HTS Rf SQUID Research Articles

Development of Metallic Contaminant Inspection System Using High-Tc rf-SQUID for Li-ion Battery Liquid Components

The use of a high-Tc (HTS) rf-superconducting quantum interference device (SQUID) for inspecting the liquid component of lithium-ion (Li-ion) batteries is proposed. It is possible that a small amount of metal can become mixed in with components of Li-ion batteries during the manufacturing process and contaminate them, with a resultant fire risk, Thus, the development of a highly sensitive inspection system is required. In this article we describe a test system, we developed using a single channel HTS rf-SQUID consisting of a cryostat, a polyethylene tube, and a permanent magnet. The SQUID needs to be installed as close to the battery liquid as possible to obtain more sensitive readings. Therefore, we designed a microscope-type SQUID cryostat in which the SQUID can approach the target as close as 1 mm. In the system, the liquid was not flowed through the polyethylene tube, but a small metallic sample stack on a fishing line was moved by a motor. A stainless steel cut wire with a nominal size of φ 50 μm was prepared. After magnetizing the sample, the stray magnetic field of the sample was measured by the SQUID. The peak-to-peak signal scaled inversely with the cube of the stand-off distance. It was found that a signal of the sample with φ 50 μm at a distance of 26 mm could be detected if the signal-to-noise ratio ( SNR ) > 3 was supposed as the threshold of the inspection. The dependence on the sample size was also investigated, using stainless steel samples with different sizes (φ 50–φ 160 μm). The peak-to-peak signal scaled with the cube of the diameter.

Fast Fourier Transform Based NDT Approach for Depth Detection of Hidden Defects Using HTS rf-SQUID

We present a new approach for depth detection of hidden defects based on the analysis of the frequency spectrum of the output waveform in a nondestructive testing (NDT) system. In our eddy current Superconducting quantum interference device (SQUID) based NDT system, we apply multiple frequencies to its single excitation coil in a magnetically unshielded environment. The excitation coil is a planar D-shaped printed circuit board coil, and a high-T c gradiometer YBCO rf-SQUID is used as the electromagnetic sensor in this system. An automated two-dimensional nonmagnetic scanning robot is used to test samples with intentional defects at different depths. In this approach, a diagram labeled “FFT relative changes” is assigned to each test sample. The optimum points of the FFT relative changes diagram occur at the optimum frequencies directly related to the depth of the corresponding defects. The relation between the optimum frequency and the corresponding depth in our system is obtained both experimentally and by simulation. An excellent agreement between the practical and the simulation results confirmed the accuracy of the proposed method.

HTS YBCO Resonator Configuration With a Coplanar Optimized Flux Concentrator Strongly Coupled to rf SQUID

We developed a novel magnetic coupling module formed of a monolayer superconducting flux concentrator, which is integrated with a coplanar resonator strongly coupled to HTS rf-SQUID. Three types of resonators, including a long stripline resonator between input loop and pick-up loop of the flux concentrator, a complementary split ring resonator (CSRR), and also a spiral shape inside the input loop are explored. The resonance quality factors as well as the coupling to the SQUID of different patterns of these three types of the resonators is evaluated using Finite Element Method (FEM) simulations. Several readout methods to couple the electronic system to the resonators are tested, including inductive (coil) and capacitive (transmission line) couplings, and the optimum readout is reported for each of the resonators. Among the evaluated resonator types, a spiral shape resonator with optimal design showing the highest quality factor (5900) together with the strongest coupling to the SQUID (-0.5 dB) at resonance frequency of 836 MHz, is fabricated using 200 nm thick superconducting YBCO on a 1 mm thick crystalline LaAlO3 substrate. The flux concentrator of the module is optimized by the variation of its linewidths and also its input loop radius to obtain maximum flux transformation efficiency.

Ultra-Low Field SQUID-NMR using LN2 Cooled Cu Polarizing Field coil

We are developing an Ultra-Low Field (ULF) Magnetic Resonance Imaging (MRI) system using a High-Temperature Superconductor superconducting quantum interference device (HTS rf-SQUID) for food inspection. The advantages of the ULF-NMR (Nuclear Magnetic Resonance) / MRI as compared with a conventional high field MRI are that they are compact and of low cost. In this study, we developed a ULF SQUID-NMR system using a polarizing coil to measure fat of which relaxation time T1 is shorter. The handmade polarizing coil was cooled by liquid nitrogen to reduce the resistance and accordingly increase the allowable current. The measured decay time of the polarizing field was 40 ms. The measurement system consisted of the liquid nitrogen cooled polarizing coil, a SQUID, a Cu wound flux transformer, a measurement field coil for the field of 47 μT, and an AC pulse coil for a 90°pulse field. The NMR measurements were performed in a magnetically shielded room to reduce the environmental magnetic field. The size of the sample was ϕ35 mm × L80 mm. After applying a polarizing field and a 90°pulse, an NMR signal was detected by the SQUID through the flux transformer. As a result, the NMR spectra of fat samples were obtained at 2.0 kHz corresponding to the measurement field Bm of 47 μT. The T1 relaxation time of the mineral oil measured in Bm was 45 ms. These results suggested that the ULF-NMR/MRI system has potential for food inspection.

Investigation on readout coil design for fluxed locked loop control of HTS rf-SQUID

We investigated the readout coil, electro-magnetically coupled with a HTS rf-SQUID for the flux-locked loop control. The design and size of the readout coil affected the SQUID performances. Among the tested combinations of different readout coils with the rf-SQUID, the rectangular coil, which just surrounded the slit in the rf-SQUID, was advantageous for the better performance. We also demonstrated the rf-SQUID operation with the rectangular coils made of the thin flexible print circuit board, which could be put on the rf-SQUID stably.

Novel Stable and Reliable Readout Electronics for HTS rf SQUID

Conventional readout electronics for radio-frequency SQUIDs exhibit long-term instabilities due to a temperature-dependent frequency drift of their voltage controlled oscillator. A novel electronics with a quartz-controlled synthesizer was developed to solve this problem. The frequency can be adjusted from 450 to 900 MHz with 2 ppm frequency stability, the amplitude range covers more than 60 dB. An adjustable phase shifter for demodulation is implemented to compensate for variable cable length. The total amplification is about 100 dB. The electronics with touchpanel and remote USB control is commercially available with up to three preamplifier units. Because thermal drift is eliminated, it is well suited for outdoor applications, e.g. in geophysics.

HTS Rf-SQUID Microscope for Metallic Contaminant Detection

We have developed metallic contaminant detection systems using HTS-SQUIDs. In this study, we designed and fabricated a single-chip HTS rf-SQUID using SQUID microscope techniques to reduce both the lift-off and cooling temperature as compared with the substrate resonator HTS rf-SQUID. The HTS rf-SQUID was a washer-type rf-SQUID without a flux focuser. By lengthening and narrowing the SQUID hole, we showed that the inductance of the SQUID ring was reduced, and the effective area and the field noise were improved consequently. By introducing a single-chip HTS rf-SQUID with hole size of 800 μm × 50 μm into the detection system, we obtained a field sensitivity of 150 fT/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> and a lift-off of 1.5 mm. Since this SQUID has advantages in the reduction of lift-off and cooling temperature, its detectability and magnetic field resolution are superior to that of the substrate resonator SQUID. Finally, we showed the single-chip SQUID can detect 20 μm in diameter iron balls.

Development of Ultra Low Field Nuclear Magnetic Resonance Imaging System Using HTS rf SQUID

We constructed and studied an ultra low-field nuclear magnetic resonance imaging system using a HTS rf SQUID and room-temperature coils. We recorded the free induction decay signals of 1H by employing a measuring field, Bm, of 44.8 μT and a polarizing field, Bp, of 36.8 mT; in the presence of ∂BZ/∂z and ∂BZ/∂y gradients of the order of 0.8 μT/m, we were able to obtain a quasi two-dimensional 4-pixels image of a simple water phantom.

Development of low field nuclear magnetic resonance system using HTS rf SQUID

We constructed a low-field (LF) nuclear magnetic resonance (NMR) system using a HTS rf SQUID and room-temperature (RT) coils. In the system, a measurement field coil, a polarizing coil and ac pulse coil were employed. In addition, gradient coils were introduced to compensate the remnant field gradient around the sample and to improve the homogeneity of Bm. With gradient coils, the spin-spin relaxation time T2* of a free induction decay (FID) signals of 1H was 2.2 s, 1.7 times longer than that without gradient coil in an experiment employing Bm of 44.8 μT and Bp of 36.8 mT. The conditions of Bac (amplitude, duration) to rotate the magnetized vector of 1H by a certain angle were experimentally studied for application of a 90° – 180° spin echo sequence. It was also found that the minimum delay time for applying the 180° pulse to obtain the spin echo signal of 1H was approximately 2.5 s.

Overview of low-field NMR measurements using HTS rf-SQUIDs

High-resolution low-field nuclear magnetic resonance (LF-NMR) investigations of liquid samples, recorded using a HTS radio frequency (rf) superconducting quantum interference device (SQUID) are overviewed in this paper. The measurements were performed either in a magnetically shielded room (MSR) or in the Earth’s Magnetic Field (EMF). In MSR, measurements with Larmor frequencies ( f L) ranging from 2 Hz to 40 kHz were demonstrated. The natural spectral linewidth of water and the scalar coupling spectra of 2,2,2-trifluoroethanol and of fluorobenzene were determined. An additional nitrogen-cooled resonant LC input circuit was also introduced in higher f L (>10 kHz) region to enhance the signal-to-noise ratio up to an order of magnitude. The influence of the speed of decay of the polarizing magnetic field ( B P) on the free-induction-decay (FID) signal was analyzed quantitatively. In EMF, NMR spectra of different liquid samples were recorded. To compensate the line broadening due to EMF fluctuations we used frequency-adjusted averaging.

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.

The set-up of a high temperature superconductor radio-frequency SQUID microscope formagnetic nanoparticle detection

SQUID (superconducting quantum interference device) microscopes are versatileinstruments for biosensing applications, in particular for magnetic nanoparticle detection inimmunoassay experiments. We are developing a SQUID microscope based on an HTS rfSQUID magnetometer sensor with a substrate resonator. For the cryogenic set-up, aconfiguration was realized in which the cryostat is continuously refilled and kept at aconstant liquid nitrogen level by an isolated tube connection to a large liquid nitrogenreservoir. The SQUID is mounted on top of a sapphire finger, connected to theinner vessel of the stainless steel cryostat. The vacuum gap between the coldSQUID and room temperature sample is adjusted by the precise approach of a50 µm thin sapphire window using a single fine thread wheel. We investigated possible sensing tipconfigurations and different sensor integration techniques in order to achieve anoptimized design. A new scheme of coupling the rf SQUID from its back to aSrTiO3 substrate resonator was adopted for the purpose of minimization of the sensor-to-sample spacing.By SQUID substrate thinning and washer size reduction, the optimum coupling conditionsfor back coupling were determined for different rf SQUID magnetometers prepared onLaAlO3 and SrTiO3 substrates. The SQUID microscope system is characterized with respect to its spatialresolution and its magnetic field noise. The SQUID microscope instrument will be used formagnetic nanoparticle marker detection.

Detection of Magnetic Contaminations in Industrial Products Using HTS SQUIDs

Many products in the pharmaceutical and food industry are packaged in metallized wrappings. With standard high-frequency search coil metal detectors, they can only be tested for metal contaminations before they are wrapped. However, a key requirement of industrial quality control is the inspection of the products at the end of the production line. We have developed an inspection system for detecting the magnetic remanence of the contaminants. The system utilizes two HTS rf SQUID magnetometers with step edge junctions immersed in liquid nitrogen. The SQUIDs are arranged such that they cover the product channel in a rotated planar electronic gradiometer configuration. In order to suppress the low-frequency magnetic disturbances typically found in industrial environment, the product channel and the SQUID system were mounted inside a coaxial three-layer Mumetal shield. In combination with the gradiometric suppression, homogeneous low-frequency disturbance fields were attenuated by a factor of 400,000. The sensitivity of the system for small magnetic particles was determined experimentally, using numerous steel balls and splinters. A stainless steel particle of 3 /spl mu/g, corresponding to a sphere diameter of 0.09 mm, was detected with and without aluminized wrapping.

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.

Saw-wave excitation eddy-current nde based on hts rf squid

Using a high-temperature superconductor radio frequency superconducting quantum interference device planar gradiometer as the sensor, we developed a saw-tooth wave excitation eddy-current nondestructive evaluation device. By analyzing the amplitudes of harmonics in the response signal of the sample, we successfully detected artificial defects in an aluminum plate and estimated the defect depth. We also demonstrated the possibility of constructing three-dimensional profiles of a defect. In our program for data processing, we employed a multiplying method to calculate the amplitudes of the harmonics, which was much faster than that using fast Fourier transforms. During the experiment, the x-y stage movement, data acquisition and data processing could be performed simultaneously.

Long baseline hardware gradiometer based on HTS rf-SQUIDs with substrate resonators

We have developed an asymmetric thin film first order gradiometer based on an HTS rf-SQUID. A coupling scheme uses an SrTiO/sub 3/ substrate resonator as an rf-tank circuit which simplifies the gradiometer balancing. Advantage of the technique is that gradiometers with very long baseline can be fabricated. The performance of 13 mm and 33 mm baseline SQUID gradiometers is presented. The gradiometers have been successfully used in applications for nondestructive evaluation of materials.

Mobile HTS rf SQUID magnetometer

Using a simple compensation method, we developed a mobile HTS RF SQUID magnetometer. The dc and low frequency (below 20 Hz) magnetic field could be compensated very well. With the compensation, the HTS RF SQUID magnetometer remained locked while being swung at large angles in the Earth's field. For frequencies greater than 20 Hz, there was no increase in the flux noise observed after moving the SQUID system in the Earth's field. Using the mobile HTS RF SQUID magnetometer, we constructed an eddy current NDE system, which demonstrated the possibility of detecting defects in ferromagnetic materials.

Low-frequency noise reduction in Y-Ba-Cu-O SQUIDs by artificial defects

We demonstrate, that extremely simple arrangements of few antidots in HTS rf-SQUIDs can significantly reduce the 1/f noise in ambient field down to the level of zero-field noise. The onset field B/sub on/ at which the low-frequency noise starts to increase, is shifted from B/sub on//spl ap/8 /spl mu/T without antidots to B/sub on//spl ap/40 /spl mu/T with antidots for field cooled measurements and to B/sub on//spl ap/25 /spl mu/T for zero field cooled experiments. The geometric arrangements of the antidots are obtained from the analysis of the current distribution in the washer and the position of penetrating vortices in the case of zero field cooled experiments.

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.

Evolution of HTS rf SQUIDs

I present the evolution of rf-SQUIDs towards modern planar thin film devices. My main objective is to describe our own planar HTS rf washer SQUIDs and their operation with coplanar resonators and multiturn flux transformers. This publication is dedicated to the memory of J.E. Zimmerman, whose ideas have guided us through the development of rf SQUID planar technology.