Preliminary Design of a High Current R&W TF Coil Conductor for the EU DEMO

Paschen failures in ITER, W7-X and JT60 superconducting coils at the acceptance tests have shown that it is highly desirable to lower the coil discharge voltage (Ud) of the DEMO Toroidal Field (TF) coils. Another benefit of lowering the Ud might be the reduction of the number of coil feeders, i.e. connect in series several TF coils to a discharge unit, which is attractive for machine integration. For a given Ampere Turn (AT), one way to reduce the Ud is decreasing the coil inductance L. Since the inductance of a coil is proportional to N2, where N is the number of turns, for a given total TF current N · Iop, decreasing the number of turns corresponds to a higher current flowing through each turn, which results this results in L ∝ I-2op to I -2op. This means that increasing the current will have a quadratic impact on L and thus a linear impact on the discharge voltage Ud making the design of a high-current (~105 kA) CICC attractive for the EUROfusion DEMO project. In the case of DEMO, increasing the operating current from 66 kA to 105 kA leads to a reduction of the TF discharge voltage of a factor 1.6. Designing a high current TF coil conductor layout includes performing mechanical studies to investigate the TF coil mechanical stability during operation. This contribution will thus present the first design for a react-and-wind TF conductor made of Nb3Sn and Cu as stabilizer designed for an operating current of 105 kA alongside the results of a dedicated 2D mechanical analysis.