Patent Number: 
Section: claims

1. A central column for a toroidal field coil, the central column comprising a plurality of segments (1001), each segment comprising first and second sets of parallel arrays of HTS tapes, the HTS tapes arranged such that c-axes of ReBCO crystal structures of the HTS tapes within each array are parallel to each other, and such that planes of the HTS layers of the HTS tapes within each segment are perpendicular to a respective radius (1010) of the central column which passes through the segment, wherein:each HTS tape has a c-angle which is an angle between a perpendicular to a plane of an HTS layer of the HTS tape and the c-axis of the ReBCO crystal structure of the tape;the HTS tapes of each array (1013) within the first set of arrays are HTS tapes having a first c-angle;the HTS tapes of each array (1011, 1012) within the second set of arrays are HTS tapes having a second c-angle which is greater than the first c-angle;wherein, within each segment, the first set of arrays are arranged closer to the respective radius than the second set of arrays. 2. A central column according to claim 1, wherein the each segment comprises one or more further sets of arrays comprising HTS tape having respective further c-angles, and wherein, within each segment, each set of arrays is arranged closer to the respective radius than sets of arrays comprising HTS tape having greater c-angles. 3. A central column according to claim 1, wherein each set of arrays is arranged such that a further radius of the central column passes through at least one array of the set of arrays, the further radius being at an angle from the respective radius equal to a c-angle of the HTS tape of that set of arrays. 4. A central column according to claim 1, and comprising two layers of segments, wherein a first layer of segments is located radially outward of a second layer of segments. 5. A toroidal field coil comprising a central column according to claim 4, wherein return limbs of the toroidal field coil which are connected to HTS assemblies of the second layer have a greater vertical extent than return limbs of the toroidal field coil which are connected to HTS assemblies of the first layer. 6. A central column according to claim 1, and comprising joints at either end of the central column. 7. A toroidal field coil comprising a central column according to claim 1. 8. A toroidal field coil comprising a central column according to claim 1, wherein each array of HTS tapes is an arc of a wound HTS coil.