An analysis of the low inductance ac-SQUID operated as a dc-magnetometer in a mixing mode

Yoshinao Mizugaki, Keiji Sugi

M. Kiviranta, Jari S Penttilä, Leif Grönberg +2 more

We have developed a set of SQUIDs optimized for readout of ac-biased transition edge sensors. Junction shunts are made of Pd and attached to cooling fins to facilitate low-noise operation at sub-kelvin temperatures. SQUID's have a low loop inductance in order to reach a large natural dynamic range even without negative feedback. The SQUIDs are intended to be used for frequency-domain multiplexing of X-ray calorimeter arrays. Both traditional dc-SQUIDs and novel un-SQUIDs are manufactured. The fabrication process and SQUID design criteria are reviewed.

R. Cantor, Tapani Ryhänen, Heikki Seppä

Qiyu Zhang, Huiwu Wang, Guanqun Li +4 more

Planar niobium nitride (NbN) dc superconducting quantum interference devices (SQUIDs) magnetometers were developed based on high-quality epitaxial NbN/AlN/NbN Josephson junctions for SQUID applications at higher operating temperatures. To improve the design of NbN dc SQUIDs, we measured the SQUID washer's inductance, which is a key SQUID parameter. A set of NbN dc SQUIDs with washers with different dimensions were implemented. The washer inductances were obtained from the periodic flux variation in the SQUIDs when a control current was injected directly into the SQUID washers; the results agree well with the ones calculated with a numerical simulation in InductEX software. We obtained an empirical expression to estimate the NbN SQUID inductance with washers with different geometric structures. Our results will be useful for designing NbN dc SQUID magnetometers in future.

Tao Hong, Hai Wang, Yi Zhang +5 more

The flux modulation scheme (FMS) is the standard readout technique of dc SQUIDs, where a step-up transformer links the SQUID to the preamplifier. The transformer's primary winding shunts the SQUID via a large capacitor while the secondary winding connects it to the preamplifier. A modulation flux having a frequency of typically 100 kHz generates an ac voltage across the SQUID, stepped up by the transformer. The SQUID with FMS is customarily operated in the current bias mode, because a constant dc bias current flows only through the SQUID due to the capacitor isolation. With FMS, however, the transformer ac shunts the SQUID so that in reality the operating mode is neither purely current-biased nor voltage-biased but rather nominal current-biased or “mixed biased.” Our objective is to experimentally investigate the consequences of ac shunting of the dc SQUID in FMS and the transformer's transfer characteristics. For different shunt values we measure the change in the SQUID bias current due to the ac shunt using another SQUID in the two-stage readout scheme, and simultaneously monitor the SQUID output voltage signal. We then explain our measurements by a simplified graphic analysis of SQUID intrinsic current-voltage (I–V) characteristics. Since the total current flowing through the SQUID is not constant due to the shunting effect of the transformer, the amplitude of SQUID flux-to-voltage characteristics V(Φ) is less as compared to the direct readout scheme (DRS). Furthermore, we analyze and compare V(Φ) obtained by DRS and FMS. We show that in FMS, the transfer characteristics of the SQUID circuit also depend on the isolation capacitance and the dynamic resistance of the SQUID.

Jason F. Ralph, Timothy P. Spiller, Terrence D. Clark +5 more