Extensive characterisation of advanced manufacturing solutions for the ITER Central Solenoid pre-compression system

Abstract

The ITER Central Solenoid (CS), positioned in the center of the ITER tokamak, will provide a magnetic field, contributing to the confinement of the plasma. The 13m high CS consists of a vertical stack of 6 independently driven modules, dynamically activated. Resulting opposing currents can lead to high separation forces. A pre-compression structure is implemented to counteract these opposing forces, by realising a continuous 180 MN coil-to-coil contact loading. Preload is applied by mechanical fastening via 9 subunits, positioned along the coil stack, each consisting of 2 outer and 1 inner tie plate. The tie plates therefore need to feature outstanding mechanical behaviour in a large temperature range. High strength, Nitronic®-50 type F XM-19 austenitic stainless steel is selected as candidate material. The linearised stress distribution reaches approximately 250MPa, leading to a required yield strength of 380MPa at room temperature. Two different manufacturing methods are being studied for the procurement of these 15m long tie plates. A welded solution originates from individual head- and slab-forgings, welded together by Gas Metal Arc Welding (GMAW). In parallel, a single piece forged solution is proven feasible, impressively forged in one piece by applying successive open die forging steps, followed by final machining. Maximum internal stress is experienced during cool-down to 4K as a result of a large difference in thermal contraction between the support system and the coils. Furthermore, the varying magnetic fields in the independently driven coils introduce cyclic loading. Therefore, assessment of the two manufacturing solutions, in terms of both static and dynamic mechanical behaviour, is performed at ambient as well as cryogenic temperature. An extensive characterisation including microstructural and mechanical examination is conducted, evaluating the comparative performance of both solutions, reporting, amongst others, yield strength reaching the requirement for both solutions.