Current leads for superconducting magnets

Abstract

A current lead arrangement for providing current to a superconducting magnet coil (10), the current lead arrangement comprising a current lead (14) and a cryogenic refrigerator (16). The current lead (14) comprises a first portion of a low temperature superconductor (LTS) wire (14a), which is bonded to a second portion of a high temperature superconductor (HTS) material (14b), which is further bonded to a third portion of a resistive material (14c). The cryogenic refrigerator (16) comprises a first cooling stage (18) and a second cooling stage (20). The lower end of the third portion (14c) and the upper end of the second portion (14b) are thermally coupled to the first cooling stage (18), and the lower end of the first portion (14a) is thermally and electrically coupled to the superconducting magnet coil (10).

Description

Technical Field

[0001] The present invention relates to superconducting magnets and, in particular, to current leads for carrying electric current between a superconducting magnet and a current source which is itself at room temperature. Background Art

[0002] Currently known superconducting materials are generally divided into "low temperature superconductors" (Low Temperature Superconductors, LTS) with a superconducting transition temperature below about 20K and "high temperature superconductors" (High Temperature Superconductors, HTS) with a superconducting transition temperature above about 20K. Summary of the invention

[0003] The present invention is particularly directed to superconducting magnets comprising coils of superconducting wires having LTS cores, but the present invention may be applicable to superconducting magnets comprising coils of superconducting wires having HTS cores. Typically, both superconducting wires having LTS cores and superconducting wires having HTS cores have copper or aluminum stabilizers and may benefit from the application of the present invention.

[0004] Figure 1 A conventional arrangement for cooling superconducting magnet coils and providing current connections to and from the magnet coils is schematically shown. Figure 1 As shown in , a magnet coil 10 consisting of a coil of superconducting wire, which itself includes one or more superconducting filaments in the form of a non-superconducting metal matrix, such as copper or aluminum, is connected to an electrical conductor 12 by a current lead 14. The current lead 14 includes a first portion 14a of an LTS superconductor, a second portion 14b of an HTS superconductor and a third portion of a non-superconducting conductor 14c such as copper, brass or aluminum. Typically, two such current lead arrangements will be provided, one for supplying current to the magnet coil 10 and one for providing a return path for current to a current supply not shown, but for ease of representation only one current lead arrangement is shown. The LTS portion 14a typically includes a superconducting wire having a stable matrix of copper or aluminum, in which a plurality of superconducting cores are provided.

[0005] The superconducting magnet coil 10 is cooled by a refrigerator 16. The refrigerator 16 has a first cooling stage 18 that cools to a first cryogenic temperature, typically in the range of 25 K to 80 K, and also has a second cooling stage 20 that cools to a second cryogenic temperature, typically about 4 K. The first cooling stage typically cools a thermal radiation shield within the cryostat, and the second cooling stage typically cools the superconducting magnet coil 10 to an operating temperature.

[0006] Thus, in operation, the third portion of the non-superconducting conductor 14 c of the current lead 14 extends between room temperature of about 300 K and a first cryogenic temperature of 25 K to 80 K. The rate of heat transfer through the third portion of the non-superconducting conductor 14 c of the current lead 14 will be determined by the material, length and cross-sectional area of ​​the third portion and the temperature difference between the two ends of the third portion.

[0007] In operation, the second portion 14b of the current lead 14 extends between a first cryogenic temperature of 25 K to 80 K and a second cryogenic temperature of about 4 K. The heat transfer rate through the second portion 14b of the current lead 14 will be determined by the material, length and cross-sectional area of ​​the second portion and the temperature difference between the two ends of the second portion.

[0008] In operation, the first portion 14a of the current lead 14 extends between the second cooling stage 20 at a second cryogenic temperature of about 4 K and the magnet coil 10, also at about 4 K. The rate of heat transfer through the first portion 14a of the current lead 14 will be determined by the material, length and cross-sectional area of ​​the third portion, but should be minimal because there should be very little temperature difference between the two ends of the third portion.

[0009] A first thermal link 22 of a conductive material, such as a solid or fluid of high purity aluminum or high purity copper or a thermosyphon or a combination thereof, thermally links the second cooling stage 20 to the magnet coil 10. Preferably, and as shown, a thermal switch 24 may be placed in the thermal path between the second cooling stage 20 and the magnet coil 10. Examples of suitable thermal switches 24 include a thermosyphon, a heat pipe, an air gap, a solid (e.g., a reluctance), or a mechanical switch.

[0010] HTS current leads, such as the second portion 14b of the current lead 14, are required for low cryogen and "dry" (no liquid cryogen path) superconducting magnet systems so that current can be transferred into and out of the superconducting magnet with minimal dissipation. As described above, and typically, the top of the HTS portion 14b is thermally anchored to the first cooling stage 18 of the cryogenic refrigerator 16 by a high thermal conductivity link, while the lower end of the HTS portion is thermally anchored to the second cooling stage 20 of the cryogenic refrigerator 16 by a high thermal conductivity link. The lower end of the HTS portion is also thermally linked to the magnet coil 10 by a high conductivity portion 14a, which is typically an LTS wire.

[0011] Heat flowing from the electrical conductor 12 through the third portion of the non-superconducting conductor 14 c of the current lead 14 at room temperature is intercepted and removed by the first cooling stage 18 of the refrigerator 16 .

[0012] Heat flowing down the second portion 14 b of the current lead 14 from the first cooling stage 18 of the refrigerator at 25 K to 80 K is intercepted and removed by the second cooling stage 20 of the refrigerator 16 .

[0013] In the event of a refrigerator 16 failure, the second cooling stage 20 of the refrigerator will warm up rapidly because of the low heat capacity of the material at the operating temperature of the second stage. In the event that the refrigerator 16 does not operate for any reason, heat from the room temperature end will be conducted through the material of the refrigerator to the second cooling stage 20 of the refrigerator. Although the thermal switch 24 preferably blocks heat conduction from the second cooling stage 20 through the first thermal link 22 to the magnetic coil 10 in the event of a refrigerator failure, heat will still be conducted from the second cooling stage 20 to the magnet coil 10 through the first portion 14a, i.e., the LTS portion, of the current lead 14. The first cooling stage 18 of the refrigerator will also warm up significantly, and heat from this first cooling stage 18 will be conducted down through the HTS portion 14b of the current lead 14 to the LTS portion, i.e., the first portion 14a, of the current lead 14, and thence to the magnet coil 10. Typically, an LTS wire, such as may be used for the LTS portion 14a of the current lead 14, comprises a plurality of superconductor filaments of small cross-section and a stable matrix material, such as aluminum or copper, of significantly larger cross-section and having considerable thermal conductivity. These heat flows to the magnet coils 10 will cause quenches, which result in lengthy shutdowns of the superconducting magnet, because the magnet coils 10 must be recooled before current can be introduced, and heat transfer at such low temperatures is often inefficient.

[0014] Conventional methods to avoid this magnet quench in the event of a refrigerator failure include:

[0015] - Providing a backup power supply and cooling water can be used to overcome the effects of a service failure and keep the chiller running. However, this arrangement is expensive because a large uninterrupted power supply or generator is required, as well as backup water chillers or air cooling compressors.

[0016] Heat capacity may be added to the magnet coils 10 to limit the temperature rise in response to a given heat influx. Example arrangements may employ a liquid cryogen such as helium, a solid with a high heat capacity such as a rare earth metal, or a more common material of greater mass such as copper. However, such materials may be very expensive and may require a large mass to significantly change the heat capacity of the magnet coils 10.

[0017] - For small magnet systems, one can simply accept that the superconducting magnet coils 10 will quench within a few minutes after the refrigerator stops operating. For small magnets that are recooled within a few hours, and /

[0018] Or in cases where user uptime is not important, this may be acceptable. For a whole body MRI system, where uptime is expected to be 99% or higher, and where recooling times may be 0.5 days to 2.0 days or more, this posture would be unacceptable.

[0019] The present invention thus provides an improved current lead for a superconducting magnet which reduces the amount of heat influx into the superconducting magnet coils in the event of a refrigerator failure.

[0020] The present invention provides an apparatus as defined in the accompanying claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The above and further objects, features and advantages of the present invention will become more apparent from the following description of specific embodiments given by way of non-limiting examples in conjunction with the accompanying drawings, in which:

[0022] Figure 1 A conventional arrangement for supplying electric current to a superconducting coil is schematically shown;

[0023] Figure 2 schematically represents an arrangement for supplying electric current to a superconducting coil according to a first embodiment of the present invention;

[0024] Figure 3 schematically shows an arrangement for supplying electric current to a superconducting coil according to a second embodiment of the present invention; and

[0025] Figure 4 An arrangement for supplying electric current to a superconducting coil according to a third embodiment of the invention is schematically shown. DETAILED DESCRIPTION

[0026] According to the present invention, an alternative arrangement for providing current to the magnet coil 10 and, correspondingly, providing a return path for the current from the magnet coil 10 is provided.

[0027] Figures 2 to 3 An alternative embodiment of the invention is illustrated.The invention provides an arrangement in which no low resistance thermal path is provided between the second cooling stage 20 of the refrigerator and the superconducting magnet coils 10 . Figures 2 to 3 of Figure 1 Features that are identical in arrangement are provided with corresponding reference numerals.

[0028] exist Figures 2 to 3 In the implementation mode, and for Figure 1In an arrangement structure of the magnet coil 10, two similar current leads will generally be provided: one to provide current to the magnet coil 10, and one to provide a return path for the current from the magnet coil 10. However, in each case, only one current lead is shown for ease of illustration.

[0029] exist Figure 2 In the embodiment of FIG. 1 , there is no thermal connection between the second cooling stage 20 and the current lead 14. The current lead 14 still includes the first LTS portion 14a, the second HTS portion 14b and the third portion of non-superconducting conductor 14c. Figure 1 In a conventional arrangement of the second HTS section 14b, the lower end of the second HTS section 14b and the upper end of the first LTS section 14a are thermally coupled to the second cooling stage 20. Figure 2 In an embodiment, there is no thermal connection between the current lead 14 and the second cooling stage 20 of the refrigerator. As schematically illustrated, this can be achieved simply by mechanically decoupling the current lead 14 from the second cooling stage 20. Alternatively, a solid thermal insulation material 260 can be provided between the current lead 14 and the second cooling stage 20 to provide mechanical support to the current lead 14 without allowing heat to be transferred from the second cooling stage 20 to the current lead. Suitable materials for use can be selected from the following materials: plastics, such as nylon, PTFE, polyester; composite materials, such as GRP, carbon fiber reinforced plastic, G10, Durastone, cotton phenolic; small cross-section stainless steel.

[0030] In an example embodiment, the first LTS portion 14a comprises NbTi superconducting filaments which remain superconducting at temperatures up to about 8K under typical current fields and background fields. In the absence of a thermal link to the second cooling stage 20, it may be necessary to increase the thermal conduction of the first LTS portion 14a of the current lead 14, since the LTS portion 14a must be cooled to superconducting temperatures - such as to 8K or less - in use by conduction through its own length and then through the main magnet thermal bus, i.e., the solid first thermal link 22. This can be achieved by adding more high thermal conductivity material, such as aluminum or copper, to the first LTS portion 14a of the current lead 14. For example, a length of copper wire of the desired cross-section can be welded in parallel to the copper sheathed superconducting wire used for the first LTS portion 14a. This copper wire can be in the form of an additional length of the sheathed LTS wire. Alternatively, a superconducting wire of the desired copper or aluminum sheath size can be used for the first LTS portion 14a.

[0031] In operation of the magnet coil 10, no current flows in the current lead 14. The current lead 14 reaches a thermal equilibrium determined by the temperature of the first cooling stage 18, the temperature of the second cooling stage 20 and the thermal resistance of the main magnet thermal bus, i.e. the solid first thermal link 22 and the magnet coil 10.

[0032] In the event of a failure of the refrigerator 16, heat conducted from room temperature through the refrigerator 16 to the second cooling stage 20 can only flow to the magnet coils 10 through the main magnet thermal bus, i.e. the solid first thermal link 22. If, as is preferred, a thermal switch 24 is provided in the solid first thermal link 22, the thermal switch 24 can be opened in the event of a refrigerator failure to prevent heat from being transferred from the second cooling stage 20 to the magnet coils 10. Thus, there will be no thermal path from the second cooling stage 20 to the magnet coils 10 or the first LTS portion 14a of the current lead 14. The heat transfer from the first stage 18 will not significantly heat the magnet coils 10 or the LTS first portion 14a of the current lead 14, because the HTS material of the second HTS portion 14b of the current lead 14 has a high thermal resistance.

[0033] exist Figure 3 In the embodiment of the invention, the second HTS portion 14b of the current lead 14 extends beyond the second cooling stage 20. Figure 1 The conventional arrangement structure, the thermal connection between the second cooling stage 20 and the current lead 14 is maintained, but Figure 3 In the embodiment of the present invention, the thermal connection between the second cooling stage 20 and the current lead 14 is partially formed along the second HTS portion 14b. Due to the high thermal resistance of the HTS material, the lower end of the second HTS portion 14b and the upper end of the first portion 14a are thermally insulated from the second cooling stage 20 by a significant thermal resistance.

[0034] In operation of the superconducting magnet, no current flows through the current leads 14, the thermal switch 24 is closed - if present, and the magnet coils 10 and the first LTS portions 14a of the current leads 14 are cooled to the LTS superconducting temperature by the second cooling stage 20 by heat conduction through the main magnet thermal bus, i.e. the solid first thermal link 22.

[0035] In the event of a refrigerator failure, the thermal switch 24 may be opened, if present. No heat will then flow from the second cooling stage 20 to the magnet coils 10 through the main magnet thermal bus, i.e. the solid first thermal link 22. Although heat will be transferred to the second cooling stage 20 and correspondingly to the current lead 14 through the structure of the refrigerator 16, the heat from the second cooling stage 20 will partially reach the current lead 14 along the second HTS portion of the current lead. Since the HTS material of the second HTS portion 14b of the current lead has a relatively low thermal conductivity, very little of the heat that reaches the second cooling stage 20 will be transferred to the current lead 14. Therefore, very little of the heat that reaches the second cooling stage 20 will be transferred to the magnet coils 10 or the first LTS stage 14a of the current lead 14. Figure 1The heat transfer to the first LTS stage 14a of the magnet coil 10 or current lead 14 is reduced compared to the conventional arrangement of the embodiment. An advantage of this embodiment is that in operation, the heat influx through the first cooling stage 18 to the second HTS portion 14b is extracted from the current lead 14 at the second cooling stage 20. Therefore, there is no need to make the first LTS portion 14a as Figure 2 The wire of the first LTS portion 14a can thus be kept relatively thin and easy to handle. Figure 2 The additional consumption of HTS materials will increase the material cost compared to the implementation method of FIG.

[0036] An example advantage of the present invention is that it allows the magnet to remain in the magnetic field for a long period of time during a cooling failure, known as the "ride-through" period, compared to conventional arrangements that do not benefit from the present invention. In certain embodiments, and preferably, this advantage allows the magnet to remain superconducting and in the magnetic field until cooling is restored.

[0037] Another exemplary advantage of the present invention is that the first LTS portion 14a remains in a superconducting state during this long period of time, so the magnet can be ramped down by orderly removing current from the magnet coils at the end of the ride-through period, thereby avoiding superconductivity. In a corresponding preferred embodiment, the superconducting magnet can remain superconducting while ramping down even when the refrigerator 16 is not operating. Because the first LTS portion 14a of the current lead 14 is not thermally attached to the cold head second cooling stage 20, the upper end of the first LTS portion 14a of the current lead 14 remains at a temperature below the superconducting transition temperature, which may be, for example, 8K, for a long period of time after the refrigerator 16 stops operating. This allows the magnet to be ramped down by orderly removing current from the magnet coils in a controlled manner, thereby extracting stored energy and avoiding superconductivity, which means that the magnet will re-cool and be ready to ramp back into the magnetic field faster after cooling is resumed than after a superconductivity.

[0038] Example materials for the first LTS portion 14a of the current lead 14 include LTS superconductors of niobium titanium or triniobium tin Nb3Sn with a base material of copper or aluminum. Suitable dimensions include any suitable length / cross-sectional area ratio. In a specific embodiment, the LTS portion may be approximately 0.7 m long and have a cross-sectional area of ​​45 mm2.

[0039] Example materials for the second HTS portion 14b of the current lead 14, preferably without a copper matrix material, include 1G or 2G HTS tapes such as BSCCO, rare earth BCO (YBCO, GdBCO) may be used.

[0040] Example materials for the third portion of non-superconducting conductor 14c of current lead 14 include brass or copper or a combination thereof. Stainless steel may also be used, but a larger cross-sectional area may be required due to the resistivity of stainless steel. According to the present invention, the low thermal resistance thermal connection between the second cooling stage 20 and the first LTS portion of the current lead 14 and the magnet coil 10 is removed and replaced by a high thermal resistance connection. The high thermal resistance may be provided by the length of the second HTS portion 14b of the current lead 14, or may be provided by, for example, Figure 3 and Figure 2 The current lead 14 provided in the embodiment of the present invention is provided by thermal separation or thermal and mechanical separation from the second cooling stage 20 .

[0041] In another embodiment of the present invention, Figure 4 As shown in , a heat discharger 26 is provided, which is in thermal contact with the first LTS portion 14a of the current lead 14 and in thermal contact with the first thermal link 22. The heat discharger 26 is electrically insulated from one or both of the first LTS portion 14a of the current lead 14 and the first thermal link 22. The heat discharger 26 intercepts the heat flowing down along the LTS portion 14a in normal operation, so that the heat flows directly into the first thermal link 22 and returns to the refrigerator second stage 20, rather than flowing into the first thermal link 22 via the magnet 10 and returning to the refrigerator second stage 20. This increases the thermal margin of the coil 10 because if the coil 10 does not transfer heat, the temperature of the coil 10 will be lower. In the example, first, the LTS portion 14a of the current lead 14 is welded to the heat discharger 26 in the form of a copper block. The copper block heat discharger 26 is bonded to the first thermal link 22 over a large area using a thin electrically insulating adhesive layer.

Claims

1. A current lead arrangement for supplying current to a superconducting magnet coil (10), the current lead arrangement comprising a current lead (14) and a cryogenic refrigerator (16), The current lead (14) comprises a first portion of a low temperature superconductor (LTS) wire (14a), the first portion of the low temperature superconductor wire (14a) being bonded to a second portion of a high temperature superconductor (HTS) material (14b), which in turn is bonded to a third portion of a non-superconducting conductor (14c), The first portion of low temperature superconductor wire (14a), the second portion of high temperature superconductor material (14b) and the third portion of non-superconducting conductor (14c) are configured to extend in this order, upwardly away from the superconducting magnet coil, The cryogenic refrigerator (16) comprises a first cooling stage (18) and a second cooling stage (20), wherein the first cooling stage is located above the second cooling stage. in, The lower end of the third portion of non-superconducting conductor (14c) and the upper end of the second portion of high temperature superconductor material (14b) are thermally coupled to the first cooling stage (18), the lower end of the first portion of low temperature superconductor wire (14a) is configured to be thermally and electrically coupled to the superconducting magnet coil (10), and A first thermal link (22) is also provided, the first thermal link (22) being in thermal contact with the second cooling stage (20) and being configured to be in thermal contact with the superconducting magnet coil (10), the first thermal link being provided with a thermal switch (24) to interrupt the thermal conductivity of the first thermal link, characterized in that the second portion of high temperature superconductor material (14b) extends downward beyond the second cooling stage (20) and is partially thermally connected to the second cooling stage (20) along the second portion of high temperature superconductor material (14b) by the provided second thermal link, and the lower end of the second portion of high temperature superconductor material (14b) and the upper end of the first portion of low temperature superconductor wire (14a) are thermally insulated from the second cooling stage (20) by the middle portion of the second portion of high temperature superconductor material (14b), and the middle portion of the second portion of high temperature superconductor material (14b) is located between the lower end of the second portion of high temperature superconductor material (14b) and the second thermal link.

2. The arrangement according to claim 1, wherein: A copper block heat discharge piece (26) is arranged in thermal contact with the second cooling stage (20) and the first portion of low-temperature superconductor wire (14a), and the copper block heat discharge piece (26) is electrically insulated from one or both of the first portion of low-temperature superconductor wire (14a) and the first thermal link (22).

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