Superconducting Quantum Interference Device System Research Articles

SQUID magnetoneurography: an old-fashioned yet new tool for noninvasive functional imaging of spinal cords and peripheral nerves.

We are engaged in the development and clinical application of a neural magnetic field measurement system that utilizes biomagnetic measurements to observe the activity of the spinal cord and peripheral nerves. Unlike conventional surface potential measurements, biomagnetic measurements are not affected by the conductivity distribution within the body, making them less influenced by the anatomical structure of body tissues. Consequently, functional testing using biomagnetic measurements can achieve higher spatial resolution compared to surface potential measurements. The neural magnetic field measurement, referred to as magnetoneurography, takes advantage of these benefits to enable functional testing of the spinal cord and peripheral nerves, while maintaining high spatial resolution and noninvasiveness. Our magnetoneurograph system is based on superconducting quantum interference devices (SQUIDs) similar to the conventional biomagnetic measurement systems. Various design considerations have been incorporated into the SQUID sensor array structure and signal processing software to make it suitable for detecting neural signal propagation along spinal cord and peripheral nerve. The technical validation of this system began in 1999 with a 3-channel SQUID system. Over the course of more than 20 years, we have continued technological development through medical-engineering collaboration, and in the latest prototype released in 2020, neural function imaging of the spinal cord and peripheral nerves, which could also be applied for the diagnosis of neurological disorders, has become possible. This paper provides an overview of the technical aspects of the magnetoneurograph system, covering the measurement hardware and software perspectives for providing diagnostic information, and its applications. Additionally, we discuss the integration with a helium recondensing system, which is a key factor in reducing running costs and achieving practicality in hospitals.

Single-trial classification of evoked responses to auditory tones using OPM- and SQUID-MEG

Objective. Optically pumped magnetometers (OPMs) are emerging as a near-room-temperature alternative to superconducting quantum interference devices (SQUIDs) for magnetoencephalography (MEG). In contrast to SQUIDs, OPMs can be placed in a close proximity to subject’s scalp potentially increasing the signal-to-noise ratio and spatial resolution of MEG. However, experimental demonstrations of these suggested benefits are still scarce. Here, to compare a 24-channel OPM-MEG system to a commercial whole-head SQUID system in a data-driven way, we quantified their performance in classifying single-trial evoked responses. Approach. We measured evoked responses to three auditory tones in six participants using both OPM- and SQUID-MEG systems. We performed pairwise temporal classification of the single-trial responses with linear discriminant analysis as well as multiclass classification with both EEGNet convolutional neural network and xDAWN decoding. Main results. OPMs provided higher classification accuracies than SQUIDs having a similar coverage of the left hemisphere of the participant. However, the SQUID sensors covering the whole helmet had classification scores larger than those of OPMs for two of the tone pairs, demonstrating the benefits of a whole-head measurement. Significance. The results demonstrate that the current OPM-MEG system provides high-quality data about the brain with room for improvement for high bandwidth non-invasive brain–computer interfacing.

Error Calibration for Crosstalk of SQUIDs in Full Tensor Magnetic Gradiometer Based on Moore–Penrose Inverse and Helmholtz Coil

In order to calibrate crosstalk error between superconducting quantum interference device (SQUID) magnetometers in a multichannel SQUID system, we propose an innovative error calibration method for crosstalk of SQUID magnetometers based on the Moore–Penrose inverse method and Helmholtz coil in this article and successfully apply it to an eight-channel SQUID magnetometers based full tensor magnetic gradiometer (FTMG). We first analyze the causes of crosstalk through working principle of the SQUID readout circuit and build the crosstalk error model of the eight-channel FTMG. Then, we introduce the solution method of 56 crosstalk coefficients based on the Moore–Penrose inverse method and the construction method of matrix needed for solution based on Helmholtz coil. The effect of nonuniformity of Helmholtz coil on the calibration of crosstalk is analyzed and calculated. The effectiveness of the proposed method is proved by repetitive simulation and comparative experiments. Compared with the traditional direct measurement method, this method has the advantage of high efficiency, and the more the channels of SQUID, the more obvious is the superiority.

A novel estimation model for hysteresis loss based on semi-minor loops

This paper proposes an estimation model of hysteresis loss based on the mapping of the hysteresis loop conducted through tiny material samples with the aid of the Superconducting Quantum Interference Device (SQUID). This approach allows precise measurement of hysteresis loops. The aim of this paper is to develop a precise estimation model for hysteresis loss, one component of core loss, so as to improve the overall estimation accuracy of the entire core loss under complex excitations. First, with the help of the SQUID system and measurements conducted on tiny samples, the proposed model manages to minimize the impact of eddy current throughout the measurement and the modeling of hysteresis loss. This contrasts with the use of the conventional Steinmetz model and its modifications. Moreover, compared with methods including the modified Steinmetz equation (MSE) and the generalized Steinmetz equation (GSE) which merely focuses on the overall iron loss under simple non-sinusoidal excitations, the model presented here enables the estimation of hysteresis loss under complex excitations where multiple peaks and DC-bias take place. This approach will be very useful in situations such as those requiring pulse-width modulation (PWM) excited converters which are quite common on occasions including wind power systems. The estimation error of this model on the actual coil is within 15% under given excitations.

Simple Polymeric Molding for Mechanical and Electrostatic Protection of Superconducting Quantum Interference Devices

For reliable and long-term operation of superconducting quantum interference devices (SQUIDs), proper protection against mechanical damage and electrostatic discharge is required. In this study, we developed a simple and practical molding method using a polydimethylsiloxane–carbon black composite to protect SQUIDs. By simply dipping SQUIDs into the molding composite precursor, they could be molded reliably with no additional mechanical damage from the molding process itself. Additionally, the molded SQUIDs showed no degradation in the flux–voltage modulation curves for ten thermal cycles between 4.2 K and room temperature. We also compared the electrostatic discharge immune characteristics of bare and molded SQUIDs by applying air discharge voltages in the range of 0.1–4.0 kV. Among the 52 SQUIDs with or without molding, all the molded SQUIDs survived up to 1.2 kV. For higher discharge voltages, the molded SQUIDs have a lower likelihood of damage than the bare SQUIDs. This study also confirmed that neither the molding material nor process increase the flux noise, indicating that the developed molding method can be used reliably for multichannel SQUID systems.

Development of SQUID based TDEM system and its utilization for field survey at Tumallapalle, Andhra Pradesh, India

This paper describes the development of low Tc SQUID (Superconducting Quantum Interference Device) based TDEM (Time Domain Electro-Magnetic) system for geophysical applications. The SQUID system has been used to perform central loop TDEM sounding measurements near Tumallapalle Uranium mine, Cuddapah (dt), AP, India, where the terrain consists of a thin conducting layer below which low grade Uraniferous Phosphatic Siliceous Dolostone is present. Locating the thin conducting layer which is covered by a thick and highly resistive layer is a challenging task due to the induction of weak eddy currents. In order to enhance the resolution of the target located at larger depth, it is essential to apply larger transmitter moment and use a sensitive receiver. In this work, we combine the use of sensitive SQUID sensor and transmitter loop with larger size to achieve better target resolution. The survey has been performed using transmitter loop sizes of 100 m × 100 m and 400 m × 400 m. In order to compare the performance of the SQUID based TDEM system, the same experiments have also been repeated using induction coil as receiver which has been built for this work. In this article, we describe in detail the results obtained from the central loop TDEM measurements.

Transforming and comparing data between standard SQUID and OPM-MEG systems.

Optically pumped magnetometers (OPMs) have recently become so sensitive that they are suitable for use in magnetoencephalography (MEG). These sensors solve operational problems of the current standard MEG, where superconducting quantum interference device (SQUID) gradiometers and magnetometers are being used. The main advantage of OPMs is that they do not require cryogenics for cooling. Therefore, they can be placed closer to the scalp and are much easier to use. Here, we measured auditory evoked fields (AEFs) with both SQUID- and OPM-based MEG systems for a group of subjects to better understand the usage of a limited sensor count OPM-MEG. We present a theoretical framework that transforms the within subject data and equivalent simulation data from one MEG system to the other. This approach works on the principle of solving the inverse problem with one system, and then using the forward model to calculate the magnetic fields expected for the other system. For the source reconstruction, we used a minimum norm estimate (MNE) of the current distribution. Two different volume conductor models were compared: the homogeneous conducting sphere and the three-shell model of the head. The transformation results are characterized by a relative error and cross-correlation between the measured and the estimated magnetic field maps of the AEFs. The results for both models are encouraging. Since some commercial OPMs measure multiple components of the magnetic field simultaneously, we additionally analyzed the effect of tangential field components. Overall, our dual-axis OPM-MEG with 15 sensors yields similar information to a 62-channel SQUID-MEG with its field of view restricted to the right hemisphere.

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.

A Practical Two-Stage SQUID Readout Circuit Improved With Proportional Feedback Schemes

Nonlinearity with multiple working points limits the practical applications of a two-stage superconducting quantum interference device (SQUID) readout circuit, which is promising for low-noise performance. To solve this problem, we proposed a simple two-stage SQUID readout circuit with two conventional dc SQUIDs improved with proportional feedback schemes, where transfer coefficient of the first-stage SQUID circuit is enhanced by additional positive feedback circuit; transfer characteristics of the second-stage SQUID circuit are linearized by an additional negative feedback circuit. Two proportional feedback schemes are cooperated together based on matching conditions to achieve both noise suppression and working point matching. Working principles were verified in experiment results. The overall transfer characteristics achieved have only one working point and are simply adjusted according to the voltage swing of an amplifier. Transfer coefficient of the first-stage SQUID circuit was raised up to 10 (20 dB), and the total flux noise of two-stage SQUID readout circuit was reduced below 1 μΦ 0 /√Hz, where Φ 0 = 2.07×10 -15 Wb. Our two-stage SQUID readout circuit is easy-to-operate and suited for high-performance practical SQUID systems.

Effects of temperature fluctuations on a SQUID-based superconducting gravimeter.

A superconducting gravimeter based on the superconducting quantum interference device system is under development. As the main source of low-frequency noise, temperature fluctuations affect the resolution of superconducting gravimeters. In this study, a set of experimental devices was built to investigate the primary coupling processes of temperature fluctuations in superconducting gravimeters. Under the temperature modulation method, the effects of temperature fluctuations can be expressed as dΦ/dT = 342(2)Φ0/K, which, according to theoretical analysis, corresponds to a displacement change of (1.38 ± 0.04) × 10-7 m/K. Based on these results, the ambient temperature is controlled to within ±100 µK, and the equivalent effect of temperature fluctuations on our superconducting gravimeter is 0.5 μGal.

Noise Compensation of a Mobile LTS SQUID Planar Gradiometer for Aeromagnetic Detection

A superconducting quantum interference device (SQUID) planar gradiometer is an extremely sensitive sensor for magnetic gradient measurements. It has been shown to have potential applications for aeromagnetic detection. The major challenge when operating an aeromagnetic SQUID system in actual environment is the motion noise, including the inherent response resulting from gradiometer imbalance and from magnetic interferences. Three orthogonal reference magnetometers are usually adopted to improve the gradiometer balance. However, magnetic interference coming from the system itself also needs to be compensated. In this article, a mathematical model of the motion noise picked up by the gradiometer is derived from the traditional magnetic total field compensation method. Based on the model, the signals from the triaxial magnetometer can also be used to compensate the eddy current contribution to the magnetic interference. To verify the compensation method, a SQUID planar gradiometer system was set up and used for flight trials. The gradient field noise and imbalance of our homemade gradiometer were measured to be 100 fT/m/rt(Hz) and 2E-4 in the lab. In flight, the motion-induced peak-to-peak output of the gradiometer was reduced from 170 to about 0.1 nT/m, so that a magnetic anomaly signal of about 2 nT/m from a cargo ship was clearly recognized.

Underwater operation of a full tensor SQUID gradiometer system

For the first time a mobile underwater full tensor magnetic gradiometer (FTMG) system based on low-Tc superconducting quantum interference devices (SQUIDs) has been deployed in order to scan the sea floor for magnetized targets. The application is mainly focused on waste deposits and unexploded ordnance (UXO), but could also include shallow geological features as well as archaeological remains. The main methods for detection and localisation of underwater UXO and waste deposits are side sonar scanning and magnetic mapping. While modern sonar scanners can achieve a very high spatial resolution and long detection range, they still have problems detecting targets under cover—for magnetic sensors a layer of non-magnetic sand or ooze does usually not have an effect on the signal apart from an increased distance. In this paper we discuss the setup of the mobile underwater FTMG SQUID system, its challenges and main performance features. It also illustrates its detection and localization capabilities in tests on known magnetic targets.

A SQUID system for geophysical measurements cooled by a pulse tube cryocooler

Geophysical measurements, which rely on the acquisition of magnetic data, can take advantage of a superconducting quantum interference device (SQUID), which offers a higher field sensitivity at lower frequencies than conventional induction coils. Although cooling a SQUID in liquid helium provides for a simple and low-interference system, in remote areas a cryocooler could reduce the expenses of operation. We studied transient magnetic measurements with a SQUID cooled by a small pulse tube cooler operated by a 1 kW compressor, which can be powered by a portable generator or large truck battery.

Superconducting Quantum Interferometers for Nondestructive Evaluation.

We review stationary and mobile systems that are used for the nondestructive evaluation of room temperature objects and are based on superconducting quantum interference devices (SQUIDs). The systems are optimized for samples whose dimensions are between 10 micrometers and several meters. Stray magnetic fields from small samples (10 µm–10 cm) are studied using a SQUID microscope equipped with a magnetic flux antenna, which is fed through the walls of liquid nitrogen cryostat and a hole in the SQUID’s pick-up loop and returned sidewards from the SQUID back to the sample. The SQUID microscope does not disturb the magnetization of the sample during image recording due to the decoupling of the magnetic flux antenna from the modulation and feedback coil. For larger samples, we use a hand-held mobile liquid nitrogen minicryostat with a first order planar gradiometric SQUID sensor. Low-Tc DC SQUID systems that are designed for NDE measurements of bio-objects are able to operate with sufficient resolution in a magnetically unshielded environment. High-Tc DC SQUID magnetometers that are operated in a magnetic shield demonstrate a magnetic field resolution of ~4 fT/√Hz at 77 K. This sensitivity is improved to ~2 fT/√Hz at 77 K by using a soft magnetic flux antenna.

SQUID-Based Magnetometric Systems for Cardiac Diagnostics

Magnetometric systems based on superconducting quantum interference devices (SQUID) were developed for bio-medical applications. The characteristics of existing SQUID systems that can be used in magnetocardiography (MCG) are discussed. The results of clinical MCG studies of a group of volunteers with paroxysmal atrial fibrillation and a group of apparently healthy persons are presented as an example.

Noise Properties of Digital SQUID Using Double Relaxation Oscillation SQUID Comparator With Relaxation Oscillation Resonant Circuit

We designed a digital superconducting quantum interference device (SQUID) system that contains a double relaxation oscillation SQUID (DROS) as a magnetic flux comparator that is connected to an additional relaxation oscillation (RO) circuit with mutually coupled inductances. In the designed digital SQUID, the RO circuit works as a resonator and increases the relaxation oscillation frequency without decreasing the input current of dc/SFQ converters. Numerical simulation shows that the digital SQUID system can operate up to 7 GHz. A fabricated digital SQUID using a DROS comparator with an RO circuit provides correct digital SQUID operation. For a magnetometer using the digital SQUID system, we can realize a magnetic field noise less than 1 pT/ $\sqrt{\mathrm{Hz}}$ with a small signal bandwidth of more than 10 MHz.

Study on Noise Matching Between SQUID Sensor and Its Readout Electronics

Noise matching between superconducting quantum interference device (SQUID) sensor and readout electronics has always been an important problem. Conventionally, in order to avoid the hysteresis, SQUIDs should be operated in the range of $\beta _{{{c}}}\, , where $\beta _{{{c}}}$ is the Stewart–McCumber parameter. Recently, Liu et al. extended the SQUID operating area to $\beta _{{{c}}}\,> \,1$ due to a large noise parameter Γ*. When $\beta _{{{c}}}\,> \,1$ , SQUID exhibits a large ∂ V /∂Φ, which is beneficial in reducing the noise contribution of the readout electronics, $\delta \Phi _{{{e}}}$ . In this work, we compare the SQUID system noise $\delta \Phi _{{{s}}}$ measured with four different readout electronics. Here, the four readout electronics employed are as follows: 1) the flux modulation scheme (FMS); 2) the direct readout scheme (DRS), which uses six parallel connected bipolar transistor as its preamplifier (DRS-PCBT); 3) the DRS, which uses a single preamplifier AD797 (DRS-AD797); and 4) the two-stage scheme. The preamplifier voltage noise of DRS-PCBT and DRS-AD797 are about 0.4 nV/√Hz and 0.9 nV/√Hz, respectively. For a two-stage scheme, the equivalent flux noise reaches about 0.25 μΦ0/√Hz, the measured SQUID intrinsic noise, δΦ $ _{{{i}}}$ , will dominate $\delta \Phi _{{{s}}}$ . In other readout schemes, however, $\delta \Phi _{{{s}}}$ depends on the value of $\beta _{{{c}}}$ . Finally, we found that FMS is suitable to match SQUIDs with $\beta _{{{c}}}\, . DRS-PCBT is a good choice for SQUIDs with $1\, whereas DRS-AD797 suffices for SQUIDs with $\beta _{{{c}}}\,> \,3$ .

HTS Multimaterial Gradiometer Modules for YBa2Cu3O7 SQUID Systems

For the measurement of very small magnetic fields, systems based on superconducting quantum-interference devices (SQUIDs) have been manufactured and employed for many years. In liquid helium operated systems with niobium SQUIDs, wire wound gradiometers can be coupled to a shielded SQUID, but in ceramic high-temperature superconductors (HTS) this is not a common setup, since appropriate thin HTS wires with a bending radius of millimeters operating at liquid nitrogen temperature and the corresponding joining technology is not easily available. We investigated the potential of additive manufacturing to fabricate hybrid plastic and ceramic structures of the insulating perovskite SrTiO3 together with the high- Tc superconductor YBa2Cu3O7, reaching a $T_{c}$ of up to 84 K in the 3D-printed polycrystalline materials. With our measurement setup, the structures are characterized in terms of $T_{c}$ and $I_{c}$ . With the successful process, now larger sample sizes and arbitrary hybrid structures can be manufactured. A gradiometric pick-up loop was fabricated and characterized. These structures can be integrated in low-noise plastic holders and encapsulated by plastic-printed capsules to protect the HTS devices from humidity. By additive manufacturing very versatile ceramic HTS multimaterial hybrid modules can be employed for novel HTS SQUID systems.

Meissner effect measurement of single indium particle using a customized on-chip nano-scale superconducting quantum interference device system

As many emergent phenomena of superconductivity appear on a smaller scale and at lower dimension, commercial magnetic property measurement systems (MPMSs) no longer provide the sensitivity necessary to study the Meissner effect of small superconductors. The nano-scale superconducting quantum interference device (nano-SQUID) is considered one of the most sensitive magnetic sensors for the magnetic characterization of mesoscopic or microscopic samples. Here, we develop a customized on-chip nano-SQUID measurement system based on a pulsed current biasing method. The noise performance of our system is approximately 4.6 × 10−17 emu/Hz1/2, representing an improvement of 9 orders of magnitude compared with that of a commercial MPMS (~10−8 emu/Hz1/2). Furthermore, we demonstrate the measurement of the Meissner effect of a single indium (In) particle (of 47 μm in diameter) using our on-chip nano-SQUID system. The system enables the observation of the prompt superconducting transition of the Meissner effect of a single In particle, thereby providing more accurate characterization of the critical field Hc and temperature Tc. In addition, the retrapping field Hre as a function of temperature T of single In particle shows disparate behavior from that of a large ensemble.

An Integrated Maximum Current Density Approach for Noninvasive Detection of Myocardial Infarction.

We present a new approach of integrated maximum current density (IMCD) for the noninvasive detection of myocardial infarction (MI) using magnetocardiography (MCG) data acquired from a superconducting quantum interference device (SQUID) system. In this paper, we investigated the relationship of the maximum current density (MCD) in the current density map and the underlying equivalent current dipole (ECD) based on a novel method of reconstructing the ECD in the extremum circle of the magnetic field map. The performance of IMCD and the integrated ECD (IECD) approaches were also evaluated by using 61-channel MCG data from 39 healthy subjects and 102 patients with ST elevation myocardial infarction (STEMI). Statistical analysis of the healthy and STEMI groups demonstrate that the IMCD approach obtains sensitivity and specificity up to 91.2% and 84.6%, somewhat higher than that of IECD, respectively. The results indicate that IMCD provides spatiotemporal information regarding cardiac electrical activity during ventricular repolarization. This approach may be helpful to diagnose MI in clinic application. The physical concept of the approach is also explained in this paper.