Highly Balanced Long-Baseline Axial Gradiometer Based on High-<tex>$rm T_rm c$</tex>Superconducting Tape

Abstract

The improving quality of high-c superconducting (HTS) tape with critical current densities larger than 1 MA cm 2 creates the possibility to construct high quality flexible superconducting electronics operating at liquid nitrogen temperatures. We patterned a symmetric flux transformer into a 700 nm thick YBa 2 Cu 3 O 7 x film on a 70 m thick, 85 mm long Hastelloy tape. The center loop of the transformer was coupled to the pickup loop of a SQUID magnetometer in a flip-chip configuration. The two outer pickup loops of the transformer were bent such that they were facing each other perpendicular to the magnetometer plane. The resulting axial gradiometer has a long baseline of 35 mm and a gradient sensitivity of 7.3 nT (cm8 0 ). A superconducting shield was used to reduce uniform magnetic fields applied perpendicular to the magnetometer plane. Common-mode rejection ratios less than 10 4 were achieved in the best case. The noise-limited gradient field resolution was approximately 330 fT (cm Hz) at a frequency of 10 Hz. This resolution was mainly limited by the flux noise level of our dc-SQUID magnetometer of approximately 45 8 0 Hz. In an unshielded laboratory environment, external noise contributions at 50 Hz were reduced by a factor of approximately 10 3 .

Preparation of a high-T c superconducting magnetic flux transformer with a 100 mm bore coil and static magnetic field transfer at 77 K.

“…However, this can be accomplished by both optimizing the design of the HTS flux transformer and modifying the magnetic coupling by direct mounting of the magnetic sensor, which is operable at 77 K, on the HTS flux transformer. This technique can be expanded to fabricate a multi-turn coil with a larger diameter and is highly applicable to the gradiometer structure of the HTS flux transformer, 26,27) which eliminates the effect of geomagnetism from the measurement of the magnetic field. HTS flux transformer is expected to be used for precise field inspection technology for non-destructive evaluation 28,29) by virtue of its convenience for refrigeration to operate the device and sensitivity to the static field, which is superior to permeability within a deep region of the object.…”

High-<i>T_c</i> Superconducting Electronic Devices Based on YBCO Step-Edge Grain Boundary Junctions

“…An alternative technique to using an array of planar gradiometers for measuring the full magnetic gradient tensor Fig. 10 (a) A rotating gradiometer consisting of a dc-SQUID magnetometer inductively coupled to a flux transformer and a planar superconducting shield, and (b) a patterned Au/YBCO flexible tape transformer [32], [33]. Fig.…”

“…was also developed by CSIRO. Rotating an axial gradiometer about its central axis enables the gradient information in a magnetic signal to be distinguished from the commonmode signal via frequency separation [32], [33]. As shown in Fig.…”

“…As shown in Fig. 10, a DC SQUID magnetometer is inductively coupled to a patterned flexible superconducting flux transformer and to a superconducting shield to form a 1st order axial gradiometer [32], [33].…”

Direct string magnetic gradiometer for space applications

“…Uniform fields contaminate the results since the common mode rejection ratio of gradiometers is finite. Typically the common mode rejection ratio is on the order of 10 4 [4][39] [40] although rotating gradiometers can do better [5]. The common mode rejection ratio tends to decrease with time [40] which introduces additional noise at very low frequencies.…”