The Garber Current Pattern: an Additional Contribution to AC Losses in Helical HTS Cables?

Conductors made of high-temperature (HTS) wires helically wound in one ormore layers on round tubes (CORT) are compact, flexible, and can carry a largeamount of current. Although these conductors were initially developed for DCapplications, e.g. in magnets, it is worth considering their use for AC, e.g.in underground cables for medium voltage grids and with currents in thekA-range. In these cases, the major challenge is reducing AC losses. Incontrast to a straight superconducting wire, in a helical arrangement, due tosuperconducting shielding, the current does not follow the direction of thewires, but takes a non-trivial zig-zag path within the individual HTS wires(Garber pattern). This includes current components across the thickness of thesuperconducting layers, so that the often used thin-shell approximation doesnot hold. In this contribution, we studied a one-layer three-wire CORT by meansof fully three-dimensional simulations, based on the H-formulation of Maxwell'sequations implemented in the commercial software package COMSOL Multiphysics.As a result of our simulations, the peculiar current profiles were confirmed.In addition, the influence of current, pitch angle, and frequency on the AClosses was studied. We found an optimum for the pitch angle and that thecurrent profiles strongly depend on frequency.