Superconducting magnet system, tokamak, proton therapy device, and method for heating a superconducting magnet

Abstract

The superconducting magnet system includes a field coil assembly including two or more coil sections connected in series, each coil section having multiple turns including superconductor material. The multiple turns in each coil section are connected by a conductive material to allow current sharing between the turns. The system further includes a magnet heating system including multiple voltage sources. Each voltage source is connected across a respective one of the coil sections of the field coil assembly to apply a voltage having an AC and / or DC component across the coil section and drive a current through the conductive material. The voltage sources are configured such that at least two of the multiple voltages have out-of-phase AC components and / or DC components of opposite polarity.

Description

[Technical field]

[0001] The present invention relates to superconductor magnets, and in particular to high temperature superconducting (HTS) magnets. More particularly, the present invention relates to a method for rapidly heating a superconductor magnet, for example in response to a quench detection, and a system for implementing the method. [Background technology]

[0002] Superconducting materials are typically classified as "high temperature superconductors" (HTS) and "low temperature superconductors" (LTS). LTS materials, such as Nb and NbTi, are metals or metal alloys whose superconductivity can be described by BCS theory. All low temperature superconductors have a 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 cannot be described by BCS theory, and many have critical temperatures well above 30 K. The most commonly used HTS materials are "copper oxide superconductors", which are ceramics based on copper oxides (compounds containing a copper oxide group) such as BSCCO or ReBCO (where Re is a rare earth element, typically Y or Gd). Other HTS materials are iron nicotides (e.g. FeAs and FeSe) and magnesium diboride (MgB 2 ).

[0003] ReBCO superconductors are typically manufactured as tapes approximately 100 micrometers thick and 2 to 12 millimeters wide. The structure of a typical tape 100 is shown in FIG. 1 and includes a substrate 101, typically an electropolished nickel-molybdenum alloy, e.g. Hastelloy®, approximately 50 micrometers thick, on which a series of buffer layers, known as a buffer stack 102, approximately 0.2 micrometers thick, are deposited. An epitaxial ReBCO·HTS layer 103 overlies the buffer stack, typically 1 micrometer thick. A 1 to 2 micrometer silver layer 104 and a copper stabilizer layer 105 are deposited on the tape, often completely enveloping it. The silver layer 104 and the copper stabilizer layer 105 extend continuously around the periphery of the tape 100 (not shown in FIG. 1 for clarity) and are therefore also referred to as "cladding". The silver layer 104 provides a low resistance electrical interface to and a hermetic protective seal around the ReBCO layer 103, while the copper layer 105 allows external connection to the tape from both sides (e.g., by soldering) and provides parallel conductive paths for electrical stabilization. "Peel-off" HTS tapes can also be produced, which lack the substrate and buffer stack.

[0004] HTS tapes and other superconducting materials are sometimes characterized by the maximum current, temperature, and magnetic field critical surface at which the superconductor transitions from the superconducting state to the normal conducting state. For example, the critical current I c is the current at which a superconductor becomes normal at a given temperature and magnetic field, and the critical temperature T c is the temperature at which a superconductor becomes normal for a given magnetic field and current. The critical temperature is often formally defined for zero magnetic field, but for convenience we use the term more generally here. The critical surface for many HTS tapes can be highly dependent on both the magnitude and direction of the magnetic field.

[0005] An HTS cable contains one or more HTS tapes connected along its length via a conductive material (usually copper). By this definition, a length of HTS tape is an HTS cable. The HTS tapes may be stacked (i.e., arranged so that the HTS layers are parallel) or may have other tape arrangements that may vary along the length of the cable.

[0006] Superconducting magnets are formed by arranging HTS cable into one or more turns of coils. A turn (or winding) of the coil is a section of HTS cable that seals the inside of the coil (i.e., can be modeled as a complete loop). Broadly speaking, there are two types of magnetic coil construction: by winding, or by assembling several sections. As shown in FIG. 2, wound coils are manufactured by winding the HTS cable 201 in a continuous spiral around a former 202. The former is shaped to give the coil the required inner circumference and may become a structural part of the final wound coil or may be removed after winding. As shown diagrammatically in FIG. 3, section coils are composed of multiple sections 301, each of which encompasses multiple cables or preformed busbars 311 and forms an arc of the entire coil (i.e., a continuous section that is smaller than a full turn). These sections are connected by joints 302 to form a complete coil. The coil or coil sections are often solidified by sealing or potting, and sometimes a sealing material fills the spaces between the turns. Suitable encapsulating materials include both insulating materials, such as epoxy, and conductive materials, such as solder.

[0007] FIG. 4 shows a cross section of a particular type of wound coil known as a "pancake coil", in which the HTS cable 401 is wound in a planar helical fashion to form a flat coil. Pancake coils can be made with an inner circumference of any two-dimensional shape. Often pancake coils are provided as "double pancake coils", as shown in cross section in FIG. 5. These include two pancake coils 501, 502 wound in opposite directions with an insulator 503 in between. The inner terminals are connected to 504. This means that one only needs to supply a voltage to the commonly accessible outer terminals 521, 522 to pass a current through the turns of the coil and generate a magnetic field. Many other coil configurations are possible, including helical solenoids.

[0008] One application of HTS field coils is in tokamak plasma chambers, including spherical tokamaks, where strong magnetic fields are required to confine and control the plasma. Another potential application of HTS field coils is in proton beam therapy (PBT) and proton boron capture therapy (PBCT) devices, where proton beams are used to treat cancer. PBT and PBCT devices require very high magnetic fields to both accelerate and steer the proton beam.

[0009] HTS coils fall into three broad classes: · Insulated, meaning that the turns have electrical insulation separating them. In this arrangement, current only flows around the turns of the coil (i.e. the helical path along the HTS cable). Non-insulated, the turns are connected with low resistance, for example by a conductive metal. This can be achieved, for example, by forming the coil with a copper stabilizer layer (or other metal cladding) connecting the turns, and / or by potting the coil with a conductive solder. · Partially insulated, where the turns are connected with a resistance intermediate between the conductor and the insulator. This may be achieved by separating the turns with a material that has a high resistance compared to copper (e.g., co-wound stainless steel tape or any layer with the desired resistance), and / or by providing intermittent insulation between the turns, and / or by providing a resistive material (which may consist of resistors or other components) along the sides of the coil connecting at least some of the turns. The resistance between turns of a partially insulated coil may be between 100 times that of copper and 10 times that of copper to achieve the desired ratio L / R of the inductance around the coil, L, to the resistance across it, R. 15 The thickness of the insulation can be controlled between 100 and 200 mm. Different forms of partial insulation are described, by way of example only, in US Pat. No. 5,399,323 and US Pat. No. 5,499,323.

[0010] A non-insulated coil may be thought of as a lower resistance case of a partially insulated coil. Generally, in both partially insulated and uninsulated pancake coils, the turns are connected by normally conducting (i.e. non-superconducting) material, or equivalently resistive (but not insulating) material, and current is shared between the turns via the conductive material. For example, in a pancake coil, current may flow radially as well as around a helical path. In a solenoid, an additional longitudinal current path is provided.

[0011] An uninsulated or partially insulated HTS coil can be modeled as having three current paths. Two current paths are a helical path that follows the HTS cable around the turns (one in the HTS and one in the metal stabilizer), and an inter-turn path that crosses the magnet between the coil terminals. For example, in a pancake coil, this inter-turn path would be a radial path through the metal stabilizer and any other resistive material that connects the turns. Although this can be modeled as a singular path, it actually represents the sum of all resistive paths across the magnet. Only the current flowing through the helical path produces a significant magnetic field. The HTS helical path can be modeled as an inductor with a large inductance and zero or negligible resistance if the tapes are all superconducting. The stabilizer helical path is in parallel with the HTS helical path and has the same inductance (in a simple model), but has significant resistance. For this reason, the current flowing through the HTS helical path can be negligible unless some part of it starts to quench. The path between the turns across the magnet can be modeled as having negligible inductance while the HTS material is superconducting, and a much larger resistance than the HTS spiral path. The current flowing in this path is negligible unless a portion of the HTS spiral path starts to quench or the current in the HTS spiral path changes (due to the large inductance of the HTS spiral path opposing the change in current). When the HTS spiral path starts to quench, the critical current I of the HTS spiral path c Any excess current above this is shared between the helical stabilizer path and the inter-turn path depending on their relative resistances and L / R time constants.

[0012] HTS field coils are generally designed so that every turn of HTS tape has a local critical current I c However, various fault conditions can cause the HTS tape current to exceed the critical current: -Failure of cooling leads to an increase in temperature (localized or global) and thus I c decreases. ·Transport current I 0Transient increase in current, e.g. overcurrent fault in power supply. Damage to HTS materials (e.g. due to stress cracking, fatigue due to thermal cycling, current cycling of magnets). - Localized energy accumulation sufficient to cause thermal runaway.

[0013] When the current through the tape exceeds the local critical current (or is close to it), a portion of the current is driven into the metal layers of the tape (mainly the copper stabilizer layer), into other normally conductive (i.e., non-superconducting) materials separating the turns of the uninsulated or partially insulated coil, and into the "spare" I of nearby HTS materials. c The current flowing through a normally conductive material generates heat and reaches a local critical current I c further reducing the power dissipation, potentially leading to thermal runaway.

[0014] The areas of the HTS tape that are first affected by a fault condition are known as "hot spots". Early detection of hot spots is important to avoid damage to the HTS magnet by "quenching" the magnet and dissipating the energy. Various approaches to detect hot spots are known, for example, temperature sensors, strain sensors, and voltage taps placed around the magnet. Large HTS magnets can store enormous amounts of magnetic energy, so if a quench occurs, that energy needs to be dissipated safely and quickly.

[0015] US Patent No. 5,399,633 describes a method for ramping down or quenching a non-insulated or partially insulated HTS magnet, in which a large "reverse" current (i.e., a current in the opposite direction to the current flowing through the magnet's HTS coils) is applied to the magnet before ramping down. This reverse current flows primarily in a radial path, causing significant heating of the magnet and resulting in a rapid quench. [Prior art documents] [Patent documents]

[0016] [Patent Document 1] International Publication No. 2019 / 150123 [Patent Document 2] International Publication No. 2020 / 079412 [Patent Document 3] International Publication No. 2020 / 104807 Summary of the Invention

[0017] According to a first aspect of the disclosure, there is provided a superconductor magnet system including a field coil assembly consisting of two or more coil sections connected in series. Each coil section has a plurality of turns including superconductor material. The turns of each coil section are connected by a conductive material such that current can be shared between the turns (i.e., current can be passed between the connected turns through the conductive material). The system further includes a magnet heating system consisting of a plurality of voltage sources. Each voltage source is connected across a respective one of the coil sections of the field coil assembly to apply a voltage having an AC component and / or a DC component across that coil section and drive a current through the conductive material. The voltage sources are configured such that at least two of the plurality of voltages have out-of-phase AC components and / or DC components of opposite polarity.

[0018] Resistive heating of the conductive material generates heat, which increases the temperature of the superconductor material, preferably above the critical temperature of the superconductor material, causing the superconductor material to lose its superconductivity.

[0019] By configuring the voltage sources such that at least two of the voltages have out-of-phase AC components and / or DC components of opposite polarity, current can be driven through the conductive material without the need to apply a large voltage across the field coil assembly. For example, by applying out-of-phase AC voltages across the coil sections, the instantaneous voltage applied across the field coil assembly can be minimized. Preferably, the instantaneous voltages applied between the coil sections can be (substantially) equal and opposite, such that the voltage applied across the field coil assembly as a whole is approximately zero (or within a predetermined tolerance). Thus, an advantage of using multiple voltage sources (as compared to a single voltage source) to heat the coils is that the voltage across the superconductor magnet can be limited to a low level, reducing the risk of voltage-induced breakdown that could damage, for example, the field coil and / or the leads supplying current to the field coil. Because the voltage sources drive current through the conductive material and not around the turns of the superconductor material, the magnet heating system can also be operated independently from the power source supplying current to the superconductor magnet. For example, the operating current from the magnet heating system (which may typically be part of the quench protection system) does not require the transport current of the superconductor magnet to be above a certain level for heating to be effective.

[0020] The conductive material is in thermal contact with the superconductor material. One arrangement that provides particularly good thermal contact is when the conductive material includes a conductive layer separating the turns. Another arrangement is to have the conductive material side-by-side with the coil.

[0021] The voltage source may be configured such that the phasor sum of the voltages applied across the coil sections has an amplitude less than the amplitude of at least one of the AC voltages applied to the coil sections, or preferably less than the amplitude of all of the AC voltages applied across the coil sections, where the phasor sum is the sum of phasors, each of which is a complex representation of one of the voltages.

[0022] The voltage sources may be configured such that the respective amplitudes of the voltages applied across the coil sections differ by less than 10%, preferably less than 5%, more preferably less than 1% of the maximum amplitude. If the voltage sources include respective AC components, the frequencies of the AC components are preferably substantially the same (e.g., the frequencies differ by less than 10%, preferably less than 5%, more preferably less than 1% of the maximum frequency).

[0023] Each voltage source comprises a capacitor or a bank of capacitors. Each voltage source may include a respective switch, preferably a thyristor, for connecting and / or disconnecting the voltage source from a respective one of the coil sections.

[0024] The field coil assembly may include a single field coil, or multiple field coils connected in series. In the case of multiple field coils, each coil section may be limited to a single field coil or may span two or more field coils. Each field coil may be a pancake coil with respective turns wound on an axis, with the turns nested one on top of the other radially relative to the axis.

[0025] The magnetic heating system may include a transformer having at least one primary coil for simultaneously inducing a respective AC voltage across two or more secondary coils of the transformer, with the voltage source including at least one secondary coil of the secondary coils each connected across a corresponding coil section, the transformer preferably being configured such that the AC voltages applied across the coil sections are out of phase.

[0026] The transformer may be a single phase transformer. Preferably, the transformer is configured such that the AC voltages applied across the coil sections by the two secondary coils are 180 degrees out of phase.

[0027] The transformer may be a three-phase transformer including three primary coils, preferably arranged in a delta configuration, and the primary and secondary coils may form a balanced three-phase system.

[0028] Each primary coil may be configured to induce a respective AC voltage in the at least two secondary coils.

[0029] The superconductor magnet system may further include a cryostat for cooling the superconductor magnet to a temperature below the critical temperature of the superconductor material. The secondary coil, and optionally the primary coil, of the transformer may be disposed within the cryostat.

[0030] The superconductor material may be a high temperature superconductor (HTS) material, such as ReBCO, or a low temperature superconductor (LTS) material.

[0031] Each coil section includes a respective number of turns that may be within 10%, preferably within 5%, and more preferably within 1% of the maximum number of turns.

[0032] The field coil assembly may include a plurality of field coils connected in series, with each coil section being provided by one or more respective field coils. Each coil section preferably includes the same number of field coils. Alternatively, the field coil assembly may include a single field coil, with each coil section including a subset of the turns of the field coil. Each coil section preferably has the same resistance.

[0033] The superconductor magnet system may further include one or more additional field coil assemblies. Each additional field coil assembly includes two or more coil sections connected in series, each coil section having a plurality of turns including superconductor material. The magnet heating system may include a plurality of voltage sources for each of the additional field coil assemblies. Each voltage source is connected across a respective one of the coil sections of the additional field coil assemblies to apply a voltage having an AC component and / or a DC component across the coil section and drive a current through the conductive material. The voltage sources are configured such that at least two voltages of the plurality of voltages have out-of-phase AC components and / or DC components of opposite polarity. The applied voltages may be different for some or all of the plurality of field coil assemblies, for example to achieve different heating rates for each field coil assembly.

[0034] According to a second aspect of the present disclosure there is provided a tokamak including one or more superconductor magnet systems according to the first aspect, wherein the field coil assembly of each superconductor magnet system may include a toroidal or poloidal field coil of the tokamak.

[0035] According to a third aspect of the present disclosure, there is provided a proton beam therapy (PBT) device including one or more superconductor magnet systems according to the first aspect, wherein the field coil assembly of each superconductor magnet system may include a field coil of an accelerator of the PBT device, or a field coil of a dipole magnet or a quadrupole magnet of a proton beam steering system of the PBT device.

[0036] According to a fourth aspect of the present disclosure, there is provided a method of heating a superconductor magnet. The superconductor magnet comprises a field coil assembly including two or more coil sections connected in series. Each coil section has a plurality of turns including superconductor material. The plurality of turns in each coil section are connected by a conductive material to allow current sharing between the turns. The method includes applying respective voltages having AC and / or DC components across each of the coil sections to drive current through the conductive material, thereby heating the superconductor material in each coil section. At least two of the plurality of voltages are configured to have out-of-phase AC components and / or DC components of opposite polarity.

[0037] These voltages may be applied across the coil sections in response to detecting a quench or a condition likely to cause a quench in one or more of the coil sections.

[0038] At least one of the amplitude, frequency, and waveform of one or more of these voltages may be adapted to heat the superconductor material in one or more coil sections from a first equilibrium temperature to a second equilibrium temperature that is greater than the first equilibrium temperature and less than the critical temperature of the superconductor material. This may, for example, reduce the critical current of the superconductor material such that the ratio of transport current to critical current in the superconductor material increases when a DC current is supplied to the field coil assembly. This may reduce or eliminate the screening current in the superconductor material.

[0039] Applying respective voltages having AC and / or DC components across each of the coil sections to drive a current through the conductive material may include applying an AC voltage to at least one primary coil of a transformer to simultaneously induce respective AC voltages across two or more secondary coils of the transformer, and applying each of the AC voltages across one of the respective coil sections, wherein the AC voltages applied across the coil sections are out of phase.

[0040] The method may further include connecting a power source across the superconductor magnet to pass a current through the superconductor material to generate a magnetic field, the power source remaining connected across the superconductor magnet after applying respective voltages across each of the coil sections at least until a temperature of some or all of the superconductor material in the coil sections exceeds a critical temperature of the superconductor material.

[0041] According to a fifth aspect of the present disclosure, there is provided a superconductor magnet system including a field coil assembly including two or more coil sections connected in series, each coil section having a plurality of turns including superconductor material, and a magnet heating system including a transformer having at least one primary coil for simultaneously inducing a respective AC voltage across two or more secondary coils of the transformer, each secondary coil being connected across a respective one of the coil sections of the field coil assembly to apply a corresponding AC voltage to that coil section, the transformer being configured such that the AC voltages applied across the coil sections are out of phase.

[0042] Embodiments of the fifth aspect may include features noted above as being optional features of the first aspect. The transformer may be a three-phase transformer including three primary coils. The three primary coils are preferably arranged in a delta configuration. The primary coils and secondary coils may form a balanced three-phase system.

[0043] According to a sixth aspect of the present disclosure, there is provided a method of heating a superconductor magnet. The superconductor magnet includes a field coil assembly including two or more coil sections connected in series. The method includes applying an AC voltage to a primary coil of a transformer to simultaneously induce respective AC voltages in two or more secondary coils of the transformer, and applying each of the AC voltages to a respective one of the coil sections. The AC voltages applied across the coil sections are out of phase.

[0044] Embodiments of the sixth aspect may include features noted above as optional features of the fourth aspect. For example, the voltages may be applied across the coil sections in response to detection of a quench or a condition likely to cause a quench in one or more of the coil sections.

[0045] According to a seventh aspect of the present disclosure, there is provided a superconductor magnet system including a magnet assembly including two or more coil sections connected in series, each coil section having a plurality of turns including superconductor material, an alternate current path across each coil section, the alternate current path including a resistive material and having a lower inductance compared to the respective coil section such that a varying current across the coil section preferentially flows through the alternate current path, and heating of the resistive material caused by current flowing through the alternate current path causes heating of the superconductor material of the respective coil section, and a plurality of voltage sources, each voltage source connected across a respective one of the coil sections and its alternate current path to apply a voltage having an AC component and / or a DC component, the voltage sources configured such that at least two of the voltages have out-of-phase AC components and / or DC components with opposite polarity.

[0046] Optional and preferred features of the seventh aspect will be apparent from the foregoing description of the other aspects. [Brief description of the drawings]

[0047] [Figure 1] FIG. 1 is a schematic diagram of an HTS tape. [Diagram 2] FIG. 1 is a schematic diagram of a wound HTS coil. [Diagram 3] FIG. 1 is a schematic diagram of a section HTS coil. [Figure 4] FIG. 2 is a cross-sectional view of a pancake coil. [Diagram 5] FIG. 2 is a cross-sectional view of a double pancake coil. [Figure 6] FIG. 1 is a circuit diagram of the HTS magnet system. [Figure 7A-7B] 7A and 7B are graphs showing simulated voltages for the HTS magnet system of FIG. [Figure 8] FIG. 1 is a circuit diagram of the HTS magnet system. [Figure 9A-9B] 9A and 9B are graphs showing simulated voltages for the HTS magnet system of FIG. [Figure 10] FIG. 1 is a circuit diagram of the HTS magnet system. [Figure 11] FIG. 11 is a circuit diagram showing an implementation of a portion of the HTS magnet system of FIG. [Figure 12] FIG. 1 is a circuit diagram of the HTS magnet system. [Figure 13] FIG. 13 is a circuit diagram showing an implementation of a portion of the HTS magnet system of FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0048] The present disclosure provides methods and systems for rapidly heating superconductor (e.g., HTS) magnets and allowing the energy stored in the magnets to be safely dissipated. Superconductor magnets, for example, include multiple field coils and require rapid simultaneous heating so that each field coil is quenched simultaneously.

[0049] To illustrate an existing system for ramping down a superconductor magnet, FIG. 6 shows an HTS magnet system 800 including an HTS magnet 802 mounted in a cryostat 803 for cooling the HTS magnet 802 so that it can sustain a superconducting current for generating a magnetic field. The HTS magnet 802 includes a partially insulated HTS field coil including a first coil section 804A and a second coil section 804B connected in series with each other. In the following description of this existing system and the present invention, it is assumed that the coil is a flat, spirally wound coil (i.e., a pancake coil) with partial insulation provided by a conductive layer separating the turns. This is purely for ease of explanation. It will be recognized that the techniques described below can be applied to many non-insulated and partially insulated coil configurations, including those described in the background introduction, and the following is only one non-limiting example.

[0050] Each coil section 804A, 804B is represented in FIG. 6 by a radial resistor 806A, 806B connected in parallel with an inductor 808A, 808B. In general, each turn of the field coil can be represented by a radial resistor and an inductor connected in parallel. That is, the field coil can be represented by a number of coil sections equal to the number of turns, each coil section including a radial resistor connected in parallel with an inductance. Thus, a coil section may include a single turn or multiple turns. "Partially insulated" means that the turns of the coil are electrically connected by a conductive material such that current can flow radially between the turns. For example, a partially insulated coil includes an insulating layer disposed between the turns of the HTS material, with one or more conductive paths extending through the insulating layer. A magnet power supply 810 is connected across the HTS magnet 802 and provides direct current to the HTS magnet 802 via first and second leads 811A, 811B. A circuit breaker 812 is also provided in series with the magnet power supply 810 to quickly disconnect the power supply 810 from the HTS magnet 802 in the event that a hot spot (or other condition that may initiate a quench) forms or begins to form in the HTS magnet 802.

[0051] The HTS magnet system 800 also includes a magnet heating system 814. The magnet heating system 814 includes a voltage source 816 (also referred to as a power source) connected in parallel with the magnet power supply 810 across the HTS magnet 802 and having a polarity opposite to that of the magnet power supply 810 to provide a "reverse" current to the HTS magnet 802. In this example, the voltage source 816 includes a pre-charged capacitor. The magnet heating system also includes a thyristor 818 (although other types of switches may be used) that is used to connect or disconnect the capacitor from the HTS magnet 802, and a current limiting inductor 820 that controls the peak current provided to the HTS magnet 802 and thus the peak terminal voltage across the magnet 802 (i.e. across the current leads 811A, 811B). The peak current may alternatively (or additionally) be controlled by adjusting the radial resistance, for example by varying the resistance of a conductive material provided between the turns. The inductor 820 is preferably tuned to achieve the lowest peak current required to raise the temperature of the HTS magnet 802 by a desired amount (e.g., about 20K to about 40K, which is the critical or "transition" temperature of the HTS material) in a desired amount of time. The time required for a safe discharge is highly dependent on the current density in the coil. For large tokamaks, 100A / mm can be achieved with additional stabilizers. 2 When operating at 600A / mm 2 For small coils operating at 100 ms this can be as long as 100 ms.

[0052] During "normal" operation of the HTS magnet 802, the circuit breaker 812 is closed and the thyristor 818 is open, thereby allowing the magnet power supply 810 to supply current to the HTS magnet 802 while the capacitor 816 is disconnected from the HTS magnet 802. The circuit breaker 812 may include a diode that prevents current from the capacitor 816 from entering the magnet power supply 810. When a ramp down of the magnet current is required, the circuit breaker 812 is opened and the thyristor 818 is closed such that current is no longer supplied from the power supply 810 to the HTS magnet 802 and a reverse current begins to flow from the capacitor 816 of the magnet heating system 814 to the HTS magnet 802. Because the superconducting spiral path of the magnet 802 has a large inductance, the reverse current flows primarily through the radial resistances 806A, 806B of the first and second coil sections 804A, 804B (and not through the inductors 808A, 808B), thereby heating the coil sections 804A, 804B. The radial resistances 806A, 806B (provided by the conductive material between the turns) are integrated into the field coil and are therefore in good thermal contact with the turns. The energy stored in the magnet is dissipated over a wide area when the magnet quenches, rather than, for example, in only one of the coil sections or in only a particular turn. Compared to other coil heating methods that rely on coupling losses between the turns to heat the superconductor material, resistive heating by current driven through the conductive layers allows more control over where the heating occurs. In addition, once the superconductor becomes normal, the very large current flowing through the superconductor is deflected through the conductive layers, rapidly heating other parts of the field coil.

[0053] 7A and 7B show how the voltage across each of the coil sections 804A, 804B (FIG. 7A) and across the entire HTS magnet 802 (FIG. 7B) varies as a function of time (seconds) after the thyristor 818 is switched to discharge the capacitor 816. In this example, the initial voltage on the capacitor 816 is about 800V and is discharged by about 30ms after the thyristor 818 is switched. Thus, there are two distinct phases of discharge: the capacitor discharge phase (until ∼30ms) and the inductor discharge phase (from about 30ms to about 100ms). During the capacitor discharge phase, energy is dissipated in the circuit resistance but is also transferred to the magnetic field of the current limiting inductor 820. The voltage across each of coil sections 804A, 804B decays as the capacitor discharges, dropping from 0 V to approximately −50 V over 40 ms, then oscillating from −50 V to −80 V and back to −50 V over approximately 70 ms. Because coil sections 804A, 804B are equivalent in this example (hence only a single curve is visible on the graph shown in FIG. 7A), the voltage across HTS magnet 802 as a whole is twice the voltage across each of coil sections 804A, 804B.

[0054] During the capacitor discharge phase, the magnitude of the (reverse) radial current through each of coil sections 804A, 804B increases to around 50 kA and then gradually decreases over the period in which the inductor discharges. This current heats up coil sections 804A, 804B, raising the temperature of the HTS material (from 20 K to around 70 K in this case) and terminating superconductivity.

[0055] A diode (not shown) may be connected across capacitor 816 to prevent capacitor voltage reversal which could cause damage to the capacitor. When the capacitor voltage reaches 0V, the diode begins to conduct and discharging is then a function of the L / R time constant of the system. This provides an effective direction to dissipate the capacitor's energy. However, the diode is not required and instead the circuit can be allowed to oscillate at the natural LC frequency.

[0056] The magnet heating system 814 can generally be straightforward to implement and tuned to give the required performance in terms of discharge time. A capacitor 816 with the required characteristics may be provided in the form of (for example) a bank of capacitors connected in parallel with each other. However, a high current circuit breaker 812 may need to be custom made, as it may be difficult to obtain or implement in some cases. The "large" voltage across the HTS magnet 802 when using the magnet heating system 814 (e.g. on the order of 200V, which is considered large in the context of the very high (e.g. kA) currents typically flowing in the coils) may also be a significant drawback in some cases.

[0057] Figure 8 shows an HTS magnet system 1000 similar to the HTS magnet system 800 of Figure 6, except that the magnet heating system 1014 includes two DC voltage sources 1014A, 1014B (one for each of the first and second coil sections 804A) and the HTS magnet system 1000 does not include a DC circuit breaker 812 (because the voltage across the HTS magnet 802 is much lower than in the HTS magnet system 800 of Figure 6, as will be described below), although the DC circuit breaker 812 may be included if desired. The first of the two voltage sources 1014A is connected across the first coil section 804A via one of the current leads 811A (i.e., to the terminal of the first coil section 804A to which the magnet power supply 810 is also connected) and the second connection is connected to a magnet "tap" (current lead) 1022 between the first and second coil sections 804A, 804B. The second of the two voltage sources 1014B is connected across the second coil section 804B via the magnet tap 1022 and the other of the current leads 811B (i.e., to the terminal of the second coil section 804B to which the power supply 810 is also connected). The DC voltage sources 1014A, 1014B are configured to be of opposite polarity. Voltage sources 1014A, 1014B each include a respective capacitor 1016A, 1016B that is switched by a respective thyristor 1018A, 1018B. In this case, voltage sources 1014A, 1014B also each have a respective inductor 1020A, 1020B to limit peak currents, as described above.

[0058] 9A and 9B show how the voltage across each of the coil sections 804A, 804B (FIG. 9A) and the total voltage across the HTS magnet 802 (FIG. 9B) change as a function of time after the thyristors 1018A, 1018B are switched to discharge the capacitors 1016A, 1016B. Since the voltages applied to the first and second coil sections 804A, 804B by the capacitors 1016A, 1016B are always equal and opposite in this case (although there may be slight differences due to variations in radial resistance, lead inductance and layer quality, etc.), their contributions to the voltage across the HTS magnet 802 cancel each other out. Thus, the total voltage across the HTS magnet 802 remains low throughout the discharge, in this case less than 4V. This low voltage means that the power supply 810 does not need to be disconnected from the HTS magnet 802 during discharge. Thus, for example, a circuit breaker 812 may not be necessary. Note, however, that the voltage at the magnet tap 1022 rises during discharge. When the capacitors 1016A, 1016B are discharged, current flows in the opposite direction through the magnet tap 1022 and radially through the first and second coil sections 804A, 804B. Thus, the net DC current measured at each terminal (i.e., current leads 811A, 811B) of the HTS magnet 802 remains approximately constant during discharge, e.g., around 7 kA in this embodiment.

[0059] Although the above examples refer to a single capacitor 816, 1016A, 1016B, each capacitor may be provided, for example, in the form of a bank of capacitors connected in parallel with each other. The capacitors may be metal film oil-filled capacitors, or so-called "supercapacitors". Other types of voltage sources may be used instead of or in addition to the capacitors, such as batteries (such as lithium polymer batteries), superconducting magnetic energy storage (SMES), flywheel energy storage, etc.

[0060] Similarly, although the magnet system 1000 is illustrated with the HTS magnet 802 including two coil sections 804A, 804B, any number of coil sections 804A, 804B connected in series may be used. More generally, a magnet may include a field coil assembly having two or more field coils connected in series. In this case, a coil section may include a subset of the turns in one coil or all the turns in one coil, or may include multiple turns from more than one coil. For example, a tokamak may in some cases include 14 toroidal field coils connected in series, and then a magnet tap 1022 is provided between the seventh and eighth toroidal field coils such that coil sections 804A, 80B are each comprised of seven field coils, so that capacitors 1016A, 1016B provide current in opposite directions through toroidal field coils 1-7 and 8-14. Preferably, the number of HTS coils is even, and the magnet tap 1022 may be located at the midpoint of the HTS magnet to ensure equal current flows on both sides of the magnet tap 1022.

[0061] A potential drawback of the magnet heating system 1014 is the use of additional magnet taps 1022 (i.e., current leads). This causes additional thermal load to the cryostat 803 and resistive load to the capacitors 1016A, 1016B. If the capacitors 1016A, 1016B are cryogenically cooled, the thermal load to the cryostat 803 may be reduced, thus allowing a "cold" connection with the magnet 802 that remains "off" while the magnet 802 is in use but no heat is transferred from the magnet 802. However, as the capacitor bank gets larger, the required additional cooling may be prohibitive in some cases (e.g., considering a typical volumetric energy density of an oil film capacitor at room temperature, a 400 kJ capacitor bank would require 2.7 m 3 will account for a certain degree of

[0062] FIG. 10 shows an HTS magnet system 1200 similar to the HTS magnet system 1000 of FIG. 8, except that the magnet heating system 1214 includes two AC voltage sources 1214A, 1214B for driving AC current through each of the coil sections 804A, 804B. The large inductance of the helical path of each of the coil sections 804A, 804B means that the current flows substantially radially through the coil sections 804A, 804B. Because the AC voltage sources 1214A, 1214B operate at the same frequency and are 180 degrees out of phase with each other, the voltage across the HTS magnet 802 remains approximately zero at all times. The use of AC current means that the coil sections 804A, 804B are neither discharged nor charged while they are being heated. That is, the DC current through the magnet 802 remains substantially constant, at least until a quench is induced in the magnet 802.

[0063] FIG. 11 shows an implementation of a magnet heating system 1214 comprising a transformer 1302 including a primary coil 1304 and two secondary coils 1314A, 1314B. The transformer 1302 acts as an AC voltage source 1214A, 1214B for applying an AC voltage across each of the coil sections 804A, 804B. The transformer 1302 is driven by an inverter (not shown) that injects an AC current that oscillates at a predetermined frequency. The inverter may be constructed, for example, using insulated gate bipolar transistors (IGBTs). A capacitor (not shown) may be connected in series with the inverter to compensate for reactive parasitics of the inductance of the transformer or leads. The resonant frequency of the system may be, for example, around 12 kHz and may be tuned by adjusting the capacitance of the capacitor. The terminals of the secondary coils 1314A, 1314B (e.g., the negative terminals of each of the secondary coils) are connected together to apply a common mode voltage to the magnetic tap 1220 disposed between the first and second coil sections 804A, 804B. Expanding the system to include more coil sections 804A, 804B only requires adding secondary windings of the transformer 1302 and does not increase the number of active components.

[0064] The transformer 1302 is preferably located inside the cryostat 803, with two current leads 1306A, 1306B used to connect to the inverter (although in some embodiments the transformer 1302 may instead be located outside the cryostat 803). By increasing the turns ratio of the transformer 1302 (i.e., the number of turns in the secondary coils 1314A, 1314B divided by the number of turns in the primary coil 1304), the transformer primary current can be reduced, and therefore the size of the current leads 1306A, 1306B can be significantly reduced.

[0065] In the example shown in Figure 11, the transformer 1302 is a single-phase transformer operating at the switching frequency of the inverter. The power provided by the transformer 1302 is pulsed at twice the switching frequency of the inverter, which means that the peak current of the transformer 1302 is approximately twice the current generated by the DC discharge magnet heating system 814, 1014 described above in connection with Figures 8 and 10. To reduce the peak current, a three-phase transformer can be used instead. In this case, the current phases are spaced 120° apart, and the instantaneous coupled power remains constant, so the root mean square (RMS) current in the primary coil can be reduced by approximately 43%.

[0066] 12 shows an HTS magnet system 1400 comprising an HTS magnet 1402 including six HTS field coils or coil sections 1404A-F connected in series, with a DC power supply 1410 connected across the coil sections 1404A-F to supply current to the coil sections 1404A-F to generate the desired magnetic field. Each of the coil sections 1404A-F has a corresponding AC power supply 1412A-F (forming part of a magnet heating system 1414) for supplying AC current to heat the coil sections 1404A-F. The AC voltage sources 1412A-F corresponding to successive pairs of coil sections (i.e., coil sections 1404A-B, 1404C-D and 1404E-F) are configured such that the coil sections in each pair are 180 degrees out of phase with each other, as described above for the transformer 1214 of FIG.

[0067] Figure 13 illustrates an implementation 1500 of the HTS magnet system 1400 shown in Figure 12. It includes three transformers 1514A-C with primary coils 1504A-C each connected in a delta configuration (i.e., connected in series with one another to form a triangle). Each primary coil 1504A-C receives AC current from one of three current leads 1506A-C, each of which has an AC waveform that is 120 degrees out of phase with the AC waveforms of the other two current leads 1506A-C. The transformers 1514A-C are configured to form a balanced three-phase system, such that the AC currents provided by the transformers 1514A-C to the coil sections 1404A-F are "balanced," i.e., sum to zero. Thus, while the coil sections 1404A-F are heated by AC current, the voltage across the HTS magnet 1402 remains generally zero (or less than a predetermined value, such as 100V, 50V, 10V, or 1V).

[0068] The transformers 1214, 1514A-C may be implemented in many ways, for example, iron core, air core, or wireless power transfer links. In some implementations, the three transformers 1514A-C may be replaced with a single three-phase transformer. However, using separate transformers 1514A-C allows for different voltages to be provided to each phase to achieve different heating strategies for the different coil sections 1404A-F.

[0069] While the above examples describe applying a voltage across a coil section of a field coil, in general a voltage may be applied across a coil section that spans multiple field coils or a group of multiple field coils connected in series or parallel. For example, for a magnet including two field coils, a first coil section may span a first coil of the field coil and a second coil section may span a second coil of the field coil. In either case, the coil sections should preferably be substantially identical. For example, a pancake coil section may be achieved by providing one or more taps such that there are the same number of turns on either side of a tap or between successive taps (or the number of turns is preferably equal within 20%, 10% or 5%, for example), or each coil section may span the same number of HTS coils as an equivalent coil connected in an equivalent manner. For example, for an HTS magnet including 14 HTS coils connected in series, each section may span 7 HTS coils connected in series. In one embodiment, two magnet taps may be used to split one HTS field coil into three (equal) sections, with each section being supplied with a voltage from a secondary coil of a three-phase power supply. Configurations such as these mean that the magnitude of the voltage applied across the HTS magnet can be kept as low as possible. Because HTS field coils are uninsulated or partially insulated, they can be heated by radial currents, i.e., currents that flow through the conductive layers between the turns, rather than along a helical path.

[0070] The magnet heating systems 814, 1014, 1214, 1414 described above may be actuated in response to detection of a quench or detection of a condition likely to cause a quench, which may be by any practical method. For example, Detection of excess voltage across HTS materials in magnets, The use of a secondary HTS tape provided adjacent to the HTS field coil and configured to quench before any of the HTS coils, for example as described in WO 2017 / 042541 or WO 2019 / 150091; By detecting temperature, strain, magnetic field or other conditions in the magnet coil, for example via Rayleigh scattering in a fiber optic cable as described in WO 2018 / 078327, or via other temperature, strain or magnetic field detectors as known in the art.

[0071] The magnet heating systems 814, 1014, 1214, 1414 described above may also be applied in other situations where the magnet is ramped down, for example, when shutting down the magnet under normal conditions in the absence of a detected quench (or conditions likely to lead to a quench), or when heating of the magnet is required for any other purpose.

[0072] The above disclosure may be applied to various HTS magnet systems. In addition to the tokamak plasma chamber mentioned as an example, it may be used in HTS coils of nuclear magnetic resonance imaging (NMR / MRI) devices, operation of magnetic devices in non-magnetic media via magnetic fields (e.g., robotic magnetic navigation systems for operating medical devices in patients), and magnets for electric motors, such as for electronic aircraft. As a further example, the present disclosure may be applied to proton therapy devices including HTS magnet systems including the disclosed features, where the HTS magnet system is used in the accelerator of a PBT device, the quadrupole or bipole steering magnet of a PBT device, or other magnets of a PBT device.

Claims

1. 1. A superconductor magnet system, comprising: a field coil assembly including two or more coil sections connected in series, each coil section having a plurality of turns including superconductor material, the turns in each coil section being connected by a conductive material to allow current sharing between the turns; a magnet heating system including a plurality of voltage sources, each connected across a respective one of the coil sections of the field coil assembly, wherein a voltage having an AC and / or DC component is applied across the respective one of the coil sections to drive a current through the conductive material, the voltage sources configured such that at least two of the voltages have out-of-phase AC components and / or DC components with opposite polarity; A superconductor magnet system comprising:

2. 2. The superconductor magnet system of claim 1, wherein the voltage source is configured such that a phasor sum of the voltages applied across the coil sections has an amplitude that is less than an amplitude of at least one of the voltages applied across the coil sections.

3. 2. The superconductor magnet system of claim 1, wherein the voltage source is configured such that a phasor sum of the voltages applied across the coil sections has an amplitude that is less than the amplitude of each of the voltages applied across the coil sections.

4. 2. The superconductor magnet system of claim 1, wherein the voltage source is configured such that a plurality of amplitudes of the voltages applied across the coil sections each differ by less than 10% of a maximum of the amplitudes.

5. 10. The superconductor magnet system of claim 1, wherein each voltage source comprises a capacitor or a bank of capacitors.

6. 10. The superconductor magnet system of claim 1, wherein each of the voltage sources includes a respective switch for connecting and / or disconnecting the voltage source from a respective one of the coil sections.

7. The superconductor magnet system of claim 1 , wherein the conductive material includes a conductive layer separating the turns.

8. 2. The superconductor magnet system of claim 1, wherein the magnet heating system includes a transformer having at least one primary coil for simultaneously inducing a respective AC voltage across two or more secondary coils of the transformer, each of the voltage sources including at least one of the secondary coils connected across a corresponding coil section to drive a current through the conductive material, and the transformer configured such that the AC voltages applied across the coil sections are out of phase.

9. 9. The superconductor magnet system according to claim 8, wherein the transformer is a single-phase transformer.

10. 10. The superconductor magnet system of claim 9, wherein the transformer is configured such that the AC voltages applied across the coil sections by two of the secondary coils are 180 degrees out of phase.

11. 9. The superconductor magnet system of claim 8, wherein the transformer is a three-phase transformer including three primary coils.

12. 12. The superconductor magnet system of claim 11, wherein the three primary coils are arranged in a delta configuration.

13. 13. The superconductor magnet system of claim 12, wherein the primary and secondary coils form a balanced three-phase system.

14. 9. The superconductor magnet system of claim 8, wherein each primary coil is configured to induce a respective AC voltage in at least two of the secondary coils.

15. 9. The superconductor magnet system of claim 8, further comprising a cryostat for cooling the superconductor material to a temperature below a critical temperature of the superconductor material, the secondary coil of the transformer being disposed within the cryostat.

16. 16. The superconductor magnet system of claim 15, wherein the primary coil of the transformer is disposed within the cryostat.

17. 2. The superconductor magnet system of claim 1, wherein each of said coil sections includes a respective number of turns, said number of turns being equal to within 10% of said maximum number of turns.

18. 10. The superconductor magnet system of claim 1, wherein the field coil assembly includes a plurality of field coils connected in series, each coil section being provided by a respective one or more of the field coils.

19. 20. The superconductor magnet system of claim 18, wherein each coil section includes an equal number of field coils.

20. 10. The superconductor magnet system of claim 1, wherein the field coil assembly includes a single field coil, and each coil section is provided by a subset of turns of the single field coil.

21. A tokamak comprising one or more superconductor magnet systems according to claim 1.

22. A proton therapy device comprising one or more superconductor magnet systems according to claim 1.

23. 1. A method of heating a superconductor magnet, the superconductor magnet including a field coil assembly including two or more coil sections connected in series, each coil section having a plurality of turns including superconductor material, the turns in each coil section connected by a conductive material to allow current sharing between the turns, the method comprising: applying respective voltages having AC and / or DC components across each of the coil sections to drive a current through the conductive material, at least two of the voltages having AC components that are out of phase and / or DC components that have opposite polarities.

24. 24. The method of claim 23, wherein the voltage is applied across one or more of the coil sections in response to detecting a quench or a condition likely to cause a quench in the coil sections.

25. 24. The method of claim 23, wherein at least one of an amplitude, frequency, and waveform of one or more of the voltages is adapted to heat the superconductor material in the one or more coil sections from a first equilibrium temperature to a second equilibrium temperature that is greater than the first equilibrium temperature and less than a critical temperature of the superconductor material.

26. applying a respective voltage having an AC component and / or a DC component across each of the coil sections to drive a current through the conductive material includes: applying an AC voltage to at least one primary coil of a transformer to simultaneously induce AC voltages across two or more secondary coils of the transformer; applying each of said AC voltages to a respective one of said coil sections; Including, 24. The method of claim 23, wherein the AC voltages applied across the coil sections are out of phase.

27. 24. The method of claim 23, further comprising connecting a power source across the superconductor material to pass a current through the superconductor material to generate a magnetic field, the power source remaining connected across the superconductor magnet after applying a respective voltage across each of the coil sections until a temperature of at least some or all of the superconductor material in the coil sections exceeds a critical temperature of the superconductor material.

28. The field coil assembly includes: a plurality of field coils connected in series, each coil section being provided by a respective one or more field coils of the plurality of field coils; or a single field coil, each coil section being provided by a subset of the turns of said single field coil; 24. The method of claim 23, comprising any one of:

29. 1. A superconductor magnet system, comprising: a magnet assembly including two or more coil sections connected in series, each coil section having a plurality of turns including a superconductor material; an alternative current path across each coil section, the alternative current path including a resistive material having a low inductance compared to the respective coil section such that a varying current across the coil section flows preferentially through the alternative current path, and heating of the resistive material caused by current flowing through the alternative current path causes heating of the superconductor material of the respective coil section; and a plurality of voltage sources, each of the voltage sources connected across a respective one of the coil sections to apply a voltage having an AC component and / or a DC component, the voltage sources configured such that at least two of the voltages have AC components that are out of phase and / or DC components that have opposite polarities; A superconductor magnet system comprising:

30. The magnet assembly includes: a plurality of field coils connected in series, each coil section being provided by a respective one or more field coils of the plurality of field coils; or a single field coil, each coil section being provided by a subset of the turns of said single field coil; 30. The superconductor magnet system of claim 29, comprising:

Images