Optimization of Structural Performance of the Toroidal Field Coil System of a Tokamak

The toroidal field (TF) coils of a tokamak are subjected to very high operational electromagnetic loads. The shape of a TF coil plays an important role for structural performance since an arbitrary shape is subjected to considerable bending. To mitigate this issue, the TF coils were designed with special “bending free” (constant tension) shapes derived analytically using a thin filament approximation. However, this approximation ignores the stiffnesses of the TF coil case, the central support, and the intercoil structures. This issue was pointed out by other authors and is studied in this paper using FEA. Based on the 2015 EU DEMO baseline design, a finite element model of a constant tension “Princeton D” TF coil was developed. Electromagnetic and structural modeling of the initial and the developed designs were conducted and the in-plane bending in the windings was analyzed. It was shown that a coil case can significantly change the bending moment distribution in the winding pack and even introduce additional bending to a “bending free” shape. On the other hand, such a shape imposes additional constraints on the magnet system design making the usage of these shapes questionable. An alternative strategy of TF coil case reinforcement is discussed.