Preliminary Design of a High Current R&W TF Coil Conductor for the EU DEMO

Abstract

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 ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$U_{d}$</tex-math></inline-formula> ) of the DEMO Toroidal Field (TF) coils. Another benefit of lowering the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$U_{d}$</tex-math></inline-formula> 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$U_{d}$</tex-math></inline-formula> is decreasing the coil inductance <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$L$</tex-math></inline-formula> . Since the inductance of a coil is proportional to N <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula> , where N is the number of turns, for a given total TF current N <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\cdot \mathrm{\mathbf {I}}_\mathbf {op}$</tex-math></inline-formula> , decreasing the number of turns corresponds to a higher current flowing through each turn, which results this results in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$L\propto I^{-2}_{op}$</tex-math></inline-formula> to I <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_\mathbf {op}^{-2}$</tex-math></inline-formula> . This means that increasing the current will have a quadratic impact on <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$L$</tex-math></inline-formula> and thus a linear impact on the discharge voltage <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$U_{d}$</tex-math></inline-formula> making the design of a high-current ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim \!\mathbf {105}\,$</tex-math></inline-formula> 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathbf {1.6}$</tex-math></inline-formula> . 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula> Sn and Cu as stabilizer designed for an operating current of 105 kA alongside the results of a dedicated 2D mechanical analysis.