Investigations on Usability of Undermatched Joint in a Superconducting Coil Case for Fusion Reactor

In a Power Fusion Reactor, a huge megnetic field is necessary for plasma confinement. This field also interacts with the high current to produce large forces tending to deform the superconducting cables ; this electro-magnetic force demands very high values of the cryogenic yield strength and fracture toughness in the supporting structural materials. It is said to be difficult to develop welds satisfing the same criteria as for the structural material. It is useful to investigate the usability of welded joint with a lower yield strength than the base metal, but associated with a higher fracture toughness.From this viewpoint, the influence of the undermatched joint on the deformation and the stressstrain property of the coil case was investigated by using a finite element method. It is concluded that (1) there is only a slight difference in the overall deformation of the coil case whether the undermatched joint is included in the calculation or not, (2) the maximum strain and the muximum plastic strain at the undermatched welds can be controlled to be less than those in the base metal with a judicious choice of weld location and yield strength level of welds.