Rapid dumping of superconducting magnets

Abstract

A superconducting magnet system includes a field coil assembly including two or more coil segments connected in series. Each coil segment has a plurality of turns including a superconducting material. The turns in each coil segment are connected by a conductive material such that current may be shared between the turns. The system also includes a magnet heating system including a plurality of voltage sources. Each voltage source is connected across a respective one of the coil segments of the field coil assembly to apply a voltage across the coil segment having an AC component and / or a DC component to drive a current through the conductive material. The voltage source is configured such that at least two of the voltages have out-of-phase AC components and / or DC components having opposite polarities.

Description

Technical Field

[0001] The present disclosure relates to superconducting magnets, in particular high temperature superconducting HTS magnets. Specifically, the present disclosure relates to a method for rapidly heating a superconducting magnet (eg, in response to quench detection) and a system for implementing the method. Background Art

[0002] Superconducting materials are usually 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. The peak critical temperature (above which the material does not superconduct even in zero magnetic field) of all low temperature superconductors is below 30K. The BCS theory does not describe the behavior of HTS materials, and many HTS materials have critical temperatures well above 30K. The most commonly used HTS materials are "cuprate superconductors" - ceramics based on cuprates (compounds including copper oxide groups), such as BSCCO or ReBCO (where Re is a rare earth element, usually Y or Gd). Other HTS materials include iron phosphides (e.g., FeAs and FeSe) and magnesium diborate (MgB2).

[0003] ReBCO superconductors are typically manufactured as ribbons with a thickness of about 100 micrometers and a width between 2mm and 12mm. Figure 1 The structure of a typical ribbon 100 is shown in FIG. 1 and includes a substrate 101 (typically an electropolished nickel-molybdenum alloy such as Hastelloy™ about 50 microns thick) on which is deposited a series of buffer layers about 0.2 microns thick (referred to as a buffer stack 102). An epitaxial ReBCO-HTS layer 103 covers the buffer stack and is typically 1 micron thick. A 1 to 2 micron silver layer 104 and a copper stabilizer layer 105 are deposited on the ribbon and typically completely encapsulate the ribbon. The silver layer 104 and copper stabilizer layer 105 extend continuously around the perimeter of the ribbon 100 (for clarity, the layers are not shown). Figure 1 The silver layer 104 forms a low resistivity electrical interface with the ReBCO layer 103 and forms a hermetic protective seal around the ReBCO layer 103, while the copper layer 105 enables external connection to the ribbon from either side (e.g., by soldering) and provides parallel conductive paths for electrical stability. It is also possible to manufacture "peel-off" HTS ribbons, which lack the substrate and 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 cis the temperature at which a superconductor becomes normal under a given magnetic field and current. The critical temperature is usually formally defined for zero magnetic field, but for convenience, this article uses the term more broadly. The critical surface of many HTS tapes can also be highly dependent on both the magnitude and direction of the magnetic field.

[0005] An HTS cable consists of one or more HTS ribbons connected along their length via a conductive material (usually copper). Under this definition, a single HTS ribbon is an HTS cable. HTS ribbons may be stacked (i.e., arranged so that the HTS layers are parallel), or they may have some other ribbon arrangement that may vary along the length of the cable.

[0006] Superconducting magnets are formed by arranging HTS cable into a coil consisting of one or more turns. A turn (or winding) of a coil is a segment of HTS cable that surrounds the interior of the coil (i.e., it can be modeled as a complete loop). Broadly speaking, magnetic coils exist in two configurations - by winding, or by assembling several segments. Figure 2 As shown, a wound coil is manufactured by winding HTS cable 201 in a continuous spiral form on a coil former 202. The coil former is shaped to provide the desired inner circumference of the coil and can be a structural part of the final wound coil or can be removed after winding. Figure 3 As shown schematically, the segmented coil consists of several segments 301, each of which may contain several cables or preformed busbars 311, and will form an arc of the entire coil (i.e., a continuous segment of less than a full turn). The segments are connected by joints 302 to form a complete coil. Coils or coil segments are usually reinforced by encapsulating or potting them, and the encapsulating material can fill the space between the turns. Suitable encapsulating materials include both insulating materials (e.g., epoxy resins) and conductive materials (e.g., solder).

[0007] Figure 4 A cross section of a particular type of wound coil known as a "pie coil" is shown, in which the HTS cable 401 is wound in a planar spiral to form a flat coil. Pie coils can be made with an inner perimeter of any two-dimensional shape. Typically, pie coils are provided as "double pie coils", such as Figure 5 501, 502, wound in opposite directions, with an insulating layer 503 in between. The inner terminals are connected 504. This means that simply supplying voltage to the outer terminals 521, 522, which are usually more accessible, can drive current through the turns of the coil and generate a magnetic field. Many other coil configurations are possible, including spiral solenoids.

[0008] One use for HTS field coils is in tokamak plasma chambers (including spherical tokamaks), where strong magnetic fields are needed to confine and control the plasma. Another potential use for 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 accelerate and guide the proton beams.

[0009] HTS coils are divided into three categories:

[0010] Insulated, with electrical insulating material between and separating the turns. In this arrangement, current can only flow around the turns of the coil (i.e., along the helical path of the HTS cable).

[0011] • Non-insulated, where the turns are connected by low resistance (e.g., by conductive metal). For example, this can be achieved by forming the coil so that a copper stabilizer layer (or other metal cladding) connects the turns and / or potting the coil with conductive solder.

[0012] Partially insulated, where the turns are connected by a resistance between the conductor and the insulator. This can be achieved by separating the turns with a material having a higher resistance than copper (e.g., a co-wound stainless steel strip or any layer with the desired resistance), and / or by providing intermittent insulation between the turns, and / or by providing resistive material (which may include components such as resistors) along the sides of the coil and connecting at least some of the turns. The resistance between turns in a partially insulated coil can be controlled to be between 100 and 10 of the resistance of copper. 15 times to achieve a desired ratio L / R between the inductance L around the coil and the resistance R across the coil. As just some examples, different forms of partial insulation are described in WO2019150123 and WO2020079412.

[0013] The non-insulated coil can be considered as the low resistance case of the partially insulated coil. Generally speaking, in both partially insulated and non-insulated pie coils, the turns are connected by normal (i.e., non-superconducting) conductive material or equivalent resistance (but non-insulating) material so that current can be shared between the turns via the conductive material. For example, in a pie coil, current can flow both radially and around a helical path. In a solenoid, an additional longitudinal current path is provided.

[0014] A non-insulated or partially insulated HTS coil can be modeled as having three current paths - two spiral paths along the HTS cable around the turns (one in the HTS and one in the metal stabilizer), and a turn-to-turn path across the magnet, between the coil terminals. In a pie-shaped coil, for example, this turn-to-turn path would be a radial path through the metal stabilizer and any other resistive material connecting the turns. Although this can be modeled as a single path, it actually represents the sum of all resistive paths across the magnet. Only the current flowing in the spiral path produces a significant magnetic field. When the strip is all superconductive, the HTS spiral path can be modeled as an inductor with large inductance and zero or negligible resistance. The stabilizer spiral path is parallel to the HTS spiral path and has the same inductance (in a simple model), but has significant resistance. For this reason, the current flowing therein can be ignored unless part of the HTS spiral path begins to quench. The turn-to-turn path across the magnet can be modeled as having negligible inductance and a resistance much greater than the HTS spiral path when the HTS material is superconductive. Unless part of the HTS spiral path begins to quench or the current in the HTS spiral path changes (due to the large inductance of the HTS spiral path hindering the change of current), the current flowing in this path is negligible. If the HTS spiral path begins to quench, the critical current I c The overcurrent is distributed between the spiral stabilizer path and the turn-to-turn path according to their relative resistances and the L / R time constant.

[0015] The HTS field coil is usually designed so that all HTS strips in all turns are below their local critical current I c Operation, the local critical current I c The current around the coil varies due to changes in the magnetic field and coil temperature. However, various fault conditions can cause the HTS strap current to exceed the critical current:

[0016] Cooling failure, causing the temperature to rise (locally or globally), thereby reducing I c .

[0017] A transient increase in the transmission current I0, such as a power supply overcurrent fault.

[0018] Damage to the HTS material (e.g., due to stress cracking, fatigue due to thermal cycling or power cycling of the magnet).

[0019] Localized energy deposition sufficient to cause thermal runaway.

[0020] If the current in any strip exceeds (or approaches) the local critical current, some current will be driven into the metal layers of the strip (primarily the copper stabilizer layer), any other normally conducting (i.e., non-superconducting) material separating the turns in an uninsulated or partially insulated coil, and any “spare” I2C of nearby HTS material. c The current flowing through the normal conductive material generates heat and further reduces the local critical current I c , which may lead to thermal runaway.

[0021] The area of ​​an HTS strip initially affected by a fault condition is known as a "hot spot". It is very important to detect the hot spot early so that damage to the HTS magnet can be avoided by causing the magnet to "quench" and dissipate its energy. Various methods are known for detecting hot spots, such as using temperature sensors, strain sensors or voltage taps distributed around the magnet. Large HTS magnets are able to store large amounts of magnetic energy which needs to be dissipated safely and quickly in the event of a quench.

[0022] WO2020 / 104807 describes a method of ramping down or quenching a non-insulated or partially insulated HTS magnet, wherein a large "reverse" current is applied to the magnet, i.e., in a direction opposite to the direction of the current flowing through the HTS coils of the magnet before ramping down. This reverse current flows primarily along a radial path, resulting in significant heating of the magnet and rapid quenching of the magnet. Summary of the invention

[0023] According to a first aspect of the present disclosure, a superconducting magnet system is provided, comprising: a field coil assembly comprising two or more coil segments connected in series. Each coil segment has a plurality of turns comprising a superconducting material. The turns in each coil segment are connected by a conductive material so that current can be shared between the turns (i.e., current can flow between the connected turns via the conductive material). The system also includes a magnet heating system comprising a plurality of voltage sources. Each voltage source is connected across a corresponding one of the coil segments of the field coil assembly to apply a voltage having an AC component and / or a DC component across the coil segment, thereby driving current through the conductive material. The voltage source is configured so that at least two of the voltages have out-of-phase AC components and / or DC components having opposite polarities.

[0024] Resistive heating of the conductive material generates heat, which raises the temperature of the superconducting material, preferably to a temperature above the critical temperature of the superconducting material, resulting in a loss of its superconductivity.

[0025] By configuring the voltage sources so that at least two of the voltages have out-of-phase AC components and / or DC components with opposite polarities, current can be driven through the conductive material without applying a large voltage across the entire field coil assembly. For example, by applying out-of-phase AC voltages across the coil segments, the instantaneous voltage applied across the entire field coil assembly can be minimized. Preferably, the instantaneous voltages applied across the coil segments can be (substantially) equal and opposite, so that the voltage applied across the entire field coil assembly is approximately zero (or within a predetermined tolerance). Therefore, an advantage of using multiple voltage sources (compared to a single voltage source) to heat the coil is that the voltage across the superconducting magnet can be limited to a lower level, which reduces the risk of voltage-induced breakdown, which may, for example, damage the field coil and / or cause current to be supplied to the field coil. Since the voltage sources drive current through the conductive material, rather than around the turns of superconducting material, the magnet heating system can also be operated independently of the power supply that supplies current to the superconducting magnet. For example, the operating current of the magnet heating system (which is typically part of a quench protection system) does not require the transmission current in the superconducting magnet to be above a certain level to effectively heat.

[0026] The conductive material is in thermal contact with the superconducting material. One arrangement which provides particularly good thermal contact is that the conductive material comprises conductive layers separating the turns. Another arrangement is to provide the conductive material alongside the coil.

[0027] The voltage source may be configured such that the magnitude of the phasor sum of the voltages applied across the coil segments is less than the magnitude of at least one of the AC voltages applied to the coil segments, or preferably less than the magnitude of all the AC voltages applied across the coil segments. Here, the phasor sum refers to the sum of phasors (i.e., phase vectors), each phasor being a complex representation of one of the voltages.

[0028] The voltage sources may be configured such that the respective amplitudes of the voltages applied across the coil segments differ by less than 10%, preferably less than 5%, or more preferably less than 1% of the maximum of the amplitudes. When 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%, or more preferably less than 1% of the maximum of the frequencies).

[0029] Each voltage source may comprise a capacitor or a capacitor bank.Each voltage source may comprise a respective switch, preferably a thyristor, for connecting and / or disconnecting the voltage source from its respective one of the coil segments.

[0030] The field coil assembly may include a single field coil or a plurality of field coils connected in series. In the case of a plurality of field coils, each coil segment may be confined to a single field coil or span two or more of the field coils. Each field coil may be a pancake-shaped coil having a respective axis around which the turns are wound, the turns being radially nested one within the other relative to the axis.

[0031] The magnet heating system may include a transformer having at least one primary coil for simultaneously inducing a corresponding 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 segment. The transformer is preferably configured such that the AC voltages applied across the coil segments are out of phase.

[0032] The transformer may be a single-phase transformer.Preferably, the transformer is configured such that the AC voltages applied across the coil segments by two of the secondary coils are 180 degrees out of phase.

[0033] The transformer may be a three-phase transformer comprising three primary coils. The three primary coils are preferably arranged in a triangle configuration. The primary coils and the secondary coils may form a balanced three-phase system.

[0034] Each primary coil may be configured to induce a corresponding AC voltage in at least two of the secondary coils.

[0035] The superconducting magnet system may further comprise a cryostat for cooling the superconducting magnet to a temperature below the critical temperature of the superconducting material. The secondary coil and optionally the primary coil of the transformer may be arranged in the cryostat.

[0036] The superconducting material may be a high temperature superconducting (HTS) material (eg, ReBCO) or a low temperature superconducting (LTS) material.

[0037] Each coil segment comprises a corresponding number of turns. The number of turns may be equal within 10%, preferably within 5%, or more preferably within 1% of the maximum number of turns among the number of turns.

[0038] The field coil assembly may comprise a plurality of field coils connected in series, each coil segment being provided by a corresponding one or more field coils in the field coils. Each coil segment preferably comprises the same number of field coils. Alternatively, the field coil assembly may comprise a single field coil, such that each coil segment comprises a subset of the turns of the field coil. Each coil segment preferably has the same resistance.

[0039] The superconducting magnet system may also include one or more additional field coil assemblies. Each additional field coil assembly includes two or more coil segments connected in series, each coil segment having a plurality of turns including superconducting material. The magnet heating system may include a corresponding plurality of voltage sources for each of the additional field coil assemblies. Each voltage source is connected across a corresponding one of the coil segments of the additional field coil assembly to apply a voltage having an AC component and / or a DC component across the coil segment to drive current through the conductive material. The voltage source is configured so that at least two of the voltages have out-of-phase AC components and / or DC components having opposite polarities. The applied voltage may be different between some or all of the field coil assemblies, for example to achieve different heating rates for each field coil assembly.

[0040] According to a second aspect of the present disclosure, a tokamak is provided, comprising one or more superconducting magnet systems according to the first aspect. The field coil assembly of each superconducting magnet system may comprise a toroidal field coil or a poloidal field coil of the tokamak.

[0041] According to a third aspect of the present disclosure, a proton beam therapy (PBT) device is provided, which includes one or more superconducting magnet systems according to the first aspect. The field coil assembly of each superconducting magnet system may include: a field coil of an accelerator of the PBT device, or a field coil of a dipole or quadrupole magnet of a proton beam steering system of the PBT device.

[0042] According to a fourth aspect of the present disclosure, a method for heating a superconducting magnet is provided. The superconducting magnet includes a field coil assembly, which includes two or more coil segments connected in series. Each coil segment has a plurality of turns including a superconducting material. The turns in each coil segment are connected by a conductive material so that current can be shared between the turns. The method includes: applying a corresponding voltage having an AC component and / or a DC component at both ends of each of the coil segments to drive current through the conductive material. This results in heating of the superconducting material of the corresponding coil segment. At least two of the voltages have out-of-phase AC components and / or DC components with opposite polarities.

[0043] In response to detecting a quench, or a condition tending to cause a quench, in one or more of the coil segments, a voltage may be applied across the coil segments.

[0044] At least one of the amplitude, frequency and waveform of one or more of the voltages may be adapted to heat the superconducting material in the one or more coil segments 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 superconducting material. This may, for example, allow a critical current of the superconducting material to be reduced so that when a DC current is supplied to the field coil assembly, a ratio of a transmission current to the critical current in the superconducting material is increased. Screening currents in the superconducting material may thereby be reduced or eliminated.

[0045] Applying a corresponding voltage having an AC component and / or a DC component across each of the coil segments to drive current through the conductive material may include: applying an AC voltage to at least one primary coil of a transformer to simultaneously induce corresponding AC voltages across two or more secondary coils of the transformer; and applying each of the AC voltages across a corresponding one of the coil segments, wherein the AC voltages applied across the coil segments are out of phase.

[0046] The method may further include connecting a power source across the superconducting magnet to cause current to flow through the superconducting material to generate a magnetic field, and after applying a corresponding voltage across each coil segment, the power source remains connected across the superconducting magnet at least until a temperature of some or all of the superconducting material in the coil segment exceeds a critical temperature of the superconducting material.

[0047] According to a fifth aspect of the present disclosure, there is provided a superconducting magnet system, comprising: a field coil assembly, comprising two or more coil segments connected in series, each coil segment having a plurality of turns comprising a superconducting material; and a magnet heating system, comprising a transformer, the transformer having at least one primary coil for simultaneously inducing a corresponding AC voltage across two or more secondary coils of the transformer. Each secondary coil is connected across a corresponding one of the coil segments of the field coil assembly to apply a corresponding AC voltage across the coil segment. The transformer is configured so that the AC voltages applied across the coil segments are out of phase.

[0048] Embodiments of the fifth aspect may include the features indicated above as optional features of the first aspect. For example, the transformer may be a three-phase transformer comprising three primary coils. The three primary coils are preferably arranged in a triangle configuration. The primary coil and the secondary coil may form a balanced three-phase system.

[0049] According to a sixth aspect of the present disclosure, a method for heating a superconducting magnet is provided. The superconducting magnet includes a field coil assembly, which includes two or more coil segments connected in series. The method includes: applying an AC voltage to a primary coil of a transformer to simultaneously induce corresponding AC voltages across two or more secondary coils of the transformer; and applying each of the AC voltages across a corresponding one of the coil segments. The AC voltages applied across the coil segments are out of phase.

[0050] Embodiments of the sixth aspect may include the features indicated above as optional features of the fourth aspect.For example, in response to detecting a quench, or a condition tending to cause a quench, in one or more of the coil segments, a voltage may be applied across the coil segments.

[0051] According to a seventh aspect of the present disclosure, a superconducting magnet system is provided, comprising: a magnet assembly comprising two or more coil segments connected in series, each coil segment having a plurality of turns comprising a superconducting material; an AC current path across each coil segment, the AC current path comprising a resistive material and having a low inductance compared to the corresponding coil segment, so that a varying current across the coil segments preferentially flows through the AC current path, wherein heating of the resistive material caused by the current flowing through the AC current path results in heating of the superconducting material of the corresponding coil segment; and a plurality of voltage sources, each voltage source being connected to a corresponding one of the coil segments and across its AC current path to apply a voltage having an AC component and / or a DC component, the voltage source being configured so that at least two of the voltages have out-of-phase AC components and / or DC components having opposite polarities.

[0052] Optional and preferred features of the seventh aspect will be apparent from the foregoing discussion of the other aspects. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 is a schematic representation of the HTS band;

[0054] Figure 2 is a schematic representation of a wound HTS coil;

[0055] Figure 3 is a schematic representation of a segmented HTS coil;

[0056] Figure 4 is the cross section of the pie-shaped coil;

[0057] Figure 5 is the cross section of a double pancake coil;

[0058] Figure 6 is the circuit diagram of the HTS magnet system;

[0059] Fig. 7A and Figure 7B It shows Figure 6 A graph of the simulated voltage of the HTS magnet system;

[0060] Figure 8 is the circuit diagram of the HTS magnet system;

[0061] Fig.9A and Fig. 9B It shows Figure 8 A graph of the simulated voltage of the HTS magnet system;

[0062] Fig.10 is the circuit diagram of the HTS magnet system;

[0063] Fig.11 yes Fig.10 A circuit diagram of an implementation of a portion of an HTS magnet system;

[0064] Fig.12 is a circuit diagram of the HTS magnet system; and

[0065] Fig.13 yes Fig.12 Circuit diagram of the implementation of a part of the HTS magnet system. DETAILED DESCRIPTION

[0066] The present disclosure provides methods and systems for rapidly heating a superconducting (eg, HTS) magnet so that energy stored in the magnet can be safely dissipated. A superconducting magnet, for example, may include multiple field coils that need to be rapidly and simultaneously heated so that each field coil quenches simultaneously.

[0067] To illustrate existing systems for ramping down superconducting magnets, Figure 6 An HTS magnet system 800 is shown, which includes an HTS magnet 802, which is disposed inside a cryostat 803 for cooling the HTS magnet 802 so that it can maintain a superconducting current for generating a magnetic field. The HTS magnet 802 includes a partially insulated HTS field coil, which includes a first coil segment 804A and a second coil segment 804B connected in series to each other. In the following description of the prior art system and the present invention, it is assumed that the coil is a planar, spirally wound coil (i.e., a pie-shaped coil), in which partial insulation is provided by a conductive layer separating the turns. This is purely for ease of illustration. It will be appreciated that the techniques described below can be applied to many non-insulated and partially insulated coil structures, including the coil structures discussed in the background technology, and the following is only a non-limiting example.

[0068] Figure 6, each of the coil segments 804A, 804B is represented 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 plurality of coil segments equal to the number of turns, each coil segment including a radial resistor connected in parallel with an inductor. Thus, a coil segment may include a single turn or multiple turns. "Partially insulated" means that the turns of the coil are electrically connected by conductive material so that current can flow radially between the turns. For example, a partially insulated coil may include an insulating layer disposed between turns of HTS material, wherein one or more conductive paths extend through the insulating layer. A magnet power supply 810 is connected across the HTS magnet 802 to supply DC current to the HTS magnet 802 via a first lead 811A and a second lead 811B. A circuit breaker 812 is also provided in series with the magnet power supply 810 to allow for rapid disconnection of the power supply 810 from the HTS magnet 802 in the event that a hot spot forms or begins to form in the HTS magnet 802 (or other conditions that may cause a quench to begin).

[0069] The HTS magnet system 800 also includes a magnet heating system 814 that is connected in parallel with the magnet power supply 810 across the HTS magnet 802 and includes a voltage source 816 (which may also be referred to as a power supply) having a polarity opposite to that of the magnet power supply 810 so as to supply 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 another type of switch may be used) that is used to connect or disconnect the capacitor and the HTS magnet 802; and a current limiting inductor 820 that controls the peak current supplied to the HTS magnet 802, thereby controlling 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 (e.g., by changing the resistance of the conductive material disposed 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., from about 20K to about 40K, the critical or "transition" temperature of the HTS material) in a desired time. The time required for a safe discharge depends largely on the current density in the coil. For large tokamaks, with the addition of stabilizers at 100A / mm 2 This time can be in the order of seconds for a 600A / mm 2 For running a small coil, this time can be around 100ms.

[0070] During "normal" operation of the HTS magnet 802, the circuit breaker 812 is closed and the thyristor 818 is open, 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 so that current is no longer supplied from the power supply 810 to the HTS magnet 802, and 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 resistors 806A of the first coil segment 804A and the radial resistors 806B of the second coil segment 804B (rather than through the inductors 808A, 808B), thereby heating the coil segments 804A, 804B. The radial resistors 806A, 806B (provided by the conductive material between the turns) are integrated into the field coils and therefore have good thermal contact with the turns. For example, when the magnet quenches, the energy stored in the magnet is dissipated over a large area, rather than just in one of the coil segments or a particular turn. Compared to other coil heating methods that rely on coupling losses between turns to heat superconducting materials, resistive heating caused by current driven through the conductive layer allows better control over where the heating occurs. In addition, once the superconducting material becomes normal, the very large current flowing through it will be diverted through the conductive layer to very quickly heat other parts of the field coils.

[0071] Fig. 7A and Figure 7B 818 to discharge the capacitor 816. Fig. 7A ) and the voltage across the HTS magnet 802 ( Figure 7B ) changes as a function of time (in seconds). The initial voltage of capacitor 816 in this example is about 800V, and it is discharged about 30ms after thyristor 818 is switched. Therefore, the discharge has two distinct phases - the capacitor discharge phase (up to about 30ms) and the inductor discharge phase (from about 30ms to about 100ms). During the capacitor discharge phase, energy is dissipated into the circuit resistance, but is also transferred to the magnetic field of the current limiting inductor 820. The voltage across each of the coil segments 804A, 804B decreases as the capacitor discharges, decreasing from 0V to about -50V at 40ms, and then undergoing an oscillation from -50V to -80V and back to -50V over the next 70ms or so. Since the coil segments 804A, 804B in this example are equivalent (this is why Fig. 7AOnly a single curve is seen on the graph shown), so the voltage across the entire HTS magnet 802 is twice the voltage across each of the coil segments 804A, 804B.

[0072] During the capacitor discharge phase, the amplitude of the (reverse) radial current flowing in each of the coil segments 804A, 804B increases to about 50 kA and then tapers off during the period of inductor discharge. This current heats the coil segments 804A, 804B to raise the temperature of the HTS material, in this case from 20 K to about 70 K, so that it is no longer superconductive.

[0073] A diode (not shown) may be connected across capacitor 816 to prevent capacitor voltage reversal, which could result in damage to capacitor 816. When the capacitor voltage reaches 0V, the diode begins to conduct and then discharges as a function of the L / R time constant of the system, providing a more efficient method of dissipating capacitor energy. However, a diode is not required, and instead the circuit may be allowed to oscillate at the natural LC frequency.

[0074] The magnet heating system 814 is generally easy to implement and can be tuned to provide the necessary performance in terms of discharge time. Capacitors 816 with the necessary characteristics can (for example) be provided in the form of a capacitor bank connected in parallel with each other. However, the high-current circuit breaker 812 may be difficult to obtain or implement in some cases, and therefore may need to be specially built. In some cases, when using the magnet heating system 814, the "large" voltage (e.g., about 200V, which can be considered large in the context of the very large currents (e.g., kA) that typically flow within the coil) may also be a significant disadvantage.

[0075] Figure 8 An HTS magnet system 1000 is shown, which is Figure 6 800, except that the magnet heating system 1014 includes two DC voltage sources 1014A, 1014B, one for each of the first coil segment 804A and the second coil segment, and the HTS magnet system 1000 does not include a DC circuit breaker 812 (because, as described below, the voltage across the HTS magnet 802 is greater than the DC voltage across the HTS magnet 802). Figure 6The HTS magnet system 800 has a much lower voltage than the HTS magnet system 800, but may also include a DC circuit breaker 812 if desired. The first of the two voltage sources 1014A is connected across the first coil segment 804A via one of the current leads 811A (i.e., to a terminal of the first coil segment 804A to which the magnet power supply 810 is also connected) and a second connection to a magnet "tap" (current lead) 1022 between the first coil segment 804A and the second coil segment 804B. The second of the two voltage sources 1014B is connected across the second coil segment 804B via the magnet tap 1022 and another current lead 811B (i.e., to a terminal of the second coil segment 804B to which the power supply 810 is also connected). The DC voltage sources 1014A, 1014B are configured to have opposite polarities to each other. Each of the voltage sources 1014A, 1014B comprises a respective capacitor 1016A, 1016B switched by a respective thyristor 1018A, 1018B. In this case, each of the voltage sources 1014A, 1014B also has a respective inductor 1020A, 1020B to limit the peak current, as described above.

[0076] Fig.9A and Fig. 9B The coil segments 804A, 804B ( Fig.9A ) and the voltage across the HTS magnet 802 ( Fig. 9B ) varies as a function of time. Since the voltages applied by the capacitors 1016A, 1016B across the first coil segment 804A and the second coil segment 804B are always equal and opposite in this case (although there may be slight differences due to changes in radial resistance, differences in the inductance of the leads, the quality of the layers, etc.), their contributions to the voltage across the HTS magnet 802 cancel each other out. Therefore, throughout the discharge process, the total voltage across the HTS magnet 802 remains low, in this case less than 4V. This low voltage can mean that the power supply 810 does not need to be disconnected from the HTS magnet 802 during the discharge. Thus, for example, the circuit breaker 812 may not be required. However, it should be noted that the voltage on the magnet tap 1022 increases during the discharge. When the capacitors 1016A, 1016B are discharged, current flows through the magnet tap 1022 and flows radially through the first coil segment 804A and the second coil segment 804B in opposite directions. Thus, the net DC current measured at each terminal of the HTS magnet 802 (ie, at the current leads 811A, 811B) remains substantially constant during discharge, such as approximately 7 kA in this example.

[0077] Although the above examples have referred to a single capacitor 816, 1016A, 1016B, each capacitor may be provided in the form of a capacitor bank, for example, which may be connected in parallel with each other. The capacitor may be a metal film oil immersed capacitor or a so-called "supercapacitor". Other types of voltage sources may also be used instead of or in addition to the capacitor, for example, batteries (e.g., lithium polymer batteries), superconducting magnetic energy storage (SMES), or flywheel energy storage.

[0078] Similarly, although the magnet system 1000 has been illustrated by an HTS magnet 802 including two coil segments 804A, 804B, any number of coil segments 804A, 804B connected in series may be used. More generally, a magnet may include a field coil assembly that may have more than one field coil connected in series. In this case, the coil segments may include a subset of the turns in a single coil or all of the turns in a single coil, or even include turns in more than one coil. For example, in some cases, a tokamak may include 14 toroidal field coils connected in series, and then the magnet tap 1022 may be disposed between the 7th toroidal field coil and the 8th toroidal field coil, so that the coil segments 804A, 80B each include 7 field coils, and the capacitors 1016A, 1016B thus supply current in opposite directions through toroidal field coils 1 to 7 and 8 to 14. Preferably, the number of HTS coils is an even number so that the magnet tap 1022 can be placed at the midpoint of the HTS magnet to ensure equal current flowing on either side of the magnet tap 1022 .

[0079] A potential disadvantage of the magnet heating system 1014 is the use of additional magnet taps 1022 (i.e., current leads), which results in an additional thermal load on the cryostat 803 and a resistive load on the capacitors 1016A, 1016B. The thermal load on the cryostat 803 can be reduced if the capacitors 1016A, 1016B are cryogenically cooled, thereby allowing a "cold" connection to the magnet 802 that remains "off" when the magnet 802 is in use and does not result in heat transfer from the magnet 802. However, in some cases, the additional cooling required may be prohibitive because the capacitor bank may be large (e.g., given the typical volumetric energy density of room temperature oil film capacitors, a 400 kJ capacitor bank would occupy approximately 2.7 m 3 ).

[0080] Fig.10 An HTS magnet system 1200 is shown, which is Figure 8802 . The HTS magnet system 1000 is similar to the HTS magnet system 1000 of FIG. 802 , except that the magnet heating system 1214 includes two AC voltage sources 1214A, 1214B for driving AC current through each of the coil segments 804A, 804B. The large inductance of the spiral path of each of the coil segments 804A, 804B means that the current flows substantially radially through the coil segments 804A, 804B. The AC voltage sources 1214A, 1214B operate at the same frequency and are 180 degrees out of phase with each other so that the voltage across the HTS magnet 802 remains approximately zero at all times. The use of AC current means that the coil segments 804A, 804B are neither discharged nor charged while they are heated, i.e., the DC current flowing through the magnet 802 remains substantially constant, at least until a quench is induced in the magnet 802.

[0081] Fig.11 An implementation of a magnet heating system 1214 is shown, which includes a transformer 1302, which includes a primary coil 1304 and two secondary coils 1314A, 1314B, which act as AC voltage sources 1214A, 1214B to apply an AC voltage across each of the coil segments 804A, 804B. The transformer 1302 is driven by an inverter (not shown), which injects an AC current that oscillates at a predefined frequency. For example, the inverter can be constructed using insulated gate bipolar transistors (IGBTs). A capacitor (not shown) can be connected in series with the inverter to compensate for any reactive parasitic components in the transformer or lead inductance. The resonant frequency of the system can be about 12kHz, for example, which can be tuned by adjusting the capacitance of the capacitor. The terminals of the secondary coils 1314A, 1314B (e.g., the respective negative terminals of the secondary coils) are connected together to apply a common mode voltage to the magnetic tap 1220 disposed between the first coil segment 804A and the second coil segment 804B. Scaling the system to include more coil segments 804A, 804B only requires additional secondary windings on the transformer 1302 without increasing the number of active components.

[0082] The transformer 1302 is preferably placed inside the cryostat 803, and the two current leads 1306A, 1306B are used to connect to the inverter (although in some implementations, the transformer 1302 can also be external to the cryostat 803). Since the transformer primary current can be reduced by increasing the turns ratio of the transformer 1302 (i.e., the number of turns of the secondary coils 1314A, 1314B divided by the number of turns of the primary coil 1304), the size of the current leads 1306A, 1306B can be greatly reduced.

[0083] exist Fig.11In the example shown, transformer 1302 is a single-phase transformer operating at the switching frequency of the inverter. The power provided by transformer 1302 pulsates at twice the inverter switching frequency, which means that the peak current in transformer 1302 is the sum of the above combined Figure 8 and Fig.10 The described DC discharge electromagnet heating system 814, 1014 produces about twice the peak current. To reduce the peak current, a three-phase transformer can be used instead, where the phases of the currents are spaced 120° apart and the instantaneous combined power is kept constant, thereby reducing the root mean square (RMS) current in the primary coil by about 43%.

[0084] Fig.12 An HTS magnet system 1400 is shown which includes an HTS magnet 1402 including six HTS field coils or coil segments 1404A to 1404F connected in series with a DC power supply 1410 connected across the coil segments 1404A to 1404F to supply them with current for producing a desired magnetic field. Each of the coil segments 1404A to 1404F has a corresponding AC power supply 1412A to 1412F (forming part of a magnet heating system 1414) for supplying AC current for heating the coil segments 1404A to 1404F. The AC voltage sources 1412A to 1412F corresponding to consecutive pairs of coil segments (i.e., coil segments 1404A to 1404B, coil segments 1404C to 1404D, and coil segments 1404E to 1404F) are configured so that the coil segments in each pair are 180 degrees out of phase with each other, as described above for Fig.11 As described in transformer 1214.

[0085] Fig.13 Shows Fig.12An implementation 1500 of an HTS magnet system 1400 is shown, which includes three transformers 1514A-1514C having respective primary coils 1504A-1504C connected in a delta configuration (i.e., connected in series with one another to form a delta), wherein each primary coil 1504A-1504C receives AC current from one of three current leads 1506A-1506C, each current lead 1506A-1506C having an AC waveform that is 120 degrees out of phase with the AC waveforms of the other two current leads 1506A-1506C. The transformers 1514A-1514C are configured to form a balanced three-phase system such that the AC currents provided from the transformers 1514A-1514C to the coil segments 1404A-1404F are "balanced," i.e., sum to zero. Thus, when the coil segments 1404A to 1404F are heated by the AC current, the voltage across the entire HTS magnet 1402 remains zero (or less than a predefined value, such as 100V, 50V, 10V, or even 1V).

[0086] The transformers 1214, 1514A to 1514C can be implemented in a variety of ways: for example, iron core, air core or wireless power transmission link are all viable solutions. In some implementations, the three transformers 1514A to 1514C can be replaced with a single three-phase transformer. However, the use of separate transformers 1514A to 1514C can allow different voltages to be provided for each phase to achieve different heating strategies for different coil segments 1404A to 1404F.

[0087] While the above examples have discussed applying voltage across coil segments of a field coil, more generally, voltage may be applied across coil segments spanning a plurality of field coils or groups of a plurality of field coils connected in series or in parallel. For example, for a magnet comprising two field coils, a first coil segment may span a first of the field coils, and a second coil segment may span a second of the field coils. In any case, the coil segments should preferably be substantially identical, for example, segments of a pie-shaped coil may be implemented by providing one or more taps such that there are an equal number of windings on either side of the tap or between consecutive taps (or the number of windings may be, for example, preferably equal within 20%, 10%, or 5%), or the HTS coils spanned by each coil segment may contain the same number of equivalent coils connected in an equivalent manner, for example, for each HTS magnet comprising 14 HTS coils connected in series, each segment may contain seven HTS coils connected in series. In one embodiment, a single HTS field coil may be divided into three (equal) segments using two magnet taps, and the voltage from the secondary coil of a three-phase power supply is applied across the respective segments. Configurations such as these mean that the amplitude of the voltage across the HTS magnet can be kept as low as possible. The HTS field coils are uninsulated or partially insulated so that they can be heated by radial currents (i.e. currents flowing through the conductive layers between the turns rather than along a helical path).

[0088] The magnet heating systems 814, 1014, 1214, 1414 described above may be triggered in response to detection of a quench or a condition that may cause a quench, which may be performed by any practical method. For example:

[0089] Detecting overvoltage across the HTS material in the magnet;

[0090] Using a secondary HTS strip that is located adjacent to the HTS field coils and configured to quench before any HTS coils, for example as described in WO2017 / 042541 or WO2019 / 150091;

[0091] Detecting temperature, strain, magnetic field or other conditions within a magnetic coil, for example via Rayleigh scattering in a fiber optic cable as described in WO2018 / 078327, or via other temperature, strain or magnetic field detectors known in the art.

[0092] The magnet heating systems 814, 1014, 1214, 1414 described above may also be applicable to other situations where the magnet is ramped down, for example, when the magnet is shut down under normal conditions, when no quench (or conditions that may cause a quench) is detected, or when heating the magnet is required for any other purpose.

[0093] The above disclosure can be applied to various HTS magnet systems. In addition to the tokamak plasma chamber mentioned above as an example, it can also be used for HTS coils in nuclear magnetic resonance imaging (NMR / MRI) equipment, magnetic devices in non-magnetic media that are manipulated via magnetic fields (e.g., robotic magnetic navigation systems for manipulating medical devices in patients), and magnets for electric motors (e.g., for electronic aircraft). As another example, the present disclosure can be applied to a proton beam therapy device that includes an HTS magnet system that includes the disclosed features, wherein the HTS magnet system is used in an accelerator of a PBT device, in a quadrupole or dipole steering magnet of a PBT device, or in any other magnet of a PBT device.

Claims

1. A superconducting magnet system, comprising: a field coil assembly comprising two or more coil segments connected in series, each coil segment having a plurality of turns comprising a superconducting material, the turns in each coil segment being connected by a conductive material so that current can be shared between the turns; as well as A magnet heating system, comprising a plurality of voltage sources, each of which is connected across a corresponding one of the coil segments of the field coil assembly to apply a voltage having an AC component and / or a DC component across the coil segment so as to drive a current through the conductive material, the voltage sources being configured so that at least two of the voltages have AC components that are out of phase and / or DC components that have opposite polarities.

2. The superconducting magnet system according to claim 1, wherein: The voltage source is configured such that a phasor sum of the voltages applied across the coil segments has a magnitude that is less than a magnitude of at least one of the voltages applied across the coil segments.

3. The superconducting magnet system according to claim 1, wherein: The voltage source is configured such that a phasor sum of the voltages applied across the coil segments has a magnitude that is smaller than a magnitude of each of the voltages applied across the coil segments.

4. A superconducting magnet system according to any one of the preceding claims, wherein: The voltage source is configured such that respective amplitudes of the voltages applied across the coil segments differ by less than 10% of a maximum of the amplitudes.

5. A superconducting magnet system according to any one of the preceding claims, wherein: Each voltage source comprises a capacitor or a bank of capacitors.

6. A superconducting magnet system according to any one of the preceding claims, wherein: Each voltage source comprises a respective switch for connecting and / or disconnecting the voltage source from its respective one of the coil segments.

7. A superconducting magnet system according to any one of the preceding claims, wherein: The conductive material includes a conductive layer separating the turns.

8. A superconducting magnet system according to any one of the preceding claims, wherein: The magnet heating system includes a transformer having at least one primary coil for simultaneously inducing corresponding AC voltages across two or more secondary coils of the transformer, each of the voltage sources including at least one of the secondary coils connected across corresponding coil segments to drive current through the conductive material, the transformer being configured such that the AC voltages applied across the coil segments are out of phase.

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

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

11. The superconducting magnet system according to claim 8, wherein: The transformer is a three-phase transformer including three primary coils.

12. The superconducting magnet system according to claim 11, wherein: The three primary coils are arranged in a triangle configuration.

13. The superconducting magnet system according to claim 12, wherein: The primary coil and the secondary coil form a balanced three-phase system.

14. The superconducting magnet system according to any one of claims 8 to 13, wherein: Each primary coil is configured to induce a corresponding AC voltage in at least two of the secondary coils.

15. The superconducting magnet system according to any one of claims 8 to 13, further comprising a cryostat for cooling the superconducting magnet to a temperature lower than a critical temperature of the superconducting material, the secondary coil of the transformer being disposed within the cryostat.

16. The superconducting magnet system according to claim 15, wherein: The primary coil of the transformer is disposed within the cryostat.

17. A superconducting magnet system according to any one of the preceding claims, wherein: Each of the coil segments includes a respective number of turns that are equal to within 10% of a maximum number of turns among the turns.

18. A superconducting magnet system according to any one of the preceding claims, wherein: The field coil assembly comprises a plurality of field coils connected in series, each coil segment being provided by a respective one or more of the field coils.

19. The superconducting magnet system according to claim 17, wherein: Each coil segment comprises the same number of field coils.

20. A Tokamak comprising one or more superconducting magnet systems according to any one of claims 1 to 19.

21. A proton beam therapy apparatus comprising one or more superconducting magnet systems according to any one of claims 1 to 19.

22. A method of heating a superconducting magnet, the superconducting magnet comprising a field coil assembly, the field coil assembly comprising two or more coil segments connected in series, each coil segment having a plurality of turns comprising a superconducting material, the turns in each coil segment being connected by a conductive material so that current can be shared between the turns, the method comprising: A respective voltage having an AC component and / or a DC component is applied across each of the coil segments to drive current through the conductive material, wherein at least two of the voltages have AC components that are out of phase and / or DC components that have opposite polarities.

23. The method according to claim 22, wherein: In response to detecting a quench, or a condition tending to cause a quench, in one or more of the coil segments, the voltage is applied across the coil segments.

24. The method according to claim 22, wherein: At least one of an amplitude, a frequency, and a waveform of one or more of the voltages is suitable for heating the superconducting material in one or more coil segments from a first equilibrium temperature to a second equilibrium temperature greater than the first equilibrium temperature and less than a critical temperature of the superconducting material.

25. The method according to any one of claims 22 to 24, wherein: Applying a respective voltage having an AC component and / or a DC component across each of the coil segments to drive current through the conductive material comprises: applying an AC voltage to at least one primary coil of a transformer to simultaneously induce corresponding AC voltages across two or more secondary coils of the transformer; and Each of the AC voltages is applied across a corresponding one of the coil segments, wherein the AC voltages applied across the coil segments are out of phase.

26. The method according to any one of claims 22 to 25, further comprising: A power source is connected across the superconducting magnet to cause current to flow through the superconducting material to generate a magnetic field, and after the corresponding voltage is applied across each coil segment, the power source remains connected across the superconducting magnet at least until the temperature of some or all of the superconducting material in the coil segments exceeds a critical temperature of the superconducting material.

27. A superconducting magnet system, comprising: a magnet assembly comprising two or more coil segments connected in series, each coil segment having a plurality of turns comprising a superconducting material; an alternating current path across each coil segment, the alternating current path comprising a resistive material and having a low inductance compared to the corresponding coil segment such that a varying current across the coil segments flows preferentially through the alternating current path, wherein heating of the resistive material caused by current flowing through the alternating current path results in heating of the superconducting material of the corresponding coil segment; as well as A plurality of voltage sources, each voltage source being connected to a corresponding one of the coil segments and both ends of an AC current path thereof to apply a voltage having an AC component and / or a DC component, the voltage sources being 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.

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