Current sharing and the stability of composite MgB2superconductors

I – V characteristics of various MgB 2 superconductors were analysed. The transition between the superconducting and normal states of an MgB 2 composite conductor is accompanied by a rapid current redistribution between MgB 2 filaments and the metallic sheath. It was clarified why the nature of the transition is different at low and at high critical currents. The heat flux and current density in MgB 2 and those in the metal sheath at the transition were determined. The transition of the filaments into a normal state is caused mainly by the heat generated in MgB 2 at high critical currents. On the other side, at low critical currents, mainly the heat generated in the sheath material is responsible for the transition of a filament. Based on the current redistribution in the matrix, the consequent heat generation and the critical current versus magnetic field dependence, various composite MgB 2 conductors were compared and discussed for their stability.