Investigations About Quench Detection in the ITER TF Coil System

Due to the large stored energy (40 GJ) and the small allowed number of fast discharges (50), severe requirements have been put in the ITER project for the Toroidal Field (TF) coil system in comparison with the Poloidal Field (PF) and Central Solenoid (CS) systems, aiming at avoiding any fast discharge not related to a quench. A very important point, which has to be examined, is whether a quench detection based on a co-wound tape, recommended in the ITER project, and located inside the conductor insulation is compulsory to ensure the inductive voltage compensation. Another possible solution is the balance of the voltage of TF coils or TF coils subcomponents. The sensitivity of the quench detection systems is examined according to the different types of flux variations experienced in the machine. During plasma discharge, the sensitivity to poloidal flux variations and plasma paramagnetism is highlighted, the last effect being illustrated for Tore Supra. It is eventually recommended to select the quench detection by balancing coils or coils subcomponents. This solution has been adopted for Tore Supra, KSTAR and in JT-60SA project. The level of refinement should be adjusted during commissioning, as a function of the required voltage detection level (0.4 V) and to the required holding time (1 s).