Preliminary studies on DEMO toroidal field circuit topology and overvoltage estimation

In the present configuration of the European demonstration nuclear fusion power plant (DEMO), the toroidal magnetic field is produced by 16 superconducting toroidal field (TF) coils. The total stored energy of 146 GJ, more than 3 times the energy stored in ITER TF coils, is quickly dissipated by suitable Fast Discharge Units (FDU) whenever a coil fast discharge is required. The voltage applied to each coil during the discharge is defined by the energy, the current and the discharge time constant; the total circuit voltage is equally distributed among the coils thanks to the TF circuit symmetry; however, in case of fault the peak value applied to the coils and at the coil terminals to ground can be much higher. This paper deals with first studies addressed to the evaluation of the relative merit of different TF circuit topologies, based on the estimation of the maximum voltage stresses on the coils in various operating and fault conditions. Aim of the study is to support the coil design work, and to explore the best compromise between requirements for the coil insulation and cost and size of FDUs. Analyses and numerical simulations have been carried out, for both the cases of TF circuit composed of 16 and 8 sectors. The results are reported and discussed.