The present application is a national stage entry of PCT application no. PCT/EP2020/070210, filed on Jul. 16, 2020, which claims the benefit of the filing date of Great Britain patent application no. GB 1912698.6, filed on Sep. 4, 2019, the contents of each of which are incorporated herein by reference in their entireties.
The present disclosure relates to superconducting magnets, and in particular to current leads for carrying electrical current between a superconducting magnet and a current source which is itself at room temperature.
Presently-known superconducting materials are commonly divided into “Low Temperature Superconductors” (LTS), which have a superconducting transition temperature below about 20K, and “High Temperature Superconductors” (HTS), which have a superconducting transition temperature above about 20K.
The present disclosure particularly relates to superconducting magnets comprising coils of superconducting wire having LTS core, but the present disclosure could be adapted for use with superconducting magnets comprising coils of superconducting wire having HTS core. Typically, both superconducting wire having LTS core and superconducting wire having HTS core both have a copper or aluminium stabiliser, and could benefit from application of the present disclosure.
The superconducting magnet coil 10 is cooled by refrigerator 16. Refrigerator 16 has a first cooling stage 18 which cools to a first cryogenic temperature, typically in the region of 25K-80K, and also has a second cooling stage 20 which cools to a second cryogenic temperature, typically approximately 4K. The first cooling stage typically cools a thermal radiation shield within a cryostat, and the second cooling stage typically cools the superconducting magnet coil 10 to operating temperature.
Accordingly, in operation, third section 14c of the current lead 14 extends between room temperature at about 300K to the first cryogenic temperature of 25K-80K. The rate of heat transfer through the third section 14c of the current lead 14 will be determined by the material, length and cross-sectional area of the third section, and the temperature difference between its two extremities.
In operation, second section 14b of the current lead 14 extends between the first cryogenic temperature of 25K-80K and the second cryogenic temperature of approximately 4K. The rate of heat transfer through the second section 14b of the current lead 14 will be determined by the material, length and cross-sectional area of the second section, and the temperature difference between its two extremities.
In operation, first section 14a of the current lead 14 extends between second cooling stage 20 which is at second cryogenic temperature of approximately 4K, and the magnet coil 10 which is also at approximately 4K. The rate of heat transfer through the first section 14a of the current lead 14 will be determined by the material, length and cross-sectional area of the third section, but should be minimal as there should be very little temperature difference between its two extremities.
A solid or fluid thermal link 22 of a conductive material such as high purity aluminium or high purity copper, or a thermosiphon, or a combination thereof thermally links second cooling stage to the magnet coil 10. Preferably, and as illustrated, a heat switch 24 may be placed in the thermal path between the second cooling stage 20 and the magnet coil 10. Examples of suitable heat switches 24 include a thermosyphon, a heat pipe, gas gap, solid (e.g. magnetoresistive), or mechanical switch.
HTS current leads, such as second section 14b of current lead 14, are required for low-cryogen and “dry” (no liquid cryogen bath) superconducting magnet systems, so that electrical current can be transferred into and from the superconducting magnet with minimal dissipation. As described above, and typically, the top of the HTS section 14b is thermally anchored by a high thermal conductivity link to the first cooling stage 18 of the cryogenic refrigerator 16, while the lower end of the HTS section is thermally anchored by a high thermal conductivity link to the second cooling stage 20 of the cryogenic refrigerator 16. The lower end of the HTS section is also thermally linked to the magnet coil 10 by a high electrical conductivity section 14a, which is typically of LTS wire.
Heat flowing through the third section 14c of current lead 14 from the electrical conductor 12 at room temperature, is intercepted and removed by the first cooling stage 18 of the refrigerator 16.
Heat flowing down the second section 14b of current lead 14 from the first cooling stage of the refrigerator 18 at 25-80K is intercepted and removed by the second cooling stage 20 of the refrigerator 16.
In the case of refrigerator 16 failure, the second cooling stage 20 of the refrigerator will warm rapidly, as materials have low heat capacity at the operating temperature of the second stage. With the refrigerator 16 inoperative 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, in case of a refrigerator failure, heat switch 24 preferably blocks thermal conduction from second cooling stage 20 to the magnet coil 10 through the thermal link 22, heat will still be conducted through the first section 14a, the LTS part, of the current lead 14, from second cooling stage 20 to magnet coil 10. First cooling stage 18 of the refrigerator will also warm significantly, and heat from this first cooling stage 18 will be conducted down through the HTS section 14b of the current lead 14 to LTS section, first section 14a of the current lead 14, and thence to the magnet coil 10. Typically, an LTS wire such as may be used for LTS section 14a of the current lead 14 includes a number of superconductor filaments of small cross-section and a stabilising matrix material such as aluminium or copper, of significantly larger cross-section, and having an appreciable thermal conductivity. These flows of heat to the magnet coil 10 will cause a quench, which leads to a lengthy down-time of the superconducting magnet since the magnet coil 10 will have to be re-cooled before electrical current can be introduced, and heat transfer at such low temperatures tends to be inefficient.
Conventional approaches to avoid such magnet quench in the case of refrigerator failure include:
The following documents provide examples of conventional arrangements:
The present disclosure accordingly provides improved current leads for superconducting magnets which reduce thermal influx to superconducting magnet coils in case of refrigerator malfunction.
The present disclosure provides apparatus as defined in the appended claims.
The above, and further, objects, characteristics and advantages of the present disclosure will become more apparent from the following description of certain embodiments, given by way of non-limiting examples, in conjunction with the accompanying drawings, wherein:
According to the present disclosure, alternative arrangements are provided for supplying current to magnet coil 10, and correspondingly providing a return path for the current from the magnet coil 10.
In the embodiments of
In the embodiment of
In an example embodiment, the first LTS section 14a includes superconducting filaments of NbTi, which remains superconducting at temperatures up to about 8K at typical currents and background fields. In the absence of a thermal link to the second cooling stage 20, it may be necessary to increase the thermal conductance of the first LTS section 14a of the current lead 14, as the LTS section 14a is, in use, cooled to a superconducting temperature—such as 8K or less—by conduction through its own length, then through the main magnet thermal bus, solid thermal link 22. This may be achieved by adding more material of high thermal conductivity such as aluminium or copper to the first LTS section 14a of the current lead 14. For example, a length of copper wire of desired cross-section may be soldered in parallel onto a copper-sheathed superconducting wire used for the first LTS section 14a. This copper wire may be in the form of extra lengths of the sheathed LTS wire. Alternatively, a superconductive wire of desired copper or aluminium sheathing dimension may be used for the first LTS section 14a.
In operation of the magnet coil 10, no current flows in current lead 14. Current lead 14 reaches a thermal equilibrium determined by the temperatures of the first cooling stage 18, the temperature of the second cooling stage 20 and the thermal resistance of the main magnet thermal bus, solid thermal link 22 and the magnet coil 10.
In case of failure of the refrigerator 16, heat conducted from room temperature through the refrigerator 16 to the second cooling stage 20 can only flow to magnet coil 10 through the main magnet thermal bus, solid thermal link 22. If, as is preferred, heat switch 24 is provided in the solid thermal link 22, this can be opened in case of refrigerator failure to prevent heat transfer from the second cooling stage 22 to the magnet coil 10. There will accordingly be no thermal path from the second cooling stage 20 to the magnet coil 10 or the first LTS section 14a of the current lead 14. Heat transfer from the first stage 18 does not significantly heat either the magnet coil 10 or the LTS first section 14a of the current lead 14 because HTS material of second HTS section 14b of the current lead 14 has a high thermal resistance.
In the embodiment of
In operation of the superconducting magnet, no current flows through current lead 14, heat switch 24, if present, is closed, and the magnet coil 10 and first LTS section 14a of the current lead 14 are cooled to LTS superconducting temperature by the second cooling stage 20 by thermal conduction through main magnet thermal bus, solid thermal link 22.
In case of refrigerator failure, heat switch 24 may be opened, if present. No heat will then flow from second cooling stage 20 to magnet coil 10 through main magnet thermal bus, solid thermal link 22. Although heat will be carried through the structure of refrigerator 16 to second cooling stage 20, and accordingly to the current lead 14, the heat from the second cooling stage 20 will reach the current lead 14 part-way along the second HTS section of the current lead. As the HTS material of second HTS section 14b of the current lead has a relatively low thermal conductivity, very little of the heat reaching the second cooling stage 20 will transfer to the current lead 14. Accordingly, very little of the heat reaching the second cooling stage 20 will transfer to the magnet coil 10 or the first LTS stage 14a of current lead 14. Heat transfer to the magnet coil 10 or the first LTS stage 14a of current lead 14 is reduced as compared to the conventional arrangement of
An example advantage of the present disclosure is that the disclosure allows the magnet to stay at field during a cooling failure for an extended period of time, known as a “ride-through” period, as compared to conventional arrangements which do not benefit from the present disclosure. In certain embodiments, and preferably, this advantage allows the magnet to remain superconducting and at field until cooling is restored.
Another example advantage of the present disclosure is in that the first, LTS section 14a remains in a superconducting state during this extended period of time so the magnet can be ramped down by orderly removal of current from the magnet coils towards the end of the ride-through period, thereby to avoid a quench. In corresponding preferred embodiments, the superconducting magnet may remain superconducting while ramping down even when the refrigerator 16 is inoperative. Because the first LTS section 14a of the current lead 14 is not thermally attached to the cold-head second cooling stage 20, the upper end of first LTS section 14a of the current lead 14 maintains a temperature below the superconducting transition temperature, which may for example be 8K, for an extended period of time after the refrigerator 16 ceases to operate. This allows the magnet to be ramped down by orderly removal of current from the magnet coils in a controlled fashion, thereby extracting stored energy and avoiding a quench, meaning the magnet will re-cool and be ready to be ramped back to field much sooner after the cooling is restored than would be the case following a quench.
Example materials for the first, LTS section 14a of the current lead 14 include LTS superconductor of niobium titanium or triniobium tin Nb3Sn, with matrix material of copper or aluminium. Suitable dimensions include any appropriate length/cross-sectional area ratio. In a specific embodiment, the LTS section may be about 0.7 m long and with a 45 mm2 cross-sectional area.
Example materials for the second, HTS section 14b of the current lead 14 include 1G or 2G HTS tape such as BSCCO, Rare-earth BCO (YBCO, GdBCO) may be used, preferably without copper matrix material.
Example materials for the third, non-superconductive 14c of the current lead 14 include brass or copper or a combination thereof. Stainless steel may also be used, but may require a larger cross-sectional area due to its resistivity. In accordance with the present disclosure, the low-thermal-resistance thermal link between the second cooling stage 20 and the first LTS section of current lead 14 and magnet coil 10 is removed and is replaced by a link of high thermal resistance. The high thermal resistance may be provided by a length of second HTS section 14b of the current lead 14, or may be provided by thermal, or thermal and mechanical, detachment of the current lead 14 from the second cooling stage 20 as provided in the embodiments of
In a further embodiment of the disclosure, as illustrated in
Number | Date | Country | Kind |
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1912698 | Sep 2019 | GB | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2020/070210 | 7/16/2020 | WO |
Publishing Document | Publishing Date | Country | Kind |
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WO2021/043486 | 3/11/2021 | WO | A |
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PCT International Search Report dated Oct. 20, 2020, Application No. PCT/EP2020/070210. |
Number | Date | Country | |
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20240096535 A1 | Mar 2024 | US |