Toroidal Coil Stress Analysis and Optimization for High-Field Tokamaks*

The expectation for fusion power plants is that overall cost will track with device volume and auxiliary power, which is why a compact, high gain device with minimal heating and current drive systems may be favorable. The development and demonstration of high temperature superconducting coils allows for higher magnetic field and reduction of the device size at a fixed fusion power. However, physical space limitations (inboard blanket thickness for sufficient moderation of fusion neutrons, magnet lifetime/neutron heating), structural limitations (mechanical and cyclic stresses on toroidal field coils), etc. ultimately limit the plasma operating space and scenario. This work focuses on toroidal high-field coil design to reduce mechanical stresses of coil structures. High-fidelity simulations were performed to assess the design using the finite element solver Elmer FEM. Elmer FEM is a multi-physics software allowing large scale analysis of complex problems. Three solvers were executed in sequence: (1) CoilSolver to evaluate coil current density within the coil; (2) MgDynamics solver to evaluate magnetic field and forces acting on the coil, and (3) linear elasticity solver to evaluate stress acting on the toroidal field coil structures. This workflow was then used to optimize the toroidal coil shape for the high-field tokamak design under assumed parametrization. This toroidal coil stress assessment workflow was developed as part of the FREDA whole-device integrated modeling framework.