Design and Properties of SQUID Sensors With Octagonal Double Transformer for Cryogenic Current Comparator

Superconducting quantum interference devices (SQUIDs) with large input inductance and octagonal double transformer were designed and fabricated using Nb/Al-AlOx/Nb Josephson junctions (JJs) for cryogenic current comparator (CCC). A 30-turn input coil of 1.5 $\mu $ H is coupled to a large octagonal transformer of 3.6 nH, and the counterpart of the transformer is a 1.5-turn coil of 3.0 nH coupled to a second-order gradiometric SQUID of 97 pH. The octagonal double transformer makes low-inductance SQUID easily match the large-inductance input coil. A current sensitivity of 1.1 $\mu $ A/ $\Phi _{0}$ , a flux noise of 3.8 $\mu $ $\Phi _{0}$ / $\surd $ Hz at 4 kHz, and 9.7 $\mu $ $\Phi _{0}$ / $\surd $ Hz at 1 Hz were achieved at 4.2 K. Another SQUID sensor with ten-turn input coil of 0.5 $\mu $ H coupled to a large octagonal transformer of 6.0 nH, which is coupled to a first-order gradiometric SQUID of 580 pH, achieves a lower-current sensitivity of 0.5 $\mu $ A/ $\Phi _{0}$ and a flux noise of 17 $\mu $ $\Phi _{0}$ / $\surd $ Hz at 4 kHz.