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ST519 - SQUIDs De-fluxing Using a Decaying AC Magnetic Field

SQUIDs De-fluxing Using a Decaying AC Magnetic Field

Andrei N. Matlashov1, Vasili K. Semenov2, William H. Anderson3

1Physics Department, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
2Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794-3800
3Senior Scientific, LLC, Albuquerque, NM 87106, USA

E-mail: [email protected]

Abstract — Flux trapping is the Achilles’ heel of all superconductor electronics. The most direct way to avoid flux trapping is a prevention of superconductor circuits from exposure to magnetic fields. Unfortunately this is not feasible if the circuits must be exposed to a strong DC magnetic field even for a short period of time. For example, such unavoidable exposures take place in superparamagnetic relaxation measurements (SPMR) and ultra-low field magnetic resonance imaging (ULF MRI) using unshielded thin-film SQUID-based gradiometers. Unshielded SQUIDs stop working after being exposed to DC magnetic fields of only a few Gauss in strength. In this paper we present experimental results with de-fluxing of planar thin-film LTS SQUID-based gradiometers using a strong decaying AC magnetic field.  We used four commercial G136 gradiometers for SPMR measurements with up to a 10 mT magnetizing field. Strong 12.9 kHz decaying magnetic field pulses reliably return SQUIDs to normal operation 50 ms after zeroing the DC magnetizing field. This new AC de-fluxing method was also successfully tested with seven other different types of LTS SQUID sensors and has been shown to dissipate extremely low energy.

Keywords (Index Terms) — SQUID, gradiometer, flux trapping, demagnetization, de-fluxing, alternating current de-fluxing, decaying magnetic field.

IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), July 2016. 
Received June 1, 2016; Accepted July 21, 2016. Reference No. ST519; Category 4.
This is a partial preview of presentation at ASC 2016.