Low-loss superconductor electrical cables

Abstract

An electrical cable having two ends and including multiple subcables extending between the two ends of the cable. The subcables are electrically insulated from one another along their length, and each subcable provides its own alternative current path between the two ends of the cable. Each subcable comprises a superconducting portion having multiple HTS tapes containing high-temperature superconducting (HTS) material, and a resistor connected in series with the superconducting portion.

Description

Technical Field

[0001] The present invention relates to a high temperature superconductor (HTS) cable including a plurality of sub-cables.

Background Art

[0002] Superconducting materials are typically divided into "high temperature superconductors" (HTS) and "low temperature superconductors" (LTS). LTS materials such as Nb and NbTi are metals or metal alloys whose superconductivity can be described by the BCS theory. All low temperature superconductors have a self-field critical temperature below about 30 K (the temperature above which the material cannot become superconducting even in zero magnetic field). In contrast, the behavior of HTS materials is not explained by the BCS theory, and such materials can have a critical temperature above about 30 K. The most commonly used HTS is a ceramic based on "cuprate superconductors" - BSCCO (bismuth strontium calcium copper oxide), or cuprates (compounds containing a cuprate oxide group) such as ReBCO (rare earth barium copper oxide, where Re is a rare earth element, typically Y or Gd). Other HTS materials include iron pnictides (e.g., FeAs and FeSe) and magnesium diboride (MgB2).

[0003] ReBCO superconductors are typically manufactured as tapes approximately 100 microns thick and 2 to 12 mm wide. A typical tape structure is shown in Figure 1, comprising a substrate 101 (typically an electropolished nickel-molybdenum alloy, e.g., Hastelloy®, approximately 50 microns thick), on which a series of buffer layers known as buffer stacks 102, approximately 0.2 microns thick, are deposited. An epitaxial ReBCO-HTS layer 103 covers the buffer stacks and is typically 1 micron thick. A 1-2 micron silver layer 104 is deposited on the HTS layer, and a copper stabilizer layer 105 is deposited on the tape, often completely encapsulating the tape. The silver layer 104 and the copper stabilizer layer 105 are also deposited on the sides of the tape 100 and the substrate 101 (not shown in Figure 1 for clarity), so that these layers extend continuously around the perimeter of the tape 100, thereby enabling electrical connections from any side of the tape 100 to the ReBCO-HTS layer 103. Thus, these layers 104, 105 may also be referred to as "cladding". The silver layer 104 forms a low-resistivity electrical interface with the ReBCO layer 103, while the copper layer 105 enables external connections to the tape (e.g., by soldering) and provides parallel conductive paths for electrical stabilization.

[0004] An HTS cable comprises one or more HTS tapes connected along its length via a conductive material (usually copper). The HTS tapes may be stacked (i.e., arranged so that the HTS layers are parallel) or may have some other tape arrangement configuration that can vary along the length of the cable.

[0005] In this specification, the following terms are used when describing coils: • "HTS cable" - A cable containing one or more HTS tapes. In this definition, a single HTS tape constitutes an HTS cable. • "Turn" (or "winding") - A section of HTS cable inside a coil that surrounds the inside of the coil (i.e., can be modeled as a complete loop). • "Arc" - The continuous length of a coil that is less than the entire field coil. • "Critical current" - The current at which the HTS becomes normal for a given temperature and external magnetic field. The HTS is considered to "normalize" at the characteristic point of the superconducting transition, where the tape generates a critical electric field of E0 volts per meter. The choice of E0 is somewhat arbitrary, but is usually interpreted as 10 or 100 microvolts per meter. • "Critical temperature" - The temperature T at which the HTS functions normally under a given magnetic field and current. C (Strictly speaking, the term "critical temperature" is formally defined relative to a zero magnetic field, but for convenience, this term is used more generally in this specification.) • "Peak critical temperature" - The temperature at which the High-Speed ​​Telescope (HTS) functions normally when there is no external magnetic field and the current is negligible. • "Generation Temperature" (or "Current Shunt Temperature") - The temperature at which the HTS material generates heat because it cannot completely carry the current flowing through it. A typical generation temperature of around 35K is when the current reaches a critical current (I C This is approximately 80% of the total, and is observed in ReBCO material within HTS field coils with a magnetic field strength of approximately 20T.

[0006] Superconducting magnets are formed by arranging HTS cables into a coil, either by winding HTS cables (or individual HTS tapes) or by providing sections of coils fabricated from HTS cables and joining them together. There are three broad classes of HTS coils. • It is insulated and has electrical insulating material between turns (so that current can only flow through the HTS cable, i.e., in a "spiral path" around the turns). • It is non-insulated, and the turns are connected radially and along the cable (for example, by connecting the copper stabilization layer of an HTS cable). • Partially insulated, the turns are connected radially with controlled resistance, either by the use of high-resistance materials (compared to, for example, copper) or by providing intermittent insulation between the coils.

[0007] Non-insulated coils can also be considered a low-resistance case of partially insulated coils. Partially insulated field coils are described, for example, in International Publication No. 2019150123.

[0008] A wound coil is manufactured by winding an HTS cable 201 in a continuous spiral around a molding tool 202, as shown in Figure 2. A section coil consists of several sections 301, each of which may contain several cables or pre-formed busbars 311, forming the arc of the entire coil, as schematically shown in Figure 3. These sections are connected by joints 302 to form a complete coil. The coil turns in Figures 2 and 3 are shown spaced apart for clarity, but generally there is a material connecting the coil turns, and for example, the turns can be reinforced by potting with a suitable material such as epoxy.

[0009] Figure 4 shows an HTS tape 400 (in this case, a ReBCO tape) illustrating the x, y, and z Cartesian coordinate system used in this document. The y-axis runs along the length of the tape (i.e., in the direction of the current when the tape is in use), the x-axis runs across the width of the tape, and the z-axis runs through the thickness of the tape (i.e., perpendicular to the plane of the tape).

[0010] Figure 5 shows a cross-section of an exemplary HTS tape in the x / z plane. The tape is shown in a simplified form having an HTS layer 501, a copper cladding 502, and a substrate 503. The crystalline structure of ReBCO has three orthogonal principal axes, referred to in the art as a, b, and c. In Figure 5, the a / b plane of the ReBCO layer 501 is shown as a single line 510 perpendicular to the c axis 520.

[0011] The critical current (and generation temperature) of a tape depends on the thickness and quality of the ReBCO crystal, the temperature and magnitude of the external magnetic field, and the orientation of the magnetic field relative to the c-axis. Typically, when the applied magnetic field vector is in the a / b plane 510, the critical current is considerably higher than when the applied magnetic field vector is aligned along the c-axis 520, and the variation between these two extremes is smooth. In practice, there may be two or more angles that show the peak, variation due to temperature and magnetic field, and / or variation within the a / b plane, but for a simple analysis, we can consider a tape with a single dominant peak that defines the optimal orientation of the applied magnetic field (B) that gives the maximum critical current. ReBCO tapes are usually manufactured so that the c-axis is as close to perpendicular to the plane of the tape as possible (i.e., parallel to the z-axis) so that the critical current is maximized when the applied magnetic field is parallel to the plane of the tape.

[0012] Because the DC resistance in the superconducting state is zero, direct current (DC) power losses in superconducting materials are generally negligible. However, power losses can still occur when superconducting materials experience time-varying magnetic or electric fields. These losses are sometimes referred to as "AC losses," but are referred to herein as "time-varying magnetic field losses" because they can be caused by dynamic or transient changes in the magnetic or electric field. Such losses are a significant problem for superconducting materials because they can generate heat, raising the temperature of the superconducting material above its current shunt temperature. Above the current shunt temperature, the current is forced to flow from the superconducting region of the material into adjacent conventional conductive material. This resistance heating of the conventional conductive material can then cause hot spots in superconducting magnets where the temperature rises dramatically. In such cases, the magnet needs to be rapidly heated or "quenched" to avoid damage. Therefore, even small amounts of power generated by time-varying magnetic field losses may be unacceptable for superconductor-based magnets.

[0013] Several mechanisms can contribute to time-varying magnetic field losses in superconducting materials. One type of time-varying magnetic field loss is called "hysteresis loss," which can occur when fluctuations in the transport current flowing through the superconducting material, or changes in the applied magnetic field, induce a voltage that drives a current loop (shielding current) within the superconducting material. Another type of time-varying magnetic field loss is called "coupling loss," which occurs when a changing magnetic field induces a voltage between two superconducting elements that drives a current through a resistive interface, or when two elements are joined together.

[0014] Hysteresis loss depends on the area of ​​superconducting material that can carry the current loop. Because typical HTS tapes have a width of several millimeters and consequently a large area, hysteresis loss is a significant factor in HTS tapes that experience rapidly changing electric or magnetic fields, such as when ramping up an HTS field coil. Using narrower HTS tape pieces reduces the cross-sectional area but also reduces the tape's current-carrying capacity. Coils wound from narrower tapes cannot generate as strong a magnetic field. To form a magnet of equivalent strength, it is typically necessary to join multiple coils via a resistive interface, which increases coupling losses. HTS tape pieces can be strung along their length to form multiple filaments connected in parallel, but strung tapes typically have high coupling losses between filaments. Cables can also be twisted or created by spirally winding HTS tape around a core, which reduces the effective area that can induce current through a changing magnetic field. One type of twisted cable is the "Conductor on Round Core" (CORC®) cable. When the cable is twisted again, an undesirable orientation is created between the c-axis of the superconducting material and the applied magnetic field, thus limiting the current capacity of the cable. [Prior art documents] [Patent Documents]

[0015] [Patent Document 1] International Publication No. 2019150123 [Overview of the project] [Problems that the invention aims to solve]

[0016] What is needed is a superconducting cable or other current-carrying assembly, particularly one made from HTS tape, that reduces both hysteresis loss and coupling loss (i.e., time-varying magnetic field loss) without sacrificing current-carrying capacity. [Means for solving the problem]

[0017] According to a first aspect of the present invention, an electrical cable is provided having two ends and including a plurality of subcables extending between the two ends of the cable. The subcables are electrically insulated from one another along their length, and each subcable provides its own alternative current path between the two ends of the cable. Each subcable comprises a superconducting portion comprising a plurality of HTS tapes containing a high-temperature superconductor (HTS) material (e.g., ReBCO), and a resistor (e.g., at least one resistor, two or more resistors, etc.) connected in series with the superconducting portion.

[0018] By including resistors in the sub-cables, it may be possible to suppress the induced current flowing between pairs of sub-cables when the cable is used in an electrical circuit. In particular, the electrical cable may be configured such that the current induced to flow through the cable between any two of the sub-cables (i.e., from one sub-cable to another within the cable) is required to flow through the resistors of the two sub-cables, i.e., such that there is no conductive loop or closed path within the cable that includes two sub-cables and does not pass through two or more of the resistors. Thereby, the coupling current between the sub-cables can be suppressed, the heating of the sub-cables can be made smaller, and the operation of the superconducting part can be made more stable. For example, the shielding current can generally be separated individually for each sub-cable. Compared with a cable of equal width having a single sub-cable, the total amount of time-varying magnetic field losses (i.e., coupling losses and hysteresis losses) in the electrical cable of the present disclosure can be reduced by a factor equal to or proportional to the number of sub-cables within the cable. For example, for a certain amount of heating P (watts), the total amount of heating in a cable having n sub-cables can be reduced to about P / n.

[0019] Generally, each sub-cable provides a conductive path that enables current to pass from one end of the cable to the other end of the cable. A single power source can be used to supply current to all the sub-cables simultaneously. In some cases, each of the sub-cables may be substantially equivalent such that the current is substantially equally divided between the sub-cables

[0020] Each sub-cable may include two resistors (i.e., a resistor and another resistor that may be the same or different from that resistor) connected in series with the superconducting part of the cable at both ends of the superconducting part of the cable. This arrangement enables heat to be dissipated from both ends of the cable, thereby reducing (e.g., halving) the cooling power required to be supplied to any one end of the cable.

[0021] Each resistor may be connected to the superconducting portion of the corresponding subcable by its respective current lead. This arrangement reduces the thermal load on the superconducting portion and facilitates cooling the resistors separately from the superconducting portion. To increase the proportion of power dissipated by the resistors, each resistor may have, or can be configured to have, a significantly larger resistance than the resistance of the current lead. In some implementations, the resistors may have a resistance of 0.1 to 100 milliohms, or 1 to 10 milliohms. The current leads may have a larger cross-sectional area than the superconducting element; for example, the current leads may have a larger diameter than the diameter of the superconducting element.

[0022] The electrical cable may further include a cooling mechanism, such as a cryostat, to cool the superconducting portion of each subcable to a temperature below the superconducting transition temperature of the HTS material in the HTS tape, while the resistor remains above the superconducting transition temperature of the HTS material. This configuration has the advantage that at least some coupling current can be dissipated in the resistor at a temperature higher than the superconducting transition temperature (using another cooling mechanism, e.g., liquid nitrogen or water cooling), thereby reducing the thermal load on the cooling mechanism used to maintain the HTS material in a superconducting state. For example, the HTS material may be maintained at a temperature of about 20K (or less), while the resistor is maintained at a temperature of about 77K (achievable with liquid nitrogen cooling), or room temperature, e.g., 300K (or more).

[0023] The superconducting portion of the sub-cable can be elongated with a maximum dimension (length) extending along the cable (as a whole). Each superconducting portion may be untwisted so as not to follow a spiral or helical path within the cable (although the cable itself may be bent or, for example, wound around an axis to form a coil). For example, the orientation of the HTS tape in the superconducting portion may remain substantially constant with respect to other portions of the sub-cable. For example, when cross-sections of the sub-cable are taken at any two points along its length, a vector perpendicular to the HTS tape in the first of the cross-sections may be substantially parallel to a vector perpendicular to the HTS tape in the second of the cross-sections.

[0024] The fact that the twist of the superconducting portion of the sub-cable is untwisted means that the HTS tape can have a constant (or nearly constant) orientation with respect to the magnetic field. In particular, in use, each superconducting portion can be arranged such that one or more major crystal axes of the HTS material have an orientation with respect to the magnetic field that remains at least substantially constant along the length of the superconducting portion (i.e., different by less than a predetermined absolute or relative amount). For example, the superconducting portion of the sub-cable may be wound around a coil (e.g., a solenoid or pancake coil) such that the major crystal axis of the HTS material is oriented perpendicular to the winding. For example, if the winding is around the axis of the coil, the major crystal axis may be oriented radially with respect to the coil axis. When the HTS material is ReBCO, such an orientation allows the magnetic field vector of the magnetic field generated by the coil to be (at least mainly) in the a / b plane of the superconducting material, and an optimal (or nearly optimal) maximum critical current can be obtained. In contrast, in the case of a cable where the sub-cables are twisted (twisted with each other and / or individually), the orientation of the HTS material varies along the length of the sub-cable, which can reduce the critical current of the HTS material as a result of the anisotropic critical current dependence of the material on the angle between the major crystal axis and the magnetic field. Therefore, the HTS material of this electrical cable can operate at a higher current compared to the HTS material of a twisted cable.

[0025] Therefore, in some implementations, the HTS tapes of each subcable may be placed in a stack and (effectively) untwisted.

[0026] Alternatively, in some implementations, the superconducting portions of the subcable may be twisted together or individually to further reduce losses. The superconducting portions may have any twist pitch length suitable for the material used, for example, the twist pitch length may be 2 to 10 times the width of the HTS tape in the direction transverse to the length of the HTS tape. The twist pitch length refers to the length measured along the cable where the HTS tapes rotate completely around each other (i.e., longitudinally with respect to the cable), i.e., the distance over which the relative orientation of the HTS tapes is repeated. In some applications, it may be preferable to have a relatively long twist pitch length, for example, more than 5 times, 10 times, or 100 times the width of the HTS tape in the direction transverse to the length of the HTS tape, in order to avoid problems associated with tightly twisting the HTS tape. In some examples, the twist pitch length may range from 10 to 1000 times, 50 to 100 times, or 500 to 1000 times the width of the HTS tape. Such cables are easier to manufacture and / or may have less strain compared to tightly twisted cables. In addition, the superconducting portions may be twisted together for only a small portion of the twist pitch length (e.g., less than 10%, less than 20%, or less than 50%), thereby allowing the HTS tape to have a certain orientation with respect to the applied magnetic field for the remainder of the twist pitch length, i.e., the superconducting portions may be untwisted together for more than 90%, more than 80%, or more than 50% of their length.

[0027] Subcables can be electrically insulated from one another along their length by an electrically insulating material (e.g., polyimide film, epoxy resin, or suitable potting or encapsulation compound) placed between them.

[0028] One or more of the subcables (for example, each of them) may be equipped with a diode configured to allow current to flow preferentially through the subcable in only one direction. Each diode limits the "reverse flow" of current through the subcable, further reducing the magnitude of coupled current flowing between the subcables and reducing the total power loss due to such current in the cable. In configurations in which two or more of the subcables are equipped with diodes, the direction in which current preferentially flows through the diodes is preferably the same for each of the subcables.

[0029] In some implementations, each superconducting portion is arranged to form a turn in a superconducting magnet coil. For example, the coil may be formed by winding the superconducting portion around an axis. As described above, the superconducting portions are preferably untwisted with each other, which may allow the HTS material to be aligned with a magnetic field (e.g., a solenoid magnetic field) generated by the coil, for example, to optimize the critical current of the HTS material and / or limit the amount of hysteresis loss (which may be generated, for example, by a magnetic field component perpendicular to the ab-plane of ReBCO).

[0030] In some implementations, the electrical cable is configured such that when the subcables experience a time-varying electromagnetic field during use, the induced current flowing between the subcables flows through at least one resistor in the subcable, and / or the induced current flowing between pairs of subcables is suppressed when the cable is used in an electrical circuit.

[0031] In some implementations, an electrical cable can be configured such that any closed loop within the electrical cable, which includes two of several sub-cables, includes the resistance of each of the two sub-cables.

[0032] In some implementations, the superconducting portion of each subcable may be wound around its respective magnetic field coil, and the magnetic field coils are positioned adjacent to each other and electrically insulated from one another.

[0033] In some implementations, the superconducting portion of each subcable is positioned such that the planes of each HTS tape in each subcable are substantially parallel to each other at each point along the length of the energizer.

[0034] In some implementations, each superconducting portion is wound around a magnetic field coil, and the plane of each subcable or each HTS tape is substantially aligned with the magnetic field generated when the field coil is used.

[0035] One or more of the resistors (e.g., each of them) may be a variable resistor. A variable resistor can be used to adjust the resistance of the subcables and control the magnitude of the coupled current that can flow between them. For example, the resistance of each subcable can be increased to decrease the magnitude of the coupled current as the current through the cable increases. The resistance of each subcable may then be reduced once a steady-state current through the cable is achieved. Alternatively or additionally, a variable resistor may be used to preferentially direct current to subcables that are substantially the same voltage as each other, so as to reduce the magnitude of any coupled current flowing between those subcables. A variable resistor may also be used to reduce the total current flowing through a damaged and / or underperforming subcable (e.g., having a lower critical current), thereby diverting the current through an undamaged and / or satisfactorily performing subcable. In some implementations, the resistance of a variable resistor can be adjusted according to a measured parameter of one or more of the subcables, such as critical current or temperature.

[0036] A second aspect of the present invention provides a superconducting magnet comprising an electrical cable according to the first aspect of the present invention. The superconducting portion of the subcable is wound around at least one coil, and for example, the magnet may comprise multiple turns or windings of each superconducting portion around an axis. The coil may be, for example, a solenoid or a pancake coil (in which the windings or turns of the coil are nested radially).

[0037] A third aspect of the present invention provides a system comprising an electrical cable according to the first aspect, or a superconducting magnet according to the second aspect, wherein one or more of the resistors are variable resistors. The system further comprises a controller for adjusting the resistance of each of the variable resistors. The controller may be configured to measure the voltage and / or current flowing through each of the subcables and to adjust the resistance of one or more of the variable resistors in response to the measured voltage and / or current difference between two or more of the subcables. The cable may be wound, for example, in a coil.

[0038] The system may further include adjustable power supplies connected to both ends of the electrical cable, and the controller is configured to adjust the voltage applied by the power supply to both ends of the electrical cable and / or the current supplied to the electrical cable by the power supply in response to the adjustment of the resistance of a variable resistor. For example, the voltage can be increased according to the total resistance of the cable to maintain a constant total current flowing through the cable.

[0039] A fourth aspect of the present invention provides a method for operating an electrical cable according to the first aspect described above. This method includes applying a voltage difference across both ends of an electrical cable to drive a current through the electrical cable, i.e., from one end of the cable to the other. The method further includes adjusting the resistance of one or more variable resistors as a function of the time after the voltage difference has been applied across both ends of the electrical cable, the current flowing through the electrical cable or through one or more of the sub-cables, the magnetic field generated by the electrical cable, and / or the voltage difference across both ends of the superconducting cable or the pair of sub-cables.

[0040] Adjusting the resistance of one or more variable resistors may involve decreasing the resistance of one or more variable resistors after the current flowing through the electrical cable and / or the magnetic field generated by the electrical cable have reached a steady state (for example, a value within a predetermined range of the target value, or a state in which the variation of the value is less than a predetermined percentage of the value).

[0041] Adjusting the resistance of one or more of the variable resistors may include adjusting the resistance of one or more of the variable resistors to bypass the current by preferentially passing through one or more of the subcables. For example, the current can be bypassed from subcables with significant coupling losses (determined, for example, by the voltage of the subcable) and through subcables with smaller coupling losses.

[0042] A further aspect of the present invention provides a superconducting cable (or current-carrying device) comprising a plurality of sub-cables extending between the ends of a cable and electrically insulated from one another along their length. The sub-cables are arranged parallel to each other so as to provide each an alternative path for current to flow between the ends of the cable. Each sub-cable comprises a superconducting element comprising a superconducting material arranged to conduct current along the cable, and at least one resistor connected in series with the superconducting element. This suppresses induced current flowing between pairs of sub-cables when the cable is used in an electrical circuit.

[0043] In another embodiment, the present invention provides a current-carrying assembly comprising a plurality of subcables, the subcables being electrically connected to one another via shared connections at both ends of the current-carrying assembly, or connectable in parallel to one another, and otherwise electrically insulated from one another, and each subcable comprising a superconducting portion comprising a superconducting material and a resistor in series with the superconducting portion, so that an induced current that flows, or would flow, between the subcables through the connection when the subcables experience a time-varying electromagnetic field during use flows through at least one resistor of the subcable. Optionally, the superconducting portion comprises at least one tape or flat wire, and each superconducting portion of the subcable is arranged adjacent to one another such that the respective planes of each subcable or each tape are substantially parallel to or aligned with each other at each point along the length of the current-carrying assembly. In other words, the tapes are parallel when viewed in cross-section of any given plane perpendicular to the tangent to the current-carrying device at that point. [Brief explanation of the drawing]

[0044] [Figure 1] This is a schematic diagram of an HTS tape. [Figure 2] This is a schematic diagram of a wound HTS coil. [Figure 3] This is a schematic diagram of the HTS coil section. [Figure 4] This figure shows the coordinate system used to explain the ReBCO tape. [Figure 5] This is a cross-sectional view of a ReBCO tape, showing the main axis of the ReBCO crystal. [Figure 6] This is a schematic circuit diagram showing an embodiment of the present invention. [Figure 7] This is a schematic circuit diagram showing another embodiment of the present invention. [Figure 8] This is a schematic cross-sectional view of multiple sub-cable superconductor sections arranged within a cable. [Modes for carrying out the invention]

[0045] Embodiments of the present invention reduce hysteresis losses in an HTS superconducting cable by arranging the HTS material as multiple subcables. This arrangement reduces the available area of ​​the HTS material where shielding currents can form compared to a full-width cable. Coupling currents between subcables are suppressed by electrically isolating the subcables from each other along their length. The subcables are electrically connected in parallel with each other, and loop currents (with associated losses) can still occur between pairs of subcables through their shared connections. These loop currents are suppressed by introducing a resistor (at least one resistor) into each subcable such that losses occur substantially only in the resistor.

[0046] Overall, embodiments of the present invention significantly reduce coupling between subcables and isolate shielding currents occurring within (narrow) subcables rather than across the full width of the cable. By reducing time-varying magnetic field losses in this way, it may be possible to avoid other (potentially unfavorable) methods of reducing this type of loss, such as twisting the subcables. Thus, embodiments of the present invention reduce or avoid the trade-off between hysteresis loss and coupling current in existing superconducting cables. This method is particularly suitable for HTS superconducting materials in the form of tape (or flat wire) because the tape can be aligned with the direction of the magnetic field generated by the field coils wound from the cable, thereby minimizing the reduction in critical current caused by unfavorable orientation.

[0047] Figure 6 shows a circuit comprising a superconducting cable 602 having multiple sub-cables 602A-C electrically connected in parallel between the two ends of the cable 602. A power supply is connected across the two ends of the cable 602 and drives the current through each of the sub-cables 602A-C. Since each sub-cable 602A-C is electrically isolated from the other sub-cables along its length, the current flow between the sub-cables caused by the potential difference between them always flows through the ends of the cable 602 to which the sub-cables are connected, rather than through any intermediate part of the cable 602 located between the ends of the cable 602. Each sub-cable 602A-C can be considered as a single sub-cable (or one containing a single sub-cable), and multiple sub-cables together form a cable, or generally, a current-carrying assembly. The cable 602 shown in Figure 6 has three sub-cables 602A-C, but generally, cable 602 can have any integer number of sub-cables, such as 2, 5, 10, 12, or 20. In some cases, each sub-cable may experience essentially the same rate of change of magnetic field (dB / dt), and thus the voltage difference between the cables is expected to be small. In the absence of resistors, even such a small voltage difference can induce a large current to flow between the sub-cables. However, in the circuit of Figure 6, these currents are limited by resistors in the sub-cables, thereby reducing the resulting power loss.

[0048] Each subcable comprises several components connected in series, which include (from left to right in the figure) a first resistor 606, a first current lead 608, a superconducting portion 610, a second current lead 612, and a second resistor 614. The first and second current leads 608 and 612 may include ordinary conductive materials, such as copper wire, or superconducting materials, but generally any form of conductor, such as superconducting wire, can be used.

[0049] Each superconducting portion 610 includes an HTS material (such as ReBCO) in the form of an elongated body, such as a wire or tape. In the superconducting cable 602 of Figure 6, each superconducting portion 610 comprises a single HTS tape, but each superconducting portion 610 may alternatively comprise multiple HTS tapes connected to one another in parallel or in series (or in some combination of series and parallel configurations). Each superconducting portion 610 may be arranged, for example, as a stack of HTS tapes.

[0050] The resistors are configured to reduce the magnitude of the coupling current flowing between the pair of subcables 602A and 602C as a result of the voltage difference between them. An example of such a coupling current is shown in Figure 6 by dashed arrows placed adjacent to the two subcables 602A and 602B. The coupling current spreads over the length of subcables 602A and 602B and flows in a closed loop passing through each of the resistors 606 and 614 at each end of subcables 602A and 602B. By including resistors 606 and 614 in the superconducting cable 602, the magnitude of the coupling current that can flow for a given voltage difference between subcables 602A and 602B is limited. The resistance of each resistor 606 and 614 is preferably greater than the resistance of each of the current leads 608 and 612, so that the heating resulting from the coupling current occurs mainly in the resistors 606 and 614 rather than in the current leads 608 and 612. The resistances of each resistor 606, 614 are also preferably selected or adjusted so that the supplied current is divided substantially equally between subcables 602A-C. Conveniently, for example, each resistance is substantially equal, and all other elements of each subcable 602A-C are substantially identical. More generally, the total resistance of each subcable 602A-C is substantially the same (though not mandatory) to achieve uniform current division between the subcables. The resistance values ​​are selected at least in part according to the desired power dissipation in use and are typically in the range of, for example, 0.1 to 100 milliohms.

[0051] The superconducting portion 610 of the subcable is placed in a desired arrangement configuration to collectively form the superconducting section 616 of the cable 602. The superconducting section 616 may be arranged, for example, as a field coil wound from adjacent subcables 602A-C. The field coil may be wound in the form of a pancake coil, a solenoid, or any other suitable arrangement configuration. Alternatively, each subcable 602A-C may be wound into a separate field coil (e.g., a pancake coil), and the field coils may be placed adjacent to each other to form a composite magnet.

[0052] If each superconducting portion 610 comprises one or more substantially flat tapes, the superconducting portions may be arranged adjacent to one another such that, at each point along the length of the superconducting section 616, all tapes are substantially aligned and lie in substantially the same plane. Thus, each individual tape, and / or the a / b plane of each tape, can all be substantially aligned with the magnetic field generated by the field coil wound from the cable. For example, in a pancake coil, all vectors perpendicular to the tape can lie in the plane of the coil, or in a linear solenoid, all vectors perpendicular to the tape can be perpendicular to the axis of the solenoid.

[0053] Furthermore, the superconducting section 616 can have a desired width formed by multiple subcables 602A-C arranged side by side. The resulting arrangement has a current density similar to that of a full-width cable, but the hysteresis loss is reduced by approximately equal to the number of subcables.

[0054] Accordingly, embodiments of the present invention provide a cable in which subcables can be substantially untwisted along their length with respect to the magnetic field direction. This allows the superconducting material within each subcable to have a larger critical current, and thus can operate at a higher transport current than a twisted tape cable, enabling the generation of a stronger magnetic field. At the same time, time-varying magnetic field losses are reduced compared to a cable without subcables and having the same or equivalent width, thereby allowing the cable embodying the present invention to be used with magnets that are intended to rapidly ramp up or ramp down, or otherwise be subjected to changing electromagnetic fields.

[0055] During use, the superconducting portion 610 is located within a cryogenic cooling region 618, such as within a cryostat, which maintains the superconducting material at a temperature below the superconducting transition temperature of the superconducting material (e.g., about 20K for some HTS materials) (however, other types of cooling systems may also be used). The resistors 606, 614 may be located outside the cryostat or within a higher temperature region of the cryostat to reduce the thermal load on the cryostat resulting from the power dissipated within the resistors 606, 614. For example, the resistors 606, 614 may be located in a region at substantially room temperature (or at least above 273K) and cooled with water.

[0056] In some implementations, some or all of the sub-cables 602A-C may consist of only a single resistor 606, 614. Alternatively, some or all of the sub-cables may consist of two or more resistors 606, 614, which is advantageous as it allows for more uniform heat distribution between the resistors and reduces the cooling power required for each resistor.

[0057] Figure 7 shows circuit 700, which is substantially the same as circuit 600 in Figure 6, except that each subcable of the superconducting cable 702 comprises a variable resistor 706 and a diode 714 (in contrast to constant resistors 606, 614). Circuit 700 also includes a controller 720 configured to adjust the resistance of the variable resistor 706 in response to the measured current flowing through each subcable and / or the measured voltage between or along the subcables. The controller may be, for example, a feedback controller (e.g., a proportional integral derivative (PID) controller). In some implementations, the controller 720 can adjust some or all of the resistance of the variable resistor 706 to ensure that any voltage difference between or along the subcables remains below a predetermined maximum voltage difference. Furthermore, the resistance of the variable resistor 706 can decrease to zero (or approach zero) when the superconducting cable 702 is not subjected to a time-varying magnetic field, for example, when the field coil is maintained in a steady state rather than being ramped up or ramped down.

[0058] The diode 714 included in the subcable can further reduce the coupling current flowing between the subcables, but the inclusion of the diode may require a higher voltage to be generated by the power supply 604 to drive the current through cable 702. In some cases, a diode may be included instead of one or more of the resistors (for example, in some cases, each of the resistors may be replaced by one or more diodes).

[0059] It may be possible to adjust the current flowing through the superconducting cables 602 and 702 by varying the voltage of power supply 604. For example, the voltage of power supply 604 may increase in response to an increase in the resistance of variable resistor 714. In some cases, the voltage of power supply 604 can be adjusted using controller 720 to ensure that the total current flowing through cable 702 remains constant (i.e., stays within a predetermined target range).

[0060] Figure 8 shows a cross-sectional view of a superconducting section 800 of superconducting cables 602, 702, including subcables, providing multiple superconducting portions 802. Each superconducting portion 802 comprises a support structure having a channel 804, which may be substantially U-shaped, for housing a stack 806 of HTS tape. The channel 804 may include, for example, copper and / or aluminum, or another metal, or may be formed from a suitable material. The HTS tape may be, for example, ReBCO tape. In some implementations, the channel 804 may include an insulating layer for electrically insulating the stack 806 of HTS tape from the channel 804 and / or adjacent channels from each other. The depth of the channel 804 and the thickness of the stack 806 can be adjusted according to the required operating current and inductance. In some examples, the channel 804 can provide a normal conductive path for current when the superconducting material becomes non-superconducting, for example in the case of quenching.

[0061] The superconducting portions 802 can be arranged in an array of superconducting portions 802, such as a 2×6 array, as shown in Figure 8, or in any other suitable arrangement configuration, with or without the presence of channels 804 or other support structures. The superconducting portions 802 may be spaced apart from each other by spaces filled with an insulating material such as epoxy resin. The insulating material can further provide structural support to the superconducting portions 802.

[0062] The superconducting cables described above can be wound into coils to produce superconducting electromagnets. Such electromagnets can be used in many different applications, including superconductor magnetic energy storage (SMES, for example, for power system stabilization), plasma confinement and control in tokamaks or stellarators, magnetic resonance imaging (MRI) and / or NMR spectroscopy, hadron therapy, and aerospace applications in aircraft, unmanned aerial vehicles, satellites, spacecraft, rocket-powered vehicles, and autonomous exploration vehicles.

Claims

1. An electrical cable having two ends, comprising a plurality of sub-cables extending between the two ends of the cable, wherein the sub-cables are electrically insulated from one another along their length, and each sub-cable provides its own alternative current path between the two ends of the cable, and each sub-cable is A superconducting portion comprising multiple HTS tapes containing high-temperature superconductor (HTS) material, A resistor connected in series with the superconductor portion and An electrical cable equipped with [a specific feature / feature].

2. The electrical cable according to claim 1, wherein each subcable is provided with two resistors connected in series with the superconducting portion at both ends of the superconducting portion of the subcable.

3. The electrical cable according to claim 1 or 2, wherein each resistor is connected by its respective current lead to the superconducting portion of the corresponding subcable.

4. The electrical cable according to claim 3, wherein each resistor has, or can be configured to have, a resistance greater than or equal to the resistance of the corresponding current lead.

5. The electrical cable according to any one of claims 1 to 4, further comprising a cooling mechanism for cooling the superconducting portion of each subcable to a temperature below the superconducting transition temperature of the HTS material of the HTS tape, while each resistor remains at a temperature above the superconducting transition temperature of the HTS material.

6. The electrical cable according to any one of claims 1 to 5, wherein the superconducting portions of each subcable are arranged parallel to each other and substantially untwisted with each other.

7. The electrical cable according to any one of claims 1 to 6, wherein the HTS tapes of each subcable are arranged in a stack and substantially untwisted.

8. The electrical cable according to any one of claims 1 to 7, wherein one or more of the sub-cables are equipped with diodes.

9. The electrical cable according to any one of claims 1 to 8, wherein each superconducting portion is arranged to form a turn of a superconducting magnet coil.

10. When the subcable experiences a time-varying electromagnetic field during use, the induced current flowing between the subcables flows through at least one resistor of the subcable, and / or When the cable is used in an electrical circuit, the induced current flowing between the pair of sub-cables is suppressed. An electrical cable according to any one of claims 1 to 9, configured as described above.

11. The electrical cable according to any one of claims 1 to 10, wherein any closed loop in the electrical cable including two of the plurality of subcables includes the resistance of each of the two subcables.

12. The electrical cable according to any one of claims 1 to 11, wherein the superconducting portion of each subcable is wound around its respective magnetic field coil, the magnetic field coils are arranged adjacent to each other and electrically insulated from each other.

13. The electrical cable according to any one of claims 1 to 12, wherein the superconducting portion of each subcable is arranged such that the planes of each HTS tape of each subcable are substantially parallel to each other at each point along the length of the energizing device.

14. The electrical cable according to any one of claims 1 to 13, wherein each superconducting portion is wound around a magnetic field coil, and the plane of each subcable or each HTS tape is substantially aligned with the magnetic field generated when the field coil is used.

15. The electrical cable according to any one of claims 1 to 14, wherein at least one of the resistors in the subcable is a variable resistor.

16. A superconducting magnet comprising an electrical cable according to any one of claims 1 to 15, wherein the superconducting portion is wound around at least one coil.

17. A system comprising an electrical cable according to claim 15, or, when dependent on claim 15, a superconducting magnet according to claim 16, and a controller for adjusting the resistance of each of the one or more variable resistors.

18. The system according to claim 17, wherein the controller is configured to measure the voltage and / or current flowing through each of the subcables and to adjust the resistance of one or more of the variable resistors in response to the voltage and / or current difference measured between two or more of the subcables.

19. The system according to claim 17 or 18, further comprising an adjustable power supply connected to both ends of the electrical cable, wherein the controller is configured to adjust the voltage applied to both ends of the electrical cable by the power supply and / or the current supplied to the electrical cable by the power supply in response to the adjustment of the resistance of the variable resistor.

20. A method for operating an electrical cable according to claim 15, or, when dependent on claim 15, a superconducting magnet according to claim 16, In order to drive the current flowing through the aforementioned electrical cable, a voltage difference is applied to both ends of the electrical cable, Adjusting the resistance of one or more of the variable resistors as a function of at least one or more of the following: the time after the voltage difference is applied to both ends of the electrical cable, the current flowing through the electrical cable or the current flowing through one or more of the sub-cables, the magnetic field generated by the electrical cable, and / or the voltage difference between both ends of the electrical cable or the pair of sub-cables. Methods that include...

21. The method according to claim 20, wherein adjusting the resistance of one or more of the variable resistors reduces the resistance of one or more of the variable resistors after the current flowing through the electrical cable and / or the magnetic field generated by the electrical cable have reached a steady state.

22. The method according to claim 20 or 21, wherein adjusting the resistance of one or more of the variable resistors is performed such that the current is diverted preferentially through one or more of the subcables.

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