Design of Double Helix Conductors for Superconducting AC Power Transmission

Coaxial cable conductors in the form of helical tape windings have been proposed in order to make ${\mathrm{Nb}}_3\mathrm{Sn}$ cables which have flexibility and the ability to take up thermal contraction. For ac power transmission the axial magnetic fields which occur in a simple helical construction produce a number of undesirable consequences. It has been shown that these problems can be avoided by using double layer windings of opposite helicity, with 45° as the optimum helix angle. However, smaller values than this are desirable for mechanical reasons, and this paper extends the theory to include pitch angles < 45°. Measurements on short cable models are shown to be in reasonable agreement with calculation. The effect of current flow around the superconductor tape edges, which occurs in helical windings, is analyzed and it is shown that appreciable ac loss can arise if laminated tape with non‐superconductive edges is used indiscriminately.