Flux kinetics and heat flow during flux jumps in type-II superconductors

The Faraday-rotation technique in combination with high-speed cinephotography has been used to study flux kinetics and heat flow associated with flux jumps in type-II superconductors. It has been shown that at ``runaway'' velocities, flux motion can be better described in terms of an exponential rather than a diffusion law. Transients observed to exist in the flux-penetrated region and thought to represent a flux relaxation or redistribution following the jump have been shown to be associated with heating effects in the Faraday glass. Heat flow and its relationship to the flux kinetics was studied for the particular case of a Nb–25%Zr disk sample at 1.4 K in different thermal environments; that is, in a vacuum or immersed in liquid-helium II. In the latter case, heat transfer to the bath was about three orders of magnitude more important than thermal conduction within the sample. Nevertheless, the assumption that flux jumps take place under essentially adiabatic conditions, made for purposes of analysis, turns out to be approximately correct; this supports the theoretical treatments of Swartz and Bean and Wipf.