Increase in coupling-current losses of superconducting cables due to first-stage cabling Part 2: optimum structure of solder-impregnated sub-cable with normal-metal strands

Abstract

We studied a.c. losses in seven-strand superconducting subcables, with normal metal strands for stabilization. In a transverse magnetic field with a trapezoidal wave form, approximate expressions for additional coupling current losses among superconducting strands through the normal metal ones were obtained by using an equivalent electric circuit model. The total losses measured in several types of the stabilized subcables were well explained by the present theoretical results. Loss-minimized strand configuration was proposed for the subcable with any given number of the normal metal strands on the basis of the present theoretical consideration. The optimum number of the normal metal strands in the subcable was also discussed under the competition between a.c. loss and current capacity based on each of a few kinds of stability conditions proposed so far. It is also pointed out that the optimized structure is strongly dependent upon cooling condition and design concepts.