Wound HTS magnet coils

Abstract

A method of manufacturing an HTS coil is provided. The method comprises winding an HTS coil cable to produce a coil having a plurality of turns. During winding of a turn of the coil, one or more HTS shunt cables are placed adjacent to the previous turn of the coil along a first arc of the coil, and then the turn is wound such that the HTS shunt cable is sandwiched between the turn and the previous turn of the coil such that current can be shared between the HTS shunt cable and the HTS coil cable.

Claims

1. A high temperature superconducting, HTS, coil comprising: an HTS coil cable comprising HTS tapes, and arranged to form a spiral having a plurality of turns; one or more HTS shunt cables comprising HTS tapes, each HTS shunt cable being arranged between a respective pair of adjacent turns, along a first arc of the coil, such that the HTS coil cable and at least one side of the HTS shunt cable are electrically connected, and such that HTS tapes of the HTS shunt cables are parallel with HTS tapes of the HTS coil cable.

2. An HTS coil according to claim 1, and comprising a plurality of HTS shunt cables arranged along the first arc of the coil, each arranged between a respective pair of adjacent turns.

3. An HTS coil according to claim 1, wherein the or each HTS shunt cable is in electrical contact with the HTS coil cable on both sides of the HTS shunt cable.

4. An HTS coil according to claim 1, wherein the HTS coil cable and/or each HTS shunt cable each comprise a stack of HTS tapes arranged such that HTS layers of the HTS tapes are parallel.

5. An HTS coil according to claim 1, and comprising further components which are one or more of: heaters; temperature sensors; magnetic field sensors; spacing elements; metallic stabilizer, such as copper, brass or steel alloy; and coolant channels; arranged between respective pairs of adjacent turns of the HTS coil cable.

6. An HTS coil according to claim 5, wherein the further components are arranged along a second arc of the coil which does not overlap with the first arc.

7. An HTS coil according to claim 1, and comprising spacing elements arranged between respective pairs of adjacent turns of the HTS coil cable, wherein the width of the spacing elements varies around the coil.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic illustration of an HTS tape;

(2) FIG. 2 is a schematic illustration of a wound HTS coil;

(3) FIG. 3 is a schematic illustration of a sectional HTS coil;

(4) FIG. 4 is a schematic illustration of a cross section of a pancake coil;

(5) FIG. 5 is a schematic illustration of a cross section of a double pancake coil;

(6) FIG. 6 is a schematic illustration of a method of winding a HTS coil;

(7) FIG. 7 is a schematic illustration of an HTS coil resulting from the method of FIG. 6;

(8) FIG. 8 is a schematic illustration of an HTS cable having an interleaved HTS shunt;

(9) FIG. 9 is a schematic illustration of an alternative construction of an HTS coil;

(10) FIG. 10 is a schematic illustration of an HTS coil having additional spacing elements; and

(11) FIG. 11 is a schematic illustration of a section of an HTS coil having HTS shunts.

DETAILED DESCRIPTION

(12) A coil construction will now be described which allows the use of grading (i.e. variable amounts of HTS in different parts of the coil) for a wound coil, particularly a pancake coil. Such a construction is of particular use for coils which would have a significantly asymmetric magnetic field when in use, be subject to a significantly asymmetric external field and/or be subject to a significant temperature gradient. For example, such a construction is particularly useful in the toroidal field (TF) coil of a tokamak, where the parts of the toroidal field coil which pass through the central column experience considerably higher magnetic field than the return limbs, and hence require considerably more HTS to carry the same transport current than the parts in the outer sections of the return limbs. The angle between the magnetic field and the ab-plane of the ReBCO must also be considered when choosing the number of HTS tapes required to carry the transport current, so the TF magnet design is complex.

(13) Grading is desirable for two reasons: (a) to minimise the amount of (expensive) HTS needed, and (b) to keep all parts of the coil at a similar fraction of critical current. The second reason is important because it ensures that the temperature margin of the coil is similar at all positions, facilitating a more uniform quench when the magnet has to be rapidly shut down by heating the coils.

(14) The manufacture of such a coil is similar to that of a conventional wound HTS coil. FIG. 6 illustrates schematically the steps of manufacturing the coil. In step 601, a former 610 is provided to define the inner perimeter of the coil. In step 602, a spool of HTS cable 611 is unwound as the former rotates such that the cable winds around the former. In step 603, during winding of a turn 612, an additional length of HTS cable, which will hereafter be referred to as an HTS shunt 613, is placed adjacent to the previous turn 614 of the HTS coil cable along an arc of the coil, so that once the turn 612 is wound (step 604), the HTS shunt 613 ends up sandwiched between the turn 612 and the previous turn 613. Electrically insulating material may be provided on one side of the shunt 613, to isolate the turns 612 and 614 from each other, but the shunt is in electrical contact to one or both of the turns 612 and 614 along its length, to allow current sharing between the shunt and the HTS coil cable. This may be repeated for multiple turns of the coil, or for all turns of the coil (with optionally either an additional shunt inside the inner turn, or an additional shunt outside the outer turn). The shunts are placed along an arc of the coil where more HTS is required.

(15) Additional components, such as sensors, coolant channels, or heaters for inducing quenches may be wound into the coil in other arcs, in a similar manner to the shunts, except that such additional components may or may not require electrical contact to the main HTS coil.

(16) FIG. 7 is a schematic illustration of the final HTS coil. While FIG. 7 shows a coil 701 with only three turns and three shunts 702, it will be appreciated that greater numbers of turns and shunts may be provided. Each of the shunts is placed along an arc of the coil, and provides a greater cross section of HTS in that arc, while still being relatively simple to manufacture compared to a sectional coil. The shunts are shown in the central column section of a TF coil, but may be placed in any location within the coil (as design considerations require). For example, shunts may be placed in the return limbs of a TF coil, as the field angle in the return limbs may be less favourable than in the central column.

(17) The HTS shunts may be made from a cable with the same structure (i.e. number and arrangement of tapes) as that of the main HTS coil, or they may be made from a cable with a different structure. HTS shunts between different turns may have different structures or be made from HTS manufactured by different methods, with varying performance and dimensions.

(18) There will be some resistance between the main HTS coil and the HTS shunts, but this will be very low as current can transfer to or from the shunts along their whole length. This is also true if the coil is provided without insulation, such that current can enter the shunts from either side—though the resistance on the substrate side of the HTS shunt would be higher than that on the HTS side. As such, when the current in the coil is such that if the critical current of the main HTS coil alone is not sufficient in the arc with the shunts to carry the transport current, then excess current will be easily shared to the HTS shunts. At currents less than the critical current of the main HTS coil in the graded region, the vast majority of the current will primarily flow in the wound HTS coil. As the wound coil current approaches the critical current of the parts of the coil experiencing higher magnetic field (or higher temperature, or magnetic field angle less well aligned with the c-axis of the ReBCO HTS layer), the HTS will generate a voltage which will drive excess current through the small resistance between the main coil and the shunt. The voltage generated per metre of HTS (E.sub.HTS) is given by

(19) $E_{HTS}={E_0\left(\frac{I}{I_c}\right)}^n$
where E.sub.0=1 μV/cm is the defined critical current criterion, I.sub.c is the critical current of the tape at this criterion, and n is an experimental parameter that models the sharpness of the superconducting to normal transition; n is typically in the range 20-50 for ReBCO. Depending on the value of n, the voltage is negligible for values of α=I/I.sub.c less than about 0.8. The excess current above the local critical current will be shared into the shunt. This will happen with minimal dissipation, and the small amount of heat generated will be accommodated by the design of the coil cooling system. The number of shunts, and the number of tapes in each shunt, can be chosen based on the amount of HTS needed to keep the ratio α approximately the same in all parts of the coil. The cable used for the main HTS coil and the cable used for the HTS shunts may have the same structure (e.g. number and arrangement of tapes), or may have different structures.

(20) Where shunts are provided along an arc of the coil, they may be provided evenly to all tapes of the coil tape (e.g. each turn of the coil tape may have an HTS shunt comprising two tapes), or the distribution of the shunts may vary across the coil cross section (e.g. providing shunts to every turn towards the outside of the central column for a TF coil, and providing shunts only to every other turn and/or shunts with fewer HTS tapes for turns towards the inside of the central column of a TF coil, as the magnetic field is lower).

(21) FIG. 8 illustrates an alternative construction to that described above. In FIG. 8, the additional HTS tapes 801, 802 in the shunts may be added between the tapes 803, 804 in the HTS cable—i.e. the shunts may be interleaved into the HTS cable in locations which will result in a graded coil when the HTS cable is wound. If substrated tapes are used, then ideally this is done such that the HTS (e.g. ReBCO) sides of the tapes in the cable face the HTS sides of the tapes in the shunts—either on one side or, as shown in FIG. 8, on both sides where the HTS shunt is provided as a type-2 pair. In parts of the coil where the shunt tapes are not needed, they can be substituted by metal spacing elements 805. This avoids steps in the cable where the shunt tapes end.

(22) This may be achieved by forming the cable during the same process as winding the cable around the former, e.g. by providing one or more spools of HTS tape, which are brought together to form a cable, which is then wound around the former in a continuous process. The HTS shunts and substituted metal layers may then be added between the HTS tapes as a part of this process.

(23) FIG. 9 shows an alternative construction having the additional HTS tapes 901, 902 of the shunts between the turns 903, 904 of the HTS cable (only one tape of each turn shown). In FIG. 9, each HTS tape 901, 902 of the shunt terminates at a different point along the length of the HTS cable. This arrangement allows more control over the critical current of the cable as the magnetic field varies along its length The metal spacing elements 905, 906 extend to abut the respective tapes of the HTS shunt.

(24) Additionally, FIG. 9 shows optional further metal tapes 907 which may be placed between the HTS shunt and the main HTS cable. These optional tapes may also be used in the construction of FIG. 8. They provide additional stability in the event of a local hot spot forming around a defect/dropout in one tape in the pair. The best quench performance (ie: slowest rise of hot spot temperature) will occur if these additional tapes are made of copper. However, alternative higher strength materials, such as a steel or nickel/tungsten alloy, may be preferred in high stress applications.

(25) The core of a spherical tokamak requires high current density in the TF coils, to minimise the space taken by windings and maximise the space available for neutron shielding. This is less important in the return limbs, where conductors can be spread out to reduce the field seen by any conductor from its near neighbours. As illustrated schematically in FIG. 10, in the parts of the coil where high current density is not needed (e.g.: in the return limbs of a TF coil), the tapes 1001, 1002 in any part of a turn can be spread apart to reduce the field on any tape, e.g. by adding further spacing elements in selected regions, in the same manner as for other components disclosed above, or by increasing the width of the metal spacing elements 1004 in the selected regions. This reduces the total number of tapes needed, as it increases the critical current per tape in those regions. Similar principles may be applied to the construction of FIG. 8, by inserting spacing elements between the turns of the HTS cable within the regions where current density is not important.

(26) Current transfer is easiest (i.e. the resistance is lower) where an HTS layer of the main coil cable faces an HTS layer of a shunt (i.e. the outer cables of the coil cable and shunt cable form a type-0 pair). As such, the HTS cables of the main coil cable and each shunt may be formed such that the outer HTS tapes of the cable have HTS layers facing outward from the cable.

(27) FIG. 11 shows an arrangement which achieves this. FIG. 11 shows a section of coil comprising a wound HTS coil 1101 and three HTS shunts 1102, 1103, 1104. Only the outer tapes of the wound HTS coil are shown, and this tape has the HTS layer facing outward. Each HTS shunt 1102-1104 is provided as a single type-2 pairs—i.e. two HTS tapes arranged such that the substrates are between the HTS layers. Where more than two additional HTS tapes are required between turns of the cable, multiple HTS shunts are provided. Spacing elements 1105 are provided to ensure an even coil cross section, as described above, though these are optional.

(28) The coil may be wound as a double pancake coil—i.e. with two coils wound in opposite sense and connected at their inner terminals. The connection can be a resistive joint, but it is possible to avoid a joint completely by winding the pair from a single length of cable, as known in prior art. The arrangement of HTS shunts in the two coils may be the same (as they are exposed to substantially the same conditions), but the heaters, sensors, and other components inserted into the coil may vary.