Design and R&D for the DEMO Toroidal Field Coils Based on Nb3Sn React and Wind Method

In 2013, the Swiss Plasma Center proposed a Toroidal Field (TF) layout for the DEMO-EUROfusion tokamak, based on a graded winding made of layers of Nb3 Sn (react&wind) and NbTi conductors. The R&D effort led in 2015 to a full size prototype conductor tested up to 82.4 kA at 12.35 T. The test continued in 2016 and new results are presented. In summer 2015 a new reference baseline was issued for the DEMO-EUROfusion tokamak, leading to an update of the TF requirements. The design update is presented in this paper, with the winding pack consisting of 12 single layers of Nb3 Sn with “invisible” (no protrusion) inter-layer joints. The high grade Nb3 Sn react&wind conductor operates at 63.3 kA, 12.23 T with T_cs > 6.5 K. A new prototype conductor is being manufactured. The main advantages of the graded approach, applied to both the superconductor and the stainless steel conduit, are a substantial space and cost saving compared to the wind&react approach with pancake winding.