Ferromagnetic forces acting on the EU-DEMO divertor

This paper presents an analysis of the mechanical loads produced by the ferromagnetic effects in the divertor of the European DEMO (EU-DEMO) reactor. The exact assessment of this force is an important prerequisite for a reliable structural design, for instance for a correct choice of the fixing supports. To this end, a 3D magnetostatic model has been built with a simplified geometry corresponding to a 22.5° angular sector of the tokamak, exploiting the toroidal symmetry and imposing periodic boundary conditions. The problem has been solved by using CARIDDI code, that implements an integral formulation and provides a significant simplification of the numerical model, since the mesh can be limited to the magnetic materials only. In our case, the main components of the model (divertor and breeding blankets) are made of a ferromagnetic material, namely, EUROFER97 steel. In presence of external static magnetic fields, these components would behave like an "electromagnet" being subjected to forces. The model considers all the sources of such magnetizing fields, namely: (i) the external toroidal field produced by the currents circulating in the external toroidal field coils, (ii) the internal field induced by the toroidal plasma current. Expected static field in the considered components ranges from 3.4 T to 8.6 T. Forces and torques have been evaluated by Kelvin method, starting from the known external magnetic fields and the computed magnetization in the ferromagnetic regions. The contribution of the static magnetic field associated to the equilibrium plasma current has been shown to be negligible.