INSTABILITY OF NbTi/CuNi SUPERCONDUCTING WIRES

Superconducting wires with a CuNi matrix for a.c. applications are not fully stabilized and normal transition is easily induced by various disturbances because the CuNi matrix has a high electrical resistivity and a very low thermal conductivity. We have investigated normal zone propagation velocities in NbTi/CuNi superconducting wires[1]. In a.c. applications of superconductivity we also need to know the thermal and the electromagnetic behaviour during such the abrupt increase of currents as short-circuit current in superconducting transformers, the current changing in superconducting current limiters and the current redistribution during quench process in multi strand cables. We measured the degradation of the quench current of NbTi superconducting wires with a CuNi matrix as a function of the current sweep rate in the range of 10 A/s to 100 kA/s and the initial current, which is a transport current just before current changing. Experimental data is compared with analytical results by the finite element method taking the magnetic and the thermal diffusion in radial direction of wire cross-section into account, From these comparisons, the relation between the rate of current changing and the degradation of the quench current in superconducting wires is discussed.