Analysis of Maximum Voltage Transient of JT-60SA Toroidal Field Coils in Case of Fast Discharge

The voltage transient appearing across and inside the toroidal field (TF) coils of JT-60SA in case of fast voltage variation, such as a safety discharge operated by the quench protection circuit (QPC), can be significantly high. In fact, the voltage distribution between coils and inside the winding can be not uniform during fast transient, being influenced by the presence of parasitic capacitances. A simplified electrical model of the TF coils has been developed to investigate this aspect. Its robustness has been proved by means of parametric sensitivity analysis, and the impact of the included simplifications has been evaluated. The obtained model has been used in conjunction with an electrical model of the TF circuit elements, including a simplified model of the QPC able to reproduce the voltage appearing across its terminals as observed during experimental operation of the QPC prototype. The worst case in terms of transient voltage applied to the winding has been identified, corresponding to a fault to ground occurring just after QPC operation. It has been verified that the resulting voltage is largely inside the coil insulation capability defined by performed insulation voltage tests.