Fast algorithm for transient 3D eddy-current calculations in the Wendelstein 7-X stellarator

The eddy-currents generated during an emergency turn-off in a magnet system of the Wendelstein 7-X stellarator, a steady-state fusion experiment being built at the Max Planck Institute of Plasma Physics, Greifswald, Germany are calculated. The magnet system of the stellarator has a non-planar construction. Therefore, computation of the eddy-currents by means of the finite element method (FEM) are difficult due to a large number of unknowns and a complicated mesh. Since the turn-off is a slowly varying transient process an indirect method can be applied. Firstly, an integral approach is used to calculate the magnetic field distribution. Then, for the given magnetic field, eddy-currents are calculated using the FEM. The indirect method gives only an approximate solution but the algorithm is much faster than for a direct volume-grid FEM. A description of the magnet system, the principles of the applied method and sample results are presented.