Transient 3-D Eddy Current Analysis in a Superconducting Magnet System Using an Integral Approach

The Wendelstein 7-X is a thermonuclear fusion experiment being built at the Greifswald Branch Institute of the Max-Planck-Institut für Plasmaphysik (IPP). The eddy current distribution is investigated during a transient process in the superconducting coils of the magnet system. Application of volume-grid methods (e.g. FEM) for calculations of the eddy currents in the magnet system is difficult due to the large number of unknowns and the complicated mesh in the air region. Therefore, the investigation of effective methods for that computational problem is useful. We apply the method based on the integral approach, where only active regions without the air embedding are taken into consideration. Such approach leads to the full coefficients matrix and conseqentially to large computational requirements. There are techniques that allow that problem to be overcome. In that approach the coupling between elements are distinguished as near and far in order to achieve the less dense resulting matrix. For far interactions the equivalent current in the coil is introduced. The principles of the applied method and example results are presented in the paper. Introduction The Wendelstein 7-X is a thermonuclear fusion experiment being built at the Greifswald Branch Institute of the Max-Planck-Institut für Plasmaphysik (IPP) [1]. Intensive research on thermonuclear fusion led to the construction of stellarator type reactors. In the stellarator the required distribution of the magnetic field is achieved by a set of non-planar coils (the magnet system) – Fig. 1.a. In order to increase the efficiency of the magnet system superconducting coils are used. Each coil embraces the winding pack and the steel encasing – Fig. 1.b [2]. During an emergency discharge of the magnet system the excitation current in winding pack JL decreases, as it is shown in Fig. 1.c. Due to changes of the magnetic field eddy currents (JE) are generated in the steel encasing. In order to assure the reliability of the overall system the knowledge of the eddy current distribution during the transient process is extremely important.