JT-60SA TF Magnet Joints Developments and Prequalification

In the framework of the so-called “broader approach” in connection with the International Thermonuclear Experimental Reactor ITER, the European and Japanese part- ners have agreed to in kind contribute to the construction of the JT-60SA tokamak in Naka (Japan). JT-60SA is a major upgrade of the former tokamak JT-60U, equipped with, in particular, superconducting coils. France and Italy, as European Voluntary Contributors, are sharing the supply of the 18 superconducting Toroidal Field coils, along with the corresponding mechanical structures and supports. These D shaped coils use a NbTi Cable-In-Conduit Conductor (CICC) and are electrically connected in series. Each coil is composed of double pancakes connected in series and two coil terminals, all located in a joint area at their outer radius at the upper part of the outer leg. The joints proposed by CEA is the twin box design which was initially developed by CEA for ITER Nb₃Sn conductors and which is now adapted to the NbTi conductor. This design relies on the use of copper/steel explosion bonded plate as base material to constitute each conductor termination, the joint being assembled by soldering of the adjacent terminations. Such a design was successfully qualified on a pre-prototype sample in the CRPP SULTAN facility. To simplify the inner joints regarding the winding pack manufacture process, an evolution towards a monobloc twin box concept is proposed. After production and qualification of the needed prototype steel/copper/steel 3-layer explosion bonded plate, a dedicated joint mock-up was used to define the corresponding manufacture process. Specific processes were also developed for the coil terminals which have to be connected in a dismountable way for the cold acceptance tests of the coils in Europe, as well as in a definitive way at the Japan site. A low cost dedicated facility was developed to measure the DC resistance in order to perform a simple prequalification in cryogen- c conditions on the produced mock-ups. A benchmark of the facility using the previously prequalified sample was done to validate this facility as a simple tool usable for this resistance measurement. The paper reports the different joint proposals and the manufacture processes developed as well as the pre-qualification results.