Low-temperature Superconducting Quantum Interference Device Research Articles

The latest trend in magnetocardiogram measurement system technology

Heart consists of myocardium cells and the electrophysiological activity of the cells generate magnetic fields. By measuring this magnetic field, magnetocardiogram (MCG), functional diagnosis of the heart diseases is possible. Since the strength of the MCG signals is weak, typically in the range of 1-10 pT, we need sensitive magnetic sensors. Conventionally, superconducting quantum interference devices (SQUID)s were used for the detection of MCG signals due to its superior sensitivity to other magnetic sensors. However, drawback of the SQUID is the need for regular refill of a cryogenic liquid, typically liquid helium for cooling lowtemperature SQUIDs. Efforts to eliminate the need for the refill in the SQUID system have been done by using cryocooler-based conduction cooling or use of non-cryogenic sensors, or room-temperature sensors. Each sensor has advantage and disadvantage, in terms of magnetic field sensitivity and complexity of the system, and we review the recent trend of MCG technology.

Magnetic susceptibility inversion method with full tensor gradient data using low-temperature SQUIDs

Full tensor magnetic gradient measurements are available nowadays. These are essential for determining magnetization parameters in deep layers. Using full or partial tensor magnetic gradient measurements to determine the subsurface properties, e.g., magnetic susceptibility, is an inverse problem. Inversion using total magnetic intensity data is a traditional way. Because of difficulty in obtaining the practical full tensor magnetic gradient data, the corresponding inversion results are not so widely reported. With the development of superconducting quantum interference devices (SQUIDs), we can acquire the full tensor magnetic gradient data through field measurements. In this paper, we study the inverse problem of retrieving magnetic susceptibility with the field data using our designed low-temperature SQUIDs. The solving methodology based on sparse regularization and an alternating directions method of multipliers is established. Numerical and field data experiments are performed to show the feasibility of our algorithm.

Capability of low-temperature SQUID for transient electromagnetics under anthropogenic noise conditions

Transient electromagnetics (TEM) is a well-established method for mineral, groundwater, and geothermal exploration. Superconducting quantum interference device (SQUID)-based magnetic-field receivers used for TEM have quantitative advantages and higher sensitivity compared with commonly used induction coils. Special applications are deep soundings with target depths [Formula: see text] and settings with conductive overburden. However, SQUIDs have rarely been applied for TEM measurements in environments with significant anthropogenic noise. We compared a low-temperature SQUID with a commercially available induction coil in an area affected by anthropogenic noise. We acquired four fixed-loop data sets with totally 61 receiver stations close to Bad Frankenhausen, Germany. The high sensitivity of the SQUID enables low noise levels, which lead to longer high-quality transient data compared with the induction coil. The effect of anthropogenic and natural noise sources is more critical for the coil than for the SQUID data. In the vicinity of the transmitter loop, systematic distortion of the coil signals occurs at early times, most probably caused by sferic interferences. We have developed 1D inversion results of both receivers that matched well in general. However, the SQUID-based models were more consistent and showed greater depths of investigation. This led to a superior resolution of deeper layers and even enabled a potential detection of thin conducting targets at up to a 500 m depth. Moreover, we find that the SQUID data inversion revealed multidimensional effects within the conductive overburden. In this regard, we applied forward modeling to analyze systematic differences between inversion results of SQUID and coil data. We determine that low-temperature SQUIDs have the potential to significantly improve the reliability of subsurface models in suburban environments. Nevertheless, we recommend combined application of both types of receivers.

Transmitting oscillation suppression of low-Tc SQUID TEM system based on RC serial and multi-parallel capacity snubber circuit

A low-temperature superconducting quantum interference device (low-Tc SQUID) can improve the depth of exploration. However, a low-Tc SQUID may lose its lock owing to oscillations in the current or the occurrence of spikes when the transmitter is switched off. If a low-Tc SQUID loses its lock, it becomes impossible for the low-Tc SQUID TEM system to function normally and stably for a long period of time. This hinders the practical use of the system. In field experiments, the transmitting current is accurately measured, the voltage overshoot and current spike data are recorded, and the gradient of the primary magnetic field at the center of the transmitting loop is calculated. After analyzing the results of field experiments, it was found that when the gradient of the primary magnetic field far exceeds the slew rate of a low-Tc SQUID, the low-Tc SQUID loses its lock. Based on the mechanisms of the transmitting oscillation, an RC serial and multi-parallel capacity snubber circuit used to suppress such oscillation is proposed. The results of simulation and field experiments show that, when using a 100 m×100 m transmitting loop, the gradient of the primary magnetic field is suppressed from 101.4 to 2.4 mT/s with a transmitting current of 40 A, and from 29.6 to 1.4 mT/s with a transmitting current of 20 A. Therefore, it can be concluded that the gradient of the primary magnetic field is below the slew rate of a low-Tc SQUID after adopting the proposed RC serial and multi-parallel capacity snubber circuit. In conclusion, the technique proposed in this paper solves the problem of a lost lock of a low-Tc SQUID, ensuring that the low-Tc SQUID TEM system functions stably for a long period of time, and providing technical assurance for ground TEM exploration at an additional depth.

Study of transient electromagnetic method measurements using a superconducting quantum interference device as B sensor receiver in polarizable survey area

Time-domain transient electromagnetic method (TEM) measurements sometimes exhibit a sign reversal in the secondary field during the off-time, which is usually attributed to the induced-polarization (IP) effect. In contrast with the conventional IP method, which uses a current source, TEM with an ungrounded transmitting loop operates using a pure voltage source, which is induced by the primary field switching on and off. We performed TEM measurements in a resistive survey area showing an IP effect, and we used a low-temperature superconducting quantum interference device (LT-SQUID) with sensitivity of [Formula: see text] as a magnetic field sensor. A sign reversal in all of our measurements was observed; furthermore, the negative amplitude reached [Formula: see text]. In-depth analysis with an extended version of a wire-filament circuit reveals that the large negative signal may be due to discharging of in-ground capacitance, an IP effect. The conduction response of the ground can be restored by subtracting the fitted discharging response (negative valued) from the observed data. To verify this operation, we compared TEM measurements with and without wire-loop targets, which can induce a conduction field with a known decay time constant during the off-time. The extracted conduction responses of the wire-loop targets match the expected ones well. This research reveals that the primary field switch-off must always be included when interpreting TEM data with sign reversal and an LT-SQUID may be a good alternative sensor for studying the IP effect in TEM.

Initial Results from SQUID Sensor: Analysis and Modeling for the ELF/VLF Atmospheric Noise.

In this paper, the amplitude probability density (APD) of the wideband extremely low frequency (ELF) and very low frequency (VLF) atmospheric noise is studied. The electromagnetic signals from the atmosphere, referred to herein as atmospheric noise, was recorded by a mobile low-temperature superconducting quantum interference device (SQUID) receiver under magnetically unshielded conditions. In order to eliminate the adverse effect brought by the geomagnetic activities and powerline, the measured field data was preprocessed to suppress the baseline wandering and harmonics by symmetric wavelet transform and least square methods firstly. Then statistical analysis was performed for the atmospheric noise on different time and frequency scales. Finally, the wideband ELF/VLF atmospheric noise was analyzed and modeled separately. Experimental results show that, Gaussian model is appropriate to depict preprocessed ELF atmospheric noise by a hole puncher operator. While for VLF atmospheric noise, symmetric α-stable (SαS) distribution is more accurate to fit the heavy-tail of the envelope probability density function (pdf).

SQUID Microscope With Hollow-Structured Cryostat for Magnetic Field Imaging of Room Temperature Samples

We have developed a high-spatial-resolution superconducting quantum interference device (SQUID) microscope for magnetic field imaging of rock samples at room temperature, using a hollow-structured cryostat. A directly coupled low-temperature SQUID with a $200 \mu\text{m} \times 200 \mu\text{m}$ pickup loop, which is mounted on a sapphire conical rod, is separated from room temperature by a thin sapphire window. Precise and repeatable adjustment of the vacuum gap between the SQUID and the sapphire window is performed by rotating a micrometer spindle connected to the sapphire rod through the hollow portion of the cryostat. A spacing of 230 $\mu\text{m}$ between the SQUID and a sample has been achieved. We demonstrated the imaging of the magnetic field on a zircon containing magnetic grains.

Room Temperature Sample Scanning SQUID Microscope for Imaging the Magnetic Fields of Geological Specimens

A microscope to image weak magnetic fields using a low-temperature superconducting quantum interference device (SQUID) had developed with a liquid helium consumption rate of ~0.5L/hour. The gradient pickup coil is made by a low-temperature superconducting niobium wire with a diameter of 66 μm, which is coupled to the input circuit of the SQUID and is then enwound on the sapphire bobbin. Both of the pickup coil and the SQUID sensor are installed in a red copper cold finger, which is thermally anchored to the liquid helium evaporation platform in the vacuum space of the cryostat. To reduce the distance between the pickup coil and sample, a 100 μm thick sapphire window is nestled up to the bottom of the cryostat. A three-dimensional scanning stage platform with a 50 cm Teflon sample rack under the sapphire window had the precision of 10 μm. To test the fidelity of the new facility, the distribution of the magnetic field of basalt slice specimens was determined. Results show that the spatial resolution of the newly-designed facility is 500 μm with a gradient magnetic field sensitivity of 380fT. This opens new opportunities in examining the distribution of magnetic assemblages in samples, which bear great geological and geophysical information.

Magnetic properties of N‐doped multi‐walled carbon nanotubes

AbstractThe magnetic properties of nitrogen‐doped multi‐walled carbon nanotubes (N‐MWCNTs) were studied in the temperature and magnetic field range of (4.2–290) K and (0.05–30) kOe, respectively. Also magnetic properties of the reference MWCNTs without nitrogen doping were investigated on the same footing for comparison. The presence of iron‐based particles inside N‐MWCNTs was detected. The low temperature SQUID (Superconducting Quantum Interference Device) magnetization measurements were supplemented with thermogravimetric analysis. The magnetic characterization provided the saturation magnetic moment MS and residual magnetization MR of N‐MWCNTs to be, respectively, 4.5 emu/g and 0.9 emu/g, whereas for the reference MWCNTs the corresponding parameters are found to be about twice higher. The coercive field value amounts to 800 Oe for both systems at low temperatures, then decreasing to about 450 Oe at 200 K.

Integration of a Cryocooler into a SQUID Magnetospinography System for Reduction of Liquid Helium Consumption

We are currently developing a magnetospinography (MSG) system for noninvasive functional imaging of the spinal cord. The MSG system is a device for observing a weak magnetic field accompanied by the neural activity of the spinal cord by using an array of low-temperature superconducting quantum interference device (SQUID) magnetic flux sensors. As in the case of other biomagnetic measurement systems such as the magnetoencephalography (MEG) system, the running cost of the MSG system is mainly dependent on the liquid helium (LHe) consumption of a dewar vessel. We integrated a cryocooler into the MSG system to reduce LHe consumption. A pulse tube cryocooler with a cooling power of 0.5Wat 4 K was placed adjacent to a magnetically shielded room and was directly connected to the thermal radiation shield of the dewar by an electrically isolated transfer tube. Cold helium gas was circulated between the cryocooler and the radiation shield. Consequently, the temperature of the radiation shield decreased below 40 K. Previous studies have shown that the detection of a weak magnetic field is often hindered by severe low-frequency band noise from the cryocooler. However, the band of the MSG signals is much higher than that of the cryocooler noise. Therefore, the noise can be filtered out and has a less detrimental effect on MSG measurement than on other biomagnetic field measurements such as MEG measurement. As a result, LHe consumption was reduced by 46%, with no increase in the noise floor.

Design and construction of a top loading dilution refrigerator probe for a superconducting quantum interference device DC magnetometer

A commercially available SQUID (Superconducting QUantum Interference Device) DC magnetometer is often limited by its relatively high temperature (≥ 1.9 K) and low magnetic field (≤ 7 T) operating environment. The need for the lower temperature and higher field DC magnetization measurements keeps growing as more materials show interesting physical phenomena with relevant energy scales that require millikelvin temperatures. To meet these needs we have developed a SQUID DC magnetometer which operates in the top loading dilution refrigerator of a 16 T superconducting magnet. An essential part of this low temperature and high field SQUID magnetometer is a specialized probe which can adapt the SQUID electronics and low friction mechanical sample shaft. The details of magnetometer probe and preliminary testing results are described in this paper.

Superconducting quantum interference device instruments and applications

Superconducting quantum interference devices (SQUIDs) have been a key factor in the development and commercialization of ultrasensitive electric and magnetic measurement systems. In many cases, SQUID instrumentation offers the ability to make measurements where no other methodology is possible. We review the main aspects of designing, fabricating, and operating a number of SQUID measurement systems. While this article is not intended to be an exhaustive review on the principles of SQUID sensors and the underlying concepts behind the Josephson effect, a qualitative description of the operating principles of SQUID sensors and the properties of materials used to fabricate SQUID sensors is presented. The difference between low and high temperature SQUIDs and their suitability for specific applications is discussed. Although SQUID electronics have the capability to operate well above 1MHz, most applications tend to be at lower frequencies. Specific examples of input circuits and detection coil configuration for different applications and environments, along with expected performance, are described. In particular, anticipated signal strength, magnetic field environment (applied field and external noise), and cryogenic requirements are discussed. Finally, a variety of applications with specific examples in the areas of electromagnetic, material property, nondestructive test and evaluation, and geophysical and biomedical measurements are reviewed.

High slew rate, ultrastable direct-coupled readout for dc superconducting quantum interference devices

A very fast and thermostable readout electronics for dc superconducting quantum interference devices (SQUIDs) is presented. The authors have applied a concept which gives them the opportunity to combine several, at first sight contradictory, important parameters for the SQUID system user. With their flux-locked-loop electronics they could reach more than 16MΦ0∕s slew rate while using a 1.3m cable between the electronics and a conventional low transition temperature SQUID (with a maximum peak-peak voltage of the flux-to-voltage transfer function of ⩾63μV). By making use of thermocurrent compensation in the first stage of the amplifier they have achieved a thermal drift of about 5nV∕K for a temperature range between 0 and 65°C. The system demonstrated a white noise voltage level of ∼0.32nV∕Hz1∕2, with a flicker noise corner frequency of about 0.1Hz.

Superconducting quantum interference devices: State of the art and applications

Superconducting quantum interference devices (SQUIDs) are sensitive detectors of magnetic flux. A SQUID consists of a superconducting loop interrupted by either one or two Josephson junctions for the RF or dc SQUID, respectively. Low transition temperature (T/sub c/) SQUIDs are fabricated from thin films of niobium. Immersed in liquid helium at 4.2 K, their flux noise is typically 10/sup -6//spl Phi//sub 0/ Hz/sup -1/2/, where /spl Phi//sub 0//spl equiv/h/2e is the flux quantum. High-T/sub c/ SQUIDs are fabricated from thin films of YBa/sub 2/Cu/sub 3/O/sub 7-x/, and are generally operated in liquid nitrogen at 77 K. Inductively coupled to an appropriate input circuit, SQUIDs measure a variety of physical quantities, including magnetic field, magnetic field gradient, voltage, and magnetic susceptibility. Systems are available for detecting magnetic signals from the brain, measuring the magnetic susceptibility of materials and geophysical core samples, magnetocardiography and nondestructive evaluation. SQUID "microscopes" detect magnetic nanoparticles attached to pathogens in an immunoassay technique and locate faults in semiconductor packages. A SQUID amplifier with an integrated resonant microstrip is within a factor of two of the quantum limit at 0.5 GHz and will be used in a search for axions. High-resolution magnetic resonance images are obtained at frequencies of a few kilohertz with a SQUID-based detector.

Monolithic low-transition-temperature superconducting magnetometers for high resolution imaging magnetic fields of room temperature samples

We have developed a monolithic low-temperature superconducting quantum interference device (SQUID) magnetometer and incorporated the device in a scanning microscope for imaging magnetic fields of room temperature samples. The instrument has a ∼100 μm spatial resolution and a 1.4 pT/Hz1/2 field sensitivity above a few hertz. We discuss design constraints on and potential applications of the SQUID microscope.

Focus on Magnetics

Focus on Magnetics

Compensation techniques for HTS-rf-SQUID magnetometers operating in unshielded environments

Abstract We have developed two methods of compensating environmental disturbances applicable to high- and low-temperature superconducting quantum interference device (SQUID) systems, operating in a magnetically unshielded environment. For testing, we used first- and second-order axial electronic gradiometer setups with rf SQUID magnetometers operating at 77 K, with different baselines between 1.8 and 8 cm. The magnetometers were single-layer washer rf SQUIDs, with bulk or thin-film magnetic flux concentrators and transformers in flip-chip geometry. The tested methods resulted in disturbance compensation levels comparable with those attained using electronically formed gradiometers. The white noise of the compensated magnetometers resulted in maximum gradiometric sensitivities of 13.5 fT/cm √Hz for first-order and 22 fT/cm2 √Hz for second-order compensation, with baselines between 7 and 8 cm down to 10 Hz. Common mode rejection was balanced to, better than 104 for homogeneous fields, and better than 200 for gradient fields, with second-order compensation. Magnetically unshielded measurements of biomagnetic signals with different baselines were demonstrated.

SQUID systems for non-destructive testing by AC field mapping

The authors describe a low-temperature liquid-helium superconducting quantum interference device (SQUID) system with high spatial resolution and a wider bandwidth than usual for an all-metal cryostat and which also provides access for a mechanism to balance the pick-up coils. They discuss the effects of these properties on AC field measurement and present experimental results from small slits which mimic growing fatigue cracks. Related work on high-temperature superconducting devices indicates that they will offer important advantages over low-temperature SQUIDs, particularly in terms of cryogenic design which has been so restrictive in low-temperature systems. The authors suggest that even relatively poor high-temperature-superconductor SQUID performance will be acceptable if these advantages can be exploited.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>