Local normal zone nucleation to ‘global’ transition: quench development in superconductors with changing current

Abstract

The quench process in a superconductor carrying a varying transport current l( t) is studied. Thermomagnetic disturbances induced by the varying current cause normal zone nucleation at non-uniformities related to inhomogeneous physical properties along the superconductor. It is shown that, depending on the value of the current rate of change .l = dl dt the quench can be either local in character or can occur over the whole sample length simultaneously due to the development of thermomagnetic instability. An equation is obtained for the threshold current rate of change . l c , above which the latter ‘global’ thermomagnetic instability occurs. The influence of non-uniformities of different types on the quench process is considered for cases of both increasing ( . l > 0) and decreasing ( . l < 0) currents. The analytical results obtained are confirmed by a numerical simulation of the quench process and they give a qualitative description of the experimental results. The processes considered may be of importance in understanding the effect of multiple zone nucleation in superconducting a.c. devices.