Highly balanced single-layer high-temperature superconductor SQUID gradiometer freely movable within the Earth s magnetic field

Abstract

We developed a gradiometer system based on a single-layer high-temperature superconductor dc superconducting quantum interference device (SQUID), which can be freely moved within the Earth's magnetic field during measurement. The problem of circumferential shielding currents in the parallel gradiometer pick-up loop is solved by the use of an appropriately designed magnetometer SQUID integrated on the gradiometer chip. The magnetometer's feedback coil of the flux-locked loop is laid out as a small Helmholtz coil pair, thus keeping the homogeneous magnetic field constant for both the magnetometer and the gradiometer. Therefore, the balance of the directly coupled gradiometer SQUID is enhanced from 100 up to 3800. The noise limited magnetic field gradient resolution of 45 pT m −1 Hz −1/2 is preserved down to frequencies of several Hz even after strong motion in the Earth's magnetic field.

Superconductive Digital Magnetometers with Single-Flux-Quantum Electronics

“…Analog SQUIDs have proven to be the most sensitive magnetometers with sensitivities of the order of 1 fT/ √ Hz [8], [9] for low-T c SQUIDs and below 40 fT/ √ Hz [1], [2], [10] for high-T c SQUID magnetometers. SQUIDs gradiometers are also at the state-of-the-art in terms of sensitivity that can reach a few tens of fT/cm/ √ Hz [10]- [12]. Though the sensitivity of analog SQUIDs is unsurpassed, a customized electronics is usually designed with a flux-locked loop [13] to obtain a stable point of operation.…”

ISEC 2005—Conference summary and comments on SQUIDs and coolers

“…Standard SQUIDs can be found in LTS as well as HTS technologies in many groups, e.g. [2][3][4][5][6][7][8]; there are even a few available as commercial products 1,2,3,4,5 . Depending on the history and application considerations different types such as RF, DC, DROS and digital SQUIDs are used.…”

A high-TcSQUID gradiometer with integrated homogeneous field compensation

“…Therefore, this directly coupled gradiometer has also a non-vanishing sensitivity to homogeneous fields, which limits the balance to values of the order of 100. This problem was solved by Schultze et al [1] by the introduction of a specially laid out magnetometer, which is integrated on the same chip as the gradiometer (figure 1, right-hand side, and figure 2, bottom). The feedback coil of the magnetometer flux-locked loop keeps the normal component of the homogeneous magnetic field constant at both sensors: magnetometer and gradiometer.…”

“…In contrast to [1], where the feedback of the magnetometer was realized with a wire-wound Helmholtz coil, here small coils realized with printed circuit boards are used. These coils were originally developed for the Earth's field compensation of SQUID magnetometers [2].…”

“…It was found to have a good balance, i.e. a strong suppression of homogeneous fields, and is freely movable within the Earth's magnetic field [1]. The key to this behaviour was the integration of a small SQUID magnetometer on the same chip as the gradiometer.…”