Electromagnetic analysis of the DEMO magnet system: electrical behavior of a Toroidal Field Coil during an electrical transient

The toroidal magnet system of DEMO includes sixteen Toroidal Field coils (TFC) for Vacuum Vessel (VV) plasma confinement. Because all coils are superconductive, during their operation, they could experience an unexpected quench (changing from a superconducting to a normal conducting state). Once the quench signals are validated, the quench protection system is activated to dump the magnetic energy into an external resistor to prevent potential damage. However, the electrical transient in the magnet system can lead to overvoltage or overcurrent across and inside each coil. To select the insulation coordination and the test voltage of the coils, the assessment of this electromagnetic load is necessary. Based on the ITER-like topology and considering the WP#1 (Winding Pack) geometry associated with the baseline (Base) scenario and the high-current (HC) scenario, a preliminary electromagnetic analysis is performed by the FEM model: the target is the knowledge of the inductance matrix of the TFC system in both scenarios. Starting from this result, lumped networks have been implemented and used to investigate the transient behavior of the TFC system. In particular, the distribution of the voltage waveform, across and among adjacent layers of each TFC, during a Fast Discharge, has been estimated for different operative conditions: Fast Discharge Units (FDUs) synchronous and asynchronous activation. The simultaneous occurrence of a fault condition during the discharge has also been considered.