G.B. Donaldson

Superconducting thin‐film gradiometer

We describe the design, fabrication, and performance of planar thin‐film dc SQUID’s and planar gradiometers in which a dc SQUID is incorporated as a null detector. Each gradiometer was fabricated on a planar substrate and measured an off‐diagonal component of changes in the magnetic field gradient. The gradiometer with the highest sensitivity had 127×33‐mm loops that could be connected in parallel or in series: The sensitivities were 2.1×10−13 and 3.7×10−13 T m−1 Hz−1/2, respectively. The intrinsic balance of the gradiometers was about 100 ppm for fields parallel to their plane, and a balance of about 1 ppm could be achieved for fields perpendicular to their plane. When the series‐loop gradiometer was rotated through 360° in the earth’s field, the output returned to its initial value to within an amount corresponding to a balance of 1 ppm. Possible improvements in sensitivity are discussed.

Antenatal fetal magnetocardiography: a new method for fetal surveillance?

Objective To establish the reliability of fetal magnetocardiography as a method of measuring the time intervals of the fetal heart during the antenatal period.

Using a 77 K SQUID to measure magnetic fields for NDE

A bare HTS SQUID of commercial design was used in 77 K experiments concerning NDE. The SQUID was operated with flux-locked instrumentation to provide a noise floor of 80 pT//spl radic/Hz. The effective sensor area was measured to be approximately 70 /spl mu/m/sup 2/ equivalent to an ideal point detector for NDE. The SQUID was used unshielded in a normal laboratory environment in a special purpose LN/sub 2/ cryostat positioned above a motorized computer-controlled scanning system. We measured magnetic fields associated with current flowing in wires and compared them with calculations. We also detected a simulated flaw in an aluminum plate using an eddy current technique and made a preliminary depth assessment by frequency sweeping. Although developments in electronic gradiometers and gradiometric SQUIDs should make the use of single bare magnetometer SQUIDs unnecessary, we show that these already have sufficient sensitivity for NDE research, even without flux-focusing washers or pick-up coils.<<ETX>>

Highly balanced long-baseline single-layer high-Tc superconducting quantum interference device gradiometer

We describe a direct-current superconducting quantum interference device (SQUID) first-order gradiometer fabricated from a single layer of YBa2Cu3O7 on a 30×10 mm2 bicrystal substrate. The device has a baseline of 13 mm and an intrinsic balance of ∼10−3. The gradient sensitivity at 77 K and 1 kHz is 50 fT/(cmHz) in magnetic shielding and 260 fT/(cmHz) when operated unshielded in our laboratory. An antiparallel two-SQUID coupling scheme is employed to optimize the device’s balance to at least 3×10−5.

SQUID-based nondestructive evaluation of carbon fiber reinforced polymer

Recent work with HTS SQUIDs in nondestructive evaluation has concentrated on the detection of flaws in aircraft-grade aluminum, with particular emphasis on surface-breaking tears beside rivets. More complex materials are now also being used in aircraft manufacture, with carbon fiber reinforced polymer (CFRP) being one of the most common. Existing technologies such as ultrasound are particularly well suited to the detection of impact-damaged sites and until now there have only been a few reports of eddy current examination of CFRP samples. Here we present results on samples with regions of heat damage, impact damage and with nonmagnetic inserts using eddy current detection techniques. We compare the signal to noise ratio and spatial resolution for a variety of sensors including HTS SQUIDs and gradiometers and conventional induction coils, and discuss variations in detection efficiency with field component measured.

Electromagnetic nondestructive evaluation: moving HTS SQUIDs, inducing field nulling and dual frequency measurements

We have previously shown that simple, single layer HTS SQUIDs can be used effectively in electromagnetic nondestructive evaluation (NDE) using eddy current techniques in a magnetically unshielded environment. HTS SQUID systems for NDE applications are expected to be small and portable allowing non-stationary measurements to be carried out in the Earths field above a stationary sample. Here we present application-oriented results showing the ability of our HTS electronic gradiometer to cope with the movement of the sensors above a series of simulated flaws in aircraft grade aluminum samples. To permit the detection of fine surface and subsurface structures we have applied active field nulling to the two SQUIDs to increase the effective signal to noise ratio. The excitation signal is applied via a non-superconducting coil to provide a lower field environment for each device. We also present results using a dual frequency eddy current technique to allow depth profiling of flaws in multilayer structures.

HTS SQUIDs for the nondestructive evaluation of composite structures

While LTS and HTS SQUIDs have successfully been applied in the detection of flaws in aircraft grade aluminium structures for well over a decade, interest has recently spread to a type of new material, namely composites. One example, carbon fibre reinforced polymer (CFRP), is increasingly being favoured by the aircraft industry because of its strength to weight ratio and the fact that it is corrosion-resistant. Material and defect characterization using SQUIDs is still at an early stage, but due to expected rapid expansion in the use of such materials, there is ample scope for the application of HTS SQUIDs. Here we have applied HTS SQUID single-layer gradiometers to investigate artificially created defects in CFRP samples.

Design and operation of SQUID-based planar gradiometers for non-destructive testing of ferromagnetic plates

A planar second-order gradiometer coupled to a SQUID has been used with a persistent mode coil generating 0.02T perpendicular to a steel plate for the detection and characterisation of defects, such as cracks. We outline design criteria for the system, which is contact free, allows stand-off distances of up to 10cm, and can accommodate intervening media, both insulating (e.g. concrete) or conducting (e.g. sea-water or aluminium cladding). The sensitivity limit appears to be set by long-range permeability variations in the material of the plates, which may be due to unrelieved stress. We explain the use of digital filtering techniques to improve the resolution. Finally we discuss the use of higher-order gradiometers to improve the discrimination of localised defects from background variations.

A comparison of the performance of planar and conventional second-order gradiometers coupled to a SQUID for the NDT of steel plates

The authors have developed a DC magnetic NDT (nondestructive testing) technique using a SQUID (superconducting quantum interference device) coupled to a planar gradiometer for the inspection of structures such as flat steel plates and steel pipes. Design criteria for the gradiometer are presented and used to demonstrate the superior performance of the planar approach compared to that of a second-order axial gradiometer of conventional design. The authors show an example of the use of system for mapping the magnetic field above a steel plate containing ideal defects. The proposed SQUID system is shown to operate normally in an unscreened laboratory close to a steel specimen in the presence of a DC magnetizing field in excess of 20 mT. >

A fully automated pulsed laser deposition system for HTS multilayer devices

For the fabrication and development of high-temperature superconducting (HTS) thin film devices, which often require multiple layers, it is essential to have control over all parameters during growth. Until recently the overwhelming majority of film growth was controlled manually. We have found this can often lead to error and irreproducibility. To overcome these problems we have designed and constructed an automated multitarget excimer pulsed laser deposition (PLD) system. We identify key elements of the system that can be utilised to address many of the problems currently encountered in HTS film growth. Optimisation using Robust Design techniques has also been used in the identification of principal parameters in growth of films and multilayers.<<ETX>>