Updating the design of the poloidal field coils for the ITER magnet system

Yoshida, Kiyoshi; Takahashi, Yoshikazu; Jong, C.*; Bessette, D.*; Mitchell, N.*

The ITER magnet system consists of 18 TF coils, 6 PF coils, 6 CS modules, 18 CC and their feeders. Because of the difficulties in replacing the PF coils, the most unreliable component (the coil insulation) is designed with extra redundancy. There are two insulation layers with a thin metal screen in between. By monitoring the voltage of the intermediate screen, it is possible to detect an incipient short, defined as a short in only one of the two insulation layers. Alternately the faulty double pancake must be disconnected and by-passed because PF conductor uses NbTi strand. This implies that the remaining pancakes are operated at a higher current as the backup mode. Jumpers are pre-installed on the coil surface to allow this reconnection with a minimum of works in the cryostat and the conductor is designed with an extra margin. This paper presents the latest design for by-pass operation in PF coil, and thermal analysis of its conductor for the backup mode. The PF coil has an enough design margin for normal and backup mode.