Properties of YBCO films at high current densities: Fault current limiter implications

We have studied the properties of superconducting strip lines, based on epi taxial YBCO thin films, at high current densities. Experiments performed wi th short constant current pulses show that a quasi spontaneous Highly Dissi pative e State (HDS) appears at high current densities. By carefully measur ing the temperature of the YBCO line, we found that its temperature is stil l below T-c when the HDS occurs, indicating that this state does not origin ate from a thermal runaway. Once initiated, this HDS starts to propagate wi th an initial velocity larger than 100 m/s, which is one order of magnitude higher than thermal velocities. We also applied constant voltage pulses to the YBCO line, simulating therefore a real short circuit. We found that th e spatial extent of the HDS along the line, fen; microseconds after the sho rt circuit, depends linearly on the applied voltage. These results allow to explain very well the fast switching properties observed in superconductin g fault current limiters (SFCL) and the fact that the peak current is limit ed at, typically, 3 times the critical current.