A narrow band pulsed eddy current instrument for materials testing

The principles of operation of an experimental prototype instrument known as J-SCAN are described along with the derivation of formulae for the rapid calculation of normalized impedances; the structure of the instrument; relevant probe design parameters; digital quantization errors; and approaches for the optimization of single frequency operation. An eddy current probe is used As the inductance element of a passive tuned-circuit which is repeatedly excited with short impulses. Each impulse excites an oscillation which is subject to decay dependent upon the values of the tuned-circuit components: resistance, inductance and capacitance. Changing conditions under the probe that affect the resistance and inductance of this circuit will thus be detected through changes in the transient response. These changes in transient response, oscillation frequency and rate of decay, are digitized, and then normalized values for probe resistance and inductance changes are calculated immediately in a micro processor. This approach coupled with a minimum analogue processing and maximum of digital processing has advantages compared with the conventional approaches to eddy current instruments. In particular there are: the absence of an out of balance condition and the flexibility and stability of digital data processing.

Quantitative pulsed phase thermography applied to steel plates

Pulsed Phase Thermography (PPT) has been proven effective on depth retrieval of flat-bottomed holes in different materials such as plastics and aluminum. In PPT, amplitude and phase delay signatures are available following data acquisition (carried out in a similar way as in classical Pulsed Thermography), by applying a transformation algorithm such as the Fourier Transform (FT) on thermal profiles. The authors have recently presented an extended review on PPT theory, including a new inversion technique for depth retrieval by correlating the depth with the blind frequency fb (frequency at which a defect produce enough phase contrast to be detected). An automatic defect depth retrieval algorithm had also been proposed, evidencing PPT capabilities as a practical inversion technique. In addition, the use of normalized parameters to account for defect size variation as well as depth retrieval from complex shape composites (GFRP and CFRP) are currently under investigation. In this paper, steel plates containing flat-bottomed holes at different depths (from 1 to 4.5 mm) are tested by quantitative PPT. Least squares regression results show excellent agreement between depth and the inverse square root blind frequency, which can be used for depth inversion. Experimental results on steel plates with simulated corrosion are presented as well. It is worth noting that results are improved by performing PPT on reconstructed (synthetic) rather than on raw thermal data.

Quantification of boundary definition using pulsed terahertz radiation for wedged geometries

The interaction of a terahertz beam with a sample containing a material boundary across the profile of the terahertz beam produces characteristic spectroscopic detail. A full vectorial model is presented to quantify boundary definition for a series of wedged geometries. As a result of this work, using simple geometric forms, we wish to be able to extend these ideas to characterize boundaries in more irregular samples, impacting most application areas of pulsed terahertz radiation.

Do in vivo terahertz imaging systems comply with safety guidelines?

Techniques for the coherent generation and detection of electromagnetic radiation in the far infrared, or terahertz, region of the electromagnetic spectrum have recently developed rapidly and may soon be applied for in vivo medical imaging. Both continuous wave and pulsed imaging systems are under development, with terahertz pulsed imaging being the more common method. Typically a pump and probe technique is used, with picosecond pulses of terahertz radiation generated from femtosecond infrared laser pulses, using an antenna or nonlinear crystal. After interaction with the subject either by transmission or reflection, coherent detection is achieved when the terahertz beam is combined with the probe laser beam. Raster scanning of the subject leads to an image data set comprising a time series representing the pulse at each pixel. A set of parametric images may be calculated, mapping the values of various parameters calculated from the shape of the pulses. A safety analysis has been performed, based on current guidelines for skin exposure to radiation of wavelengths 2.6 µm–20 mm (15 GHz–115 THz), to determine the maximum permissible exposure (MPE) for such a terahertz imaging system. The international guidelines for this range of wavelengths are drawn from two U.S. standards documents. The method for this analysis was taken from the American National Standard for the Safe Use of Lasers (ANSI Z136.1), and to ensure a conservative analysis, parameters were drawn from both this standard and from the IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields (C95.1). The calculated maximum permissible average beam power was 3 mW, indicating that typical terahertz imaging systems are safe according to the current guidelines. Further developments may however result in systems that will exceed the calculated limit. Furthermore, the published MPEs for pulsed exposures are based on measurements at shorter wavelengths and with pulses of longer duration than those used in terahertz pulsed imaging systems, so the results should be treated with caution.

Exploitation of a pulsed neutron source for the structural study of anisotropic polymer films

A novel but simple time-of-flight neutron scattering geometry which allows structural anisotropy to be probed directly, simultaneously and thus unambiguously in polymeric and other materials is described. A particular advantage of the simultaneous data collection when coupled to the large area of the beam is that it enables thin films (< 10 μm < 10 mg) to be studied with relative ease. The utility of the technique is illustrated by studies on both deformed poly(styrene) glasses and on thin films of electrical conducting polymers. In the latter case, the power of isotopic substitution is illustrated to great effect. The development of these procedures for use in other areas of materials science is briefly discussed.

Combined use of two genetic fingerprinting methods, pulsed-field gel electrophoresis and ribotyping, for characterization of Escherichia coli O157 isolates from food animals, retail meats, and cases of human disease

Two genetic fingerprinting techniques, pulsed-field gel electrophoresis (PFGE) and ribotyping, were used to characterize 207 Escherichia coli O157 isolates from food animals, foods of animal origin, and cases of human disease (206 of the isolates were from the United Kingdom). In addition, 164 of these isolates were also phage typed. The isolates were divided into two general groups: (i) unrelated isolates not known to be epidemiologically linked (n = 154) and originating from food animals, foods and the environment, or humans and (ii) epidemiologically related isolates (n = 53) comprised of four related groups (RGs) originating either from one farm plus the abattoir where cattle from that farm were slaughtered or from one of three different English abattoirs. PFGE was conducted with the restriction endonuclease XbaI. while for ribotyping, two restriction endonucleases (PstI and SphI) were combined to digest genomic DNAs simultaneously. The 207 E. coli O157 isolates produced 97 PFGE profiles and 51 ribotypes. The two genetic fingerprinting methods had similar powers to discriminate the 154 epidemiologically unrelated E. coli O157 isolates in the study (Simpson's index of diversity [D] = 0.98 and 0.94 for PFGE typing and ribotyping, respectively). There was no correlation between the source of an isolate (healthy meat or milk animals, retail meats, or cases of human infection) and either particular PFGE or ribotype profiles or clusters. Combination of the results of both genetic fingerprinting methods produced 146 types, significantly more than when either of the two methods was used individually. Consequently, the superior discriminatory performance of the PFGE-ribotyping combination was proven in two ways: (i) by demonstrating that the majority of the E. coli O157 isolates with unrelated histories were indeed distinguishable types and (ii) by identifying some clonal groups among two of the four RGs of E. coli O157 isolates (comprising PFGE types different by just one or two bands), the relatedness of which would have remained unconfirmed otherwise.

Complex extreme learning machine applications in terahertz pulsed signals feature sets

This paper presents a novel approach to the automatic classification of very large data sets composed of terahertz pulse transient signals, highlighting their potential use in biochemical, biomedical, pharmaceutical and security applications. Two different types of THz spectra are considered in the classification process. Firstly a binary classification study of poly-A and poly-C ribonucleic acid samples is performed. This is then contrasted with a difficult multi-class classification problem of spectra from six different powder samples that although have fairly indistinguishable features in the optical spectrum, they also possess a few discernable spectral features in the terahertz part of the spectrum. Classification is performed using a complex-valued extreme learning machine algorithm that takes into account features in both the amplitude as well as the phase of the recorded spectra. Classification speed and accuracy are contrasted with that achieved using a support vector machine classifier. The study systematically compares the classifier performance achieved after adopting different Gaussian kernels when separating amplitude and phase signatures. The two signatures are presented as feature vectors for both training and testing purposes. The study confirms the utility of complex-valued extreme learning machine algorithms for classification of the very large data sets generated with current terahertz imaging spectrometers. The classifier can take into consideration heterogeneous layers within an object as would be required within a tomographic setting and is sufficiently robust to detect patterns hidden inside noisy terahertz data sets. The proposed study opens up the opportunity for the establishment of complex-valued extreme learning machine algorithms as new chemometric tools that will assist the wider proliferation of terahertz sensing technology for chemical sensing, quality control, security screening and clinic diagnosis. Furthermore, the proposed algorithm should also be very useful in other applications requiring the classification of very large datasets.

Modelling signatures of pulsed magnetopause reconnection in cusp ion dispersion signatures seen at middle altitudes

It is shown that the open magnetosphere model can reproduce both the down-going and the up-going magnetosheath ions seen in the cusp and mantle regions by the Polar satellite at middle altitudes. ?he pass studied shows a series of discontinuities in the ion dispersion, most of which are shown to arise from pulses of magnetopause reconnection rate. A total of 9 pulses are detected in an interval estimated to be about 30 min long, giving a mean repetition period of about 3 min: they vary in length between 0.5 min and 3.5 min and are separated by periods of much slower reconnection of duration 1-3 min. One step is not as predicted for reconnection rate pulses but is explained in terms of compressive motions caused by a pulse of solar wind dynamic pressure. The reconnection site is found to be 16 +/- 3 R-E from the ionosphere along the separatrix field line, placing it at low latitudes on the dayside magnetopause.

Predicted signatures of pulsed reconnection in ESR data

Early in 1996, the latest of the European incoherent-scatter (EISCAT) radars came into operation on the Svalbard islands. The EISCAT Svalbard Radar (ESR) has been built in order to study the ionosphere in the northern polar cap and in particular, the dayside cusp. Conditions in the upper atmosphere in the cusp region are complex, with magnetosheath plasma cascading freely into the atmosphere along open magnetic field lines as a result of magnetic reconnection at the dayside magnetopause. A model has been developed to predict the effects of pulsed reconnection and the subsequent cusp precipitation in the ionosphere. Using this model we have successfully recreated some of the major features seen in photometer and satellite data within the cusp. In this paper, the work is extended to predict the signatures of pulsed reconnection in ESR data when the radar is pointed along the magnetic field. It is expected that enhancements in both electron concentration and electron temperature will be observed. Whether these enhancements are continuous in time or occur as a series of separate events is shown to depend critically on where the open/closed field-line boundary is with respect to the radar. This is shown to be particularly true when reconnection pulses are superposed on a steady background rate.

Occurrence probability, width and number of steps of cusp precipitation for fully pulsed reconnection at the dayside magnetopause

We discuss the characteristics of magnetosheath plasma precipitation in the “cusp” ionosphere for when the reconnection at the dayside magnetopause takes place only in a series of pulses. It is shown that even in this special case, the low-altitude cusp precipitation is continuous, unless the intervals between the pulses are longer than observed intervals between magnetopause flux transfer event (FTE) signatures. We use FTE observation statistics to predict, for this case of entirely pulsed reconnection, the occurrence frequency, the distribution of latitudinal widths, and the number of ion dispersion steps of the cusp precipitation for a variety of locations of the reconnection site and a range of values of the local de-Hoffman Teller velocity. It is found that the cusp occurrence frequency is comparable with observed values for virtually all possible locations of the reconnection site. The distribution of cusp width is also comparable with observations and is shown to be largely dependent on the distribution of the mean reconnection rate, but pulsing the reconnection does very slightly increase the width of that distribution compared with the steady state case. We conclude that neither cusp occurrence probability nor width can be used to evaluate the relative occurrence of reconnection behaviors that are entirely pulsed, pulsed but continuous and quasi-steady. We show that the best test of the relative frequency of these three types of reconnection is to survey the distribution of steps in the cusp ion dispersion characteristics.

Ionospheric Signatures of Pulsed Magnetopause Reconnection

The concept of zero-flow equilibria of the magnetosphere-ionosphere system leads to a large number of predictions concerning the ionospheric signatures of pulsed magnetopause reconnection. These include: poleward-moving F-region electron temperature enhancements and associated transient 630nm emission; associated poleward plasma flow which, compared to the pulsed variation of the reconnection rate, is highly smoothed by induction effects; oscillatory latitudinal motion of the open/closed field line boundary; phase lag of plasma flow enhancements after equatorward motions of the boundary; azimuthal plasma flow bursts, coincident in time and space with the 630nm-dominant auroral transients, only when the magnitude of the By component of the interplanetary magnetic field (IMF) is large; azimuthal-then-poleward motion of 630nm-dominant transients at a velocity which at all times equals the internal plasma flow velocity; 557.7nm-dominant transients on one edge of the 630nm-dominant transient (initially, and for large |By|, on the poleward or equatorward edge depending on the polarity of IMF By); tailward expansion of the flow response at several km s-1; and discrete steps in the cusp ion dispersion signature between the polewardmoving structures. This paper discusses these predictions and how all have recently been confirmed by combinations of observations by optical instruments on the Svalbard Islands, the EISCAT radars and the DMSP and DE satellites.