Experimental results of stability and current sharing of NbTi cable-in-conduit conductors for the poloidal field coils

Japan Atomic Energy Research Institute has developed and tested the 30-kA NbTi Demo Poloidal Coils whose stored energy and inner diameter were 30 MJ and 1 m, respectively. The conductor of these coils consists of Formvar-coated NbTi strands inside the stainless steel conduit. During the charging test, an instability was observed. Stability and current sharing of the cable-in-conduit conductor were studied to solve the problem. These experimental results clarified that the cause of the instability was the low stability margin of the conductor due to the nonuniformity of current distribution between the strands. Therefore, the surface of the strand of the new conductor was designed to be chromium instead of the Formvar. Two types of the new 30-kA conductors were fabricated. One of these conductors was tested, and good results were obtained. These new conductors can be used for the conductor of the outer ring coils of the International Thermonuclear Experimental Reactor (ITER) with minor modifications.<>