Development of Low Temperature SQUID Gradiometer Array for Metallic Magnetic Microcalorimeters

We report on the performance of two types of SQUID gradiometers developed for the readout of magnetic calorimeters. Our previously developed low dissipation SQUID gradiometer optimized for low temperature operation has demonstrated the flux noise level of under a magnetic field of 2.5 mT and 150 mK. With a cylindrical Au:Er paramagnetic sensor mounted inside the octagonal pickup washer of the SQUID gradiometer, we succeeded in detecting X-ray signals. However, our achieved energy resolution was 47.2±2.1 eV at 5.9 keV limited by the high operating temperature of 150 mK and by a magnetic field, small for that temperature, due to the limited critical current of the field coils. Based on these results, we fabricated new arrays of SQUID gradiometer by tuning the line width and the number of turns of the field coils and shunt resistance to realize a lower noise level and a larger magnetic field. Furthermore, arrays of SQUID gradiometer with meander patterned pickup washer was fabricated which provides a stronger coupling between the paramagnetic sensor and the pickup washer, and a larger magnetic field at the sensor.      

Electric current injection NDE using a SQUID magnetometer

We built a portable low-cost superconducting quantum interference device (SQUID) magnetometer to be used as the sensing element of an electric current injection NDE system. Using this system, we measured the magnetic fields associated with aluminum plate with single flaws in the millimeter range (2 to 8 mm), at lift-off distances up to ten times the size of the flaw, with injection currents in the order of a few amps. Because the system is at a relatively large distance from the test plate, the signal generated by the flaw is masked by the sinal generated by the edges of the plate. This is independent of the amount of current injected. Due to this, visual inspection is difficult when the distance from the sensor to the flaw exceeds five times its size. To intensify the magnetic field disturbance due to the flaws, we used digital image-enhancement techniques. Applying the enhancement algorithm, magnetic field disturbances were visualized at distances up to ten times the size of the flaw.     

Planar HTS structures for high-power applications in communication systems

The use of rotational symmetric modes of superconductive disk (ring) resonators in order to increase the power handling capability of planar filters has been proposed. Several YBCO disk resonators on LaAlO₃ and sapphire substrates have been measured as one- and two ports. The unloaded Q-factor of the resonators has been measured to be greater than 10⁵ up to the oscillating power of 50 kW. Experimental results obtained show that a disk resonator can be used as a building block for low insertion loss, sharp skirt high-power planar filters at low GHz frequency range and temperatures of around 60 K.     

Integrated Statistical Processing of NDE Signals for Class Discrimination

Abstract

An integrated methodology for the analysis of experimental NDE signals for the classification of test-objects according to their mechanical characteristics is presented. The methodology consists of several techniques, adopted from various multi-disciplinarial fields, and incorporates concepts from the theory of information, pattern recognition techniques, cluster analysis, and data compression. Statistical tests are performed to verify the significance of the differences between the signals obtained from different groups and to estimate the confidence level associated with the classification procedure. The techniques are outlined and discussed, and experimental examples are presented to demonstrate their successful application.     

Design of Trimmingless Narrowband Planar HTS Filters

The use of high temperature superconductor (HTS) films provides a higher selectivity of planar filters. The lower out-of-band interference increases the sensitivity of receiving systems and allows reduction of transmitter radiated power. Our goal was to show how these filters can be designed with a desired accuracy, and to give evidence for a real possibility of designing the trimmingless planar HTS filters. The analysis of filter performance is based on accurate models of the characteristics of the HTS planar transmission lines. The HTS film and substrate characteristic tolerance sensitivity is investigated. The design procedure of the trimmingless filter is proposed. Mass production of such filters requires a specific certification of the substrate materials and HTS films used.     

Integrated statistical processing of NDE signals for class discrimination

An integrated methodology for the analysis of experimental NDE signals for the classification of test-objects according to their mechanical characteristics is presented. The methodology consists of several techniques, adopted from various multi-disciplinarial fields, and incorporates concepts from the theory of information, pattern recognition techniques, cluster analysis, and data compression. Statistical tests are performed to verify the significance of the differences between the signals obtained from different groups and to estimate the confidence level associated with the classification procedure. The techniques are outlined and discussed, and experimental examples are presented to demonstrate their successful application.     

Cryogenic Integrated Offset Compensation for Time Domain SQUID Multiplexing

Superconducting QUantum Interference Device (SQUID) multiplexing is a common technique in the use of large arrays of Transition Edge Sensors (TES). A?Time Domain Multiplexer (TDM) combines input TES signals into one output signal using several SQUIDs. Different TES, SQUID and amplifier characteristics induce unavoidable different offsets on the multiplexed signal. Additionally, given the periodicity of the SQUID characteristic, the Flux Locked Loop (FLL) operating point is only defined modulo?Φ ₀. This can lead to a large output offset. In multiplexed mode, the difference between offsets associated with different pixels can induce a parasitic signal which is often larger than that of the TES. These offset signals drastically constrain the readout dynamic range and thus the maximum gain allowed. They also limit the signal-to-noise ratio, the FLL stability and the multiplexing frequency. Offsets in SQUID readout are discussed and offset compensation for TDM is presented. The dynamic calibration and compensation on a simplified 4:1?TDM are demonstrated in simulation. Dynamic offset compensation is being implemented on a cryogenic SiGe integrated circuit operated at 4?K for 128:1?TDM.     

Integrated niobium DC SQUID gradiometers for non-destructive evaluation and biomagnetism

We describe the design and fabrication of thin-film Nb gradiometers with integrated DC SQUIDs for use in non-destructive evaluation (NDE) and biomagnetism. Issues of sensitivity, imbalance and field response are considered. Results are presented from eddy-current NDE in an unshielded environment of aluminium plates with sub-surface flaws, and from biomagnetic measurements of spinal and peripheral nerve evoked fields.     

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, B _m, of 44.8???T and a polarizing field, B _p, of 36.8?mT; in the presence of ? B _Z /? z and ? B _Z /? y gradients of the order of 0.8???T/m, we were able to obtain a quasi two-dimensional 4-pixels image of a simple water phantom.     

A Novel HTS Shuttle-Shape Resonator for High-Power Application

A novel shuttle-shape microstrip resonator for high-power high-temperature superconductor (HTS) filters is presented. By transmission line theory analysis, it can improve the power handling capability up to 20%, depending on the configuration of the resonator. To confirm this analysis result, two 2-pole HTS filters based on shuttle-shape resonator and rectangle resonator were simulated. Considering the limitation of computer memory, a moderate configuration was chosen for HTS filter design. By computer simulation, a comparison between filters with shuttle-shape and rectangle resonators showed a 10% increase in power handling capability. The HTS shuttle-shape filter was fabricated and tested. It has a center frequency of 2022 MHz with a 2.5% bandwidth. The measured power level was over 34 dBm at 70 K with sufficiently low insertion loss.     

Study of Anisotropy Effect in Planar Lenses for Integrated Optics

Abstract

The extraordinary-ray ray-tracing method for planar waveguides on uniaxial crystals is developed. Using the method and the extended equivalence principle, the effect of anisotropy on the performance of planar lenses is investigated in detail. The agreement between theoretical and experimental results is very good.     

Cryostats for SQUID magnetometers

A non-metallic and non-magnetic cryostat, with a very low thermal budget and a container type autonomy was developed, to condition SQUID magnetometers whose maximum sensitivity reaches 10^?14$\text{T Hz}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}$. This instrumentation uses composite materials, thermal shock and vibration resistant, multilayer thermal radiative insulation, to detect vibrations with thermal equivalent emissivity lower than 10^?3.     

Fiber optic sensing for ultrasonic NDE

An innovative approach to nondestructive evaluation (NDE) using noncontacting optical sensors has demonstrated. In this effort a single mode optical fiber interferometer (OFI) was used to sense the presence and form of Rayleigh waves traveling along the surface of a steel test bar at a velocity of nearly 3mm/µs. Acousto-optic time-domain data was successfully used to detect the presence and locate the position of a test flaw (a machined slot) in the bar, and spectrum analysis was used to estimate its geometry and size. This approach has many potential applications in the ultrasonic evaluation of real flaws in structures with complex geometries. Coupled with the authors' earlier work demonstrating the feasibility of generating acoustic waves in metals using laser light pulses transmitted through the fiber optic probes, this latest achievement points to the development of a fully noncontacting, fiber optic based thermal-acousto-photonic (TAP) NDE system, with potential applications to the reliability testing of many important structures where composition, scale, geometry, or restricted access preclude the use of conventional NDE techniques.     

DC SQUID Spectrometers for NMR

We report experiments in which we have used DC SQUIDs in pulsed NMR spectrometers to observe directly the free precession of nuclear spins. A broadband spectrometer, which operates in flux-locked loop mode with a bandwidth of 3.4 MHz using a SQUID with additional positive feedback and an untuned superconducting input circuit, has been used to observe NMR signals from platinum powder at frequencies of 38, 65, 85, 240 and 513 kHz. The performance is compared with a second system in which a DC SQUID is operated open loop as a small signal rf amplifier with a series tank input circuit tuned at 1 MHz.     

Return-flux corrections for SQUID susceptometry

A temperature controlled sample in a SQUID magnetometer or susceptometer cannot fill the cross-section of the gradiometer pickup due to the requirement of a cryogenic insulating space between the sample and the superconducting pickup. Return-flux corrections beyond those accounted for by the ordinary demagnetizing factor are thus required. Such corrections as applicable to spheroidal samples are calculated herein, and some of their applications and consequences discussed.     

SQUID magnetometers for low-frequency applications

We present a novel formulation for SQUID operation, which enables us to evaluate and compare the sensitivity and applicability of different devices. SQUID magnetometers for low-frequency applications are analyzed, taking into account the coupling circuits and electronics. We discuss nonhysteretic and hysteretic single-junction rf SQUIDs, but the main emphasis is on the dynamics, sensitivity, and coupling considerations of dc-SQUID magnetometers. A short review of current ideas on thin-film, dc-SQUID design presents the problems in coupling and the basic limits of sensitivity. The fabrication technology of tunnel-junction devices is discussed with emphasis on how it limits critical current densities, specific capacitances of junctions, minimum linewidths, conductor separations, etc. Properties of high-temperature superconductors are evaluated on the basis of recently published results on increased flux creep, low density of current carriers, and problems in fabricating reliable junctions. The optimization of electronics for different types of SQUIDs is presented. Finally, the most important low-frequency applications of SQUIDs in biomagnetism, metrology, geomagnetism, and some physics experiments demonstrate the various possibilities that state-of-the-art SQUIDs can provide.     

Flux-Ramp Modulation for SQUID Multiplexing

SQUIDs are typically operated in a flux-locked loop to linearize their response to input flux. However, a flux-locked loop is not possible with a microwave SQUID multiplexer. We describe an alternative technique called flux-ramp modulation and report its successful implementation.     

Characterization of capacitive array for NDE applications

We have developed a capacitive array sensor which responds to the complex dielectric constant of an interrogated material. The sensor requires only single-sided access and operates in a differential mode for detection of discontinuities in the relative dielectric constant, _r , or in the absolute mode where the interest is in absolute quantities. The device in general is noncontacting but can be fabricated as an embedded sensor.Various proof-of-concept studies have been performed to explore possible applications of the device. In the differential mode, small surface features (notches) were detected in a conductor; in dielectrics, both surface and subsurface features were detected. The probe was sensitive to change in impedance caused by a curing epoxy, viewed through a graphite-epoxy composite panel.A study was performed to assess the use of the probe for noncontact characterization of sintering of ceramics. In this (absolute) mode, the effects of liftoff and dielectric constant must be separated. We propose a scheme based on the ability to multiplex the capacitive array probe to accomplish this. Preliminary investigation shows that the effect of parasitic capacitance between the probe and ground points in the environment must be suppressed.Contribution of the National Institute of Standards and Technology (formerly National Bureau of Standards); not subject to copyright.     

Characterization of Capacitive Array for NDE Applications

Abstract

We have developed a capacitive array sensor which responds to the complex dielectric constant of an interrogated material. The sensor requires only single-sided access and operates in a differential mode for detection of discontinuities in the relative dielectric constant, ε _r , or in the absolute mode where the interest is in absolute quantities. The device in general is noncontacting but can be fabricated as an embedded sensor.

Various proof-of-concept studies have been performed to explore possible applications of the device. In the differential mode, small surface features (notches) were detected in a conductor; in dielectrics, both surface and subsurface features were detected. The probe was sensitive to change in impedance caused by a curing epoxy, viewed through a graphite-epoxy composite panel.

A study was performed to assess the use of the probe for noncontact characterization of sintering of ceramics. In this (absolute) mode, the effects of liftoff and dielectric constant must be separated. We propose a scheme based on the ability to multiplex the capacitive array probe to accomplish this. Preliminary investigation shows that the effect of parasitic capacitance between the probe and ground points in the environment must be suppressed.     

Laser excitation through fiber optics for NDE

This paper describes experiments designed to generate acoustic waves by using a laser pulse, transmitted through fiber optics, to thermally shock the surface of a steel specimen. The purpose of this effort was to explore the noncontacting generation of Rayleigh surface waves appropriate to the interrogation of structures for the detection of subcritical defects, with the ultimate goal of developing an efficient laser-based nondestructive evaluation technique utilizing flexible fiber optics.