Thermal exchanges and transition of bulk YBCO conductors

The influence of heat transfer during the transition of bulk YBCO was studied within the framework of the resistive fault current limiter. The aim of this work is to determine whether the cryogenic surroundings has an influence on the limitation ability and process. Samples are meanders (section: 4 mm2) cut in YBCO single domain pellets. All the electrical measurements were carried out above 90 K. Firstly, the sample is introduced into a vacuum container. The magnetic penetration (AC losses signal), the transition (power law) and the thermal runaway are successively observed. The current slope influence on the V(I) characteristic during the transition is significant. The thermal runaway occurs for a small energy of 25 mJ cm-3. Secondly, the sample was plunged directly in a pressurised liquid nitrogen and its transition was compared to the vacuum experiment. It was observed that the transition depends on heat transfer conditions. The transition in a liquid is finally analysed and its influence on the current distribution is discussed