Octagonal Double Transformer SQUID sensors with Large Input Inductance for Cryogenic Current Comparator

Superconducting quantum interference devices (SQUIDs) with large input inductance and octagonal double transformer were designed and fabricated using Nb/Al-AlO
x
/Nb Josephson junctions for cryogenic current comparator (CCC). A 30-turn input coil of 1.5 µH is coupled to a large octagonal transformer of 3.6 nH. The counterpart of transformer is a 1.5turn coil of 3.0 nH coupled to a second-order gradiometric SQUID of 64 pH. The octagonal double transformer makes lowinductance SQUID easily match to the large-inductance input coil. At 1 kHz, a current sensitivity of 1.1 µA/Φ
0
, a flux noise of 3.7 µΦ
0
/√Hz and a current noise of 4.1 pA/√Hz were achieved at 4.2 K. Another SQUID sensor with 10-turn input coil of 0.5 µH 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 µA/Φ
0
and a current noise of 5.9 pA/√Hz.