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 (U-d) of the DEMO Toroidal Field (TF) coils. Another benefit of lowering the U-d 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 U-d is decreasing the coil inductance L. Since the inductance of a coil is proportional to N-2, where N is the number of turns, for a given total TF current N.I-op, decreasing the number of turns corresponds to a higher current flowing through each turn, which results this results in L proportional to I-op(-)2 to I (-)(2 )(op)This means that increasing the op op current will have a quadratic impact on L and thus a linear impact on the discharge voltage U-d making the design of a high-current (similar to 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.