superconducting electromagnet

Abstract

An electromagnet having one or more grooves, each groove having a conductor element disposed therein, the conductor element including one or more layers of superconductor material for conducting current along the axis of the groove, the conductor element being arranged to contact the sidewalls of the groove through first and second wedge surfaces that are inclined relative to one another, such that a force biasing the conductor element in a direction perpendicular to the axis generates an opposing contact force on the wedge surfaces that acts to compress the conductor element along a direction having a component perpendicular to the force.

Description

[Technical Field]

[0001] The present invention relates to electromagnets comprising superconducting material, particularly but not exclusively to electromagnets used in plasma chambers such as those used in Tokamak fusion reactors, for example, electromagnets used in the central pillars of toroidal field coils for Tokamak plasma chambers. [Background technology]

[0002] Tokamaks are characterized by a strong toroidal magnetic field, high plasma current, and usually a large plasma volume combined with significant auxiliary heating to provide a hot, stable plasma. This allows them to create the conditions for nuclear fusion to occur. Auxiliary heating (e.g., by injection of tens of megawatts of high-energy H, D, or T neutral particle beams) is required to raise the temperature to a sufficiently high value required for fusion to occur and / or to maintain the plasma current.

[0003] The problem is that the large size, large magnetic fields, and high plasma currents typically required make construction and running costs high, and the engineering must be robust to deal with the large stored energies present in both the magnet system and the plasma, which is at risk of "collapse" - when megaampere currents can be reduced to zero in a few thousandths of a second in a violent instability.

[0004] This situation can be improved by shrinking the doughnut-shaped torus of a conventional tokamak to its limit, so that it looks like a cored apple - a "spherical" tokamak (ST). The first realization of this concept, at the START tokamak in Culham, UK, has shown a significant improvement in efficiency. Above The magnetic field required to confine the high-temperature plasma can be reduced by a factor of 10. This also improves plasma stability and reduces construction costs.

[0005] To obtain the fusion reaction necessary for economical power generation (i.e., much more power output than power input), conventional tokamaks must be massive so that the energy confinement time (roughly proportional to the plasma volume) can be long enough for the plasma to become hot enough for thermonuclear fusion to occur.

[0006]

[0004] Patent Document 1 describes an alternative approach involving the use of a compact spherical tokamak for use as a neutron or energy source. The low aspect ratio plasma shape in a spherical tokamak improves particle confinement time and allows for net power generation in a much smaller machine. However, a small diameter central column is required, which presents challenges to the design of the plasma confinement vessel and associated magnets.

[0007] Tokamak electromagnetic coils can be divided into two groups: Poloidal field coils are horizontal circular coils wound with their centers located at the tokamak's central column, generating a poloidal magnetic field (i.e., a magnetic field substantially parallel to the central column); and Toroidal field coils are wound vertically through the central column and around the outside of the plasma chamber (the "return limb"), generating a toroidal magnetic field (i.e., a circular magnetic field around the central column). The combination of the poloidal and toroidal fields generates a helical magnetic field within the plasma chamber, keeping the plasma contained.

[0008] The currents required for the toroidal magnetic field are very large. For a compact spherical tokamak, the diameter of the central pillar needs to be as small as possible. This presents a conflicting requirement, as even superconducting materials are limited in the current densities that can be achieved.

[0009] The large currents, combined with the tokamak's large magnetic field, can generate large Lorentz forces that can deform or otherwise damage parts of the tokamak.

[0010] 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 explained by BCS theory. All low-temperature superconductors have a critical temperature below approximately 30 K (the temperature above which the material cannot become superconducting, even in zero magnetic field). The behavior of HTS materials is not explained by BCS theory; such materials may have critical temperatures above approximately 30 K. (Note, however, that it is not the critical temperature that defines HTS and LTS materials, but rather the physical differences in superconducting behavior and composition.) The most commonly used HTS materials are "cuprate superconductors," ceramics based on copper oxides (compounds containing a copper oxide group), such as BSCCO or ReBCO (where Re is a rare-earth element, usually Y or Gd). Other HTS materials include iron pnictides (e.g., FeAs and FeSe) and magnesium diboride (MgB2).

[0011] ReBCO is typically manufactured as a tape, which is approximately 100 microns thick overall and includes a substrate (usually electropolished Hastelloy, approximately 50 microns thick) on top of which a series of buffer layers, known as a buffer stack, is deposited by IBAD, magnetron sputtering, or other suitable techniques, each approximately 0.2 microns thick. An epitaxial ReBCO-HTS layer (deposited by MOCVD or other suitable technique) covers the buffer stack and is typically 1 micron thick. A 1-2 micron silver layer is deposited on the HTS layer by sputtering or other suitable technique, and a copper stabilization layer is deposited on the tape by electroplating or other suitable technique, often completely encapsulating the tape.

[0012] The substrate provides a mechanical backbone that can be fed through the manufacturing line and enable the growth of subsequent layers. The buffer stack is required to provide a biaxially oriented crystalline template on which to grow the HTS layers, preventing chemical diffusion of elements from the substrate into the HTS that would impair its superconducting properties. The silver layer is required to provide a low-resistance interface from the ReBCO to the stabilization layer 105, which provides an alternative current path if any portion of the ReBCO stops superconducting (becomes "normal").

[0013] Commercially available HTS tapes begin to degrade at internal stresses greater than about 400 MPa. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] International Publication No. 2013 / 030554 Summary of the Invention [Problem to be solved by the invention]

[0015] It is an object of the present invention to provide a central pillar for a toroidal field coil that addresses or at least mitigates the above problems. [Means for solving the problem]

[0016] According to a first aspect of the present invention, there is provided an electromagnet including one or more grooves, each having a conductor element disposed in the groove and including one or more layers of superconductor material for conducting current along an axis of the groove, the conductor element being arranged to contact sidewalls of the groove through first and second wedge surfaces that are inclined relative to one another such that a force biasing the conductor element in a direction perpendicular to the axis (e.g., into the groove) generates an opposing contact force on the wedge surfaces that acts to compress the conductor element along a direction having a component perpendicular to the force.

[0017] The electromagnet can be configured such that the stress generated during operation of the electromagnet does not exceed a threshold stress associated with the superconductor material, which can be determined, for example, based on degradation of the HTS material and / or a reduction in the ability of the HTS material to function as a superconductor.

[0018] The or each groove may be an "open" groove such that the conductive element is not enclosed by the walls of the groove. Alternatively, the groove may be "closed" such that the walls of the groove surround the conductive element, for example, the groove may be a through hole in the support member.

[0019] In some cases, the electromagnet can have a central axis, and the axis of one or more grooves can be arranged parallel to the central axis. In this case, one or more layers of superconductor material are arranged to conduct current parallel to the central axis. A force biasing the conductor element in a direction perpendicular to the axis can be directed toward the central axis.

[0020] Each conductor element can include a stack of one or more high temperature superconductor (HTS) tapes, each HTS tape including a layer of HTS material extending along the grooves. The HTS material can be ReBCO.

[0021] Each conductor element can include a wedge member disposed between the stack of HTS tapes and one of the sidewalls of the groove, the first wedge surface being disposed on the wedge member. The wedge member can be secured to the stack of HTS tapes.

[0022] The wedge member includes a surface that contacts the stack that is substantially perpendicular to the layers of HTS material.

[0023] For one or more of the conductor elements, the acute angle between the first wedge surface and the surface in contact with the stack can be greater than 1 degree, greater than 3 degrees, or greater than 5 degrees.

[0024] Each conductor element can include another wedge member attached to a stack of one or more HTS tapes, with the second wedge surface being provided on the other wedge member.

[0025] The coefficient of static friction between each of the wedge surfaces of the conductor element and the sidewalls of the groove may be from 0.1 to 0.3, or greater than 0.3, or greater than 0.4.

[0026] Each of the conductor elements may be potted (embedded in an insulator).

[0027] The electromagnet further includes a support member, and the groove is disposed in the support member. The support member may include one or more through-holes extending in a direction having a component parallel to one or more of the grooves. The one or more through-holes may be disposed adjacent to one or more sidewalls of the groove. The one or more through-holes may be used to cool the electromagnet.

[0028] According to a second aspect of the present invention, there is provided a central pillar for a toroidal field coil of a tokamak plasma chamber. The central pillar includes a plurality of electromagnets according to the first aspect of the present invention. Grooves are spaced about a central axis, and one or more layers of superconductor material are arranged to conduct current parallel to the central axis. The force is a radial force biasing the conductor elements toward the central axis.

[0029] The grooves may be provided in a single support member.

[0030] The central pillar can include one or more holes extending through the or each support member, each hole extending parallel to the central axis. The one or more holes can be disposed radially outward of the radially inner edges of the conductor elements. The holes can be used to cool the central pillar, for example, by flowing a coolant through the holes. Despite the one or more holes, the toroidal field coils can operate without deforming the central pillar as a result of lower internal stresses in the central pillar compared to known central pillar designs.

[0031] The or each support member may include one or more removable angle segments, and each angle segment may include one or more of the plurality of grooves.

[0032] According to a third aspect of the present invention, there is provided a central pillar for a toroidal field coil of a tokamak plasma chamber. The central pillar includes a support member having a plurality of grooves spaced about a central axis. Each groove has a conductor element disposed therein, the conductor element including one or more layers of superconductor material for conducting current parallel to the central axis. The conductor element is arranged to contact sidewalls of the groove through first and second wedge surfaces that are inclined relative to each other, such that a radial force biasing the conductor element toward the central axis generates an opposing contact force on the wedge surfaces that acts to compress the conductor element along a direction having a component perpendicular to the radial force.

[0033] Each conductor element may include a stack of one or more high temperature superconductor (HTS) tapes, each HTS tape including a layer of HTS material extending parallel to a central axis.

[0034] The HTS material can be ReBCO.

[0035] The layers of HTS material in each stack may be substantially perpendicular to radial lines extending from the central axis and through the corresponding conductor elements in a plane transverse to the central axis.

[0036] For one or more of the conductor elements, the acute angle between the first wedge surface and the radial line may be greater than 1 degree, greater than 3 degrees, or greater than 5 degrees.

[0037] Each conductor element can include a wedge member attached to a stack of HTS tapes, with a first wedge surface provided on the wedge member. Each conductor element can include other wedge members attached to a stack of one or more HTS tapes, with a second wedge surface provided on the other wedge members.

[0038] The coefficient of static friction between each of the wedge surfaces of the conductor element and the sidewalls of the groove may be from 0.1 to 0.3, or greater than 0.3, or greater than 0.4.

[0039] The central post may include one or more holes extending through the support member parallel to the central axis, and the one or more holes may be disposed radially outward of the radially inner edges of the conductor elements.

[0040] The support member can include one or more removable angle segments, and each angle segment can include one or more of the plurality of grooves.

[0041] The conductive elements may be potted.

[0042] According to a fourth aspect of the present invention there is provided a toroidal field coil for a tokamak, comprising a central column according to the first, second or third aspect.

[0043] According to a third aspect of the present invention, there is provided a tokamak including a toroidal field coil according to the fourth aspect. [Brief explanation of the drawings]

[0044] [Figure 1] 1A is a schematic cross-sectional view of a tokamak. FIG. 1B is a schematic axial cross-sectional view of the central column of the tokamak of FIG. [Figure 2] 1 is a schematic axial cross-section of an angular segment of a central pillar of a toroidal field coil. [Figure 3] 1 is a schematic axial cross-section of a portion of an angular segment of a central pillar of a toroidal field coil according to the present invention; FIG. [Figure 4] 1 is a graphical plot of the results of a finite element method (FEM) calculation of radial stress (normal stress) for different wedge angles and friction coefficients. [Figure 5] 1 is a contour plot showing stresses obtained from FEM calculations. [Figure 6] This is a contour plot of the von Mises stress calculated by FEM calculation. DETAILED DESCRIPTION OF THE INVENTION

[0045] 1A shows a vertical cross section of a spherical tokamak 100 including toroidal field coils 101, poloidal field coils 103, and a toroidal plasma chamber 105 disposed within toroidal field coil 101. Tokamak 100 also includes a central pillar 107 that extends through the centers of plasma chamber 105 and toroidal field coils 101 and poloidal field coils 103. Each of toroidal field coils 101 includes a straight portion 109 that extends along axis A-A' of central pillar 107 and a curved portion 111 that is electrically connected to both ends of straight portion 109 to form a closed loop.

[0046] FIG. 1B shows an axial cross section of the central pillar 107 as viewed along axis A-A'. The example tokamak 100 includes twelve toroidal field coils 101, each with its respective linear sections 109 angularly spaced equiangularly about the axis A-A' of the central pillar 107. The central pillar includes a support member 113 made of copper (although other metals and / or alloys may be used) that extends along axis A-A' and has a plurality of grooves 115 into which the linear sections 109 of the toroidal field coils 111 are received. The linear sections 109 include a stacked arrangement of HTS tapes, each with a length extending along the central pillar 107, arranged to contact each other through each of their respective faces. The HTS layers of the tapes extend parallel to axis A-A'.

[0047] FIG. 2 shows an axial cross-section of an angular segment of the central pillar 107, illustrating one of the straight sections 109 of the toroidal field coil 101. In this example, the straight section 109 includes six HTS tape stacks 201A-F, with four inner stacks 201B-E having the same thickness (i.e., the same length along a radial line B-B' of the central pillar 107 that passes through the center of the straight section 109 and crosses the tapes) sandwiched between two outer stacks 201A, F, each of which has a smaller thickness than the inner stacks 201B-F. The stacks 201A-F are bonded together with their outermost radial edges aligned. Each groove 115 has a cross-sectional shape that corresponds to the profile of the corresponding straight section 109 of the toroidal field coil 101 (i.e., the shape of the outer periphery of the groove 115 in a cross section perpendicular to line A-A').

[0048] During use, the HTS tape stacks 201A-F, carrying an axial current in an azimuthal magnetic field, are subjected to Lorentz forces 203 acting toward the center of the pillar 107 (i.e., along the radial direction indicated by dashed line B-B'). These Lorentz forces act radially on the HTS tapes 201A-F, pressing the HTS tapes 201A-F against the support member 113 of the central pillar 107, thereby generating contact forces 205 between the radially innermost edges of the HTS tape stacks 201A-F and the walls of the grooves 115. The combination of Lorentz forces 203 and contact forces 205 therefore generates compressive stresses within the HTS tapes 201A-F that may damage the HTS tapes 201A-F or otherwise reduce their ability to carry large currents.

[0049] FIG. 3 is similar to FIG. 2 except that it shows a central pillar 300 in which wedge members 301A-D are secured to the outermost surfaces of stacks 201A, 201B, 201E, and 201F, forming the circumferentially outer portions of straight sections 109 of toroidal field coil 101. Grooves 115 through support member 113 are enlarged to accommodate the increased size of straight sections 109. Wedge members 301A-D in this example are triangular prisms extending with translational symmetry parallel to axis A-A'. The triangles forming the cross sections of the wedge members can be substantially right-angled triangles, and the wedge members can be positioned such that the largest outer surface of each triangular prism (i.e., the hypotenuse of the triangular cross section) is away from HTS tapes 201A-F, with the narrower (pointed) end of the triangular prism positioned radially inward of the wider end of the triangular prism (i.e., the side of the triangle farthest from the acutest angle of the triangle). That is, wedge members 301A-D taper radially inward along the B-B' direction. Each of grooves 115 also tapers radially inward so that the sidewalls of groove 115 are parallel to the corresponding outer surfaces of wedge members 301A-D. In fact, the sidewalls of groove 115 are tangent to the corresponding outer surfaces of wedge members 301A-D. In this example, wedge angle 305 (the interior angle between the two large faces of wedge members 301A-D at the narrower ends of the triangular prism) is 5 degrees.

[0050] During use, Lorentz forces 203 acting radially on the straight portions 109 of HTS tapes A-F generate contact forces 303 between the outer surfaces of wedge members 301A-D and the sidewalls of groove 115. Contact forces 303 are directed perpendicular to the outer surfaces of wedge members 301A-D; i.e., contact forces 303 have a radial component (along radial axis B-B') and a circumferential component (directed toward radial axis B-B'). Wedge members 301A-D therefore provide a mechanism by which radial stresses within central post 101 are traded for hoop stresses, i.e., the radial stresses within the central post decrease while the hoop stresses increase. This tradeoff allows stacks of HTS tapes 201A-F to conduct higher currents without mechanically damaging the HTS tapes.

[0051] Figure 4 shows a graphical plot of the results of a finite element method (FEM) calculation of the radial stress generated within the stack of HTS tapes 201A-F when operating the tokamak with a current of 28 mA through the central pillar 101. This current results in a peak magnetic field of 26.9 T at the radially outermost edge of the HTS tape. The magnetic field decays monotonically away from the central pillar 101, having a magnitude of 4 T at a distance of 1.4 m from the central pillar 101. The plotted results show the radial stress (MPa) in the coils (vertical, Y-axis) as a function of the coefficient of friction (horizontal, X-axis) between the outer surfaces of the wedge members 301A-B and the sidewalls of the corresponding grooves 115.

[0052] The two top sets of data points 401, 403 ( Black and white circles, respectively Figure 4 (shown) shows the calculated radial stresses in the outer laminations 201A, F for wedge angles of 1 degree and 5 degrees, respectively (the same wedge angle is used for each wedge element 301A-D). These results indicate that the larger the wedge angle, the lower the radial stress, and that for both angles, the radial stress decreases monotonically with increasing coefficient of friction. Similar results (not shown) are obtained for other angles ranging from 1 to 5 degrees, with calculated radial stresses having intermediate values ​​for the sets of data points 401, 403. Angles greater than 5 degrees can also be used, for example, up to 10, 30, 45, or 60 degrees. Depending on the desired radial stress, the coefficient of friction can vary from 0.1 to 0.3, from 0.2 to 0.4, or greater than 0.4, for example, up to 1.0.

[0053] The two lowest sets of data points 405, 407 ( The black and white triangles respectively Figure 4 (shown in red) shows the calculated radial stresses in the inner laminations 201B-E for wedge angles of 1 degree and 3 degrees, respectively (the same wedge angle is used for each wedge member 301A-D). The calculated radial stress decreases monotonically with increasing coefficient of friction for the 1 degree wedge angle (data point 405), but reaches a minimum at a friction coefficient of 0.2 for the 3 degree wedge angle. Calculations of the radial stress for wedge angles of 2, 4, and 5 degrees yield values ​​roughly between those obtained for the 1 and 3 degree wedge angles.

[0054] A target maximum radial stress of 350 MPa in the outer HTS tape stack 201A,F can be achieved using a friction coefficient of 0.3 and a wedge angle greater than 1 degree.

[0055] 5 and 6 show contour plots of the calculated radial and von Mises stresses in the axial cross section of the central pillar 300, calculated for a wedge angle of 3 degrees and a friction coefficient of 0.3. For computational efficiency, calculations are performed for half the angle segment shown in FIG. 3, and boundary conditions are applied to enforce symmetry of the FEM solution with respect to the symmetry of the central pillar 300.

[0056] The radial stress within the laminate of HTS tape 107 is calculated to be less than 400 MPa. The maximum radial stress values ​​for each of HTS tape laminates 201A-C are located on the radially innermost surface of the laminate, and are calculated to be 272.4 MPa for laminate 201A, 313.43 MPa for laminate 201B, and 327.51 MPa for laminate 201C.

[0057] The calculated von Mises stress in the support member 113 is mostly below about 240 MPa adjacent the wedge members 301A, B. This is low enough that cooling holes extending parallel to the axis A-A' can optionally be provided in this portion of the central post 300, i.e., between the grooves 115, without unduly weakening the central post.

[0058] While the above description focuses on a central post containing HTS tape, other forms of HTS material, such as wire, can be used, and LTS material can be used instead of or in addition to the HTS material. HTS tape can be "potted," for example, by encasing a stack of HTS tape in solder.

[0059] While the present disclosure is exemplified by a central pillar for a toroidal field coil (such as for a tokamak plasma chamber), many other applications are possible that do not involve a toroidal field coil. For example, electromagnets can be used in aerospace applications, including in aircraft, unmanned aerial vehicles, satellites, spacecraft, rocket-powered vehicles, and autonomous probes. Indeed, it will be appreciated that the use of wedging surfaces / wedge members described above can be used to reduce (or at least redistribute) stresses on electrical conductors that typically press against a support member. The use of such wedging surfaces / wedge members is particularly beneficial for superconductor (LTS and / or HTS) materials that degrade or fail to function effectively above a threshold stress.

Claims

1. 1. An electromagnet including one or more grooves, each groove having a conductor element disposed therein, the conductor element comprising one or more layers of superconductor material for conducting current along an axis of the groove, the conductor element being arranged to contact first and second side walls of the groove through first and second wedge surfaces, respectively, that are inclined relative to one another, such that a Lorentz force biasing the conductor element in a direction perpendicular to the axis generates an opposing contact force on the first and second wedge surfaces acting to compress the conductor element along a direction having a component perpendicular to the Lorentz force.

2. 10. The electromagnet of claim 1, wherein each conductor element comprises a stack of one or more high temperature superconductor (HTS) tapes, each HTS tape comprising a layer of HTS material extending along the axis of the groove.

3. The electromagnet of claim 2 , wherein the HTS material is ReBCO.

4. 3. The electromagnet of claim 2, wherein each conductor element includes a first wedge member disposed between the stack of HTS tapes and a first sidewall of the groove, the first wedge surface being disposed on the first wedge member.

5. The electromagnet of claim 4 , wherein the first wedge member is fixed to the stack of HTS tapes.

6. The electromagnet of claim 4 , wherein the first wedge member includes a surface that contacts the laminations and is substantially perpendicular to the layers of HTS material.

7. 7. The electromagnet of claim 6, wherein for one or more of the conductor elements, the acute angle between the first wedge surface and a surface that contacts the lamination is greater than 1 degree, greater than 3 degrees, or greater than 5 degrees.

8. 8. The electromagnet of claim 7, wherein each conductor element includes a second wedge member disposed between the stack of HTS tapes and a second sidewall of the groove, the second wedge surface being disposed on the second wedge member.

9. 2. The electromagnet of claim 1, wherein the coefficient of static friction between each of the wedge surfaces of the conductor element and each of the side walls of the groove is between 0.1 and 0.3, greater than 0.3, or greater than 0.

4.

10. The electromagnet of claim 1 , wherein each of said conductor elements is potted.

11. 2. The electromagnet of claim 1, further comprising a support member, the grooves being disposed in the support member, the support member including one or more through holes extending in a direction having a component parallel to one or more of the grooves.

12. The electromagnet of claim 11 , wherein the one or more holes are disposed adjacent one or more sidewalls of the groove.

13. 10. A central pillar for a toroidal field coil of a tokamak plasma chamber, the central pillar comprising a plurality of electromagnets according to claim 1, wherein the grooves are spaced apart about a central axis, the one or more layers of superconductor material are arranged to conduct current parallel to the central axis, and the Lorentz force is a radial force urging the conductor elements toward the central axis.

14. The center post of claim 13 , wherein the groove is provided in a support member.

15. The central post of claim 14 including one or more holes extending through said support member, each hole extending parallel to said central axis.

16. A central pillar as described in Claim 15, wherein the one or more holes penetrating the support member are positioned radially outward from the radial inner edge of the conductor element.

17. The central post of claim 14 , wherein the support member includes one or more removable angle segments, each angle segment including one or more of the plurality of grooves.

18. 1. A central pillar for a toroidal field coil of a tokamak plasma chamber, the central pillar comprising: a support member having a plurality of grooves spaced about a central axis, each groove having a conductor element disposed therein, the conductor element comprising one or more layers of superconductor material for conducting electrical current parallel to the central axis, the conductor element being positioned to contact first and second sidewalls of the groove through first and second wedge surfaces that are inclined toward one another, such that a radial Lorentz force urging the conductor element toward the central axis generates an opposing contact force on the first and second wedge surfaces acting to compress the conductor element along a direction having a circumferential component perpendicular to the radial Lorentz force.

19. A toroidal field coil for a tokamak, comprising the central column of claim 13.

20. 20. A tokamak comprising the toroidal field coil of claim 19.

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