HIGH TEMPERATURE SUPERCONDUCTOR FIELD COIL

Abstract

A high temperature superconductor, HTS, field coil having turns comprising HTS material wound about an axis of the coil. The turns are separated from one another along a direction perpendicular to the axis by a layer of insulator material having a first resistivity at a temperature less than a generation temperature of the HTS material and a lower, second resistivity at a second temperature greater than the generation temperature of the HTS material. The HTS field coil further comprises one or more electrical conductor elements arranged to provide one or more electrically conductive pathways extending from one turn to an adjacent turn and through the layer of insulator material. In response to a voltage difference generated across the one or more electrically conductive pathways, electrical current is driven through the one or more electrical conductor elements to heat the layer of insulator material.

Claims

1-17. (canceled)

18. A high temperature superconductor, HTS, field coil having turns comprising HTS material wound about an axis of the coil, the turns being separated from one another along a direction perpendicular to the axis by one or more layers comprising magnetoresistive material having an electrical resistivity that, at a temperature less than a generation temperature of the HTS material, decreases in response to an applied magnetic field, whereby the electrical resistance between the turns along the direction perpendicular to the axis decreases as electrical current is supplied to the coil.

19. An HTS field coil according to claim 18, wherein the magnetoresistive material is or comprises europium dicarbide, EuC2.

20. An HTS field coil according to claim 18, wherein the magnetoresistive material has a first, metallic phase at a temperature below a phase transition temperature and a second, insulator or semiconductor phase at a temperature above the phase transition temperature, the phase transition temperature being less than the generation temperature of the HTS material.

21. An HTS field coil according to claim 18, wherein the temperature at which the magnetoresistive material has an electrical resistivity that decreases in response to an applied magnetic field is greater than the phase transition temperature.

22. An HTS field coil according to claim 20, wherein the magnetoresistive material has a first resistivity at a temperature less than the generation temperature of the HTS material and a lower resistivity at a second temperature greater than the generation temperature of the HTS material.

23. An HTS field coil according to claim 20, wherein the turns are formed from one or more lengths of HTS tape comprising the HTS material as a layer on a flexible substrate.

24. An HTS field coil according to claim 23, wherein each layer comprising the magnetoresistive material is provided on a flexible substrate that is co-wound about the axis of the coil with the one or more lengths of HTS tape.

25. A high temperature superconductor, HTS, cable for winding into an HTS field coil, the cable having a stack of layers comprising a flexible substrate, a layer of HTS material and a layer of magnetoresistive material, the magnetoresistive material having an electrical resistivity that, at a temperature less than a generation temperature of the HTS material, decreases in response to an applied magnetic field.

26. An electromagnet comprising one or more HTS field coils according to claim 18.

27. A system comprising a plasma vessel and a set of field coils for generating a magnetic field within the plasma vessel, each field coil being an HTS field coil according to claim 18.

28. A tokamak fusion reactor comprising an HTS field coil according to claim 18, wherein the HTS field coil is one of a toroidal field coil or a poloidal field coil.

29. A high temperature superconductor, HTS, magnet system comprising: a high temperature superconductor, HTS, field coil according to claim 18; a detection unit configured to detect a loss of superconductivity in the HTS material of at least a portion of one of the turns of the HTS field coil; a quench protection unit configured to transfer electrical current from the HTS field coil to an external resistive load in response to said detection.

30. An HTS magnet system according to claim 29, wherein the external resistive load is a varistor.

31. A method of protecting a high temperature superconductor, HTS, field coil according to claim 18 from damage following a loss of superconductivity in one or more of turns of the HTS field coil, the method comprising: detecting a loss of superconductivity in the HTS material of at least one of the turns of the HTS field coil; and electrically connecting the HTS field coil to an external resistive load in response to said detection.

32-36. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0100] FIG. 1 is a schematic illustration of a known HTS tape;

[0101] FIG. 2 is a schematic radial cross section view of a coil according to an embodiment of the present invention;

[0102] FIG. 3A is a schematic illustration of an insulator tape according to an embodiment of the present invention;

[0103] FIG. 3B is a schematic side view of the insulator tape of FIG. 3A;

[0104] FIG. 4 is a schematic radial cross section view of a coil according to another embodiment of the present invention;

[0105] FIG. 5 is a schematic cross section view of a tokamak fusion reactor;

[0106] FIG. 6 is a graph showing the results of a calculation of the energy in a toroidal field coil during dumping of the magnet energy to an external resistance;

[0107] FIG. 7 is a graph showing the results of a calculation of the temperature of HTS and insulation tape during dumping of the magnet energy to an external resistance; and

[0108] FIG. 8 is a circuit diagram of a quench protection system.

DETAILED DESCRIPTION

[0109] Partial Insulation with Electrical Conductor Elements

[0110] A field coil is proposed here, in which adjacent turns of an HTS field coil are separated from one another by a “partially insulating” layer having an electrical conductivity that is able to increase rapidly as a result of electrical current flowing between the turns during or after a hotspot is formed. The partially insulating layer comprises a layer of insulator material and electrical conductor elements that provide electrically conductive pathways through the layer of insulator material. Hotspot formation in one of the turns of the coil causes electric current to bypass the affected turn, at least to some extent, thereby causing current to flow through the conductor element(s) from the turn to an adjacent turn (or turns). The insulator material is selected as having a first resistivity at a temperature less than the generation temperature of the HTS material and a lower, second resistivity at a second temperature greater than the generation temperature of the HTS material. Resistive heating of the conductor element(s) increases the temperature of the surrounding insulator material, thereby decreasing the turn-to-turn resistance of the coil.

[0111] FIG. 2 shows a radial cross section view of a coil 200 formed by winding an HTS tape 100 (in this case ReBCO tape) about an axis Z (the axis of the coil), in a similar manner to a spool of ribbon. For clarity, only two turns (windings) 201A, B are shown in the figure, but any number of windings can be used. The turns 201A,B are nested within one another to form a generally planar coil 200, often referred to as a “pancake” coil. The HTS tape 100 may be an HTS tape as described above in relation to FIG. 1 (or an exfoliated version thereof). The inner perimeter of the coil 200 is circular, but can in general be any two dimensional shape, e.g. a “D” shape.

[0112] An insulator tape 203 is co-wound with the HTS tape so that each of the turns 201A, B of the HTS tape 100 are separated from one another in a radial direction (i.e. a direction that is perpendicular to and passes through the axis Z) by a turn of the insulator tape 203. The insulator tape 203 has a width (i.e. length along the Z-axis) substantially equal to that of the HTS tape (in this case 12 mm). The insulator tape 203 comprises insulator material 207 that electrically insulates the turns 201A,B from one another when the coil is operated under usual, low temperature conditions (i.e. in the absence of any hotspots). In this example, the insulator material is europium dicarbide (EuC.sub.2), but other suitable insulator materials can also be used in addition to or instead of europium dicarbide. For example, the insulator material can be a vanadium oxide of various stoichiometries (e.g. V.sub.2O.sub.3, V.sub.3O.sub.5, V.sub.2nO.sub.2n+1 . . . VO) depending on the required temperature performance of the material. Other insulator materials that can be used, depending on requirements, include semiconductors, such as germanium, or other known metal insulator transition (MIT) materials, such as Fe.sub.3O.sub.4, RNiO.sub.3 (R=La, Sm, Nd, Pr), La.sub.1-xSr.sub.xNiO.sub.4, NiSe.sub.1-xSe.sub.x, and BaVS.sub.3. Alternatively or additionally, carbides, oxides or even nitrides of other transition metals or rare earth elements may be suitable insulator materials in some cases.

[0113] Europium dicarbide is a preferred insulator material as it has high electrical resistivity at low temperatures, but much lower resistivity at high temperatures. In particular, europium dicarbide has a first resistivity at a temperature less than the generation temperature of the HTS material (ReBCO) and a much lower resistivity at a second temperature greater than the generation temperature of the HTS material (the first resistivity and the second resistivity being for europium dicarbide 207 in a constant applied magnetic field, e.g. at zero applied magnetic field). For example, the electrical resistivity of europium dicarbide (measured in zero magnetic field) has a maximum electrical resistivity of around 10.sup.7 Ohm cm at a temperature of 15 K (less for non-zero magnetic fields), but an electrical resistivity of less than around 1 Ohm cm at a temperature of 300 K. This behaviour allows the EuC.sub.2 207 to electrically insulate the turns 201A, B from one another when the coil is operated at low temperatures (e.g. temperatures below 40 K), i.e. the EuC.sub.2 207 substantially prevents electrical current from flowing in a radial direction between the turns, so that a high proportion (e.g. >95%) of the current flows circumferentially around the turns. At higher temperatures, the EuC.sub.2 207 provides a substantially lower resistance between the turns 201A, B, thereby allowing greater current to flow radially between the turns when there is a loss of superconductivity (and hence increased electrical resistance) in the HTS material layer 103.

[0114] The insulator tape 203 further comprises copper bridges 205A-C that pass through the europium dicarbide 207 in a radial direction with respect to the coil axis Z to electrically connect the turns 201A,B. The copper bridges 205A-205C may extend uninterruptedly within the insulator tape in the length direction, such that it is arranged circumferentially around each turn of the insulator tape 203. i.e. such that it subtends at least 360 degrees about the coil axis Z. Alternatively, the copper bridges 205A-C may extend only by a short distance in the circumferential direction, i.e. each copper bridge 205A-205C fills a respective window (through hole) in the insulator material 207, and there may be copper bridges 205A-205C spaced apart circumferentially. In the latter case, there may be radial cross-sections containing no copper bridges (i.e. the insulator material 207 extends uninterruptedly from the top to bottom of the field coil in these radial cross-sections). Although only three copper bridges are visible in the radial cross section shown in FIG. 2, it will be understood that there may be any number of bridges in such a cross-section. The insulator tape 203 as a whole therefore acts as a “partial” insulator at low temperatures because of the relatively high resistivity of the europium dicarbide 207 together with the relatively high conductivity of the copper bridges 205A-C.

[0115] In the event of a hotspot forming in the HTS material layer 103 of one of the turns 201A,B, at least some of the current flowing in the affected turn bypasses the hotspot and is transferred to the adjacent turn through the copper bridges 205A-C as a result of the voltage generated between the turns, i.e. the copper bridges 205A-C transfer the current radially between the turns 201A,B. The copper bridges 205A-C are rapidly heated by the radial current, which increases the temperature of the europium dicarbide 207 surrounding the copper bridges 205A-C, and leads to a rapid decrease in the turn-to-turn resistance for at least a portion of the coil 200 adjacent to the hotspot. Thereafter, the magnetic energy stored in the coil 200 drives radial current through the insulator material 207 to quickly turn the HTS material in the turns 201A,B normal. This process then continues until the HTS material becomes normal for the bulk of the coil.

[0116] The greatly reduced resistivity of the europium dicarbide 207 following heating by the copper bridges 205A-C causes at least some of the radial turn-to-turn current to flow through the europium dicarbide 207 instead, i.e. the current carried by the copper bridges 205A-C is reduced relative to the current that would have been carried if the resistivity of the europium dicarbide 207 remained very high. This reduction greatly reduces or eliminates the risk of the copper bridges 205A-C (or another part of the coil 200) being damaged or “burnt out” by very large currents flowing through the resistive material of the copper bridges 205A-C.

[0117] Limiting the current flowing in the copper bridges 205A-C is particularly important to avoid excessive heat generation because the resistance of the copper bridges 205A-C increases with temperature (e.g. by a factor of around 15 between 20 K and 200 K). These concerns are paramount for very large coils, which can store huge amounts of magnetic energy, e.g. tokamak TF coils having a height of around 6 m, which may store around 1.5 GJ of magnetic energy. Some of the problems with larger coils can be understood by considering how the properties of the coil vary as a characteristic linear dimension (e.g. radius) of the coil is increased. The stored magnetic energy rises with linear dimension cubed, but the coil's mass, which has to absorb that energy, only rises linearly or quadratically with the coil dimension. Therefore, it is crucial to ensure that that the heat during a quench is evenly distributed through the coil.

[0118] The combination of the europium dicarbide 207 and the copper bridges 205A-C greatly increases the speed at which the insulator tape 203 is able to respond to localised heating caused by a hotspot forming (or beginning to form), i.e. the rate at which the resistance of the insulator tape 203 decreases is enhanced by the copper bridges 205A-C heating the europium dicarbide 207. For existing forms of temperature dependent turn-to-turn insulation that lack copper bridges, more heat must be generated by current flowing in the cladding (e.g. the silver layer 104 or copper layer 105) of the HTS tape 100 before the insulation is heated sufficiently to cause a significant reduction in the turn-to-turn resistance of the coil. Such localised and relatively prolonged heating may lead to increased damage to the HTS tape 100. Another advantage arising from the improved (i.e. decreased) “response time” of the insulator tape 203 is that there is less time for unbalanced forces to build up between different parts of the HTS field coil 200, which may result in mechanical damage to the HTS magnet.

[0119] The turn-to-turn resistance of the HTS field coil 200 can be varied in a number of different ways, such as changing number density of the bridges 205A-C within the insulator tape 203, varying the thickness of the insulator tape 203, or using resistive materials other than copper for the bridges, such as brass, stainless steel, Hastelloy, nickel or a semiconductor material. The bridges may have a cross sectional area that is substantially constant along their lengths (i.e. along the horizontal, radial direction shown in FIG. 2) and may take any shape. Increasing (or decreasing) the cross sectional area of the bridges allows the turn-to-turn resistance to be increased (or decreased) as necessary. In some cases, some of the bridges may extend to an edge of the insulator tape 203, i.e. so that each of these bridges is not surrounded by the insulator material on one side.

[0120] The length of the conductive pathways through the insulator material can also be increased by configuring the bridges 205A-C to follow a meandering path within the insulator material, e.g. a winding or “serpentine” path that loops back on itself one or more times within the insulator material. This kind of arrangement allows heat generated by the bridges to be transferred to the insulator material over a larger area compared to non-winding paths.

[0121] The metal bridges described above may be varied along the insulator tape 203 to ensure that the turn-to-turn resistance provided by the insulator tape 203 remains (at least approximately) constant for each turn.

[0122] Two coils 200 may be arranged as a “double pancake coil” with one coil 200 stacked on the other coil 200 with an insulated layer between them. Each of the two pancake coils 200 is wound in an opposite sense and the inner terminals of the coils 200 (i.e. the end of the innermost turn of each coil) connected together to allow current to be supplied to the coil by applying a voltage across the respective outer terminals of the coils 200 (i.e. the end of the outermost turn of each coil).

[0123] The insulator tape 203 may comprise a substrate on which the europium dicarbide 207 (or other insulator material) is provided as a layer, with through holes (i.e. windows) allowing the copper bridges 205A-C to pass through both the substrate and the europium dicarbide 207. The substrate may be formed from or comprise another thermally conductive material, such as copper, in order to facilitate heat transfer from the copper bridges 205A-C to the europium dicarbide 207. The substrate may also be electrically conductive, in which case, the copper bridges 205A-C may each be in electrical contact with the substrate such that through holes in the substrate are not necessary. In one example, the substrate is made of Hastelloy and has a thickness of 100 microns, whilst the EuC.sub.2 layer 207 has a thickness of 20 microns. The thickness of the EuC.sub.2 layer 207 can be adjusted according to the desired turn-to-turn resistance and may be from 1 micron to 100 microns, for example. Optionally, an EuC.sub.2 layer 207 may be provided on both sides of the substrate. The EuC.sub.2 layer(s) 207 can be formed, for example, by the reaction of elemental europium with graphite at 1673 K, as described by Wander et al., Inorg. Chem (2010) 49, 1, 312-318.

[0124] Alternatively (or additionally), rather than providing the insulator tape 203 separately from the HTS tape 100, the EuC.sub.2 layer 207 and copper bridges 205A-C may be bonded to one side of the HTS tape 100, or even provided on both sides of the HTS tape 100, i.e. such that there are respective EuC.sub.2 layers 207 and copper bridges 205A-C provided on each side of the HTS tape 100. A cable comprising such a tape or tapes and one or more HTS tapes 100 in a stacked arrangement (i.e. arranged such that the HTS layers in each of the tapes are parallel) can also be made. Such cables may be particularly beneficial for producing large field coils.

[0125] Another method of forming an insulator tape 203 comprises depositing a mixture of insulator material (e.g. europium dicarbide) and metal particles (e.g. copper powder) on to a substrate, such a copper layer or HTS tape 100. The metal particles are dispersed within the europium dicarbide to form miniaturised conductive “tracks” that provide conductive pathways through the europium dicarbide. This method may be readily incorporated into existing ways of manufacturing HTS tape 100, e.g. REBCO tape. The number density and/or size of the metal particles within the europium dicarbide may be non-uniform so that certain portions of the insulator tape 203 are more conductive. The number density and/or size of the metal particles may also be varied so that the insulator tape 203 provides a (substantially) constant turn-to-turn resistance when it is wound into an HTS field coil 200.

[0126] FIGS. 3A and 3B illustrate an insulator tape 300 in which a metal (e.g. copper) strip 301 (which can be considered a substrate, as discussed above for the insulator tape 203) is provided with a thin coating of insulator material (e.g. EuC.sub.2) 302 on at least the sides that will face the HTS cables when a coil is wound. The insulating coating is removed (or not applied) over windows or “through holes” 303 at intervals on each side of the metal strip 301. The windows can have any shape and can optionally extend to the edges of the insulator tape 300. The location of windows 303 on either side of the metal strip 301 are staggered (i.e. the windows are offset from one another), as shown in FIG. 3B, so that the path 310 for current passing from one window to another includes a portion running parallel to the length direction of the metal strip 301. This arrangement increases the resistance across the insulator tape 300 (compared to an uninsulated strip, or to a strip where the windows on each side were directly opposite each other) and leads to greater heating of the insulator material coating.

[0127] The metal strip can be soldered to the HTS cables (e.g. during winding) to ensure good electrical contact. Alternatively, contact may be achieved merely due to the pressure in the coil once wound. As a further alternative, additional electrically conductive inserts may be added into the windows, or the metal strip may have protrusions which extend into the windows. The inserts or protrusions may fill the entire window, or may fill only a portion of the window. For example, where the window is a “lane”, the inserts may be provided at intervals along that lane, so that the inserts, in effect, reduce the size of the window, and can be used to tune the turn-to-turn resistance provided by the insulator tape 303.

[0128] Insulator tapes similar to the insulator tape 300 of FIG. 3 may be manufactured as a flexible printed circuit board (PCB), with the insulator material (e.g. EuC.sub.2) 302 being bonded to the metal strip by adhesive, and then etched to form the windows, with any further metal elements being bonded to the insulating coating or the metal strip as appropriate, such that they are in electrical contact with the metal strip. Alternatively the coating of insulator material may have the windows pre-cut (or be sized so as to provide “lanes” when applied to the strip), and then be bonded to the metal strip during winding by adhesive. Other manufacturing methods may also be used.

[0129] As noted above, HTS coils may be non-insulated, partially insulated or insulated, depending on the material(s) provided between adjacent turns of the HTS material. These different types of HTS coil may have significantly different behaviours following hotspot formation in part of the HTS material and different approaches are required to respond to the hotspot formation safely and effectively.

[0130] In the case of a fully insulated magnet, formation of a hotspot in a part of an HTS tape 100 causes current to bypass the normal zone in the HTS material and flow preferentially in another layer of the HTS tape or HTS cable. For example, in a magnet wound using stacked tape cable with no additional so-called “stabilizer” metal (such as copper or aluminium), the transport current flows preferentially in the copper in the outer shell of the HTS tape (if present) and the Hastelloy (or nickel-tungsten) substrate Joule heating within the other layer(s) of the HTS tape or HTS cable will cause the temperature of the hotspot to rise rapidly, typically exceeding the melting point of copper within 10-1000 milliseconds, depending on the construction of the cable and the amount of magnetic energy stored in the magnet. If enough stored magnetic energy is available, the turn will vaporise at its hottest point and a destructive arc will form, permanently damaging the magnet. This problem is particularly severe in HTS magnets because they can remain superconducting at higher temperatures (generally up to 30-60 K, depending on magnet design). At these temperatures most materials have greatly increased heat capacity compared to lower temperatures, such as 1.2-4.2 K, the temperatures at which conventional, low temperature superconducting (LTS) magnets are operated. This higher heat capacity means that a hotspot typically propagates around the turn and between adjacent turns very slowly (e.g. with a velocity of around 10-100 mm/s), and a large temperature gradient forms between the surrounding HTS material, which is still superconducting and dissipation-free, and the peak temperature of the hotspot.

[0131] These problems can be mitigated for fully insulated magnets by adding additional high conductivity normal (i.e. non-superconductor) metal (e.g. copper) to the HTS cable to reduce Joule heating and slow down the rate of temperature rise, thus allowing more time to detect the hotspot and initiate a safe shutdown of the magnet. However, large magnets require a considerable amount of additional high conductivity metal to be added to each turn to give an acceptable time window to detect a hotspot and initiate a quench. The additional metal significantly reduces the winding pack current density, i.e. the current carried by the cable divided by cross-sectional area of the cable, but serves no purpose until a hotspot occurs. Furthermore, the best metals for this purpose (copper and aluminium) have relatively low Young's moduli, so it is often necessary to encase the cable in a jacket of high strength material to withstand the high electromagnetic stresses seen, further reducing the current density of the HTS cable. These problems can be avoided using field coils such as the field coils 200, 400 described above, in which the layer(s) of insulator material between the turns of HTS tape 100 are relatively thin, e.g. having a thickness of less than 1 mm or from 5 microns to 100 microns.

[0132] In the case of the centre column of a toroidal field (TF) magnet of a compact spherical tokamak (ST), the resultant current density is typically <50 A/mm.sup.2, and certainly <100 A/mm.sup.2. However, the required average current density for a larger ST is much higher. For example, a ST with a 1.4 m major radius (R.sub.0) and a 4 T field at R.sub.0 requires an average current density of the order of 400 A/mm.sup.2. Adding large amounts of additional conductor is therefore not practical for larger STs. Field coils such as the field coils 200 described above (and also the field coils 400 described below) are therefore particularly suitable for compact spherical tokamaks.

[0133] HTS Field Coils Comprising Magnetoresistive Materials

[0134] Another solution to some of the problems described above is proposed here, in which a magnetoresistive material layer is used to provide turn-to-turn electrical insulation for an HTS coil. The electrical resistivity of the magnetoresistive material decreases in response to an applied magnetic field at a temperature that is less than a generation temperature of the HTS coil. The magnetoresistive material layer provides a high electrical resistance between the turns of the HTS coil (i.e. the coil is an “insulated” coil) when the amount of electrical current supplied to the HTS coil is low (and hence the magnetic field generated by the coil is low). However, as more current is supplied to the HTS coil, the electrical resistance of the magnetoresistive material layer decreases (the coil becomes a “non-insulated” or only “partially insulated” coil) because of the magnetic field generated by the HTS coil. This behaviour allows electrical current to be supplied to the coil rapidly during the initial stages of ramp-up (i.e. as the electrical current is raised from a low value to a higher value) and then at a lower rate as the turn-to-turn resistance of the coil decreases (and hence the ratio L/R increases) to a lower final resistance. The overall ramp-up time of the HTS coil is therefore decreased compared to other HTS coils that have the same turn-to-turn resistance as the final turn-to-turn resistance reached by the HTS coil, but which do not comprise magnetoresistive materials.

[0135] For example, supposing an HTS coil has an inductance L and that a final turn-to-turn resistance, R.sub.1, is desired, e.g. to allow current to flow between turns if a hotspot forms in the HTS layer 103. An HTS coil comprising a magnetoresistive material layer has an initial turn-to-turn resistance, R.sub.0 (at zero current), that is higher than the final turn-to-turn resistance, R.sub.1. Therefore, the mean time constant of the HTS coil will be between L/R.sub.0 and L/R.sub.1. By contrast, an equivalent HTS coil having a constant turn-to-turn resistance, R.sub.1, will have a larger time constant, L/R.sub.1. Preferably, the magnetoresistive material may exhibit colossal magnetoresistance (CMR). For example, the percentage decrease in electrical resistivity of the magnetoresistive material on going from an applied magnetic field of strength 0 T to an applied magnetic field of strength 1 T may be greater than 5%.

[0136] FIG. 4 shows a radial cross section view of a coil 400 which comprises a plurality of turns 401A,B of HTS tape 100. This coil is identical to the coil 200 described above with respect to FIG. 2, except that the insulator tape 403 does not comprise copper bridges, i.e. the insulator tape 403 comprises an electrically conductive (e.g. copper or Hastelloy) substrate 405 and a layer of insulator material 407, e.g. europium dicarbide.

[0137] The insulator material is a magnetoresistive material, i.e. a material that exhibits magnetoresistance, a change in electrical resistivity upon application of an external magnetic field. Europium dicarbide is a preferred choice of magnetoresistive material because it exhibits so-called colossal magnetoresistance (CMR), in which is the change in electrical resistivity upon applications of large magnetic fields is enormous. For example, at a temperature of 20 K, europium dicarbide has an electrical resistivity of around 10.sup.7 Ohm cm in zero magnetic field, but a resistivity of around 10.sup.4 Ohm cm in a magnetic field of 1 T, and a resistivity of 10 to 100 Ohm cm in a magnetic field of 14 T.

[0138] The magnetoresistance of the insulator tape 403 means that electrical current can be supplied to the coil 400 rapidly at first, and then at a lower rate as the turn-to-turn resistance of the HTS coil 400 decreases in response to the magnetic field generated by the HTS field coil 400. The much lower turn-to-turn resistance when the HTS coil 400 is operated at high electric currents (i.e. high magnetic fields) also allows some sharing of the electric current between adjacent turns 401A,B of the HTS tape, which makes the HTS coil 400 more resilient against hotspot formation, as described below.

[0139] If a region of one of the turns 401A,B of the HTS tape 100 forms a “hotspot”, i.e. becomes warmer than the rest of the HTS tape 100, the hotspot may spread around the turn until sufficient heat is generated to cause the HTS tape 100 to cease being superconducting, i.e. for a generation temperature of the HTS material 103 to be exceeded. However, the insulator tape 403 helps mitigate against this problem because the heat generated in the hotspot makes the insulator tape 403 adjacent to the hotspot (e.g. on either side of the hotspot) become more electrically conductive, which causes some of the current in the turn 401A,B containing the hotspot to bypass the hotspot and transfer into the adjacent turns 401A,B (i.e. the current flows in a radial direction).

[0140] As discussed above, the magnetic field generated by current flowing through the HTS coil 100 decreases the resistance of the magnetoresistive layer 407 substantially. This decreased resistance means that only a relatively small increase in temperature is required before the electrical resistance of the magnetoresistive layer 407 is low enough to allow current to transfer between adjacent turns 401A,B during or after hotspot formation. The magnetoresistance of the layer 407 therefore helps to prevent excessive localised heating before the current can be substantially diverted away from a hotspot. This behaviour can be contrasted with the behaviour of non-magnetoresistive materials, which may require a comparatively larger increase in temperature before the resistance of the material is sufficiently low for significant current transfer between adjacent turns to take place.

[0141] In the embodiment shown in FIG. 4, each turn 401A,B has a single layer of HTS tape, but the coil 400 may also be formed from a cable comprising two or more layers of HTS tape 400 stacked in a face-to-face arrangement, so that each turn 401A,B comprises more than one layer of HTS tape. Preferably, the cable comprises HTS tapes arranged such that the HTS layers 103 are facing one another (i.e. the HTS layers 103 are between the respective hastelloy layers 101 of the HTS tapes 100).

[0142] Europium dicarbide has a resistivity of around 10.sup.6 Ohm cm at 20 K and around 10.sup.3 Ohm cm at 35 K. A hotspot that develops in the HTS tape will therefore initially tend to spread relatively slowly (e.g. at a speed of around 10 to 50 cm/s) around a turn 401A,B as the turn-to-turn resistance provided by the EuC.sub.2 layer 407 remains relatively high. This means that the electrical current continues to flow through the turn and the magnetic field generated by the coil 400 remains approximately constant at around 20 T. As the temperature of the hotspot increases, the temperature of the EuC.sub.2 layer 407 increases, reducing its resistivity, e.g. to around 10 Ohm cm at around 70K. The lower resistance of the EuC.sub.2 layer 407 allows a significant amount of electrical current to pass radially from the turn 401A,B in which the hotspot has formed, through the insulator tape 403 and into one or more adjacent turns 401A,B of the HTS tape. The portions of these turns near the hotspot then exceed critical current and turn normal themselves. The hotspot is therefore able to expand radially across the coil 200 (i.e. in a direction perpendicular to both the Z-axis and a tangent to one or more of the turns) before a quench occurs.

[0143] By contrast with other metal insulator transitions (MIT) materials such as vanadium oxide (e.g. V.sub.2O.sub.3), the decrease in electrical resistivity in europium dicarbide is not confined to a narrow temperature range and starts at lower temperatures, in particular, temperatures around 15-25 K, where the best compromise between cooling cost and conductor cost occurs for REBCO-based magnets. In particular, europium dicarbide has a phase transition temperature of around 15 K, which is significantly lower than that of other metal insulator transition materials, with the phase transition occurring from a metallic phase below the transition temperature to an insulator or semiconductor phase above the transition temperature. The electrical resistivity of the europium dicarbide in this latter phase decreases markedly with increasing temperature, as discussed above. Consequently, turn-to-turn current leakage occurs more rapidly with europium dicarbide after the generation temperature is exceeded (and the HTS material transitions from the superconductor state to the normal state). This allows the europium dicarbide layer 407 to have a better “braking” effect on the rising hotspot temperature than e.g. vanadium oxide. Whilst the disclosure in this section of the application has focused on europium dicarbide, other magnetoresistive materials could be used (depending on the generation temperature of the HTS material). For example, other carbides, oxides or even nitrides of other rare earth elements or transition materials may be suitable insulator materials in some cases.

[0144] Quench Protection

[0145] As described above, partially insulated coils provide a way of reducing the heat generated in the vicinity of hotspots by reducing the resistance of the insulation between turns, so that current can bypass a turn with a hotspot by flowing radially into adjacent turns. Therefore, if the magnet has partial insulation between turns, the rate at which the temperature of a local “hotspot” (normal zone) rises is significantly reduced because current can transfer between turns as well as between tapes in one turn. This reduction allows significantly more time to detect the hotspot and take protective action before a potentially damaging quench occurs.

[0146] When the presence of a growing local hotspot is detected in a non-insulated or partially insulated magnet the sequence of events is different from those that occur in a fully insulated coil. The option of non-insulation is only applicable to coils with very low inductance. This is because the charge (or ramp) time of the magnets is determined by the ratio of the inductance of the superconducting spiral path to the resistive radial path, the L/R time constant. The radial resistance R depends on the inverse of magnet dimension (e.g. the length of the turn), whereas L scales as the cube of magnet dimensions, hence L/R scales as the fourth power of linear magnet dimension. Inductance can be reduced by choosing to operate at very high transport current, I.sub.0, but there is a practical limit (˜100 kA) to this approach because the current leads connecting the cryogenic coil to the room-temperature power supply become too large (leading to an unacceptable heat leak). In the case of a tokamak, the L/R time constant of non-insulated TF coils would be too long to ramp the magnet in a practical timeframe, even if a very high transport current were used. In practice, a compromise must be found between a reasonable ramp time (requiring higher R) and slowing the rate of hotspot temperature rise to allow a reasonable detection window (requiring lower R). Finding the appropriate compromise value for R (call this R.sub.c) requires complex modelling using a thermal-electric network model that takes account of critical current in each element.

[0147] The shutdown generally involves disconnecting the power supply and redirecting the magnet current via a resistive shunt external to the magnet, to absorb and dissipate the energy stored in the magnetic field. The rate of hotspot heating falls rapidly as the magnet energy is discharged to the external dump. The magnet designer has to ensure that the peak temperature of the hotspot will not exceed the maximum allowed temperature (normally chosen to be 100 K, or at most 200K), to avoid excessive differential thermal stress. This typically involves designing a quench protection system to detect the hotspot and activate an energy dump within a fixed time window (typically less than a second in an HTS magnet). The stored energy depends on the size and shape of the magnet (being proportional to the integral of the square of magnetic flux over volume).

[0148] Partial insulation has the further advantage that it allows current to divert from the turn containing the hotspot at all points around the turn, not just at the location of the hotspot. When the presence of a hotspot is detected and a magnet dump is initiated this causes distributed heating around the whole turn, leading to a faster and better-distributed propagation of the magnet quench. This is of particular importance in magnets in which the local magnetic field value, field angle or temperature vary at different points around the coil, changing the critical current, I.sub.c. This is the case with TF coils in a tokamak. When the dump phase is activated, the quench will start at the point where laic is highest, and needs to propagate fast to avoid a local burn-out. Hence, the distributed heating around the turn afforded by partial insulation is an important feature.

[0149] The behaviour of a non- or partially-insulated magnet when the power supply unit (PSU) is disconnected and/or replaced with a dump resistance R.sub.d is complex. Each turn is connected in series with the next turn, closely magnetically coupled to all other turns, and connected back on itself by R.sub.tt, forming a shorted loop. Therefore, current in the turn in the simple case of an axi-symmetric coil, where all points around the circumference of each turn are at nominally equal IA, the inductance current in each turn will connect back to the turn via the turn-turn resistance, R.sub.tt. The coil therefore behaves like a set of nested, resistive loops, with high mutual inductance. The Joule heating caused by current flowing in R.sub.tt in each loop will cause each turn to become quenched with slightly different timing. A normal “shockwave” will propagate radially, inwards and outwards, until the whole coil is quenched. It is important to note that the effective inductance of the magnet is significantly reduced in this mode. This means that the voltage developed across the coil is also substantially reduced compared to a quenching insulated coil with the same dimensions and amp-turns, by a factor roughly equal to 1/N, where N is the number of turns. This behaviour offers a number of very significant advantages for superconducting magnets, such as reduced ground wrap breakdown insulation voltage rating (useful in a nuclear device where organic insulators degrade in effectiveness due to neutron exposure).

[0150] A potential disadvantage of partial insulation that has a nominally fixed turn-turn resistance value, is that it becomes impossible to divert all of the magnet's stored energy to an external resistor when a dump is initiated. If the PSU is disconnected and an external dump resistor, R.sub.d, is connected across the coil terminals, R.sub.d appears in parallel to the radial resistance R. Hence magnet current splits between R and R.sub.d. Energy dissipated in R heats the magnet cold mass, while energy dissipated in R.sub.d is dissipated outside the magnet. In some cases, it is difficult to find a compromise value R.sub.c that simultaneously satisfies all the constraints.

[0151] This problem can be mitigated if the value of the turn-turn resistance of the partial insulation can be made to vary significantly with temperature and/or magnetic field, e.g. the partial insulation described above with reference to insulator tapes 203, 303, 403. The turn-turn surface resistivity, ρ, is simply the radial resistance, R, divided by the number of turns in the coil and multiplied by the area of one turn. If we denote ρ.sub.c as the surface resistivity for the compromise R.sub.c achieved with temperature and magnetic field insensitive insulation, then it is particularly beneficial if ρ>ρ.sub.c) when the local coil temperature T<T.sub.c, and ρ<ρ.sub.c when T>T.sub.c. This arrangement facilitates sharing of current between turns only in the vicinity of the hotspot, which achieves the aim of slowing the rate of temperature rise in the hotspot without compromising the ramping time of the magnet. It also allows most of the magnet energy to be dumped into an external resistor rather than a significant proportion being dumped in the magnet, potentially causing it to overheat in the dump phase. This reduces overall magnet risk and also reduces the time needed to recover from a protection dump (because the magnet needs less time to re-cool before re-ramping).

[0152] We now consider the use of HTS field coils, such the HTS field coils 200, 400 described above in relation to FIGS. 2 and 4 (and also field coils formed using the insulator tape 300 described in connection with FIG. 3), in tokamak fusion reactors and plasma chambers, particularly in spherical tokamaks. A tokamak has two sets of magnets: poloidal field coils, which are aligned to produce a poloidal field and are generally circular; and toroidal field coils, which comprise a central column and a plurality of return limbs, and are arranged to produce a toroidal field. HTS magnets can be used for either set of field coils, but are particularly useful for toroidal field coils in small tokamaks, as such field coils have tight space limitations. The improved current density and/or reduced cooling requirements of HTS can help significantly with such limitations.

[0153] FIG. 5 shows a vertical cross section through a spherical tokamak 500 comprising toroidal field (TF) coils 501, poloidal field (PF) coils 503 and a toroidal plasma chamber 505 located within the toroidal field coils 501. The tokamak 500 also comprises a central column 507, which extends through the centres of the plasma chamber 505 and the toroidal 501 and poloidal 503 field coils. Each of the generally D-shaped toroidal field coils 501 comprises turns arranged such that each coil 501 has a “straight” section 509 that extends along the axis A-A′ of the central column 507 and a curved section 511 that is electrically connected to either end of the straight section 509 to form the D-shape coil. In practice, the straight section 509 may be slightly curved, e.g. with a radius of curvature that is greater (e.g. more than twice, five times or ten times larger) than the radius of curvature of the curved section 511.

[0154] Conventionally, toroidal field coils 503 may include extra copper material (in addition to the copper cladding of the HTS tape 100) to temporarily carry the heat generated in a normal zone formed in the HTS tape 100. The thicker the copper layer, the slower the rate of temperature rise in the hotspot (i.e. the section of tape containing the normal zone). If there is no hotspot, all transport current flows in the superconducting layer with no current in the copper layer. The latter only carries current for a brief period when the hotspot forms. If the local cooling is inadequate to stabilize the hotspot temperature, a thermal runaway will occur, ultimately resulting in the copper layer melting, and an arc forming. The time taken for this to occur can be extended by making the copper layer thicker, but this carries several penalties: reduced current density in the HTS cable (a critical factor in tokamak size), and reduced composite Young's modulus of the cable due to the relatively very low modulus of high purity copper compared to Hastelloy. However, the thickness of this extra copper material may be reduced, or the copper material may be removed entirely, if the insulator tapes such as those described above (e.g. insulator tapes 207, 407) are used. More compact, higher current density TF coils are therefore possible.

[0155] Considering the TF coils 501, the magnetic energy stored in a TF coil 501 may be several GJ. If a hotspot is detected then, in the case of insulated coils, this energy may be dumped to an external resistor (preferably a varistor) when the power supply unit powering the insulated TF coil 301 disconnected. Assuming the external resistor has a resistance of 0.167 Ohm and the maximum voltage drop across the TF coil 501 is 5 kV, then the time constant associated with removing energy from the TF coil 501 is around 20 s, and the overall dumping process may take of the order of 100 s. This timescale may be too long to prevent the hotspot from damaging the HTS tape 100.

[0156] FIG. 6 is a graph showing the results of a calculation of the magnetic energy stored in a TF coil 501 during a quench. The TF coil 501 includes leaky insulator tape 407 and HTS tape 100 as described above for HTS field coil 400, with the following parameters: [0157] Europium dicarbide layer 407 thickness: 20 microns [0158] Substrate 105 thickness: 100 microns [0159] Insulator tape 403 width (i.e. length along the coil axis): 12 mm [0160] Turn circumference (length): 16 m [0161] Transport current: 30 kA [0162] Number of HTS tapes: 43 [0163] Thickness of HTS tape: 110 microns [0164] Current per tape: 700 A [0165] Stored energy: 1.5 GJ [0166] Inductance 3.3 H

[0167] The graph has a curve 600 showing the decay in magnet energy and a curve 602 showing the rise in the energy dissipated in the external resistor over time (in seconds). These curves 600 and 602 are associated with the left-hand vertical axis, which has units of GJ. Another curve 604 shows the energy dissipated internally in the TF coil 501 over time and is associated with the right-hand axis, which has units of kJ. Most of the energy previously stored in the magnetic field is dissipated in the external resistance during a time of around 20 s. The energy dissipated in the TF coil 501 reaches only around 2 kJ at this time.

[0168] FIG. 6 is a graph showing the results of a calculation of the temperature (in K) of the HTS tape 100 and leaky insulator tape 407 in the region of a hotspot (of initial length 1 cm) as a function of time (in milliseconds). The temperature of the insulator tape 407 (line 500) rises from around 25 K initially to around 150 K after around 40 ms, whilst the temperature of the HTS tape 100 (line 502) increases from around 25 K to around 50 K in this time.

[0169] The external resistor may be a varistor (voltage dependent resistor) to allow the external resistance to be varied over time to control the proportion of energy dissipated from the TF coil 501 in the varistor and in the magnet cable and EuC.sub.2 insulation (or another insulating material, as described above). Alternatively or additionally, the external resistor may be initially disconnected from the TF coil 501 and then connected a short time after a hotspot has formed (so that electrical current then flows from the TF coil through the external resistor). This provides a way of providing a very high (effectively infinite) resistance initially and then a lower (finite) resistance after hotspot formation. More generally, the external resistor may comprise a plurality of resistors arranged in parallel with one another that can each be connected or disconnected from the other resistors using a respective switch, such that the overall resistance across the plurality of resistors can be varied by switching each of the switches. A quench detection system for initiating the energy dissipation after detection of a loss of superconductivity in the HTS material of at least a portion of one of the turns of the HTS field coil is described in WO2018/078327, for example.

[0170] Another way to reduce the peak coil temperature, which may be used in combination with using an external resistor (such as a varistor) as described above, is to couple the magnet current inductively into a secondary coil that is tightly inductively (magnetically) coupled to the magnet coil. Such a secondary coil may be referred to as a “close coupled secondary” (CCS) coil. In this method, the secondary coil is typically a single turn of normal metal, such as copper or aluminium. The two coils act as a transformer. During slow ramping of the primary coil, current is induced in the secondary coil, but decays quickly because it is resistive. However, if the primary coil turns fully (or mostly) normal, the collapsing magnetic field induces current in the secondary coil. The effect is for current to rapidly transfer (or “jump”) from the primary coil to the secondary coil, reducing the heating in the hotspot in the primary coil. This method requires the flux coupling coefficient, k, between the primary and secondary coils to be very high (>0.9), meaning the coils have to be in very close proximity so that most of the flux generated by the primary coil links the secondary coil.

[0171] This protection method may be suitable for LTS magnets, which have fast normal zone propagation, but it is not generally suitable for use in insulated HTS coils, which display very slow normal zone propagation. However, the partially insulated HTS coils 200, 400 described above (and other field coils comprising the insulator tape 300 described above), allow the normal zone to propagate much faster, which makes quench protection using a CCS coil much more viable.

[0172] FIG. 8 shows a quench protection circuit 800 for dissipating the energy of a superconducting field coil 802 following loss (or partial loss) of superconductivity within the HTS material of the field coil 802. The field coil 802 comprises turns of HTS tape wound about an axis to create a spiral path for electrical current supplied to the field coil 802. When the coil is in use, circulation of the electrical current around the spiral path of the turns generates a magnetic field. The field coil 802 is partially insulated, i.e. the turns of the coil are electrically connected to one another radially. This is achieved by providing the field coil 802 with electrical conductor elements (such as copper bridges 205A-C) or conductive “tracks” that extend through a layer of insulator material located between the turns to provide electrically conductive pathways from one turn to an adjacent turn. The field coil 802 may, for example, be a field coil 200, 300 as described above in connection with FIGS. 2, 3A, and 3B. When the resistance of the spiral path increases as a result of a hotspot forming, at least some of the electrical current circulating in the coil 802 is therefore able to flow radially into an adjacent turn and bypass the affected part of the turn.

[0173] The field coil 802 is represented in the circuit diagram by an inductor 802A and a resistor 802B. Electrical current flowing through the inductor 802A corresponds to electrical current flowing around the spiral path provided by the turns of the field coil 802, whereas electrical current flowing through the resistor 802B corresponds to electrical current flowing radially between two or more turns of the field coil 802. When the coil 802 is fully superconducting, the electrical current flows predominantly around the spiral path of the coil 802. When the coil 802 ceases superconducting, the resistance of the spiral path increases such that the electrical current flows preferentially radially between the turns. In this case, the resistance of the resistor 802B in FIG. 8 corresponds, to a first approximation, to the number of turns of the field coil 802 multiplied by the turn-to-turn or “radial” resistance between pairs of adjacent turns, i.e. the combined resistance of the conductor elements (such as copper bridges 205A-C) connecting adjacent turns.

[0174] The quench protection circuit 800 further comprises a power supply 804 for supplying electrical current to the field coil 802. A circuit breaker 806 is provided to allow the power supply 804 to be disconnected in the event of a hotspot forming within the field coil 802. The circuit breaker may preferably be operated automatically by a quench detection system as described above. A dump resistor 808, preferably a varistor, is connected across the power supply 804 and circuit breaker 806, in parallel with the field coil 802. The dump resistor 808 may be thermally decoupled from the field coil 802 such that heat generated within the dump resistor 808 does not lead to any (or at most minimal) heating of the field coil 802.

[0175] The quench protection circuit 800 also comprises (in this example) a close-coupled secondary (CCS) coil 810 that is represented in FIG. 8 by an inductor 810A and a resistor 810B. The CCS coil 810 is inductively coupled to the field coil 802 such that a change in the magnetic field produced by the field coil 802 induces an electrical current within the CCS coil 810.

[0176] When the field coil 802 is in use, the circuit breaker 806 is initially closed so that electrical current is supplied to the superconducting field coil 802 from the power supply 804. As described above, formation of a “hotspot” caused by loss of superconductivity within a portion of one or more turns of the coil causes some of the electrical current to flow radially from the affected turn to the neighbouring turn or turns (e.g. via copper bridges 205A-C (or other such bridges made of a resistive material other than copper) in the field coil). When a hotspot is detected, the power supply 804 is disconnected by opening the circuit breaker 806. Electrical current may then flow from the field coil 802 (i.e. from the inductor 802A) and through the dump resistor 808. However, at least some of the electrical current from the field coil 802 is able to flow back into the field coil 802 by flowing radially between the turns of the coil. This current corresponds in FIG. 8 to the electrical current flowing from the inductor 802A, through the resistor 802B, and back to the inductor 802A. The proportion of the current that flows along this path (i.e. through the resistor 802B) depends inversely on the resistance of the dump resistor 808 relative to the resistance of the resistor 802B. The resistance of the dump resistor 808 is selected such that enough of the electrical current flows radially between the turns of the coil 802 (i.e. through resistor 802B in FIG. 8) to heat the conductor elements (e.g. copper bridges 205A-C) in order to cause more of the HTS material in the coil 802 to turn normal (e.g. as described above for the first aspect of the invention). For example, the resistance of the dump resistor 808 may be larger than the resistance of the resistor 802B (at least initially), such that a significant proportion (e.g. more than 10% or more than 50%) of the electrical current flows radially between two or more of the turns of the field coil 802. This radial or “turn-to-turn” electrical current heats the conductor elements, causing their temperature to rise and their resistivity to increase. In general, the conductor elements have relatively low electrical resistance at the operating temperature of the HTS field coil 802, but much higher resistance at higher temperatures. For example, when the conductor elements are made of copper, the turn-to-turn resistance of the field coil 802 may increase by around a factor of 15 when the temperature is increased from 20 K to 200K.

[0177] As the turn-to-turn (radial) resistance of the field coil 802 increases, a greater proportion of the electrical current flows out of the coil 802 to the dump resistor 808, thereby reducing the amount of heating provided to the coil 802. As the dump resistor 808 can be configured to dissipate heat effectively (e.g. by attaching it to a heat sink or providing high rates of cooling), any rise in temperature of the dump resistor 808 can be minimised or otherwise constrained to an acceptable range. Thus, the heating of the conductor elements by the turn-to-turn current creates a form of negative feedback in which the level of heating of the conductor elements is reduced as a result of the increased turn-to-turn resistance of the coil 802 relative to the resistance of the dump resistor 802. This process avoids the conductor elements (e.g. copper bridges 205A-C) in the coil 802 from being “burned out” by the electrical current, whilst still allowing rapid heating of the coil 802 following hotspot formation. The duration and magnitude of the heating pulse may be optimised so that the turns of the coil 802 are heated above the generation temperature of the HTS material as rapidly as possible, whilst ensuring that the temperature of the conductor elements does not exceed a threshold value.

[0178] Preferably, the resistance of the dump resistor 808 varies with time in order to further limit the electrical current flowing between the windings of the coil 802 (i.e. through resistor 802B). This may be accomplished by, for example, by using a varistor as the dump resistor 808, with the varistor being configured such that its resistance decreases as the voltage across coil 802 increases following hotspot formation. Suitable varistors are available from Metrosil™, for example.

[0179] In response to the heating caused by the radial electrical current and the hotspot, more of the HTS material in the coil turns normal (i.e. becomes non-superconducting) and the electrical current circulating around the turns of the field coil 802 decays, leading to a collapse of the magnetic field generated by the coil 802. As this happens, energy is transferred inductively from the field coil 802 to the CCS coil 810 and is ultimately dissipated in the resistor 810B of the CCS coil 810 circuit. The dump resistor 808 and the CCS coil 810 therefore act in a synergistic way to dissipate the electromagnetic energy stored in the field coil 802 efficiently. In particular, by limiting the turn-to-turn electrical current, the dump resistor 808 prevents the temperature of the field coil 801 from increasing too much before significant inductive transfer of energy to the CCS coil 810 can occur. Conversely, the rate at which the field coil 802 is heated further decreases as energy is transferred to the CCS coil 810, further limiting the temperature increase of the field coil 802 to an acceptable level.

[0180] The above disclosure can be applied to a variety of HTS magnet systems. In addition to the tokamak fusion reactor mentioned above as an example, it may be used for HTS coils in nuclear magnetic resonance imaging (NMR/MRI) devices, manipulation of magnetic devices within a non-magnetic medium via magnetic fields (e.g. robotic magnetic navigation systems for manipulating medical devices within a patient), and magnets for electric motors, e.g. for electronic aircraft. As a further example, the disclosure may be applied to proton beam therapy (PBT) devices comprising HTS magnet systems which include the disclosed features, where the HTS magnet systems are used within the accelerator of the PBT device, the quadrupole or dipole steering magnets of the PBT device, or any other magnet of the PBT device.

[0181] While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It would be apparent to one skilled in the relevant art(s) that various changes in form and detail could be made therein without departing from the spirit and scope of the invention.