Flexible HTS current leads

Abstract

According to a first aspect, there is provided a HTS current lead. The HTS current lead comprises: a HTS cable comprising a plurality of HTS tapes; a braided sleeve surrounding the HTS cable; and a stabilizer material impregnating the HTS cable and the braided sleeve. The stabilizer material has a melting point higher than a critical temperature of the HTS tapes and lower than a thermal degradation temperature of the HTS tapes.

Description

[0001] This application is a divisional application of Chinese invention patent application No. 201980071080.0 (titled "Flexible HTS Current Lead") filed on April 27, 2021. Technical Field

[0002] This invention relates to superconducting devices. In particular, this invention relates to current leads comprising high-temperature superconductors. Background Technology

[0003] Superconducting materials are typically classified 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 BCS theory. All low-temperature superconductors have a critical temperature below approximately 30 K (above this critical temperature, the material cannot superconduct even in zero magnetic field). The behavior of HTS materials is not described by BCS theory, and such materials can have critical temperatures above approximately 30 K (however, it should be noted that physical differences in superconducting operation and composition define HTS materials, not the critical temperature). The most commonly used HTS are "copper oxide superconductors"—ceramics based on copper oxides (compounds containing copper oxide groups), such as BSCCO or ReBCO (where Re is a rare earth element, typically Y or Gd). Other HTS materials include iron phosphides (e.g., FeAs and FeSe) and magnesium diborate (MgB2).

[0004] ReBCO is typically manufactured to have, for example, the following characteristics. Figure 1 The structure shown is a strip. Such a strip 100 is typically about 100 micrometers thick and includes a substrate 101 (typically an electropolished Hastelloy about 50 micrometers thick), onto which a series of buffer layers (known as a buffer stack 102 about 0.2 micrometers thick) are deposited by IBAD, magnetron sputtering, or another suitable technique. An epitaxial ReBCO-HTS layer 103 (deposited by MOCVD or another suitable technique) overlaps the buffer stack 15 and is typically 1 micrometer thick. A 1-2 micrometer silver layer 104 is deposited on the HTS layer by sputtering or another suitable technique, and a copper stabilizer layer 105 is deposited on the strip by electroplating or another suitable technique, the copper stabilizer layer 105 often completely encapsulating the strip.

[0005] The substrate 101 provides a mechanical backbone that can be fed through a fabrication line and allows for the growth of subsequent layers. The buffer stack 102 is required to provide a bidirectional textured crystal template for growing the HTS layer and to prevent chemical diffusion of elements from the substrate to the HTS (which would compromise its superconducting properties). The silver layer 104 is required to provide a low-resistance interface from the ReBCO to the stabilizer layer, and the stabilizer layer 105 is required to provide an alternative current path in the event that superconductivity ends (enters a "normal" state) at any point in the ReBCO.

[0006] A common challenge in the design of superconducting systems (such as magnets) is obtaining high currents at cryogenic temperatures with minimal thermal loads. Typically, high currents are generated at room temperature (i.e., outside of the cryogenic environment) and transmitted to a cryogenic holder via current leads. These current leads are typically mixtures of copper (for temperatures around 300 K), brass (for temperatures between 300 K and 77 K), and high-temperature superconductors (HTS) (for temperatures below 77 K). Current leads can also be used to transmit current in different regions of the system at a single temperature.

[0007] Other solutions can be used, such as having a switching power supply deliver power to the cryogenic holder, but these solutions will generally still involve HTS current leads located between the power supply and the superconducting system.

[0008] The HTS current lead can be rigid or flexible. Rigid leads may cause mechanical problems due to thermal contraction when the cryogenic holder is cooled. Flexible leads must be handled carefully because if the HTS strip is bent to too small a radius, the HTS strip will develop cracks, and if the HTS strip is subjected to strain due to bending during operation, the HTS strip will have a reduced critical current. Summary of the Invention

[0009] According to a first aspect, an HTS current lead is provided. The HTS current lead includes: an HTS cable comprising a plurality of HTS strips; a braided sleeve surrounding the HTS cable; and a stabilizer material impregnating the HTS cable and the braided sleeve. The stabilizer material has a melting point higher than the critical temperature of the HTS strips and lower than the thermal degradation temperature of the HTS strips.

[0010] According to a second aspect, a method for manufacturing HTS current leads is provided. An HTS cable comprising a plurality of HTS strips is provided. A braided sleeve is placed around the cable. A sealing sheath is placed around the braided sleeve. The sealing sheath is filled with a stabilizer material having a melting point higher than the critical temperature of the HTS strips and lower than the thermal degradation temperature of the HTS strips. The sealing sheath is then filled.

[0011] According to a third aspect, a method is provided for reshaping an HTS current lead as described in the first aspect, further comprising a sealing sheath surrounding the stabilizer material. The HTS current lead is heated to a temperature above the melting point of the stabilizer material and below the degradation temperature of the HTS tape. The HTS current lead is bent into a desired shape. The HTS current lead is allowed to maintain its desired shape while cooling. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of an HTS tape;

[0013] Figure 2 This is a schematic diagram of an exemplary construction of the HTS current lead;

[0014] Figure 3 This is a schematic diagram of an exemplary construction including the wiring of the HTS current leads. Detailed Implementation

[0015] exist Figure 2 The diagram illustrates the construction of an HTS current lead. The HTS current lead comprises a plurality of HTS strips 201 held within a braided sleeve 202. The strips 201 and the braided sleeve 202 are encapsulated in wax 203, which is held in place by a sheath 204.

[0016] The HTS tapes may be stacked (i.e., all parallel) or arranged in some other orientation (e.g., double-layered or braided cable) in the cable.

[0017] The braided sleeve is used to make the current leads thicker, meaning that it is very difficult to bend the current leads to a bending radius that would damage the strip, compared to simply bending the cable. The braided sleeve may include metal to also act as an electrical "stabilizer" for the HTS strip, i.e., as an alternative current path if the current through the HTS strip exceeds a critical current. Suitable metals for the braided sleeve include brass or copper, and the wires may be coated with another metal (such as PbSn tin).

[0018] The braided sleeve can be configured to completely prevent bending below a certain radius of curvature.

[0019] The wax is used to maintain the structural stability of the current leads during processing of the system and operation of the superconducting system. The current leads can be heated to melt the wax, allowing the wax to be bent into a desired shape, and then allowed to cool to maintain that desired shape. A suitable wax will have a melting point above the expected operating temperature of the current leads (e.g., 77 K, or the critical temperature of the HTS strip) or above room temperature (290 K, to ensure the wax is stable at room temperature) but below the degradation temperature of the HTS (i.e., the temperature at which the HTS will suffer permanent damage, typically around 200 °C). If the melting point of the braided sleeve, sheath, or any solder in the HTS current leads is below the HTS degradation temperature, the melting point of the wax may also be below the HTS degradation temperature—or heat may be applied to prevent lower-melting-point components from reaching the same temperature as the wax (e.g., avoiding soldering areas).

[0020] Although wax is used as an example in this article, other substances with melting points above room temperature and below the degradation temperature of the HTS would also be suitable as stabilizing materials for impregnating the HTS current leads.

[0021] The properties of the wax during cryogenic cooling can also be considered as follows: if cooled to a low temperature and then heated back to room temperature, some waxes will decompose into powder and will not be suitable for applications with anticipated cooling and heating cycles (but will be suitable for devices that permanently maintain low temperatures). Waxes that perform well at low temperatures can be determined through routine experiments.

[0022] The stiffness of the wax at room temperature and at low temperatures will also be considered based on the design constraints of the system. A more rigid wax will provide better structural protection during operation (e.g., against Lorentz forces), but may suffer from mechanical problems due to thermal shrinkage. A more flexible wax will provide less protection against Lorentz forces, but will be more easily bent to account for thermal shrinkage.

[0023] One type of wax that meets the above conditions is beeswax (melting temperature of 60°C and stable throughout the low-temperature cycle). When used herein, the term "wax" can also refer to different waxes or mixtures of other materials to achieve the desired properties.

[0024] The sheath 204 may be a heat-shrinkable material, such that when the wax is heated to reshape the cable, the heat-shrinkable material will shrink to facilitate the wax filling any gaps in the structure. Once the HTS current leads have been shaped into the appropriate position or orientation and the wax has solidified, the sheath 204 can be removed (if desired) or retained for additional electrical insulation.

[0025] Many materials that are otherwise suitable for providing a sealing sheath will become brittle at low temperatures. Therefore, the sheath 204 may include a first “room temperature” sheath and a second “low temperature” sheath surrounding the “room temperature” sheath, the first “room temperature” sheath being sealed at room temperature and at the temperature required to melt the wax, and the second “low temperature” sheath maintaining structural integrity at low temperatures.

[0026] Figure 3 It is based on Figure 2 This is a schematic diagram of the connection between the HTS current lead and the junction box. The junction box can be used to electrically, thermally, and structurally connect the HTS current lead to other components.

[0027] The HTS current lead 200 has the same components as before—namely, the HTS tape, braided sleeve 202, wax 203, and sheath 204. The HTS tape 201 extends beyond the braided sleeve 202 and into the junction box 300. Additional copper strips can be placed between the HTS tapes, further separating them within the junction box and providing extra copper for higher current applications—the junction box may undergo resistance heating, which reduces the critical current of the tapes and leads to a greater risk of quenching; the excess copper helps mitigate this.

[0028] The junction box may be formed of an upper portion 301 and a lower portion 302, with the HTS tape held between the upper portion 301 and the lower portion 302. Alternatively, the junction box may be formed as a single unit having a recess or through-hole for receiving the HTS tape. Other suitable constructions may also be used. The recess or through-hole in the junction box (or the space between the two portions in the holding example) can mount the HTS tape within 0.1 mm to allow the HTS tape to be easily soldered to the junction box.

[0029] The braided sleeve 202 surrounds a protrusion 303 located on the side of the junction box, and the sheath 204 also extends around this protrusion. The braided sleeve 202, the HTS tape 201, and the junction box 300 can be joined by soldering, for example, by immersing the entire junction box and the protrusion in solder (keeping the sheath 204 and wax 203 away from the hot solder). A solder with a melting point lower than the degradation temperature of the HTS should be selected. A sealant can be applied to join the junction box and the sheath 204.

[0030] The HTS current lead, including the junction box, can be manufactured using the following steps:

[0031] 1. Cut the HTS tape to a certain length and assemble the HTS tape into a cable (e.g., by stacking).

[0032] 2. Place the first junction box on one end of the cable.

[0033] 3. Cut the braided sleeve to the desired length and slide it on the cable until it reaches the first junction box and around the protrusion.

[0034] 4. Immerse the first junction box in solder to secure the HTS cable and braided sleeve to each other and to the first junction box.

[0035] 5. Slide the sheath on the braided sleeve and seal the sheath at the end adjacent to the first junction box.

[0036] 6. Fill the sheath with wax and temporarily fix the sheath at the end farther from the first junction box, so that the end is held slightly away from the position where the second junction box will be attached.

[0037] 7. Place the second junction box on the free cable end and allow the braided sleeve to slide on the protrusion of the second junction box.

[0038] 8. Immerse the second junction box in solder to secure the HTS cable and braided sleeve to each other and to the second junction box.

[0039] 9. Slide the sheath down into the second junction box and seal the sheath at the end.

[0040] 10. Heat the assembly to melt the wax (and if the sheath uses the heat-shrinkable material, shrink the heat-shrinkable material).

[0041] Besides about Figure 3 Beyond the described junction box, these steps can be appropriately varied for each wiring connection. Typically, forming the HTS current leads will require the following steps:

[0042] 1. Provides HTS cables with multiple straps.

[0043] 2. Place a braided sleeve around the cable.

[0044] 3. Place a protective sleeve around the braided sleeve.

[0045] 4. Fill the sheath with wax (molten wax or wax in the form of small balls).

[0046] 5. Seal the sheath.

[0047] 6. If using small balls, melt the wax.

[0048] These steps can be rearranged or broken down into sub-steps as needed to connect the HTS cable to the terminal used.

[0049] When using HTS cables that include overlay tape (and possibly for other cable designs), the relative orientation of the junction box will be limited to approximately the orientation of the junction box when the cable was manufactured (e.g., if the junction box is parallel, it will remain nearly parallel when the HTS current leads are bent). This is because a significant change in the relative orientation of the junction box would require the HTS tape on one side of the overlay to extend relative to the HTS tape on the other side of the overlay. This can be mitigated by pre-bending the overlay HTS cable before attaching the second junction box, so that the junction box is in the desired relative orientation.

[0050] The method of manufacturing the HTS current lead, the method of forming the HTS current lead, and the HTS current lead of this disclosure can be summarized according to the following numbered aspects:

[0051] 1. A high-temperature superconducting current lead, namely an HTS current lead, the HTS current lead comprising: an HTS cable including a plurality of HTS strips; a braided sleeve surrounding the HTS cable; and a stabilizer material impregnating the HTS cable and the braided sleeve, the stabilizer material having a melting point higher than the critical temperature of the HTS strips and lower than the thermal degradation temperature of the HTS strips.

[0052] 2. The HTS current lead according to aspect 1, wherein the stabilizer material has a melting point higher than 290K.

[0053] 3. The HTS current lead according to aspect 2, wherein the stabilizer material is wax.

[0054] 4. The HTS current lead according to any one of aspects 1 to 3, wherein the braided sleeve comprises metal and is electrically connected to the HTS cable.

[0055] 5. The HTS current lead according to aspect 4, wherein the metal is copper or brass.

[0056] 6. The HTS current lead according to any one of aspects 1 to 5, the HTS current lead comprising a corresponding junction box located at each end of the HTS cable, each junction box being electrically connected to the HTS strip, and the HTS current lead being configured for electrical connection to an external component.

[0057] 7. The HTS current lead according to aspect 6, wherein each junction box is soldered to the HTS cable.

[0058] 8. The HTS current lead according to aspect 6 or 7, wherein each junction box is attached to the braided sleeve.

[0059] 9. The HTS current lead according to aspect 7, wherein each junction box is soldered to the braided sleeve.

[0060] 10. The HTS current lead according to aspect 8 or 9, wherein each junction box includes a protrusion extending around a portion of the HTS cable, and the braided sleeve extends around the protrusion.

[0061] 11. The HTS current lead according to any one of aspects 1 to 10, wherein the HTS current lead includes a sealing sheath surrounding the stabilizer material.

[0062] 12. The HTS current lead according to aspect 11, wherein the sealing sheath is removable.

[0063] 13. The HTS current lead according to aspect 11, wherein the sealing sheath comprises an inner sheath formed of a sealing material and an outer sheath formed of a material that maintains structural integrity after cooling to below 77K.

[0064] 14. A method of manufacturing an HTS current lead, the method comprising: providing an HTS cable including a plurality of HTS strips; placing a braided sleeve around the HTS cable; placing a sealing sheath around the braided sleeve; filling the sealing sheath with a stabilizer material having a melting point above the critical temperature of the HTS strips and below the thermal degradation temperature of the HTS strips; and sealing the sealing sheath.

[0065] 15. The method according to aspect 14, wherein the stabilizer material has a melting point higher than 290K.

[0066] 16. The method according to aspect 15, wherein the step of filling the sealing sleeve with a stabilizer material comprises:

[0067] The stabilizer material pellets are embedded into the sealing sleeve;

[0068] The small ball is melted.

[0069] 17. The method according to any one of aspects 14 to 16, the method comprising: attaching the HTS cable to a junction box at at least one end.

[0070] 18. The method according to aspect 17, the method comprising: attaching the braided sleeve to the junction box.

[0071] 19. The method according to aspect 18, wherein attaching the braided sleeve and the HTS cable to the junction box comprises: immersing the terminal block, a portion of the HTS cable, and a portion of the sleeve together in a solder bath.

[0072] 20. The method according to any one of aspects 17 to 19, the method comprising: bending the HTS cable before attaching the second junction box to the HTS cable such that the first junction box and the second junction box are at an angle relative to each other when they are attached.

[0073] 21. The method according to any one of aspects 14 to 20, wherein the sealing sleeve is formed of a heat-shrinkable material, and the method comprises: heating the sealing sleeve to melt the stabilizer material and shrink the sealing sleeve after the step of filling the sealing sleeve with a stabilizer material.

[0074] 22. A method of reshaping an HTS current lead according to any one of aspects 11 to 13, the method comprising: heating the HTS current lead to a temperature above the melting point of the stabilizer material and below the degradation temperature of the HTS band; bending the HTS current lead into a desired shape; and allowing the HTS current lead to cool while maintaining the desired shape.

Claims

1. A high-temperature superconducting current lead, the high-temperature superconducting current lead comprising: High-temperature superconducting cable, wherein the high-temperature superconducting cable comprises multiple high-temperature superconducting strips; A braided sleeve surrounding the high-temperature superconducting cable; A stabilizer material is used to impregnate the high-temperature superconducting cable and the braided sleeve. The stabilizer material has a melting point higher than 290 K and lower than the thermal degradation temperature of the high-temperature superconducting strip.

2. The high-temperature superconducting current lead according to claim 1, wherein, The stabilizer material is wax.

3. The high-temperature superconducting current lead according to any of the preceding claims, wherein, The braided sleeve comprises metal and is electrically connected to the high-temperature superconducting cable.

4. The high-temperature superconducting current lead according to claim 3, wherein, The metal is copper or brass.

5. The high-temperature superconducting current lead according to claim 1, wherein the high-temperature superconducting current lead includes a corresponding junction box located at each end of the high-temperature superconducting cable, each junction box being electrically connected to the high-temperature superconducting strip, and the high-temperature superconducting current lead is configured for electrical connection with an external component.

6. The high-temperature superconducting current lead according to claim 5, wherein, Each junction box is welded to the high-temperature superconducting cable.

7. The high-temperature superconducting current lead according to claim 5, wherein, Each junction box is attached to the braided sleeve.

8. The high-temperature superconducting current lead according to claim 6, wherein, Each junction box is welded to the braided sleeve.

9. The high-temperature superconducting current lead according to claim 7 or 8, wherein, Each junction box includes a protrusion extending around a portion of the high-temperature superconducting cable, and the braided sleeve extends around the protrusion.

10. The high-temperature superconducting current lead according to claim 1, wherein the high-temperature superconducting current lead includes a sealing sheath surrounding the stabilizer material.

11. The high-temperature superconducting current lead according to claim 10, wherein, The sealing sleeve is removable.

12. The high-temperature superconducting current lead according to claim 10, wherein, The sealing sheath includes an inner sheath formed of a sealing material and an outer sheath formed of a material that maintains structural integrity after cooling to below 77 K.

13. A method for manufacturing a high-temperature superconducting current lead, the method comprising: A high-temperature superconducting cable is provided, the high-temperature superconducting cable comprising multiple high-temperature superconducting strips; A braided sleeve is placed around the high-temperature superconducting cable; A sealing sleeve is placed around the braided sleeve; The sealing sheath is filled with a stabilizing material having a melting point above 290 K and below the thermal degradation temperature of the high-temperature superconducting band; Seal the sealing sleeve.

14. The method according to claim 13, wherein, The step of filling the sealing sleeve with a stabilizer material includes: The stabilizer material pellets are embedded into the sealing sleeve; The small ball is melted.

15. The method according to claim 13 or 14, wherein the method comprises: The high-temperature superconducting cable is attached to a first junction box at at least one end.

16. The method of claim 15, wherein the method comprises: The braided sleeve is attached to the first junction box.

17. The method according to claim 16, wherein, Attaching the braided sleeve and the high-temperature superconducting cable to the first junction box includes immersing the first junction box, a portion of the high-temperature superconducting cable, and a portion of the braided sleeve together in a solder bath.

18. The method of claim 15, wherein the method comprises: The high-temperature superconducting cable is bent before the second junction box is attached, such that the first junction box and the second junction box are at an angle relative to each other when they are attached.

19. The method according to claim 13, wherein, The sealing sleeve is formed of a heat-shrinkable material, and the method includes: heating the sealing sleeve to melt the stabilizer material and shrink the sealing sleeve after the step of filling the sealing sleeve with a stabilizer material.

20. A method for reshaping a high-temperature superconducting current lead, the high-temperature superconducting current lead comprising: High-temperature superconducting cable, wherein the high-temperature superconducting cable comprises multiple high-temperature superconducting strips; A braided sleeve surrounding the high-temperature superconducting cable; A stabilizer material is used to impregnate the high-temperature superconducting cable and the braided sleeve. The stabilizer material has a melting point that is higher than the critical temperature of the high-temperature superconducting strip and lower than the thermal degradation temperature of the high-temperature superconducting strip. A sealing sleeve encapsulates the stabilizer material; The method includes: The high-temperature superconducting current lead is heated to a temperature higher than the melting point of the stabilizer material and lower than the thermal degradation temperature of the high-temperature superconducting band. The high-temperature superconducting current lead is bent into the desired shape; and While maintaining the desired shape of the high-temperature superconducting current lead, the high-temperature superconducting current lead is allowed to cool.

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