Fault analysis and overvoltage estimation in the DEMO Toroidal Field circuit

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, has to be quickly dissipated in case of quench by suitable Fast Discharge Units (FDU). The energy, the current and the discharge time constant define the voltage to be applied to the coils; however, the peak value at the coil terminals during the fast transient phase at the beginning of the discharge or in case of faults can be much higher. This paper deals with first studies addressed to estimate maximum voltage stresses on the coils in various operating and fault conditions and to evaluate the relative merit of different TF circuit topologies to support the design work, aimed at finding the best compromise between requirements for the coil insulation and cost and size of FDUs, busbars and current leads. 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.