Progress in assembling coated conductor cables by the Roebel technique (RACC)

Abstract

High temperature superconducting cables for transport currents well above 1 kA, assembled from a number of coated conductor tapes, are required for application in transformers, generators and, with further increased currents, for future fusion reactor coils. At present the assembling of meander shaped tapes by the Roebel technique seems to be the most promising way of manufacturing high current cables. With short cable samples it was previously shown that the problem of the in the plane inflexibility of the tapes can be overcome by this technique. For some ac applications the losses are however still too high. Since the hysteresis losses in the tapes are dominant in the frequency range below 100 Hz, the main emphasis should be directed to reduce this loss component. Several routes to achieve this goal are investigated: striations in the individual tapes reduce the effective width of the superconducting layer and hence the hysteresis losses, but the problem of twisting of these striations is not yet solved. A second route is the reduction of the meander shaped Roebel tape width to a value in the region of 1 mm which allows the assembling of a correspondingly larger number of tapes, e.g. 30 or 40. This would lead to a cable approaching a square cross section with reduced losses and, as another important advantage, with a certain in-plane flexibility. The critical current degradation of tapes with 1 and 2 mm width is investigated and ac-loss results of cables with 2 mm meander tapes are presented.