Nitronic 50 Steel as a Structural Material for EU-DEMO's Toroidal Field Coils

We report the results of an exploratory design study for Nitronic 50 (N50) Toroidal Field (TF) coils for the EU-DEMO fusion power plant. N50 is a super-austenitic steel that has a yield stress ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\sigma _y})$</tex-math></inline-formula> of up to 1500 MPa at 4 K, 50% higher than the conventional structural steels (e.g., 316LN) that have been considered for EU-DEMO so far. 2D mechanical analyses have been performed on the inner legs of 30 different layer-wound and graded TF coil designs from the Swiss Plasma Center (SPC), with different current per turn's ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${I_t}$</tex-math></inline-formula> 's) and discharge time constants ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\tau _d}$</tex-math></inline-formula> 's) while maintaining the same ampere-turns in the Winding Pack (WP). We show that there is a wide range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${I_t}$</tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\tau _d}$</tex-math></inline-formula> parameter space that allows N50 coils with radial thicknesses of <1100 mm, using materials that are available today. We also show that material and manufacturing development programmes for the insulation and N50 coil case would be needed to unlock all the radial space reductions on offer from N50. If such programmes were successful, then thicknesses as low as 710 mm may be possible with N50 (∼20% lower than the thinnest possible 316LN design). The designs that we have found fit comfortably in the 1400 mm space envelope assigned to the TF coil inner leg in the 2018 EU-DEMO magnet baseline design.