Studies of YBCO strip lines under voltage pulses: optimization of the design of fault current limiters

We present experimental results on the behavior of a superconducting YBCO/Au meander of length L submitted to short circuit tests with constant voltage pulses. The meander, at the beginning of the short-circuit, is divided in two regions; one, with a length L₁ proportional to the applied voltage, which first switches into a highly dissipative state (HDS) while the rest remains superconducting. Then the rest of the meander will progressively switch into the normal state due to the propagation of this HDS (few m/s) from both ends. The part L₁ has to initially support a power density proportional to ρ·J_p² (ρ is the resistivity of the bilayer and J_p the peak current density). To avoid local excessive dissipation of power and over heating on one part of the wafer in the initial period, we have developed a novel design in order to distribute the dissipating section of the meander into many separated small dissipative zones. Furthermore the apparent propagation velocity of these dissipative zones is increased by the number of propagation fronts. We will show results obtained on 3 kW (300 V, 10 A) FCL on a 2" wafer which confirm the benefits of this new design.