Current Sources (current + source)

Distribution by Scientific Domains

Selected Abstracts

Electromagnetic fields in a steel-cased borehole

Ki Ha Lee
ABSTRACT The development of an electromagnetic numerical modelling scheme for a magnetic dipole in an arbitrary casing segment in an inhomogeneous conductivity background has been difficult, due to the very high electrical conductivity and magnetic permeability contrasts between the steel casing and the background medium. To investigate the effect of steel casing efficiently, we have developed an accurate but simple finite-element modelling scheme to simulate electromagnetic fields in a medium of cylindrically symmetric conductivity structures. In order to preserve the cylindrical symmetry in the resulting electromagnetic fields, a horizontal loop current source is used throughout. One of the main advantages of the approach is that the problem is scalar when formulated using the azimuthal electric field, even if the casing is both electrically conductive and magnetically permeable. Field calculations have been made inside the cased borehole as well as in another borehole which is not cased. Careful analyses of the numerical modelling results indicate that the anomaly observed in a cross-borehole configuration is sensitive enough to be used for tomographic imaging. [source]

Towards a circuit theory for metallic single-electron tunnelling devices

J. HoekstraArticle first published online: 20 APR 200
Abstract A circuit theory for metallic single-electron tunnelling (SET) junctions is presented. In detail circuits with a single SET junction in arbitrary environments are described. Based on the conservation of energy in the circuits,a fundamental circuit theorem,equivalent circuit elements are proposed and possible physical justifications are presented. The resulting model represents the tunnel event by an impulse current source, the junction by a charged capacitor, and the tunnelling condition as a discrete process based on local circuit parameters,and may include a tunnelling time. Simple examples illustrate Coulomb blockade, Coulomb oscillations, and continuous direct tunnelling. Copyright 2007 John Wiley & Sons, Ltd. [source]

Development of 3-D equivalent-circuit modelling with decoupled L-ILU factorization in semiconductor-device simulation

Szu-Ju Li
Abstract In this paper, we develop a three-dimensional (3-D) device simulator, which combines a simplified, decoupled Gummel-like method equivalent-circuit model (DM) with levelized incomplete LU (L-ILU) factorization. These complementary techniques are successfully combined to yield an efficient and robust method for semiconductor-device simulation. The memory requirements are reduced significantly compared to the conventionally used Newton-like method. Furthermore, the complex voltage-controlled current source (VCCS) is simplified as a nonlinear resistor. Hence, the programming and debugging for the nonlinear resistor model is much easier than that for the VCCS model. Further, we create a connection-table to arrange the scattered non-zero fill-ins in sparse matrix to increase the efficiency of L-ILU factorization. Low memory requirements may pave the way for the widespread application in 3-D semiconductor-device simulation. We use the body-tied silicon-on-insulator MOSFET structure to illustrate the capability and the efficiency of the 3-D DM equivalent-circuit model with L-ILU factorization. Copyright 2007 John Wiley & Sons, Ltd. [source]

Clinical Use of Intracardiac Impedance: Current Applications and Future Perspectives

ARTHUR, W., et al.: Clinical Use of Intracardiac Impedance: Current Applications and Future Perspectives. For over 40 years the measurement of intracardiac impedance has been proposed as a method of assessing the contractile state of the heart muscle. This technique requires the positioning of one or more intracavitary electrodes and the generation of an electric field from an alternating current source. Variations in the calculated impedance signal reflect changes in the ventricular blood pool volume adjacent to the electrodes. Intracardiac impedance measurement has been successfully developed as a research tool to assess myocardial contractility, and from this, clinical uses have evolved. Commercial rate responsive pacing systems use intracardiac impedance to assess the inotropic state of the heart. Further development of this technology might allow hemodynamic discrimination of cardiac arrhythmias. [source]

3D resistivity inversion using 2D measurements of the electric field

P.D. Jackson
Field and ,noisy' synthetic measurements of electric-field components have been inverted into 3D resistivities by smoothness-constrained inversion. Values of electrical field can incorporate changes in polarity of the measured potential differences seen when 2D electrode arrays are used with heterogeneous ,geology', without utilizing negative apparent resistivities or singular geometrical factors. Using both the X - and Y -components of the electric field as measurements resulted in faster convergence of the smoothness-constrained inversion compared with using one component alone. Geological structure and resistivity were reconstructed as well as, or better than, comparable published examples based on traditional measurement types. A 2D electrode grid (20 10), incorporating 12 current-source electrodes, was used for both the practical and numerical experiments; this resulted in 366 measurements being made for each current-electrode configuration. Consequently, when using this array for practical field surveys, 366 measurements could be acquired simultaneously, making the upper limit on the speed of acquisition an order of magnitude faster than a comparable conventional pole,dipole survey. Other practical advantages accrue from the closely spaced potential dipoles being insensitive to common-mode noise (e.g. telluric) and only 7% of the electrodes (i.e. those used as current sources) being susceptible to recently reported electrode charge-up effects. [source]

Knowledge, Market Structure, and Economic Coordination: Dynamics of Industrial Districts

Ron A. Boschma
The industrial rise of the Third Italy has been characterized by the growth of dynamic networks of flexible small and medium,sized enterprises (SMEs) that are spatially concentrated in specialized industrial districts. This network type of coordination has been associated with horizontal, trust,based relations rather than vertical relations of power and dependency between local organizations. This would lower transaction costs (essential for local systems with an extreme division of labor), facilitate the transmission and exchange of (tacit) knowledge (and thus, learning and innovation), encourage cooperation mechanisms (such as the establishment of research centers), and stimulate political,institutional performance (e.g. through regulation of potential social conflicts). From an evolutionary perspective, the focus is on the dynamics of industrial districts drawing from current experiences in Italy. In this respect, this paper concentrates on two main features of industrial districts that have largely contributed to their economic success in the past, that is, their network organization and the collective learning process. The evolution of industrial districts is described in terms of organizational adjustments to structural change. The way in which the size distribution of firms has changed is discussed (in particular the role of large companies), how the (power) relationships between local organizations have evolved, what are the current sources and mechanisms of learning, and to what extent institutional lock,in has set in. Finally, a number of trajectories districts may go through in the near future are presented. [source]

Mental health training and development needs of community agency staff

Jenny Secker
Abstract Emphasis has long been placed in UK national policy on providing ,seamless' mental health services to meet both the health and social care needs of service users. While attention has been paid to the training required by specialist mental health and primary care staff in order to achieve this, the needs of other community agency staff have received less attention. The present article describes a study designed to identify the training needs of staff working within a broad range of agencies. Focus group discussions were used to explore participants' experiences of mental health problems amongst clients, their confidence in dealing with these, current sources of support and perceived training needs. The results indicate that participants in all agencies routinely encountered a range of problems. Colleagues were the main source of support, followed by line managers, but supervision structures and wider organisational support were lacking in some cases. Joint working with specialist mental health services was almost universally problematic and all groups identified a range of training needs. On the basis of the results, the present authors put forward suggestions as to how these needs might be met. [source]

Improved EEG source analysis using low-resolution conductivity estimation in a four-compartment finite element head model

Seok Lew
Abstract Bioelectric source analysis in the human brain from scalp electroencephalography (EEG) signals is sensitive to geometry and conductivity properties of the different head tissues. We propose a low-resolution conductivity estimation (LRCE) method using simulated annealing optimization on high-resolution finite element models that individually optimizes a realistically shaped four-layer volume conductor with regard to the brain and skull compartment conductivities. As input data, the method needs T1- and PD-weighted magnetic resonance images for an improved modeling of the skull and the cerebrospinal fluid compartment and evoked potential data with high signal-to-noise ratio (SNR). Our simulation studies showed that for EEG data with realistic SNR, the LRCE method was able to simultaneously reconstruct both the brain and the skull conductivity together with the underlying dipole source and provided an improved source analysis result. We have also demonstrated the feasibility and applicability of the new method to simultaneously estimate brain and skull conductivity and a somatosensory source from measured tactile somatosensory-evoked potentials of a human subject. Our results show the viability of an approach that computes its own conductivity values and thus reduces the dependence on assigning values from the literature and likely produces a more robust estimate of current sources. Using the LRCE method, the individually optimized four-compartment volume conductor model can, in a second step, be used for the analysis of clinical or cognitive data acquired from the same subject. Hum Brain Mapp, 2009. 2008 Wiley-Liss, Inc. [source]

The reduced scalar potential in regions with permeable materials: Reasons for loss of accuracy and cancellation

S. Balac
Abstract Practical three-dimensional magnetic field problems usually involve regions containing current sources as well as regions with magnetic materials. For computational purposes, the use of the reduced scalar potential (RSP) as unknown has the advantage to transform a problem for a vector field throughout the space into a problem for a scalar function, thus reducing the number of degrees of freedom in the discretization. However, in regions with high magnetic permeability the use of the RSP alone usually results in severe loss in accuracy and it is recommended to use both the RSP and the total scalar potential. Using an asymptotic expansion, we investigate theoretically the underlying reasons for this lack of accuracy in permeable regions when using the RSP as a unique potential. Moreover, this investigation leads to an efficient numerical method to compute the magnetic field in regions with high magnetic permeability. Copyright 2007 John Wiley & Sons, Ltd. [source]