Washer designs for SQUIDs with several junctions in a parallel array

We report on washer designs for superconducting quantum interference devices (SQUIDs) which include more than two Josephson junctions in a parallel array, and on calculations of the operating characteristics of such devices in the resistively-shunted junction model. Coupling of the operating current to the multi-junction device suppresses the voltage biased at constant current. The coupling decreases as the ratio of the mutual inductance between adjacent loops to the self inductance of a single loop approaches 0.5. In various washer designs, however, the practical limit for the ratio is no greater than about 0.4. At this value, the simulations indicate that significant reductions in voltage modulation occur for even a small number of junctions. Also, the flux responsivity increases with the number of junctions only slightly more rapidly than does the total loop area, so the sensitivity to a current in a coupling coil is nearly the same as that of a single-loop device. This trend may ultimately limit the usefulness of these devices.<>