Low input coil inductance SQUIDs for cryogenic current comparator applications

Dc SQUIDs with an optimal input coil inductance have been developed for a C ryogenic Current Comparator (CCC) that is used for the calibration of elect rical standards, We studied a series of SQUIDs with input inductances in th e range from 20-160 nH. The electrical properties like input current noise and flux-to-voltage transfer have been investigated, The CCC is an overlapp ing tube configuration and the tube itself is used as the pick-up coil of t he flux transformer circuit of the SQUID. The coupling between CCC and flux transformer is in this case ideal and should have an optimal value when th e effective overlapping tube inductance, typically in the range from 10-100 nH, equals that of the SQUID input coil (flux transformer theory), To comp are with theory, sensitivity measurements on the SQUID-CCC have been perfor med in a special set-up where the effective overlapping tube inductance can be modified placing the CCC in a superconducting shield at various distanc es.