WASHER DESIGNS FOR SQUIDS WITH SEVERAL JUNCTIONS IN A PARALLEL ARRAY

We report on washer designs for superconducting quantum interference d evices (SQUIDs) which include more than two Josephson junctions in a p arallel array, and on calculations of the operating characteristics of such devices in the resistively-shunted junction model. Coupling of t he operating current to the multi-junction device suppresses the volta ge modulation when the device is biased at constant current. The coupl ing decreases as the ratio of the mutual inductance between adjacent l oops to the self inductance of a single loop approaches 0.5. In variou s washer designs, however, the practical limit for the ratio is no gre ater than about 0.4. At this value, the simulations indicate that sign ificant reductions in voltage modulation occur for even a small number of junctions. Also, the flux responsivity increases with the number o f junctions only slightly more rapidly than the does the total loop ar ea, so the sensitivity to a current in a coupling coil is nearly the s ame as that of a single-loop device. This trend may ultimately limit t he usefulness of these devices.