Three SQUID gradiometer

Abstract

We have invented a three superconducting quantum interference device (SQUID) gradiometer (TSG) that uses three SQUID magnetometers and a novel feedback method to measure magnetic field gradients. One SQUID, designated the reference SQUID, operates normally except that its feedback loop output is directed to all three SQUIDs through identical nonsuperconducting coils around each SQUID. The feedback loops for the remaining two SQUIDs, the sensor SQUIDs, measure the differences between the magnetic field at the reference SQUID location and those at the sensor SQUID locations. The voltage difference between the two sensor SQUID outputs divided by the gradiometer base line, the distance between the sensor SQUIDs, represents the average magnetic field gradient. We have measured gradient sensitivities of 10−12 and 10−10 T/m√Hz for TSGs made from bare low-Tc and high-Tc SQUIDs. An advantage of a TSG is that a sensitive gradiometer, free of hysteresis error, can be made using relatively small substrates.

High-transition-temperature superconducting quantum interference devices

“…None of these difficulties arises with superconducting gradiometers, which thus have an inherent advantage over electronic cancellation. However, the three-SQUID gradiometer (TSG) of Koch et al (1993), shown in Fig. 31, also circumvents these problems by using electronic cancellation and subtraction.…”

Actively Shielded, Adaptively Balanced SQUID Gradiometer System for Operation Aboard Moving Platforms

“…Another barrier to the gradiometric flux transformer approach is that devices made from HTS materials show hysteresis if exposed to the full earth's field [2]. A good gradiometer configuration has been demonstrated by Koch [3] where a reference SQUID is used to negatively feed back magnetic field to the two other SQUIDs that form the gradiometer. This dramatically reduces the dynamic range requirements on the outer SQUIDs by (largely) canceling the earth's field at the sensors.…”

Long Baseline 2nd-Order High-$T_{\rm c}$ SQUID Gradiometer Made From a Single-Layer ${\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{7}$ Film

“…Electronic gradiometers have also been developed. Koch et al invented a three-SQUID gradiometer with a reference SQUID for baseline subtraction [5]. Tarvin et al developed a second-order gradiometer composed of three sets of SQUID magnetometers [6].…”