CURRENT LEADS FOR SUPERCONDUCTING MAGNETS

Abstract

A current lead arrangement for supplying current to a superconducting magnet coil, comprising a current lead and a cryogenic refrigerator. The current lead comprises a first section of low temperature superconductor (LTS) wire, joined to a second section of a high temperature superconductor (HTS) material, in turn joined to a third section of a resistive material. The cryogenic refrigerator comprises a first cooling stage and a second cooling stage. A lower end of the third section and an upper end of the second section are thermally linked to the first cooling stage, a lower end of the first section is thermally and electrically connected to the superconducting magnet coil.

Claims

1.-5. (canceled)

6. A current lead arrangement for supplying current to a superconducting magnet coil, comprising: a current lead comprising a first section of low temperature superconductor (LTS) wire joined to a second section of a high temperature superconductor (HTS) material, which is in turn joined to a third section of a resistive material, wherein each of the first section, the second section, and the third section is configured to extend, in that order, upward and away from the superconducting magnet coil; a cryogenic refrigerator comprising a first cooling stage and a second cooling stage, the first cooling stage being located above the second cooling stage, wherein a lower end of the third section and an upper end of the second section are thermally linked to the first cooling stage, wherein a lower end of the first section is configured to be thermally and electrically connected to the superconducting magnet coil; and a first thermal link thermally in contact with the second cooling stage and configured to be thermally in contact with the superconducting magnet coil, the first thermal link being provided with a heat switch to interrupt a thermal conductivity of the first thermal link, wherein the second section extends downwards beyond the second cooling stage and is thermally linked to the second cooling stage by a second thermal link provided partially along the second section of the high temperature superconductor, and wherein a lower end of the second section and an upper end of the first section are thermally isolated from the second cooling stage by an intermediate portion of the second section, which is located between the lower end of the second section and the second thermal link.

7. An arrangement according to claim 6, further comprising: a copper block in thermal contact with the second cooling stage and the first section, the copper block being electrically isolated from either or both of the first section and the first thermal link.

Description

DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0029] 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:

[0030] FIG. 1 schematically represents a conventional arrangement for supplying current to a superconducting coil;

[0031] FIG. 2 schematically represents an arrangement for supplying current to a superconducting coil, according to a first embodiment of the present disclosure;

[0032] FIG. 3 schematically represents an arrangement for supplying current to a superconducting coil, according to a second embodiment of the present disclosure; and

[0033] FIG. 4 schematically represents an arrangement for supplying current to a superconducting coil, according to a third embodiment of the present disclosure.

DETAILED DESCRIPTION

[0034] 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.

[0035] FIGS. 2-3 illustrate alternative embodiments of the present disclosure. The present disclosure provides such arrangements in which no low-resistance thermal path is provided between the second cooling stage 20 of the refrigerator and the superconducting magnet coil 10. Features of FIGS. 2-3 which are in common with the arrangement of FIG. 1 carry corresponding reference numerals.

[0036] In the embodiments of FIGS. 2-3, and for the arrangement of FIG. 1, typically two similar current leads will be provided: one to supply 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.

[0037] In the embodiment of FIG. 2, there is no thermal link between the second cooling stage 20 and the current lead 14. Current lead 14 still comprises a first LTS section 14a, a second HTS section 14b and a third non-superconductive section 14c. In the conventional arrangement of FIG. 1, a lower end of the second HTS section 14b and an upper end of the first LTS section 14a are thermally linked to the second cooling stage 20. In the embodiment of FIG. 2, there is no thermal link between the current lead 14 and the second cooling stage 20 of the refrigerator. As schematically illustrated, this may be simply achieved by mechanically detaching the current lead 14 from the second cooling stage 20. Alternatively, a solid thermal insulator material 260 may 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 from the second cooling stage to be transferred to the current lead. Suitable materials to use may be selected from among the list: plastics such as nylon, PTFE, polyester; composites such as GRP, carbon fibre reinforced plastic, G10, Durastone, cotton phenolic; small cross-section stainless steel.

[0038] 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 temperaturesuch as 8K or lessby 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.

[0039] 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.

[0040] 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.

[0041] In the embodiment of FIG. 3, the second HTS section 14b of the current lead 14 is extended beyond the second cooling stage 20. The thermal link between second cooling stage 20 and current lead 14 is maintained, corresponding to the conventional arrangement of FIG. 1, but in the embodiment of FIG. 3, the thermal link between the second cooling stage 20 and the current lead 14 is made part-way along the second HTS section 14b. Due to the high thermal resistivity of the HTS material, the lower end of the second HTS section 14b and the upper end of the first section 14a are thermally isolated from the second cooling stage 20 by a significant thermal resistance.

[0042] 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.

[0043] 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 FIG. 1. An advantage of this embodiment is in that, in operation, heat influx reaching the second HTS section 14b through first cooling stage 18 is extracted from the current lead 14 at the second cooling stage 20. Therefore, there is no need to make the first LTS section 14a highly thermally conductive, as is the case in the embodiment of FIG. 2. The wire of the first LTS section 14a therefore may remain relatively thin and easy to handle. On the other hand, the added consumption of HTS material will add material costs as compared to the embodiment of FIG. 2.

[0044] 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.

[0045] 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.

[0046] Example materials for the first, LTS section 14a of the current lead 14 include LTS superconductor of niobium titanium or triniobium tin Nb.sub.3Sn, 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 mm.sup.2 cross-sectional area.

[0047] 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.

[0048] 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 FIG. 3 and FIG. 2, respectively.

[0049] In a further embodiment of the disclosure, as illustrated in FIG. 4, a thermal dump 26 is provided, in thermal contact with first, LTS section 14a of current lead 14 and with thermal link 22. Thermal dump 26 is electrically isolated from either or both of the first, LTS section 14a of current lead 14 and thermal link 22. The thermal dump 26 intercepts the heat flowing down LTS section 14a in normal operation, so that it flows directly into the thermal link 22 back to the refrigerator second stage 20, rather than via the magnet 10. This increases the thermal margin of the coil 10 because its temperature will be lower if it is not transferring heat. In an example, first, LTS section 14a of current lead 14 is soldered to thermal dump 26 in the form of a copper block. The copper block, thermal dump 26, is bonded to thermal link 22 over a large area with a thin electrically insulating adhesive layer.