Fault current limiting properties of YBCO-films on sapphire substrates

We have studied the switching of YBCO thin film resistive fault current lim iting devices. Films of 300 nm thickness were deposited on 2 inch and 4 inc h sapphire substrates by thermal co-evaporation. Bridges 10 mm wide and 22 mm long (2 inch) or 42 mm long (4 inch) were structured by standard photoli thography. Contacts were made by in-situ gold overlayers and soft solder. T he gold film was removed from the switching area so that the YBCO film was not shunted. The films were tested by 30 Its DC pulses and 50 Hz AC pulses for 50 mu s. We find evidence that at the AC tests heat propagates over sev eral cm under these conditions in sapphire so that hot spots can be avoided even without shunt layer with the prospect of higher switching power. The highest destruction free switching power - the rms critical current times r ms voltage after switching - was 57 kVA. The highest switching power densit y achieved was 2.5 kVA/cm(2). To our knowledge, these are the highest switc hing power densities observed so far.