BENT TOROIDAL FIELD COILS

Abstract

A toroidal field coil. The toroidal field coil comprises a central column and a plurality of return limbs. Each return limb comprises a plurality of double pancake, DP, coils, the DP coils comprising high temperature superconducting, HTS, tapes. The DP coils are arranged such that a section of the DP coil which passes through the central column is positioned such that the tapes are aligned substantially with the local magnetic field during operation of the toroidal field coil. At least two DP coils at the outside of each return limb are bent about an axis parallel to the central column such that they each curve inwards towards each other.

Claims

1. A toroidal field magnet comprising a central column and a plurality of return limbs, wherein: each return limb comprises a plurality of double pancake, DP, coils, the DP coils comprising high temperature superconducting, HTS, tapes; the DP coils are arranged such that a section of the DP coil which passes through the central column is positioned such that the tapes are aligned substantially with the local magnetic field during operation of the toroidal field coil; at least two DP coils at the outside of each return limb are bent about an axis parallel to the central column such that they each curve inwards towards each other.

2. The toroidal field magnet according to claim 1, wherein a central DP coil of each return limb is planar.

3. The toroidal field magnet according to claim 1, wherein each DP coil that is bent is bent over a section of the DP coil extending at least 10% of the length of the DP coil outside of the central column.

4. The toroidal field magnet according to claim 1, wherein each DP coil that is bent is bent in a horizontal section of the DP coil.

5. The toroidal field magnet according to claim 1, wherein each DP coil that is bent is bent in a curved section of the DP coil adjacent to the central column.

6. A method of manufacturing a toroidal field magnet, the method comprising: manufacturing a plurality of DP coils, the plurality of DP coils including DP coils bent about an axis parallel to a central column section of the DP coil; assembling the DP coils into return limbs, each return limb comprising a plurality of DP coils such that at least two DP coils at the outside of each return limb are bent such that they each curve inwards towards each other assembling the return limbs into a toroidal field coil, such that, for each DP coil, a section of the DP coil which passes through the central column is positioned such that the tapes are aligned substantially with the local magnetic field during operation of the toroidal field coil.

7. The method according to claim 6, wherein manufacturing the plurality of DP coils comprises, for each bent DP coils, winding the DP coil as planar and subsequently bending the DP coil out-of-plane.

8. The method according to claim 6, wherein manufacturing the plurality of DP coils comprises, for each bent DP coil, winding the DP coil on a former shaped to provide the required bend.

9. The method according to claim 8, wherein the former is a rocking winding table.

10. The method according to claim 6, wherein manufacturing the plurality of DP coils comprises, for each bent DP coil, winding the DP coil with a rocking taping head.

11. The method according to any of claim 6, and comprising one or both of: impregnating each bent DP coil with epoxy resin, solder or other suitable bonding agent; enclosing each bent DP coil in a casing.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a diagram of a ReBCO tape;

[0023] FIG. 2 shows an toroidal field coil;

[0024] FIG. 3 shows an alternative toroidal field coil;

[0025] FIG. 4 shows a cross section of the central column of the TF coil of FIG. 3;

[0026] FIGS. 5A and B show an exemplary TF coil;

[0027] FIG. 6 shows a cross section of the central column of the TF coil of FIGS. 5A and B;

[0028] FIG. 7 shows a double pancake of the TF coil of FIGS. 5A and B;

[0029] FIGS. 8A and B show an alternative exemplary TF coil; and

[0030] FIG. 9 shows a double pancake of the TF coil of FIGS. 8A and B

DETAILED DESCRIPTION

[0031] An alternative design of TF coil is proposed below, to allow for both a high level of field uniformity within the central column and easy access to components within the TF coil (e.g. the plasma chamber).

[0032] The principle of the alternative design lies in providing double pancake coils (DP) which are curved out of the plane of the coil, such that the tapes of each DP are aligned substantially parallel to the magnetic field at the central column, and the DPs curve away towards the outer radius of the TF coil to increase the space available for access to components within the TF coil.

[0033] One example construction is shown in FIG. 5A, and in plan view in FIG. 5B. The TF coil 500 comprises twelve return limbs 501, each comprising four DP coils 502. In the central column, each DP is “field aligned”, i.e. arranged such that the HTS tapes are substantially parallel to the magnetic field. The individual DPs are curved out-of-plane (or “bent”) such that the DPs of each return limb bunch together at the outer radius of the TF coil, so that each return limb forms a “petal shape”, resulting in the “flower-like” top view of FIG. 5B

[0034] Field lines through the central column section are shown in FIG. 6. Each DP 502 is aligned such that the HTS tapes are substantially parallel to the magnetic field, resulting in higher critical current for the DPs.

[0035] An example construction for a DP for use in the example of FIGS. 5A and B is shown in FIG. 7. The DP is shaped as a rounded rectangle, with a central column section 701, four “corner sections” 702, an outer vertical section 703, and two horizontal sections 704, 705 (note that the sections are entirely notional for the sake of description—the DP may be wound continuously, or provided with joints which divide it in other ways). The central column sections, corner sections, and outer vertical sections are structured the same as they would be for a conventional “flat” DP. The horizontal sections 704, 705 each bend through an angle α. α is chosen to provide the required shape for the return limbs, and will be different for DPs at different positions in the respective return limbs.

[0036] For example, for a spherical tokamak with 12 return limbs, and a return limb radius of 4 m, a may be 15 degrees (assuming a typical width for the DPs, and as such a typical inner radius for the return limbs at the central column). For a DP wound with 25 mm width tape, this would give a bending strain of 0.21%, which is well within the limit recommended by the manufacturers of some HTS tape (the value of the maximum strain of the HTS tape will vary depending on the tape used).

[0037] Such a coil can be wound on a “rocking” winding table, and then bonded by resin impregnation or soldering. Alternatively, the coil may be wound using a rocking taping head (i.e. the spool from which the tape is dispensed may rock, instead of rocking the table). As a further alternative, the coil can be wound as a planar coil, and then subsequently bent and encapsulated with resin or solder. The coil may be enclosed in an external casing or other structural support to ensure that it keeps its shape, and remove any need to “overbend” the coil to compensate for its tendency to straighten out.

[0038] A similar construction can be used toroidal field magnets with “D-shaped” DP coils, i.e. without a horizontal section, or other shapes of DP coils, where the bend occurs over a significant portion, e.g. at least 10%, of the length of the DP coil outside of the central column.

[0039] An alternative construction is shown in FIGS. 8A (isometric view) and B (equatorial cross section) and FIG. 9 (single coil). Each DP is shaped as a rounded rectangle with a central column section 801, four “corner sections” 802a-d, an outer vertical section 803, and two horizontal sections 804, 805 (note that the sections are entirely notional for the sake of description—the DP is wound continuously). The horizontal sections 804, 805, outer corner sections 802c, 802d, and outer vertical section 803 are structured the same as they would be for a conventional “flat” DP. The inner corner sections 802a, 802b are structured such that the central column section 801 is rotated around a vertical axis compared to its orientation in a “flat” DP, i.e. the inner corner sections have both a 90° bend about a horizontal axis and a twist about the vertical axis.

[0040] When assembled into a full TF coil, the central column section 801 of each DP is aligned with the HTS tapes perpendicular to the radius of the central column, and the DPs are arranged in return limbs 811 (e.g. pairs as shown in FIGS. 8A and 9B) so as to leave gaps 812 where the DPs on either side of the gap bend away from the gap.

[0041] In a further example, the DPs of FIG. 9 may be arranged with three DPs to each return limb, with the outer DPs of the return limb being DPs as shown in FIG. 9 (one being a reflection of the other), and the central DP being a flat DP.