High-temperature superconducting magnet and coil

Abstract

To provide a high temperature superconductor (HTS) magnet system including an HTS field coil and a power supply, which can be used in a nuclear fusion reactor and a proton beam therapy device.SOLUTION: An HTS field coil includes a plurality of turns including an HTS material and a metal stabilizer, and a conductive layer separating the turns to distribute a current among the turns through the conductive layer. A power supply unit (PSU) provides a first current through a radial path to the HTS field coil during ramp-up of the HTS field coil and a second current through the radial path opposite in direction to the first current through the radial path to the HTS field coil during ramp-down of the HTS field coil.SELECTED DRAWING: Figure 9

Description

[Technical field]

[0001] The present invention relates to high temperature superconducting (HTS) magnets, and more particularly to a method for ramping down an HTS magnet, for example in response to a quench detection, and an apparatus for carrying out the method. [Background technology]

[0002] Superconducting materials are generally 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 explained by the BCS theory. All low temperature superconductors have a critical temperature (the temperature above which the material does not become superconducting, even in zero magnetic field) below about 30 K. The behavior of HTS materials is not explained by the BCS theory, and such materials may have critical temperatures above about 30 K (it should be noted, however, that it is the physical differences in composition and superconducting behavior, not the critical temperature, that define HTS and LTS materials). The most commonly used HTS are "cuprate superconductors", ceramics based on copper oxides (compounds containing a copper oxide group), such as BSCCO or ReBCO (Re is a rare earth element, typically Y or Gd). Other HTS materials include iron pnictides (e.g., FeAs and FeSe) and magnesium diborate (MgB2).

[0003] ReBCO is typically manufactured as a tape having the structure shown in Figure 1. Such a tape 100 is typically about 100 microns thick and includes a substrate 101 (typically electropolished Hastelloy about 50 microns thick) on top of which is deposited by IBAD, magnetron sputtering, or other suitable technique a series of buffer layers known as a buffer stack 102 about 0.2 microns thick. An epitaxial ReBCO-HTS layer 103 (deposited by MOCVD or other suitable technique) covers the buffer stack and is typically 1 micron thick. A 1-2 micron silver layer 104 is deposited on the HTS layer by sputtering or other suitable technique, and a copper stabilization layer 105 is deposited on the tape by electroplating or other suitable technique, which often completely encapsulates the tape.

[0004] The substrate 101 provides a mechanical backbone that can be fed through the manufacturing line and allow for the growth of subsequent layers. The buffer stack 102 is required to provide a biaxially oriented crystalline template on which to grow the HTS layers, preventing chemical diffusion of elements from the substrate into the HTS that would impair its superconducting properties. The silver layer 104 is required to provide a low resistance interface from the ReBCO to the stabilization layer, and the stabilization layer 105 provides an alternative current path in case any portion of the ReBCO stops superconducting (becomes "normal").

[0005] Additionally, a "peeled-off" HTS tape can be fabricated that lacks the substrate and buffer stack, but instead has a silver layer on either side of the HTS layer. The tape with the substrate is referred to as a "substrate-attached" HTS tape.

[0006] HTS tapes can be arranged in HTS cables. HTS cables contain one or more HTS tapes connected along their length by a conductive material (usually copper). The HTS tapes can be stacked (i.e. arranged so that the HTS layers are parallel) or they can have other arrangements of tapes that can vary along the length of the cable. A notable special case of HTS cables is a single HTS tape and HTS pair. An HTS pair contains a pair of HTS tapes arranged so that the HTS layers are parallel. When substrate-attached tapes are used, the HTS pair can be type 0 (HTS layers facing each other), type 1 (HTS layer of one tape facing the substrate of the other tape), or type 2 (substrates facing each other). Cables containing three or more tapes can have some or all of the tapes arranged in HTS pairs. Laminated HTS tapes can contain either various arrangements of HTS pairs, most commonly a stack of type 1 pairs, or a stack of type 0 pairs (or equivalently type 2 pairs). HTS cables can contain a mixture of substrate-attached and release tapes.

[0007] Broadly speaking, there are two types of magnetic coil constructions: winding and assembly of multiple sections. As shown in FIG. 2, a winding coil is manufactured by winding the HTS cable 201 around a former 202 in a continuous spiral. The former is shaped to provide the required inner circumference of the coil and can be a structural part of the final winding coil or can be removed after winding. A section coil is composed of several sections 301, as shown diagrammatically in FIG. 3, each of which can contain several cables or preformed busbars 311, forming an arc of the entire coil. These sections are connected by joints 302 to form a complete coil. The coil turns in FIG. 2 and FIG. 3 are shown spaced apart for clarity, but there is generally a material connecting the coil turns, for example the coil turns can be solidified by potting with epoxy resin.

[0008] FIG. 4 shows a cross-section of a particular type of wound coil known as a "pancake coil" where the HTS cable 401 is wound to form a flat coil similar to a single turn of ribbon. Pancake coils can be made with any internal circumference of two-dimensional shape. Often, pancake coils are provided as "dual pancake coils," which, as shown in the cross-sectional view of FIG. 5, include two pancake coils 501, 502 wound in opposite directions, insulation 503 between the pancake coils, and an internal terminal 504 connected to each other. This means that a voltage only needs to be applied to the generally more accessible external terminals 521, 522 to pass current through the turns of the coils and generate a magnetic field.

[0009] The coils may be "insulated", having electrically insulating material between the turns of the coil, or "non-insulated", where the turns of the coil are electrically connected radially as well as along the cable (e.g., by connecting the copper stabilization layers of the cable by soldering or direct contact). The resistance between the turns is less than the resistance of conventional insulators such as ceramic or organic insulators, but greater than the resistance between the tapes in the coil cable, e.g., 100 to 10 times the resistance between the tapes in the cable. 15 "Partially insulated" coils are also possible, with two turns between each. No or partial insulation between the turns allows the current to be distributed between the turns, slowing the rate at which local "hot spots" (normal conducting regions) heat up.

[0010] A non-insulated HTS coil can be modeled as having three current paths: two helical paths following the HTS tape (one in the HTS and one in the metal stabilizer), and a radial path through the metal (and other resistive materials) connecting the non-insulated or partially insulated HTS cable turns between the coil ends (which can be modeled as a single path, but actually represents the sum of all radial resistive paths through the magnet). Only the current flowing through the helical path produces a large axial magnetic field in the center of the coil. The HTS helical path can be modeled as an inductor with a large inductance and zero or negligible resistance if the tapes are all superconducting. The copper stabilizer helical path is parallel to the HTS path and has the same inductance, but has significant resistance. Because of this, only a small amount of current flows through the copper stabilizer helical path unless some of the HTS helical path begins to quench. In this case, the excess current above the critical current Ic of the HTS helical path is distributed between the helical stabilizer path and the radial path according to their relative resistances and L / R time constants. The radial current path can be modeled as having negligible inductance and a much larger resistance than the helical path while the HTS is superconducting throughout.

[0011] One potential application of HTS field coils is in tokamak plasma chambers, especially for fusion reactors. US Patent No. 5,999,943 describes an approach involving the use of compact spherical tokamaks for use as neutron or energy sources. The low aspect ratio plasma shape in spherical tokamaks improves particle confinement times and allows for net power generation in a much smaller machine. However, a small diameter central column is required, which presents a challenge for the design of plasma confinement magnets. High temperature superconductor (HTS) field coils are a promising technology for such magnets.

[0012] Another potential application for HTS field coils is in proton beam therapy devices. Proton beam therapy (PBT, also known as proton therapy) is a type of particle beam therapy used to treat cancer (and other conditions that respond to radiation therapy). In PBT, a beam of protons is directed at the treatment site (e.g., a tumor).

[0013] Other similar treatments involve introducing boron-11 into the target area and then using proton beams to create p+ 11 The first is proton-boron capture therapy (PBCT), which initiates the B→3α reaction. The same equipment can be used to provide protons for either PBT or PBCT.

[0014] Proton beams for PBT and PBCT are generated by particle accelerators such as cyclotrons or linear accelerators. Accelerators typically used for PBT and PBCT produce protons with energies typically ranging from 60 to 250 MeV, with the most powerful facilities currently in operation having a maximum energy of 400 MeV.

[0015] Broadly speaking, there are two types of PBT device designs that allow for beam angle variation. In the first type of design, as shown in Figure 6, the accelerator 3001 is mounted on a gantry 3002 that allows the accelerator 3001 to rotate (usually around a horizontal axis) around the patient 3003. The patient is placed on a movable bed 3004 that provides additional degrees of freedom (e.g. translation and rotation around a vertical axis).

[0016] The second type of design is shown in Figure 7. The accelerator 4001 is fixed and the beam is directed to the patient via steering electromagnets 4002 (which typically include both quadrupole and dipole magnets), at least a portion of which is located in a gantry 4003, allowing the beam to be rotated (e.g., about a horizontal axis) around the patient 4004. The patient is placed on a movable bed 4005.

[0017] Both designs require a gantry to hold electromagnets capable of directing protons at beam energies that can be as high as 400 MeV. This requires very high magnetic fields, so the use of HTS field coils can significantly reduce the mass and size of the electromagnets and the gantry required to move them. HTS field coils can be used in the accelerator, the quadrupole magnets of the steering magnet, or the dipole magnets of the steering magnet. [Prior art documents] [Patent documents]

[0018] [Patent Document 1] International Publication No. 2013 / 030554 Summary of the Invention [Means for solving the problem]

[0019] The invention is defined in the appended claims. [Brief description of the drawings]

[0020] [Figure 1] FIG. 1 is a schematic diagram of an HTS tape. [Diagram 2] FIG. 1 is a schematic diagram of a wire-wound HTS coil. [Diagram 3] FIG. 1 is a schematic diagram of a section HTS coil. [Figure 4] FIG. 2 is a cross-sectional view of a pancake coil. [Diagram 5] FIG. 2 is a cross-sectional view of a dual pancake coil. [Figure 6] FIG. 1 is a schematic diagram of a first PBT device. [Figure 7] FIG. 2 is a schematic diagram of a second PBT device. [Figure 8] The simulation results of a partially insulated coil are shown. [Figure 9] 4 illustrates the results of a simulation of an exemplary magnet system. [Figure 10]4 shows a graph of the magnetic field and temperature of the coil after simply disconnecting the power supply. [Figure 11] 10 shows the voltage across the coil during the same process as shown in FIG. [Figure 12] 12A and B show an example of leaky insulation. [Figures 13A-13C] A further example of leaky insulation is shown. [Figure 13D] A further example of leaky insulation is shown. [Figure 13E] A further example of leaky insulation is shown. [Figure 14] The equivalent circuit of a two-turn coil is shown. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Figure 8 shows the current, voltage and power of the non-insulated coil during ramp-up and steady-state operation. During ramp-up of the non-insulated coil, the current initially flows mainly in the radial path (period A in Figure 8) and then stabilizes. The amount of current flowing in the radial path is higher at faster ramp rates (due to the higher voltage (L.dI / dt) developed across the helical path, this is period B). At the end of the ramp, dI / dt drops to zero and the current is transferred to the HTS helical path with time constant L / R (period C). The current is transferred mostly to the helical path with a few (approximately 5) L / R time constants after the end of the ramp. For this reason, the time constant should be selected to provide a reasonable ramp-up time, for example a time constant of 5-10 hours is appropriate for a tokamak TF coil (providing a ramp-up time of approximately 1-2 days).

[0022] In large magnets, active quench protection schemes can also be implemented to avoid damage from quench (either insulated or non-insulated coils). In this scheme, the magnet's stored energy can be dumped to some component outside the quench region of the magnet before a sufficient temperature rise occurs in the quench region to cause damage. The other component can be an external resistance, or a separate part of the magnet that is quenched with a larger percentage of the magnet's cold mass (thus distributing the magnet's stored energy over a larger volume, reducing the overall temperature rise). However, active approaches require that the time between the onset of the normal conduction region (also called the "hot spot") and the induction of the ramp-down ("dump") of the magnet current be short enough that the terminal temperature of the hot spot is less than the temperature at which coil damage can occur, e.g., about 200K. Such approaches can also be used in small magnets to provide additional protection against quenches.

[0023] The above and a description of suitable structures for achieving the desired time constant are described in more detail in co-pending application GB18817.7.

[0024] Although the use of PI coils extends the time available to dump the magnet current, it is still important to perform this action as quickly as possible after the detection of a hot spot. In the literature, there is relatively little discussion of current dumping techniques for PI coils because PI coils are generally only used for small coils where the total energy in the coil is relatively low and quenches tend to propagate relatively quickly throughout the coil, i.e., the energy dump spreads throughout the coil. In addition, PI coils can often be operated without quench protection, since they are inherently more stable compared to insulated coils, and therefore less risky. However, active quench protection is important for large coils with significant stored energy, coils with geometries that allow hot spots to cover all turns in only a relatively small percentage of the coil windings, and / or coils intended for long-term operation in harsh environments (e.g., field coils for fusion reactors).

[0025] The use of a PI coil provides an additional advantage in dumping energy from the magnet. Figure 10 shows a graph of the magnetic field (proportional to the current in the helical path) and temperature of the coil after simply cutting off the power (i.e., opening circuiting). During period A, the power is on (indicated by current 1001). At the beginning of period B, the power is cut off, causing the inductance of the coil to develop a voltage that continues to drive the current in each turn in a closed loop, shorting it through the resistance between the turns to the beginning of the turn. This causes ohmic heating that reduces the critical current of the HTS. This process continues for period B (depending on many factors, typically several seconds, 14 seconds in this experiment) until the HTS of the turn quenches and generates enough voltage to dump its loop current into the metal stabilizer of the helical path (period C). Because it has a much higher resistance than the resistance between the turns, the magnetic field energy of the turn is rapidly converted to heat in the stabilizer, and the temperature of the turn rises uniformly as the current in the helical path rapidly drops to 0 (<1s).

[0026] However, for effective quench protection, a longer period B is still undesirable. In the event of a quench, this period may be long enough that significant local heating occurs in local hot spots elsewhere in the magnet, resulting in unacceptably high peak temperatures at the hot spots. Furthermore, magnets such as the TF magnets of a tokamak may contain several coils that are thermally and magnetically poorly coupled (e.g., separate salients), and when inducing a dump by turning off the power, it is desirable for all coils to experience the same delay before quenching. Manufacturing differences in the local temperature of the magnetic field difference may result in variations between the coils. If the coils do not quench simultaneously, the resulting very large electromagnetic forces between the coils may damage the magnet's mechanical support structure and / or the coils themselves. To minimize this, it is desirable to minimize the variation between the delay between PSU off and quench in each coil, which in turn minimizes the variation between the delay periods. The length of this period can be reduced by increasing the resistance of the partial insulation layer (and thereby increasing the heating due to the current traveling in a radial path in period B), but this has a knock-on effect on other electrical properties of the coil, for example changing the time constant for ramping up the coil or making current distribution between the turns more difficult (making it more likely that a hot spot will cause a global quench).

[0027] Instead, rather than cutting off the power, it has been proposed to force a large reverse current through the magnet coils (i.e. in the opposite direction to the current flowing through the coils before ramp-down), for example by using a four-quadrant PSU that can sink current from the magnet coils. Since the superconducting path has a large inductance, this reverse current flows mainly in the radial path of all coils of the TF magnet. This large increase in radial current would cause significant heating of all coils and rapidly quench the entire magnet (thus spreading the energy dump over a wide area).

[0028] While it may seem counterintuitive to run a reverse current rather than simply shutting down the PSU, the key difference is that the excess radial current heats the entire bulk of the magnet. This means that a quench spreads quickly throughout the magnet, and the dump of energy spreads throughout the volume (or at least a significant percentage of it). A concentrated energy dump, such as would occur without intervention, would cause an unacceptable temperature rise in that small area, damaging the HTS. Once a significant portion of the magnet is quenched, the same amount of energy (plus a small contribution from the reverse current itself) spreads throughout the magnet, limiting the temperature rise of the HTS. Additionally, by heating the magnet more uniformly, it prevents the formation of a steep temperature gradient within the magnet. If the temperature gradient is too high, the different thermal expansion of nearby areas of the magnet would cause structural damage.

[0029] In existing magnets, uniform heating is achieved through the use of a "quench heater", a heating element located adjacent to the HTS cable that can be turned on to supply heat to the coil. However, such heaters take up space, reducing the space available for the HTS conductor or metal stabilizer. The "reverse current" method effectively uses the radial conduction path as a "quench heater", meaning that heating is uniformly distributed through the coil and no additional heating element is required.

[0030] The reverse current can be limited or set to the operating current of the magnet. In this way, maximum heating is achieved by the reverse current compared to that required for normal magnet operation without exceeding the design parameters of the magnet's external components.

[0031] It should be noted that although the above has been described with respect to quench protection, the energy dumping techniques described above may also be applicable to other situations in which a magnet is ramped down, such as, for example, shutting down the magnet under normal conditions in the absence of a detected quench (or conditions that could lead to a quench).

[0032] 9 shows simulation results for an exemplary partially insulated coil during ramp-up, steady-state operation, and ramp-down. In this case, the PSU (power supply) is modeled as a current source, i.e., the current drawn from the PSU is set in the simulation and the voltage across the PSU is calculated.

[0033] During ramp-up, the PSU current is gradually increased at a constant rate to 2.2kA. The PSU voltage is positive and around 0.1V. The radial path current is approximately proportional to the PSU voltage (since the radial path can be modelled as a simple resistor) and the helical path current increases at a constant rate. At time T1, once the desired current is reached, the magnet is switched to steady state operation, the PSU is set to a constant current and the radial path current increases with the time constant L / R as described above. radial As the radial path current decays, the PSU voltage decreases to a value on the order of a few millivolts because the helical path has very little resistance. (In this simulation, the helical path is modeled as having zero resistance, so the PSU voltage tends to zero. In reality, the PSU voltage typically settles on the order of tens to hundreds of millivolts.) During ramp-up and steady-state operation, the temperature of the HTS remains essentially constant at less than 20 K.

[0034] At time T2, a magnet dump is initiated (in response to quench detection or otherwise). The PSU supplies a reverse current (with a fast current ramp, modeled in this case as a falling current ramp 10 times faster than the initial ramp-up), which flows primarily in a radial path. The PSU voltage is negative during this current supply, around -0.5V. The HTS temperature rises rapidly. When the HTS temperature reaches about 55K, the entire coil quenches and the simulation is terminated because the temperature rise is too fast for the model used. However, in reality the magnet's stored energy is rapidly converted to heat, which is spread relatively evenly throughout the magnet, allowing it to safely shut down.

[0035] The reverse current may be supplied for a set time or until a particular condition is reached, for example, upon detection of a quench in a bulk of the magnet, detection of a particular temperature in a bulk of the magnet, or detection that the current in the helical path (or the magnetic field generated by the coil) has decreased below a threshold value.

[0036] The rate of ramp down is determined by the rate of the PSU's reverse current ramp.

[0037] It is desirable to be able to control the rate of change of current during both the ramping and dumping phases of the magnet. For this reason, a PSU with a feedback controlled current output is preferred. The PSU current can be controlled based on the current in the helical path, the temperature of the magnet, the magnetic field produced by the coil, or any other suitable characteristic of the coil.

[0038] The power supply can include multiple power supply units, each power supply unit supplying power to the coil during different time periods. In particular, the power supply can include a first unit supplying power to the coil during ramp-up and steady state operation, and a second unit supplying a reverse voltage to the coil during ramp-down.

[0039] The power supply (or one or more power units of the power supply) may be located partially within the cryostat containing the HTS magnet and may include a transformer arranged to transmit power across the cryostat without cables passing through the cryostat, as described in PCT / GB2018 / 050337.

[0040] Where magnet ramp-down is triggered in response to detection of a quench or a condition that may lead to a quench, this detection may be by any practical method. For example, Detection of excess voltage across HTS materials in magnets, The use of a secondary HTS tape adjacent to the main coil and configured to quench before the main coil, for example as described in International Patent Application PCT / GB2016 / 052712 or UK Patent Application GB1812120.2; Detection of temperature, strain, magnetic field or other conditions in an electromagnetic coil, for example by Rayleigh scattering in a fiber optic cable as described in International Patent Application PCT / GB2017 / 053066, or by other temperature, strain or magnetic field detectors known in the art.

[0041] It is important not to run reverse current for long periods of time; otherwise the active dump could heat the magnet above 200K and cause problems. Ideally, the dump system should limit the total energy applied to the magnet to the energy required to raise the temperature of the entire magnet above its critical temperature (i.e. to change all coils from superconducting to normal conducting). This is a small fraction of the total energy required to bring the entire magnet up to about 200K. As the coils begin to quench, the stored energy in the magnet itself dissipates, promoting a global quench.

[0042] A simple way to add the right amount of energy is to discharge a capacitor bank into the magnet. This also eliminates the need for a four-quadrant PSU. A single-quadrant PSU can be used to ramp the magnet. When the magnet needs to be dumped, it is simply disconnected, using an active switch and a pre-charged capacitor bank connected across the magnet to drive a radial current in the reverse direction. Note that no significant voltage is created by disconnecting the PSU, as the large inductance of the magnet is shunted by its radial resistance.

[0043] While most of the above disclosures have focused on ramping down a magnet after quench detection where the speed of ramp down is critical, there are also techniques using the same basic principles that are applicable to controlling magnets in conditions where ramp down time is not a major factor.

[0044] As an example, the PSU can be configured to provide a ramp-down current that is smaller than the current in the magnet, but in the same direction. This will cause a current equal to the difference between the magnet and PSU currents to flow in the radial path, heating the magnet as before. This will slow the ramp-down compared to a reverse current (or simply disconnecting the PSU), reducing the temperature rise of the magnet.

[0045] As a further example, the PSU can be configured to supply an AC current that is superimposed on the DC supplied to the coils (either during steady state, ramp-up, or ramp-down). If the period of the AC current is significantly shorter than the time constant L / R, this AC current will flow entirely in the radial path. This will heat the magnet without affecting the current in the helical path (compared to when only DC current is supplied).

[0046] The superimposed AC current can also be used in combination with any of the previous examples. For example, a combination of DC countercurrent and AC current (i.e., the total current is a sinusoidal current with an average value opposite in sign to the coil current and a period smaller than the magnet's time constant) can be used to ramp down the coil with additional heating. Alternatively, the AC current can be combined with a DC ramp-down current that is smaller than the current in the magnet but in the same direction (i.e., the total current is a sinusoidal current with an average value smaller than the coil current and a period smaller than the magnet's time constant). As a further alternative, a pure AC current can be provided to ramp down the magnet, which in addition to the AC current, will cause a current equal to the magnet current to flow in the radial path. In either case, the addition of the AC current will cause a larger current to flow in the helical path (hence a faster ramp-down) compared to using only DC current.

[0047] A surprising feature of ramping down partially insulated coils (by shutting down the PSU or supplying a modified current) as described in the examples above is that the voltage across the turns of the coil remains low (on the order of a few volts even for large coils) throughout the process. Large insulated superconducting coils require heavy-duty insulation capable of withstanding very high voltages, but the small voltages experienced by partially insulated coils can be effectively insulated by a much wider variety of materials (or even simple vacuum or air gaps).

[0048] FIG. 11 shows the voltage across each coil of the same magnet (containing six pancake coils and with a total inductance of 0.12H) used in FIG. 10. The peak voltage occurring at the time corresponding to the onset of the global quench (i.e., the end of period B and the beginning of period C in FIG. 10) is about 0.1V. In contrast, the expected voltage for a comparable insulated coil under the same conditions is on the order of 2kV, and for the large coil it is at least 5kV (the voltage can be calculated as V=-LdI / dt, where dI / dt is roughly the transport current (1.4kA for the test coil) divided by the time it takes to quench (0.1kA for the test coil). With partially insulated coils, the coils can be operated at high transport currents and inductances without particularly high voltages developing, which is most practical when operating large coils at low transport currents (a few kA versus typically 50kA) and high inductances (i.e., more turns).

[0049] The peak voltage developed between the two turns of a partially insulated coil during a current dump with an open circuit PSU is V = I0R stab where I0 is the transport current and R stab is the resistance of the resistive material in the coil's helical path. The quench propagates between the coils by mutual inductance, which means that the peak coil voltage will not exceed the peak voltage of one turn. For practically large coils, the peak voltage will not exceed 10V.

[0050] Figure 14 shows the equivalent circuit for a two-turn coil (which can be extended to more turns by adding more coils in series and adding mutual inductance between each coil). In this diagram, I0 is the transport current, R tt is the resistance between turns (i.e., the resistance of the radial path divided by the number of turns), R stab is the resistance of the resistive spiral path, L turn is the induction for each turn, R HTS is the resistance of the HTS in each turn (i.e., zero during normal operation, non-zero only during quench or near quench), M is the mutual induction between the two turns, and K is the coupling coefficient between the turns.

[0051] This is particularly important for the insulation of the coil, both to insulate the entire coil from other components ("ground wrap"), and for the design of partial insulators that use insulating material with many conductive channels through it (called "leaky insulators" and described in more detail below). The low voltage means that the insulator does not need to be a robust material such as Kapton (as used in conventional insulated magnets), but simpler materials such as paint, varnish, or paper can be used, or the insulating material can be replaced by an air or vacuum gap (with appropriate support structure and, if necessary, insulation).

[0052] Insulating structures can be characterized by a "breakdown voltage," above which the structure ceases to be insulating and the resistance of the structure drops from the order of a few megaohms to the order of a few ohms or milliohms. For insulated coils, this breakdown voltage must be at least 2 kV (at least 5 kV for large coils, e.g., with a radius greater than 50 cm), severely limiting the materials that can be used while keeping the insulation reasonably compact (the breakdown voltage is roughly proportional to the thickness of the material, and the constant of proportionality (the "dielectric constant") varies from material to material).

[0053] For partially insulated coils, only a breakdown voltage of greater than 10V or so is required, which allows the use of any material that can reasonably be called an "insulator".

[0054] Alternatively, in environments hostile to the material (e.g., in a fusion reactor where the material is bombarded with neutrons), this allows the insulator to be used for a longer period of time before it degrades to the point where it no longer insulating, since neutron bombardment tends to cause changes in the insulator that lower its dielectric constant or introduce physical gaps.

[0055] An example of leaky insulation is shown in Figures 12A and B. A metal strip 901 has a thin insulating coating 902 on at least the side facing the HTS cable, with the insulating coating removed or missing over windows (or "through holes") 903 spaced apart on each side of the metal strip. The windows can have any shape and can extend to the edge of the tape. The location of the windows on both sides of the metal strip are staggered as shown in Figure 12B, which increases resistance (compared to a non-insulated strip or compared to a strip with windows on both sides directly opposite each other) since the current must take a path 910 along part of the length of the metal strip.

[0056] By varying the spacing of the windows so that they are closer together at the return limb and further apart at the core, the required difference in turn-to-turn resistance can be achieved between the return limb and the core. Further adjustments can be achieved by using different metals for the metal strip in the core versus the return limb, or by changing other aspects of the shape of the strip.

[0057] To allow further adjustment, rather than a metal strip of the same material throughout, a layer having a number of metal tracks can be used, effectively forming an insulating layer having conductive radial tracks disposed therein, with the spacing and length of the tracks determining the resistance of the partial insulating layer.

[0058] 13A-E show a further example of a leaky insulator layer. The leaky insulator layer comprises five layers in the following order: a first metal connection layer 1611; a first insulating layer 1621; Conductive layer 1630; a second insulating layer 1622; · A second metal contact layer 1612 .

[0059] Figures 13A-C respectively show the layout of first metal contact layer 1611, conductive layer 1630, and second metal contact layer 1622. Figures 13D and E are cross-sectional views along lines D and E of Figures 13A-C.

[0060] The connection layer is present to facilitate attachment to the HTS cable by soldering.

[0061] In contrast to the previous embodiment where the conductive layer is a continuous metal strip, in this embodiment the conductive layer is divided into a number of conductive regions. These regions are of two types: square regions 1631 (which can actually be any shape) that are connected to only one of the metal connection layers by vias 1606. These regions do not affect the electrical properties of the partial insulating layer, but provide a thermal path through the respective insulating layer. By varying the size of these regions and the number of connections between these regions and the metal connection layers, the thermal properties of the partial insulating layer can be changed independently of the electrical properties.

[0062] Each of the other regions 1632 connects a window 1601 in the first insulating layer 1621 to a window 1602 in the second insulating layer 1622. The resistance between the windows can be controlled by varying the geometry of the regions 1632, for example if the regions 1632 include elongated tracks 1633 as shown in Figure 13B, increasing the width of the tracks decreases the resistance between the windows and increasing the length of the tracks (e.g. by providing a non-linear track or moving the windows) increases the resistance between the windows.

[0063] A window 1601 in the first insulating layer is formed by the first connection layer and a via drilled through the first insulating layer, which is then plated with metal 1603 (or other conductive material) to connect the first connection layer and the conductive layer. A window 1602 in the second insulating layer is formed by drilling a via 1602 through all layers, which is then plated with metal 1604 (or other conductive material). To prevent a connection being made to the first connection layer through the window 1602 in the second insulating layer, the first connection layer is etched around the via 1602 to electrically insulate it, and an insulating cap 1605 is placed on the end of the via 1602 to ensure that no bridging occurs due to soldering or contact with the HTS cable.

[0064] Alternatively, the window 1602 can instead be drilled from the other side of the partial insulating layer, passing through the second connecting layer, the second insulating layer, and the conductive layer, but not (or not completely) through the first insulating layer. As a further alternative, all windows can be formed from vias that go through all layers, with an etch of the second connecting layer and an insulating cap of the second connecting layer being used for the window 1601 in the first insulating layer.

[0065] Another unexpected benefit of partially insulated coils is that the added quench stability allows for a wider choice of materials for the non-superconducting conductive elements in each cable. In conventional coils, both the HTS stabilizer (i.e., the thin metal layer or metal cladding on each tape) and any material connecting the tapes are copper. This is because copper has a very low resistivity and higher resistivity materials would cause excessive heating. However, copper is also a relatively soft metal, so under high pressure it can be extruded from the tape or deformed under shear stresses that can cause damage to the HTS layer after quenching.

[0066] Therefore, it is preferable to reduce or eliminate copper from the coil turns and partial insulation. The reduced copper can be, for example, less than 10 microns thick copper per HTS tape in the coil (i.e., reduced compared to conventional HTS tapes), or less than 5 microns thick copper (i.e., less than half that). The metal or other conductor used in place of copper can have one or more of the following: Reduced ductility compared to copper, Increased shear modulus compared to copper, Increased Young's modulus compared to copper, Increased bulk modulus compared to copper; Increased Brinell hardness number compared to copper.

[0067] Suitable materials include stainless steel.

[0068] The above disclosure can be applied to various HTS magnet systems. In addition to the tokamak fusion reactor mentioned above as an example, the disclosure can be used in HTS coils in nuclear magnetic resonance imaging (NMR / MRI) machines, the operation of magnetic devices in non-magnetic media by magnetic fields (e.g., robotic magnetic navigation systems for operating medical devices in patients), and magnets for electric motors, for example, for electronic aircraft. As a further example, the disclosure can be applied to proton therapy machines including HTS magnet systems including the disclosed features, where the HTS magnet system is used in the accelerator of a PBT machine, the quadrupole or dipole steering magnet of a PBT machine, or any other magnet of a PBT machine.

Claims

1. 1. A high temperature superconductor (HTS) magnet system including an HTS field coil and a power supply, The HTS field coil is a plurality of turns including an HTS material and a metal stabilizer; a conductive layer separating the turns such that current can be distributed between the turns through the conductive layer; Including, The power source includes: providing a first current to the HTS field coil during ramp-up of the HTS field coil; and an HTS magnet system configured to provide a second current to the HTS field coil during ramp-down of the HTS field coil, the second current being opposite in direction to the first current;

2. a quench detection system configured to detect a quench of the HTS material and / or to detect a condition that may cause a quench of the HTS material; 10. The HTS magnet system of claim 1, wherein the power supply is configured to ramp down the HTS field coils in response to detection of a quench or a condition that may cause a quench by a quench detection system.

3. 3. The HTS magnet system according to claim 1 or 2, wherein the second current is a DC current.

4. 3. The HTS magnet system according to claim 1 or 2, wherein the second current is a combination of DC and AC currents such that the second current varies sinusoidally with an average value opposite in sign to the first current and with a period smaller than a time constant of the HTS field coils.

5. 4. The HTS magnet system according to claim 1, wherein the power supply comprises a four-quadrant power supply unit (PSU).

6. 4. The HTS magnet system of claim 1, 2 or 3, wherein the power supply includes a single-quadrant power supply unit (PSU) and a capacitor, the single-quadrant PSU configured to provide the first current and the capacitor configured to provide the second current.

7. The power source includes: detecting a quench in a predetermined portion of the HTS field coil; Detecting a particular temperature at a predetermined portion of the HTS field coil; detecting that the magnetic field generated by the magnet has decreased below a threshold; and Detecting that the current in the HTS material of the HTS field coil has decreased below a threshold value.

7. The HTS magnet system according to claim 1 , configured to supply the second current up to

8. The power source includes: a current in the HTS material of the magnet; the temperature of the HTS field coil; and Magnetic field of the HTS field coil 8. The HTS magnet system of claim 1 , further comprising a feedback system configured to control the current supplied to the magnet based on one or more of:

9. 1. A high temperature superconductor (HTS) magnet system including an HTS field coil and a power supply, The HTS field coil is a plurality of turns including an HTS material and a metal stabilizer; a conductive layer separating the turns such that current can be distributed between the turns through the conductive layer; and Including, The power source includes: providing a first current to the HTS field coil during ramp-up of the HTS field coil; and 2. An HTS magnet system configured to provide a second current to the HTS field coil during ramp-down of the HTS field coil, the second current being in the same direction as the first current and less than the current in an HTS material of the field coil.

10. 10. The HTS magnet system of claim 9, wherein the second current is a combination of DC and AC current such that the second current varies sinusoidally with an average value greater than zero and less than the first current and with a period less than a time constant of the HTS field coil.

11. 1. A high temperature superconductor (HTS) magnet system including an HTS field coil and a power supply, The HTS field coil is a plurality of turns including an HTS material and a metal stabilizer; a conductive layer separating the turns such that current can be distributed between the turns through the conductive layer; and Including, The power source includes: providing a DC current to the HTS field coil; and configured to supply an AC current to the HTS field coil in addition to the DC current to heat the field coil; The HTS magnet system, wherein the AC current has a period smaller than a time constant of the field coil and an amplitude smaller than the DC current.

12. 12. A tokamak comprising a magnet system according to any one of claims 1 to 11, wherein an HTS field coil of the magnet system is one of a toroidal field coil or a poloidal field coil of the tokamak.

13. A proton beam therapy (PBT) device including a magnet system according to any one of claims 1 to 10, wherein the HTS field coils of the magnet system are A field coil of an accelerator of the PBT device; A dipole magnet or a quadrupole magnet of the proton beam steering system of the PBT device One of these is the PBT device.

14. 1. A method of ramping down a high temperature superconductor (HTS) field coil, comprising: the HTS field coil includes a plurality of turns including an HTS material and a metallic stabilizer and conductive layers separating the turns such that current can be shared between the turns through the conductive layers; The method includes passing a second current through the HTS field coil opposite to a first current through the HTS material.

15. 15. The method of claim 14, wherein the second current is caused to flow in response to detecting a quench or a condition that may cause a quench in the HTS field coil.

16. The second current is detecting a quench in a predetermined portion of the HTS field coil; Detecting a particular temperature at a predetermined portion of the HTS field coil; detecting that the magnetic field generated by the magnet has decreased below a threshold; and Detecting that the current in the HTS material of the HTS field coil has decreased below a threshold value. The method according to claim 14 or 15, wherein the liquid is passed through either

17. 17. The method of any one of claims 14 to 16, wherein the second current is a combination of DC and AC current such that it varies sinusoidally with an average value opposite in sign to the first current and with a period smaller than a time constant of the HTS field coil.

18. 1. A method of ramping down a high temperature superconductor (HTS) field coil, comprising: the HTS field coil includes a plurality of turns including HTS material and a metallic stabilizer and conductive layers separating the turns such that current can be shared between the turns through the conductive layers; The method includes passing a second current through the HTS field coil, the second current being less than the first current through the HTS material.

19. 20. The method of claim 18, wherein the second current is a combination of DC and AC current such that the second current varies sinusoidally with an average value greater than zero and less than the first current, and with a period less than a time constant of the HTS field coil.

20. 1. A method for heating a high temperature superconductor (HTS) field coil, comprising: the HTS field coil includes a plurality of turns including HTS material and a metallic stabilizer and conductive layers separating the turns such that current can be shared between the turns through the conductive layers; The method includes passing a combination of DC and AC current through the HTS field coils.

21. 21. The method of claim 20, wherein the DC current is equal to the current in the HTS material of the HTS field coil.

22. 1. A high temperature superconducting (HTS) magnet, comprising:

1. A high temperature superconducting (HTS) coil, a plurality of turns including an HTS material and a metal stabilizer; a conductive layer separating the turns and providing electrical connections between the turns; and an HTS coil comprising: an insulating structure substantially surrounding the HTS coil and having a breakdown voltage less than 5 kV and greater than 10 V; 2. An HTS magnet comprising:

23. 23. The HTS magnet of claim 22, wherein the insulating structure is a coating of paint or varnish.

24. 24. The HTS magnet of claim 22 or 23, wherein the insulating structure includes a gas or vacuum filled gap and a support structure configured to structurally connect the HTS coil to other components of the magnet.

25. 25. The HTS magnet of any one of claims 22 to 24, wherein the insulating structure has a breakdown voltage of less than 2 kV, less than 1 kV, less than 500 V, or less than 100 V.

26. 1. A high temperature superconducting (HTS) coil, a plurality of turns including an HTS material and a metal stabilizer; a partial insulating layer separating the turns, a conductive material that provides an electrical path between adjacent turns; an insulating structure that limits the electrical path through the partial insulating layer; A partial insulating layer including Including, The insulating structure has a breakdown voltage less than 5 kV and greater than 10 V.

27. 27. The HTS coil of claim 26, wherein the insulating structure comprises a gap filled with a gas or vacuum.

28. 28. An HTS coil as claimed in claim 26 or 27, wherein the insulating structure does not include materials having a dielectric constant greater than 5 kV volts divided by the minimum spacing between adjacent turns.

29. The conductive material is A conductive layer having insulating structures on both sides; a plurality of conductive links extending through the insulating structure and connecting the conductive layer to respective turns; 29. The HTS coil of any one of claims 26 to 28, formed as follows:

30. 30. The HTS coil of claim 29, wherein the conductive layer is divided into regions separated by portions of the insulating structure, each region connecting a conductive link connected to a first coil to a conductive link connected to a second coil.

31. 1. A high temperature superconducting (HTS) coil, a plurality of turns including an HTS material and a metal stabilizer; a partial insulating layer separating the turns, a conductive material that provides an electrical path between adjacent turns; a gap filled with a gas or vacuum, the gap being arranged to limit an electrical path through the partial insulating layer; A partial insulating layer including An HTS coil comprising:

32. 1. A high temperature superconducting (HTS) coil, a plurality of turns including an HTS material and a metal stabilizer; a partial insulating layer separating the turns; Including, 1. An HTS coil, wherein the turns are copper-free or the turns contain copper with a total thickness per HTS conductor within the turn of less than 10 microns.

33. 33. The HTS coil of claim 32, wherein the partial insulation layer is copper-free.

34. The turns and / or partial insulating layers are Reduced ductility compared to copper, Increased shear modulus compared to copper, Increased Young's modulus compared to copper, Increased bulk modulus compared to copper, Increased Brinell hardness number compared to copper 34. The HTS coil of claim 32 or 33, comprising a conductor having one or more of:

35. 35. The HTS coil of claim 34, wherein the turns and / or partial insulation layers comprise steel.

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