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.