Low input coil inductance SQUIDs for cryogenic current comparator applications

Dc SQUIDs with an optimal input coil inductance have been developed for a Cryogenic Current Comparator (CCC) that is used for the calibration of electrical standards. We studied a series of SQUIDs with input inductances in the range from 20-160 nH. The electrical properties like input current noise and flux to voltage transfer have been investigated. The CCC is an overlapping tube configuration and the tube itself is used as the pick-up coil of the 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 the effective overlapping tube inductance, typically in the range from 10-100 nH, equals that of the SQUID input coil (flux transformer theory). To compare with theory, sensitivity measurements on the SQUID-CCC have been performed in a special set-up where the effective overlapping tube inductance can be modified placing the CCC in a superconducting shield at various distances.