ITER magnet design and R&D

Abstract

The magnet design described in the ITER final design report (FDR) has a ‘bucked’ configuration, with the toroidal field (TF) coils resting against the central solenoid (CS). The TF conductor design uses a circular Incoloy 908 jacket, containing a cable of Nb 3Sn strands. The conductor is insulated and embedded into a groove cut in each side of a steel plate to form a double pancake. The winding pack is formed from a stack of such plates. The CS, to provide a smooth cylindrical outer surface to support the high pressure applied by the TF coils, is layer wound using a square shaped Nb 3Sn conductor to form a single monolithic coil. Overturning forces on the TF are reacted by crowns resting above and below the TF coil inner leg, as well as an outer intercoil structure and a massive case surrounding the winding pack. The design was conceived as a compact structure for optimum cost and reliability under the engineering design activity (EDA) requirements. The magnets are required to operate for 50 000 full power plasma pulses. The bucked concept suppresses fatigue in the CS, but the overturning forces applied on the TF coil inner leg cases, as they press on the CS, induce limiting shear stresses in the CS. The plasma position and shape can be adequately controlled by the poloidal field coil system, including a single monolithic CS coil. R&D for the magnets has concentrated on the production of two large model coils representative of the CS and TF configurations, but has also included extensive investigations into superconducting cable behaviour, insulation and structural material properties. Testing of the model coils will not occur before 1999 but many of the principles included in the magnet design and operation have already been confirmed. Valuable data on production costs and critical tolerances has also been obtained, for strand, conductor, windings and structures.