Impact of ferritic steel blankets on the magnetic field configuration of the helical fusion reactor

Efficient thermal conversion, sufficient tritium breeding ratio, adequate neutron shielding, and low waste disposal are critical issues required in the design of blankets for fusion reactors. The choice of the blanket material and a proper design can greatly enhance the longevity of blankets and superconducting coils, thereby promoting the commercialization of fusion reactors. Reduced Activation Ferritic/Martensitic (RAFM) steel has been a choice for the structural material of blankets to date. However, it can induce large field distortions in the magnetic configuration. This issue is studied in the present work where F82H ferritic steel is selected as a structural material for the blankets of the FFHR-d1 helical fusion reactor. To understand the impact of this material on the magnetic field over the plasma region, a full 3D model was built in the commercial Finite Element solver COMSOL Multiphysics®. It includes the helical and vertical field coils, as well as the blankets. Distribution of the magnetic field over the plasma confining region is presently computed with a precisely-modeled thin and thick structural walls for the blankets using a non-linear B - H curve. In the present work, the magnetic field calculations are restricted to vacuum conditions. Hence, induced fields are estimated from external coils and ferromagnetic materials without plasma currents. The results show discrepancies compared to the previous work that assumed thick blanket walls and a linear magnetic material property. It demonstrates the need for nonlinear, more-realistic models simulating magnetic blankets.