Broadband Electromagnetic Modeling of the Superconducting Toroidal Field Magnets of JT-60SA: Transient Simulation and Fault Detectability

The superconducting toroidal field (TF) magnets of JT-60SA are designed to conduct a 25.7 kA current to generate a steady magnetic field for plasma confinement. In case of loss of their superconductive status, a fast discharge of the current is operated and the TF magnets are subjected to electromagnetic transient phenomena in a range of frequencies up to several tens of kHz. The TF coils show an involved frequency-dependent behavior as a consequence of eddy currents induced in the casing, stray capacitances, as well as electrical skin-effect in the jacket enclosing the strands; therefore, electrical models derived considering the coil characteristics at a single frequency may not be suitable to perform reliable analyses of the fast discharge event. A broadband electromagnetic model was generated and validated against experimental data. Such a model can reproduce with sufficient accuracy the behavior of the TF magnets in the frequency range of interest. This model was employed to simulate the transient of the voltage distribution inside the coils during a fast discharge and to evaluate the maximum voltage difference between adjacent turns. The model was also employed to make predictions about the detectability of internal short-circuit in a coil by means of impedance spectrum measurements. It was possible to conclude that the highest sensitivity to short circuits can be achieved looking for impedance variations in proximity of the antiresonant frequency.