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-AlO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub>/Nb Josephson junctions for cryogenic current comparator. 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 1.5-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 to the large-inductance input coil. A current sensitivity of 1.1 μA/Φ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub>, a flux noise of 3.8 μΦ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub>/√Hz at 4 kHz and 9.7 μΦ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub>/√Hz at 1 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<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and a flux noise of 17 μΦ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub>/√Hz at 4 kHz.