Rf SQUID Gradiometer Research Articles

Non destructive testing (NDT) with high Tc RF SQUIDs

A high Tc rf SQUID nondestructive evaluation (NDE) set-up has been realized to measure the eddy current induced field in different aluminum samples, under a weak AC and low frequency magnetic excitation. An efficient and simple analytic modeling method has been also developed for computing the magnetic field created by the induced eddy current in the samples and compare it with experimental results. The modeling results for different samples with various calibrated defects correlate well with the experimental data obtained with the rf SQUID gradiometers in an unshielded environment.

Recording fMCG and adult MCG using multi-channel HTS rf SQUID gradiometers

Abstract We report our recent work to set up a multi-channel high temperature superconducting (HTS) radio-frequency (rf) SQUID gradiometer and to perform adult and fetal magnetocardiography (MCG). We have compared the adult MCG measurement results using a magnetometer and different gradiometers in a standard magnetically shielded room. The special requirements of a system for fMCG measurements will be discussed. We show that our multi-channel HTS rf SQUID gradiometers are capable of recording the cardiac activity of two fetuses (31st and 33rd week of gestation) in real-time.

Foetal magnetocardiography with a multi-channel HTS rf SQUID gradiometer

Foetal magnetocardiography (fMCG) is a promising technique for prenatal cardiac diagnosis.In this paper, we discuss the special requirements of a system for fMCG measurements. ASQUID system incorporating five HTS rf SQUID magnetometers has been developed andconstructed. Four magnetometers are arranged at the bottom of the cryostat in a4 cm × 4 cm square configuration. One reference magnetometer is located 20 cm above the plane to formfour first-order axial gradiometers with the four bottom magnetometers. Themagnetometer with a 18 mm diameter flux focuser, which reaches a field sensitivity of20–30 fT Hz−1/2, proved sufficient for recording foetal magnetocardiograms in a magneticallyshielded room. The cardiac activities of two foetuses (31st and 33rd weeks ofgestation) were recorded in 90 Hz bandwidth. The foetal QRS peak signals rangedfrom 4 to 7 pT and could easily be identified in the real-time gradiometer outputswith a signal-to-noise ratio of up to five. Furthermore, the averaged fMCG dataenabled the determination of de- and repolarization time intervals. Overall, thefMCG signals proved of sufficient quality to perform foetal heart diagnostics.

SQUID method of lung contamination testing

We are reporting on the development of a SQUID magnetometric method of ferromagnetic dust quantification in the human lungs. In order to solve this problem we utilize a forward method of magnetized ferromagnetic particle (dipole) distribution 3D modeling in human lung torso and in an arc welder’s lungs. We also solve the inverse problem, by which the amount of dust in the lungs is estimated using the results of the remanent magnetic induction Br measurement upon the human chest. We state the formula for SQUID measured output voltage U to Br conversion for the second order gradiometer, which is in a highly dipole position and density dependent. We utilize a low-Tc second order rf SQUID gradiometer with the sensitivity of 10−14T in the unit frequency range.

Multi-Channel HTS rf SQUID Gradiometer System Recording Fetal and Adult Magnetocardiograms

We report on experiments with a multi-channel HTS radio-frequency (rf) SQUID gradiometer for recording fetal and adult magnetocardiograms. Four sensing SQUID magnetometers and two common reference SQUID magnetometers form a 4 channel electronic gradiometer system of either first or second order. The magnetometers consist of HTS step edge SQUIDs and flux concentrators fabricated from YBaCuO thin films, with dielectric substrate resonators serving as tank circuits. With a washer area of 18 mm in diameter, all six magnetometers reached a field sensitivity of 20-30 fT//spl radic/Hz. Each gradiometer channel is formed using two or three such magnetometers with individual readouts in electronic difference. The dc and rf crosstalk between any channel pair was measured. In ordinary operation we did not find any noise contribution from neighboring channels, even though the resonant frequencies of the resonators are closely spaced. In a standard magnetically shielded room, using a first-order gradiometer configuration with an ultra-long baseline of about 20 cm, we demonstrated 4 channel real-time heart signal recordings of a fetus in the 33rd week of gestation.

Multiplexed SQUID array for non-destructive evaluation of aircraft structures

SQUID sensors offer a significant advantage for eddy-current (EC) testing of aircraft components for material flaws hidden deeply in the tested structure. However, the requirement to take maps of the magnetic field, usually by meander-shaped scans, leads to unacceptably long measurement times. Due to their inductive coupling to a tank circuit, several rf SQUID sensors may be read out sequentially by selectively coupling to their tank circuits, using only one electronics with a multiplexer. The multiplexed operation of three planar HTS rf SQUID gradiometers with one electronics and one cable is shown, demonstrating the advantage of lower liquid nitrogen boil-off. Independent operation and switching is confirmed using local coil excitation of the individual SQUIDs. We report on the implementation of two multiplexed SQUID sensors in conjunction with an EC excitation and lock-in readout at unshielded laboratory environment. Scanning is performed while continuously switching the operating SQUID, thus obtaining two traces simultaneously. The applicability to EC testing of riveted sections of aircraft fuselage is discussed.

Measurements of the magnetocardiography in unshielded environment using high-T c rf SQUID gradiometer

We recorded the 9-point magnetocardiography (MCG) of a normal person in an unshielded environment by using a first-order gradiometer made by subtracting the outputs of two high-T c YBCO rf SQUID magnetometers. A compensation method was used to cancel the 50Hz line interference. After data processing an averaging, the fine structures of the heart signal (P-wave, Q,R,S-complex, T-wave) could be seen clearly and a SNR of 120 could be obtained.

Application of a two-hole HTS RF-SQUID gradiometer to non-destructive evaluation

A high-temperature superconducting (HTS) RF-SQUID gradiometer was used to make a non-destructive evaluation (NDE) device. A two-hole HTS RF-SQUID gradiometer fabricated with TBCCO was used to measure directly the magnetic field without using flux transformer coils. We incorporated an exciting coil in our NDE system to induce the eddy current in the sample. Small cracks in the aluminium at a depth in below the surface could be detected. Also the reinforcing bars in concrete blocks could be located with our NDE device. A NDE device made with a HTS RF-SQUID could make a significant impact on metal fatigue detection technology.

Measurements of Biomagnetism with a Magnetic Shield for a High-Tc Superconductor

We developed a helmet-sized superconducting magnetic shield for a high-T c Bi-Pb-Sr-Ca-Cu-O superconductor. The shield is 32 cm in diameter, 60 cm in depth, and 2.5 cm thick. It was experimentally confirmed that the container can reduce a weak magnetic field by ?100 dB or to 1/100,000. The shielding effect of the superconducting container does not decrease even at frequencies as low as 0.2 Hz. Using equipment consisting of a superconducting container, a cryostat to keep the container at liquid nitrogen temperature, and an rf SQUID gradiometer, we successfully detected the magnetic fields of a human brain. We detected somatosensory magnetic fields by stimulating the tibial nerves of the patient with current pulses.

High temperature RF SQUID gradiometer applied to non-destructive testing

This paper reports the utilization of a high temperature superconducting (HTS) RF SQUID gradiometer in non-destructive testing (NDT). A two-hole HTS RF SQUID, capable of working in a noisy environment without any magnetic shielding, is employed as a gradiometer sensor without using flux transformer coils. This gradiometer, maintained at LN 2 temperature, is sensitive enough to detect small mock cracks in an aluminium alloy slab covered by another 25.4mm thick solid aluminium alloy slab. Such an NDT device should have a significant impact on metal fatigue detection technology and demonstrates an important practical usage of high temperature superconductors.

高温超電導体磁気シールドと生体磁気計測

We developed a helmet-sized superconducting magnetic shield for a high-Tc Bi-Pb-Sr-Ca-Cu-O superconductor. The shield is 32 cm in diameter, 60 cm in depth, and 2.5 cm in thickness. It was experimentally confirmed that the container can reduce a Weak magnetic field to -100 dB or 1/100000. The shielding effect of the superconducting container does not decrease even at frequencies as low as 0.2 Hz. Using equipment consisting of a superconducting container, a cryostat to keep the container at liquid nitrogen temperature, and an rf SQUID gradiometer, we successfully detected the magnetic fields of a human brain. We detected somatosensory magnetic fields by stimulating the tibial nerves of the patient with current pulses.