HTS Dc SQUID Research Articles

Characteristic Analysis of Directly Coupled HTS dc-SQUID Magnetometer With Superconducting Film Magnetic-Shield Considering Josephson-Junction Resistance

A high temperature superconducting quantum interface device (HTS SQUID) is an extremely sensitive magnetic sensor. The HTS SQUID is required to have a high robustness with respect to magnetic noise for stable operation outside a magnetic shielding room. In order to achieve the high robustness of HTS SQUID, use of a magnetic shield with a superconducting film has been proposed, and its effectiveness has been confirmed in the experiments. In this paper, an electromagnetic field simulation was performed using the 3-D edge finite element method to confirm the effectiveness of the superconducting film magnetic-shield. Since both the SQUID magnetometer and the superconducting film magnetic-shield are made of HTS, it is required to consider the HTS characteristics in the electromagnetic field simulation. Additionally, the SQUID magnetometer has Josephson junction. In this paper, the property of the Josephson junction is also considered. The simulation accuracy depending on the type of the finite element mesh is also investigated.

Magnetic Shielding Simulation of Superconducting Film Magnetic Shield Covering Directly Coupled HTS dc-SQUID Magnetometer

An HTS SQUID magnetometer is a highly sensitive magnetic sensor. The SQUID magnetometer is, however, susceptible to magnetic noise. Hence, to enhance the robustness of the SQUID magnetometer with respect to magnetic noise, a cover with a superconducting film magnetic shield has been proposed. The superconducting film magnetic shield improves the robustness with respect to magnetic noise and widens the effective area of the SQUID sensor. It was also confirmed that the shape of the superconducting film magnetic shield affected the effectiveness in the magnetic shield in experiments. In our research, we have carried out a magnetic shielding simulation of the SQUID magnetometer covered with the superconducting film magnetic shield by using a 3D edge finite element method. To clarify the influence of the shape of the superconducting film magnetic shield on the effective area of the SQUID sensor, the simulation concerning a few shapes of the superconducting film magnetic shield was carried out. In order to evaluate the effectiveness of the superconducting film magnetic shield, the supercurrent inside the SQUID magnetometer and the superconducting film magnetic shield are simulated. From the simulation results, it is confirmed that the effective area of the SQUID sensor is widened by attaching the superconducting film magnetic shield over the SQUID ring.

Magnetic shield effect simulation of superconducting film shield covering directly coupled HTS dc-SQUID magnetometer

An HTS SQUID is a high sensitive magnetic sensor. In recent years, the HTS SQUID is widely used in various applications. In some applications, high robustness with respect to magnetic noise is required to realize stable operation at outside of a magnetic shielding room. The target of this paper is a directly coupled HTS dc-SQUID magnetometer. To enhance the robustness of the SQUID magnetometer, use of a superconducting thin film shield has been proposed. The magnetic field directly penetrating the SQUID ring causes the change of the critical current of Josephson junction, and then the SQUID magnetometer transitions into inoperative state. In order to confirm the magnetic shield effect of the superconducting film shield, electromagnetic field simulation with 3D edge finite element method was performed. To simulate the high temperature superconductor, E–J characteristics and c-axis anisotropy are considered. To evaluate the effect of the superconducting film shield, an external magnetic field which is supposed to be a magnetic noise is applied. From the simulation results, the time transition of the magnetic flux penetrating the SQUID ring is investigated and the effect of the superconducting film shield is confirmed. The amplitude of the magnetic flux penetrating the SQUID ring can be reduced to about one-sixth since the superconducting film shield prevents the magnetic noise from directly penetrating the SQUID ring.

Scanning SQUID Microscope NDT System Based on Eddy Current Testing

A scanning SQUID microscope based on HTS dc SQUID has been developed. The SQUID is mounted inside the insulation vacuum of a cryostat, which is separated from room temperature samples by a 65 μm thick sapphire window. Operating with a double-D exciting coil, it could be assembled to a scanning SQUID microscope Nondestructive Testing (NDT) system based on eddy current testing. The current is excited by an appropriate sinusoidal alternating (double-D coil) current in the conducting material, and then it induces a corresponding magnetic field. The vertical component of the field is then detected by the scanning SQUID system. The distortion of the field could be detected at the regions of discontinuities, such as flaws. Thereby, the defects inside the materials will be detected with neither contact nor destruction.

Detection of Magnetomechanical Effect in Structural Steel Using SQUIDs and Flux-gate Sensors

In this work we present a non-destructive technique to detect the magnetization changes due to mechanical stress produced in steel notched plates under tensile loads. The measurements have been carried out using, separately, a magnetic field probe based on HTS dc SQUID and a flux-gate gradiometer probe. The aim of this work is to demonstrate that it is possible to localize the dislocation density change in the samples and to correlate it with the change of magnetization using a contact-less and non-invasive technique.

DC SQUID Modulation Electronics for Operation With HTS DC SQUID Magnetometers in the Unshielded Environment

The new variant of DC SQUID modulation electronics for functioning with HTS DC SQUID magnetometers in the unshielded environment, was designed, manufactured and tested. The electronics was optimized for operation with new ultrasensitive HTS DC SQUID magnetometer providing magnetic field resolution of about 15 fT/radicHz at frequencies above 10 Hz and 30 fT/radicHz at 1 Hz. The central commutation core of electronics was based on a complex programmable logic device (CPLD). It has allowed to include into the SQUID-electronics the flexible system of a bias reversal which provides a stable magnetometer operation for various configurations of wiring in the cryostat and various HTS DC SQUID topologies. The electronics was manufactured in one compact box with size 110 mm times60 mm times 15 mm. The channel has standard bandwidth of about 100 kHz with greatly increased dynamic range due to high coupling (8 mV/Phi0) of feedback signal with main SQUID's loop.

Real-time monitoring of fatigue damage in Carbon Fiber Reinforced Polymers for aeronautical applications using HTS SQUID magnetometer

The main advantage of using SQUIDs in NDE is its unrivalled magnetic sensitivity down to very low frequencies, which allows the detection of weak magnetic fields due to defects also in very low electrical conductive materials like CFRP, commonly used in aerospace structures. In this work the fatigue damage evaluation on CFRP laminates, cross-ply or quasi-isotropic, in real-time during quasi-static loading is monitored. The material responses are detected by using an Eddy Current technique with HTS dc SQUID magnetometer.

Magnetic shielding effects of a superimposed cylinder composed of BPSCCO and soft-iron cylinders: Dependencies of the air gap of the superimposed cylinder and the thickness of the soft-iron cylinder

Abstract The behavior of the magnetic flux density B in within a superimposed cylinder, constructed by the superposition of a soft-iron cylinder over a BPSCCO cylinder, is unknown. The present research examines and clarifies the effect of B in on the changes of the wall thickness W of the soft-iron cylinder, and the air gap distance δ between the soft-iron and BPSCCO cylinders. In addition, the magnetic noise power spectra of B in are examined for various values of δ and W , by the use of an HTS dc-SQUID system and a spectrum analyzer. The results lead to an important criterion for the design of an effective magnetic shielding vessel.

A low-cost magnetic shield consisting of nonoriented silicon steel

This paper presented a double cylindrical magnetic shield with nonoriented silicon steel plate rings. The total weight of cylinders was about 14 kg, and its corresponding material cost was less than 50 USD. A relatively low magnetic noise of 2 pT/Hz/sup -1/2/ at 10 Hz and 20 pT/Hz/sup -1/2/ at 1 Hz was measured with a HTS dc-SQUID sensor inside the shield. The transverse shielding factor was measured with a fluxgate sensor, and the value was about 500 when the ranges of frequency and amplitude were 10 Hz-1 kHz and 100 nT-10 /spl mu/T. The value dramatically improved to as high as 20 000 by magnetic shaking enhancement. If magnetic shaking was not applied to the inner cylinder, this cylinder would reduce the leakage field of the magnetic shaking from the outer cylinder to less than 300 nT. The cost performance of the multicylindrical shield was also discussed. To provide the most outer cylinder with magnetic shaking allowed the cost performance to be twice as good as cylinders that did not use magnetic shaking.

Electronic Axial HTS dc SQUID Gradiometer Cooled by Pulse-Tube Cooler

An electronic axial high-T/sub c/ dc SQUID gradiometer cooled by pulse-tube cooler was developed. In order to extend the operating times, a method of temperature control was used to stabilize the operating temperature of the SQUID. We used the SQUID gradiometer cooled by pulse-tube cooler to perform eddy-current nondestructive evaluation and corrosion signal detection in an unshielded environment.

Detection of Mobile Targets from a Moving Platform Using an Actively Shielded, Adaptively Balanced SQUID Gradiometer

Tests have been performed with a moving HTS dc SQUID gradiometer mounted on a nonmetal motion table. The system operated with sensitivity of 80 pT/m//spl radic/Hz (1 Hz) and 1 pT/m//spl radic/Hz (white) whilst undergoing pitch, roll and yaw motions of /spl plusmn/5/spl deg/ in an outside environment. Gradient anomalies of /spl ges/ 1 nT/m were detected with 12 dB signal-to-noise ratio and a measurement bandwidth of 1 kHz. In this paper, we describe the performance of the gradiometer at measuring remote moving ferromagnetic targets whilst it is in motion. These measurements have an important bearing on understanding the viability of SQUID-based systems as magnetic anomaly detectors, when mounted on airborne or seaborne platforms. In one early test, a target of approximately 300 Am/sup 2/ travelling at 2-5 m/s was detected at 25 m range with a signal to noise ratio of 4:1, whilst the gradiometer was undergoing motions of /spl plusmn/5/spl deg/ in pitch roll and heading.

SCANNING SQUID MICROSCOPE AND ITS APPLICATION IN DETECTING WEAK CURRENTS

A scanning SQUID microscope based on HTS dc SQUID has been developed. One of the applications of this microscope is to detect weak currents inside the sample. Considering that what being detected by the SQUID is the vertical component of the magnetic field on a plan where the SQUID lies, whereas the current which produces the magnetic field is actually located in a plan below the SQUID, a TWO PLAN model has been established. In this model Biot-Savart force laws and Fourier transformation were used to inverse the detected magnetic field into the underneath weak current. It has been shown that the distance between the current and the SQUID and the noise ratio of the experimental data have significant effects on the quality of the inverse process.

Experimental determination of HTS dc-SQUID amplifier inductance and noise performance

The inductance L/sub sq/, transimpedance Z, transfer function V/sub /spl Phi// and flux noise S/sub /spl Phi// have been experimentally determined, in open loop mode, for a series of simple dc-SQUID amplifiers which varied only in the length of the loop. The SQUID design had no input flux transformer but included a current path for directly injecting flux into the loop. Correlations between these parameters were obtained which compare favorably with theoretical estimates. We confirm that the best device performance (small S/sub /spl Phi//) is obtained if L/sub sq/ is kept small whilst V/sub /spl Phi// is maximized; the latter was achieved in one device by a novel ion-beam trimming technique.

Sensitive HTS DC-SQUID gradiometers for magnetic evaluation applications

An axial HTS DC-SQUID gradiometer formed by electronic subtraction with a 10 cm baseline and a gradient resolution better than 1 fT/cm Hz at 77 K has been developed and demonstrated. The best achieved gradient resolution for the planar HTS DC-SQUID gradiometer was of about 30 fT/cm Hz for both shielded and unshielded operation. A comparison of the characteristics of the gradiometers with ones of competitive room temperature sensors like fluxgates is discussed.

HTS dc SQUID based rf amplifier: development concept

We present a concept of a rf amplifier based on a directly coupled dc SQUID with bicrystal junctions, which have high saturation power and can be used with SIS mixers or possibly for satellite and cellular phone communications. A novel input resonant circuit is proposed using single layer of HTS. Estimated parameters are (per stage): central frequency≈11 GHz, bandwidth≈400 MHz, noise temperature≈10 K, gain≈10 dB and input saturation≈1000 K GHz.

SQUID array for magnetic inspection of prestressed concrete bridges

For detection of tendon ruptures in prestressed members of bridges, a four-channel SQUID system was developed. The tendons are magnetized by scanning a yoke electromagnet over the concrete surface along the hidden member. Four HTS dc-SQUID magnetometers with ramp-type junctions, optimized for high-field performance, are mounted in an orientation-independent liquid nitrogen cryostat. The SQUIDs are integrated as a linear array within the yoke and operated in magnetic fields up to 15 mT, recording the stray field during magnetization as well as the remanent field after switching off the excitation. Unwanted signals from stirrups of the mild steel reinforcement are suppressed with two types of techniques: either the comparison of remanent field signals after changing the magnetization direction of the stirrups, or a best fit of typical stirrup signals to the stray field signal and their subtraction. Subsequent correlation analysis with the dipolar signal of a typical void yields rupture signal amplitudes. A finite element program was written to simulate stray field and remanent field traces of typical steel configurations. Excellent agreement with measured data was found. Results of measurements on a prestressed highway bridge are presented. Signal amplitudes above the threshold values were verified as originating from ruptures of the steel tendon by opening the bridge deck.

HTS dc SQUID behavior in external magnetic fields

Several types of HTS dc SQUID magnetometers - directly coupled magnetometers with either a conventionally solid pickup loop or with 17 narrow loops in parallel and flip-chip magnetometers-are tested concerning their behavior in external fields. The stability against flux penetration was measured by the determination of the magnetic hysteresis. This stability is increased by steep structure edges in the SQUID and in the pickup loop. The intrinsic noise of SQUIDs during operation in external fields was determined by simultaneous measurement of the spectra of two SQUIDs. A directly coupled magnetometer with narrow parallel loops showed much smaller intrinsic noise after movement in the Earth's magnetic field than one with a solid pickup loop.

HTS dc SQUID systems for geophysical prospection

We describe the use of high T/sub c/ flip-chip dc SQUID magnetometers in systems for several geophysical prospection methods. For use in the Transient Electro Magnetics (TEM) method, a one-channel SQUID magnetometer system (vertical field component) was adapted to rough conditions in the field. We report on measurements in South Africa over a geophysically interesting target in real production mode. Transients up to 200 ms could be recorded, which is about a factor of 100 more than with conventional coils. For investigation of the magnetic field gradient a two-channel system, built as a first-order electronic gradiometer with a baseline up to 1 m, was designed and manufactured. It can be used, e.g., for surface exploration or for archeometry. The system can be moved in the Earth's magnetic field. We achieve a common mode rejection of 100. The reasons for the imbalance and the possibility of further improvement of the system are discussed.

Low noise HTS dc-SQUID flip-chip magnetometers and gradiometers

We have fabricated HTS dc-SQUID flip-chip sensors with a large area multilayer flux transformers. Different layouts of the flux transformers provide a large variety of magnetometers and planar gradiometers. For the magnetometers a resolution /spl sim/6 fT//spl radic/Hz and the planar gradiometers a resolution of about /spl sim/30 fT/cm./spl radic/Hz were routinely obtained at 77 K. The noise was nearly white down to frequencies of few Hz. The sensors were vacuum-tight encapsulated together with a heater and a feedback coil. This makes the handling of the sensors more reproducible and convenient. Production of the magnetometers and gradiometers in small series was proven.

An HTS dc SQUID system for geomagnetic prospection

We have developed a highly sensitive dc SQUID system for geomagnetic prospection operating at 77 K. Because of the large voltage swing of our flip chip magnetometers it is possible to use a directly coupled SQUID electronics. Thus, the requirements for TEM measurements can be fulfilled: slew rate >1 mT s-1 and dynamic range >120 dB. In measurements above real geological targets the significantly better performance of the SQUID compared to conventional coils has been demonstrated. The lower noise of the SQUID system at late times allows us to measure the transients for 5 to 10 times longer, which results in an increase of the investigation depth by a factor of 2-3. For the same data quality a reduction of the measurement time between 25 and 100 compared to coils is possible.