Operating modes of high-Tc composite superconductors and thermal runaway conditions under current charging

Abstract

The operating thermal and electric modes of ahigh-Tc superconducting composite in partially and fully penetrated states induced bythe charging current are investigated. They were studied under conditions inwhich the current charging rate, the volume fraction of the superconductor in acomposite or the temperature of the cooling bath were changed. The transientbehaviour of the voltage–current dependence, which is characteristic during stable andunstable increases in electric field inside the composite under a continuous currentcharging, is discussed. Simulations were done using zero- and one-dimensionalsteady and unsteady thermoelectric models with a power equation describing thevirgin voltage–current characteristic of a superconductor. It is found that somethermoelectric trends underlie the shape of the voltage–current characteristic of thehigh-Tc superconducting composite. These have to be considered during experiments in whichthe critical or quench currents are defined. First, in the initial stage of the fullypenetrated regime (in the low voltage range), the electric field distribution does nothave a uniform character. These states depend on the volume fraction of thesuperconductor and the current charging rate: the higher these quantities, thehigher the heterogeneity of the electric field. Second, during the stable over-criticalregime (in the high voltage range) occurring in complete penetration modes, theevolution of the electric field may depend on the relevant temperature increase of acomposite according to the corresponding increase in its temperature-dependent heatcapacity. Consequently, the shape of the voltage–current characteristic of a compositehigh-Tc superconductor during continuous current charging, both before and after thermal runaway,has only a positive slope. Moreover, it is proved that the growth of the fully penetrated part ofthe voltage–current characteristic becomes less intensive when the current charging rate orthe coolant temperature increase. That is why the voltage–current characteristic of ahigh-Tc superconductor cannot determine the boundary of onset of thermal runaway. It is alsoconfirmed that there is a thermal degradation mechanism of the current-carrying capacityof the composite. In particular, according to this mechanism, the quench currents do notincrease proportionally to an increase in the amount of superconductor in a composite.