AC loss and electrical resistance of the RW3 diffusion-bonded joint

Abstract

For the European DEMO fusion reactor, several options of the Toroidal Field (TF) coil design were proposed. The winding of the TF coil option 1 is based on React&Wind (RW), Nb3Sn, graded conductors in order to optimize the amount of superconductor according to the amplitude of the magnetic field in individual layers, as well as to limit the stress applied to the conductor using thicker jacket where the Lorentz forces are higher.In order to splice two different grades of Nb3Sn conductors, a joint is made to insure the adequate thermal and electrical contact between both conductors ends. For the good operation of the magnet, both electrical resistance and AC loss of the joint must be sufficiently low. A novel conductor splicing technique has been developed at Swiss Plasma Center (SPC) based on diffusion bonding and copper spray coating. The first joint prototype based on this technology was manufactured and tested successfully in 2019.The scopes of this paper are as follows: a) to present the design, the manufacture and the test of the second joint prototype based on diffusion bounding, b) to address the issue of joint resistance degradation during cyclic loading, experienced by the first prototype and c) to investigate on the reproducibility of the joint technique testing a second prototype.