 604A - 604C compared to the center of the stack. This greater curvature means that the magnetic field is generally not as well aligned with the crystal axes (e.g., the ab - axis) of the HTS material in the first and third field coils 604A - 604C located at the ends of the stack, as compared to the second field coil 604B located at the center of the stack. Thus, the HTS material within the second field coil 604B generally has a higher critical current than the HTS material of the first field coil 604A and the third field coil 604C, and can therefore accommodate a larger transport current without losing superconductivity. The secondary current source 612 can have the same polarity as the primary current source 610, such that the current flowing within the HTS material in the turns of the second field coil 604B is greater than the current flowing within the HTS material in the turns of the first field coil 604A and the third field coil 604C. One or both of the current sources 610, 612 can be adjustable such that they can vary the absolute and / or relative amounts of the current supplied by the primary current source 610 and the secondary current source 612. For example, the current can be adjusted such that the ratio of the transport current to the critical current in the HTS material of each of the field coils 604A - 604C is approximately constant, thereby enabling efficient use of the superconducting "capacity" of the field coils and / or reducing the magnitude of the screening current within the HTS material.Alternatively, the absolute and / or relative amount of the transport current flowing through the field coils 604A - 604C can be adjusted to control the contribution to the magnetic field provided by each of the field coils 604A - 604C, thereby changing the magnitude and / or shape of the magnetic field generated by the superconducting magnet 602 as a whole. Such adjustment can eliminate the need for separate "shim" coils to achieve a desired (e.g., more uniform) magnetic field.

[0055] FIG. 7 shows a circuit diagram of another superconducting magnet system 700 that is identical to the superconducting magnet system 600 of FIG. 6, except that the secondary current source 712 is configured to supply an AC current to the second field coil 604B. In the partial insulation magnet of this example, the AC current preferentially flows between the turns of the HTS material through the conductive layer of the partial insulation coil 604B, which is represented by the resistor 606B in FIGS. 6 and 7. In contrast, since the field coil 604B has a long time constant with respect to the frequency of the AC current, there is little AC current flowing around the superconducting "helical" path within the field coil 604B, as represented by the inductor 608B. In other implementations where an insulated coil is used (the inter-turn resistance is very high), it will be understood that the AC current flows within the helical path of the coil. The resistive heating of the conductive layer (i.e., the resistor 606B) by the AC current raises the temperature of the HTS material of the turns on both sides of the layer. Since the critical current of the HTS material decreases as its temperature rises, the ratio of the critical current of the transport current in the HTS material increases after the AC current is applied, which means that the screening current of the HTS material is suppressed. Thus, the magnetic field generated by the superconducting magnet 602 can more closely match the magnetic field intended by the designer of the magnet system, be more stable, and / or reduce the "drift" of the magnetic field. One or more parameters of the AC current, such as its amplitude, frequency, and / or waveform, can be varied to control the temperature of the HTS material and thus the ratio of the critical current of the transport current in the HTS material.

[0056] In some embodiments, the secondary current sources 612, 712 may be configured to provide both DC current and AC current to the second field coil 604B either simultaneously or separately. For example, the secondary current sources 612, 712 can provide a DC current to the second field coil 604B to adjust (e.g., maximize) the transport current in the turns of the HTS material of the second field coil 604B while simultaneously providing an AC current that reduces the critical current of the HTS material. Thus, the local ratio of the transport current to the critical current can be increased (i.e., brought closer to 1 without quenching either the magnet or the field coil) in different coils of the magnet. In certain applications, the primary current source 610 may also be configured to supply an AC current in addition to the DC current.

[0057] Generally, most of the current supplied to the coils is supplied by the primary current source 610, and the secondary current sources 612, 712 provide a smaller amount of current to enable relatively small corrections or modifications to the magnetic field generated by the superconducting magnet 602.

[0058] The superconducting magnet systems 600, 700 can be housed within a cryostat (not shown) that cools the superconducting magnet 602 such that the superconducting material becomes and remains in a superconducting state. Power can be supplied to the primary and secondary current sources 610, 612, 712 through feedthroughs that pass from the relatively high temperature exterior of the cryostat to the lower temperature interior of the cryostat. Separate pairs of feedthroughs can be provided for both of the current sources 610, 612, 712, or a single pair of feedthroughs can be used to supply power to both the primary and secondary current sources 610, 612, 712.

[0059] Generally, the superconducting magnet 602 can have any number of field coils 604A - 604C greater than 1 such that the secondary current sources 612, 712 can be connected across a subset of the field coils (a subset that includes at least one but not all of the field coils, i.e., a proper subset). For example, the superconducting magnet 602 may have only two field coils 604A - 604C, or may have 3, 4, 5, or 10 or more field coils 604A - 604C. The field coils 604A - 604C do not have to be identical to each other, although in some embodiments and use cases this may be preferred. Two or more secondary current sources 612, 712 can also be provided, with each secondary current source 612, 712 connected across a different respective subset of the field coils. Such a configuration can, for example, allow for greater control over the magnetic field generated by the superconducting magnets 600, 700 and / or more effective cancellation of screening currents. The subsets may overlap such that one or more of the field coils belong to two or more subsets and thus receive DC and / or AC current from two or more secondary current sources. In some cases, the secondary current sources 612, 712 may be connected across a subset of the field coils, and another secondary current source 612, 712 may be connected across a part (i.e., a proper subset of the subset) but not all of the field coils within the subset. This type of "nested" configuration of the secondary current sources 612, 712 can make it possible to continuously provide a larger current to the field coils near the center of the stack without exceeding the critical current of the HTS material in any of the field coils towards the ends of the stack (when the critical current is lower).

[0060] FIG. 8 shows a superconducting magnet system 800 comprising a stack of ten pancake coils 804A - 804J connected in series with each other via a joint 814 arranged such that current flows continuously around the turns of each coil, and with the direction of current with respect to the central axis A - A' reversing as it passes from one coil to the next. A primary current source 810 is connected across the stack of pancake coils 804A - 804J using a pair of conductive plates 811A - 811B acting as terminals enabling electrical connection to the radially outermost ends of each of the coils 804A, 804J at both ends of the stack. Four secondary current sources 812A - 812D are connected in parallel across different subsets of the coils. The first of the secondary current sources 812A is connected across all except the two end boundary field coils 804A, 804J of the stack, the second of the secondary current sources 812B is connected across all except the four end boundary field coils 804A - 804B, 804I - 804J of the stack, and so on. In this example, the electrical connection to these coils 804B - 804I is made using a series of conductive plates intervening between the coils. The superconducting magnet 802 is cooled by a cryostat 816 thermally coupled to the pancake coils 804A - 804J by a series of plates intervening between the coils and by the plates 811A - 811B.

[0061] The cross-sectional area of the lead wires used to connect the current sources 810, 812A to 812D across the coils 804A to 804J can be different from each other depending on the amount of current that each source is required to supply to the field coils. For example, since the primary current source 810 supplies most of the current (e.g., 400 A in this example), a lead wire having a larger cross-sectional area can be used compared to the lead wires used for the secondary current sources 812A to 812D that supply less current (e.g., 100 A). In the example shown in FIG. 8, the primary and secondary current sources are arranged outside the cryostat, and thus each pair of lead wires places a thermal load on the cryostat 816. However, since each pair of lead wires carries only a portion of the total current required to power the magnet 802, the amount of resistive heating caused by using two pairs of lead wires can remain the same as when only a single pair of lead wires is used to supply current to the magnet 802. Therefore, by using multiple current sources, it may be possible to increase the flexibility of the magnet design and operation without adversely affecting the cooling and temperature stability of the magnet.

[0062] In this example, in order to measure the temperatures of the field coils 804A to 804J, temperature sensors T1 to T5 (such as thermocouples) are provided at various positions within the magnet 802. Measurement values from one or more of the temperature sensors T1 to T5 are provided to a feedback controller 818 (such as a proportional integral derivative (PID) controller) that controls one or more of the current sources 810, 812A to 812D so as to maintain the temperature of the field coil. For example, as shown in FIG. 8, the temperature sensor T1 provided on the conductive plate 811A can be used to measure the temperature of the first field coil 804A, and the measured temperature value can be provided to the controller 818, which adjusts the primary current source 810 based on the measured value to maintain the temperature of the field coil 804A at a specific set point. The field coils 804A to 804J may each include one or more heaters (not shown), and each heater has an associated feedback controller for helping to maintain the temperature of the field coils 804A to 804J. In such an implementation, the associated feedback controller and the controller 818 may be configured such that an increase in the radial current supplied to the field coils 804A to 804J is compensated by a decrease in the current supplied to the heaters of the coils (and vice versa) in order to maintain the set point (i.e., target) temperature.

[0063] Those skilled in the art will understand that various modifications can be made to the above-described embodiments without departing from the scope of the present invention.

Claims

1. A high-temperature superconductor (HTS) magnet system for nuclear magnetic resonance (NMR), wherein the HTS magnet system comprises: An HTS magnet comprising multiple field coils connected in series, wherein each field coil has multiple turns containing HTS material, A primary current source connected between the ends of the field coils is used to supply DC current to the plurality of field coils and generate a magnetic field. A secondary current source connected in parallel with the primary current source between the ends of the subset of field coils is provided to supply an additional DC current to each field coil in the subset of field coils in order to modify or correct the magnetic field, A control system for adjusting the additional DC current supplied by the secondary current source to enhance the uniformity of the magnetic field in the target region of space, An HTS magnet system equipped with this.

2. The HTS magnet system according to claim 1, further comprising a magnetic field sensor for measuring one or more parameters of the magnetic field generated by the HTS magnet within the HTS magnet or in a region of space adjacent to the HTS magnet.

3. The HTS magnet system according to claim 2, wherein the control system is configured to adjust the additional DC current supplied by the secondary current source in order to reduce the absolute difference between the measured gradient of the magnetic field and the corresponding target gradient.

4. The HTS magnet system according to claim 1, wherein when the DC current from the primary current source is supplied to the field coils, the HTS material of each field coil in the subset has a higher critical current than the HTS material of the field coils not in the subset.

5. The HTS magnet system according to claim 1, wherein the primary current source and the secondary current source are configured such that the additional DC current supplied by the secondary current source is less than the DC current supplied by the primary current source.

6. The HTS magnet system according to claim 1, wherein the field coils include a stack of planar coils, and the subset of field coils includes one or more individual adjacent field coils in the stack.

7. The HTS magnet system according to claim 6, wherein the subset of the field coils excludes one or both of the field coils at both ends of the stack.

8. The HTS magnet system according to claim 1, wherein each of the turns of the field coil is connected by a conductive material so that current can be shared between the turns of the field coil.

9. The HTS magnet system according to claim 1, wherein each field coil has an alternative current path that crosses it, the alternative current path comprises a conductive material, and has a lower inductance compared to each of the field coils such that any change in the current flowing through the field coil preferentially flows through the alternative current path.

10. The HTS magnet system according to claim 8, wherein the secondary current source can be configured to pass an additional AC current through the conductive material of each field coil in the subset, thereby heating the HTS material of each field coil in the subset by resistive heating of the conductive material.

11. The HTS magnet system according to claim 1, further comprising a cryostat housing the HTS magnet, wherein the cryostat is configured to maintain the HTS material at a temperature below the critical temperature of the HTS material during the operation of the HTS magnet, the primary current source and the secondary current source are housed within the cryostat, the cryostat includes feedthroughs for supplying power to the primary current source and the secondary current source, and the primary current source and the secondary current source are configured to receive power from different feedthroughs.

12. The HTS magnet system according to claim 1, further comprising a further secondary current source connected between the ends of a further subset to supply additional DC and / or AC current to the field coils in a further subset of the field coils.

13. The HTS magnet system according to claim 12, wherein the secondary current source and the further secondary current source are connected in parallel between the ends of the further subset of the field coil.

14. A method for generating a magnetic field for nuclear magnetic resonance (NMR) using a high-temperature superconductor (HTS) magnet having multiple field coils connected in series, wherein each field coil has multiple turns containing HTS material, and the method is: A primary current source connected between the ends of the field coils is used to supply DC current to the plurality of field coils and generate a magnetic field, A secondary current source connected in parallel with the primary current source between the ends of the subset of field coils is used to supply an additional DC current to each field coil in the subset of field coils in order to modify or correct the magnetic field, Adjusting the additional DC current supplied by the secondary current source to increase the uniformity of the magnetic field in the target region of space. Methods that include...

15. The method of claim 14, further comprising obtaining measurements of one or more parameters of the magnetic field generated by the HTS magnet, and adjusting the additional DC current supplied by the secondary current source to reduce the absolute difference between the measured gradient of the magnetic field and the corresponding target gradient.

16. The method according to claim 14, wherein the additional DC current supplied by the secondary current source is smaller than the DC current supplied by the primary current source.

17. The method according to claim 16, wherein the additional DC current supplied by the secondary current source is adjusted such that the difference in the ratio of the maximum transport current to the critical current of the HTS material in each of the field coils is less than 20%.

18. The method according to claim 14, wherein each of the turns of the field coil is connected by a conductive material so that current can be shared between the turns of the field coil, and the method further comprises using the secondary current source to supply an additional AC current flowing through the conductive material of each field coil in the subset, thereby heating the HTS material of each field coil in the subset by resistive heating of the conductive material.

19. The method according to claim 14, wherein each field coil in the subset has a time constant defined by the ratio of the inductance of the field coil to the radial resistance of the field coil, and the additional DC current is maintained for a time multiple times the time constant.

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