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
$\cdot \mathrm{\mathbf {I}}_\mathbf {op}$
, decreasing the number of turns corresponds to a higher current flowing through each turn, which results this results in
$L\propto I^{-2}_{op}$
to I
$_\mathbf {op}^{-2}$
. This means that increasing the 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 (
$\sim \!\mathbf {105}\,$
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
$\mathbf {1.6}$
. 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 Nb
$_{3}$
Sn and Cu as stabilizer designed for an operating current of 105 kA alongside the results of a dedicated 2D mechanical analysis.