Effect of voltage–current characteristic smoothness on the stability margin of cable-in-conduit conductors

Experimental and numerical investigation of the AC losses induced by the transient magnet field (TMF) in cable-in-conduit conductors (CICC) revealed some interesting results. First of all, the critical current in the strands could be exceeded as a result of simultaneous increase of the strand temperature due to AC losses and the addition of the induced shielding currents to the transport current. Secondly, after the sample reaches its critical state the energy stored in the loops formed by the superconducting strands and normal “matrix” quickly dissipates in the normal matrix of the strands and in the normal “matrix” of the bundle. This results in a step increase of the conductor temperature. It was logical to assume that during the process preceding the occurrence of the first flux-jump, the maximum temperature and shielding current could depend on the smoothness of the voltage–current characteristic (VCC) of the superconducting strands. In other words, the smoothness of the VCC of the superconducting strands should influence the stability margin of a conductor. A numerical investigation of the VCC smoothness effect on stability margin was carried out for a wide range of the Stekly parameter, α, and varying the dimensionless transport current i = It/Ic (0 ⩽ α < 50; 0 ⩽ i < 1). As a result of these calculations, it was found that the energy necessary for the first flux-jump to occur, grows as the VCC smoothness increases, when the Stekly parameter lies in the range 0 ⩽ α < 1, and the stability margin of a conductor decreases as the VCC smoothness increases, when the Stekly parameter α ⩾ 1.