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 theUdmight 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 theUdis decreasing the coil inductanceL. 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 inL∝ I⁻²ₒₚto Iₒp⁻². This means that increasing the current will have a quadratic impact onLand thus a linear impact on the discharge voltageUdmaking 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 factor1.6. 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.