A discrete HTS Josephson flux-flow structure with gain at 77 K

We have developed flux-flow devices based on parallel arrays of HTS bi-crystal grain-boundary Josephson junctions (GBJs) with current gains well above unity (1.6-1.8) at 77 K. The symmetric control characteristics are similar to those of the LTS SuperCIT with the critical current maximum at zero control current and decreasing linearly with increasing control currents. Our design featured a control line that is defined by a parallel array of long slots in the base electrode. The slots confined the control current to within 2 /spl mu/m of the grain-boundary junctions resulting in tight coupling of the control fields to the junction array. Furthermore, the slots allowed the bias current to remain uniform as it crossed the control line into the base electrode. This combination was necessary to produce SuperCIT-like control characteristics as well as useful current gains in single-layer device structures. Device transresistance and usable output voltage range (typically 0.2-0.3 /spl Omega/ and 0.1 mV, respectively, for YBCO devices at 77 K) were limited by the low intrinsic shunt resistance and I/sub c/R/sub n/ product of the bi-crystal GBJs. Better device characteristics can be expected with improved junction qualities.