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Roebel cables made of high temperature superconducting (HTS) coated conductors can carry high currents with a compact design and reduced ac losses. Therefore, they are good candidates for manufacturing coils for HTS applications such as motors and generators. In this paper, we present the experimental dc and ac characterization of several coils assembled from a 5-m-long Roebel cable built at Karlsruhe Institute of Technology, which differ in the number of turns and turn-to-turn spacing. Our experiments, which are supported by finite-element-method calculations, show that a more tightly wound Roebel coil, despite having a lower critical (and therefore operating) current, can produce a higher magnetic field than a loosely wound one. For a given magnetic field produced at the coil's center, all the coils have similar ac losses, with the exception of the most loosely wound one, which has much higher losses due to the relatively large current needed to produce the desired field. The experiments presented in this paper are carried out on the geometry of pancake coils made of Roebel cables, but they are exemplary of a more general strategy, based on coupling experiments and numerical simulations, that can be used to optimize the coil design, with respect to different parameters, such as tape quantity, size, or ac loss, the relative importance of which is dictated by the specific application.

Coated conductor applications such as fusion magnets, particle accelerator magnets and generator windings require high current-carrying capabilities. This requirement can be fulfilled by various cable concepts using commercial long length REBCO coated conductors with high current-carrying performance. In the past few years, our group has successfully developed the Roebel cable concept for coated conductors. The design advantages of such a cable are high current-carrying capability and low alternating current (AC) losses. Unfortunately, for large-scale applications, the possibilities of a simple scale-up of the Roebel geometry are limited and additional design ideas are needed. One way to reach the required high currents is the Rutherford cable concept. In this concept a conductor is wound with transposition on a flat metal former. In order to design the former, the bending properties of the Roebel assembled coated conductor cables (RACC) must be measured and characterized. This allows the identification of a destruction-free interval for the Roebel cable, in terms of bending angle and transposition length. In this work we designed and assembled a demonstrator of a coated conductor Rutherford cable (CCRC) with three RACC cables. We measured the critical current and the AC losses of the cable demonstrator. Our results show that, despite still needing efforts in terms of reproducibility of the assembly process and of AC loss reduction, this design is a promising and viable solution for high current-capacity cables made of coated conductors.