Quench properties of Y-Ba-Cu-O films after overpowering quenches [superconducting fault current limiters]

The authors present the property degradation and quench behavior of a current limiting element under repeated quenches. The current limiting element was meander type YBa/sub 2/Cu/sub 3/O/sub 7/ stripes deposited on Al/sub 2/O/sub 3/ substrates and coated with a Au shunt. The films showed reproducible quench properties under repeated quenches when the gold layer was coated in-situ on the YBCO film. The films, however, revealed apparent degradation in properties even after the second quench when the gold layer was coated ex-situ on the YBCO film. The authors speculate that incomplete contact between the YBCO and the gold layer produced negative effect on the limiter property. The quench current density J/sub q/ was improved at source voltages up to 300 V/sub rms/ and saturated afterwards when the voltage was increased stepwise by 10 V/sub rms/ from 100 V/sub rms/ to 350 V/sub rms/. It is believed that the thermal shock in a rapid superconducting-normal transition gives the annealing effects to the sample. X-ray diffraction and Rutherford backscattering spectroscopy data did not change considerably after repeated quenches but some minor change appeared. This change might be related to the improvements in the quench current density.