Publication
One of the main milestones of the EUROfusion roadmap to fusion is the demonstrator reactor DEMO. Like ITER, the EU DEMO will be a large-scale tokamak with superconducting toroidal field (TF) coils to generate a strong magnetic field for plasma confinement. Even though no voltage should arise across the superconducting TF coils during DC operation, a large voltage develops during a fast safety discharge, due to the rapid exponential decay of the current. This discharge process is necessary to protect the coil in case of a quench (sudden loss of superconductivity). Considering the magnets in a large-scale tokamak like DEMO store approximately 10 GJ of energy, the maximum discharge voltage of one of these coils can reach tens of kilovolts, posing significant challenges. In the vacuum environment where the TF coils operate, such a high voltage requires exceptional insulation of the magnet to prevent Paschen breakdown, which could cause severe damage to the coils. This risk can be substantially mitigated by reducing the discharge voltage of a single TF coil below 5 kV. To reach this goal, a high-current Nb3Sn winding pack (WP) was designed for the TF coils of the EU DEMO. The proposed WP has an operating of ~105 kA, increased from the EU DEMO baseline of 66 kA. This higher operating current allows to reduce the coil inductance by reducing the number of turns from 226 in the nominal design to 142, while maintaining the total ampere-turn in the magnet. Due to the strong dependency of the discharge voltage on the operating current and the number of turns, the proposed design has a resulting discharge voltage of 4.23 kV, much smaller than the one of the nominal design 6.7 kV. Furthermore, the proposed high-current WP design of the DEMO TF coils also makes use of the react&wind (RW) technique and layer winding, allowing for grading, which ensures the optimal use of steel and Nb3Sn in each layer of the winding pack. The work conducted during this thesis proves that the proposed high-current WP results in a radial gain of almost 400 mm compared to the reference WP design. Before this thesis, the highest current conductor proposed for the EU DEMO operated at ~82 kA. Since this is the first time that a conductor exceeding 100 kA is considered for the EU DEMO, a full-scale conductor prototype, named RW4, was developed and tested in SULTAN. This test was used to validate its design principles by performing a DC and an AC characterization of the cable-in-conduit-conductor (CICC) foreseen for the first layer of the TF WP. The DC characterization of the RW4 conductor prototype was affected by sudden quenches. A significant part of this thesis is devoted to a detailed evaluation of the causes behind this unstable behavior. An in-depth study of the AC losses confirmed that the proposed RW design developed at SPC allows to design of low-loss CICCs.