Impressed Current Density Research Articles

Evaluation of inverse field solutions with biomedical applications

We applied minimum norm estimations using different regularization techniques to the solution of the biomagnetic inverse field problem. Using magnetic field data measured with a multi‐channel‐SQUID‐sensor‐system we computed reconstruction of the impressed current density distributions which were generated by extended current sources placed inside a human torso phantom. The common inverse techniques usually applied in modern biomedical investigations in bioelectricity or biomagnetism are compared, and their aptitude for reconstruction of 3D current sources in space was evaluated. We analyzed the impact of using magnetic data, electrical data, and combination of both respectively on the localization of an equivalent current dipole (ECD). Finally, we use a visualization tool which enables a comparison of current density reconstruction. The study is, in parts, related to the new TEAM problem No. 31.

The evaluation of bond degradation between rebar and concrete due to the impressed cathodic current using interface parameters

In this study, bond degradation between rebar and concrete due to impressed cathodic current is evaluated using four interface parameters. It was found that the shear stiffness per unit length, the critical debond shear force per length and the surface energy decreased as the impressed cathodic current density and/or the polarization time increased; however, the remaining shear force ratio, D, did not vary with the cathodic current density and the polarization time. A parameter, ψ, which combined the effects of the current density and the polarization time, is proposed, and unified regression curves between this parameter and interface parameters have been obtained.

Reconstruction of extended current sources in a human body phantom applying biomagnetic measuring techniques

We have prepared a human body phantom for experimental verification of inverse solution techniques which are applied to magnetic (and electric) measuring data. Physical models of extended primary current sources were used to generate these fields. Magnetic field maps closed to the phantom surface were recorded by means of multi-channel biomagnetic measuring systems. Different deterministic optimization techniques were applied to both measured and simulated data to reconstruct the impressed current density distribution. We have found that all common used minimum norm methods (L/sub p/-norms, 1/spl les/p/spl les/2) cannot reconstruct the extension of current source distributions satisfactorily. The physical phantom was found to be a suitable tool for validation of source reconstruction techniques.

Rigorous computation of time-dependent electromagnetic fields in gyrotron cavities excited by internal sources

Computation in frequency, as well as in time domain of the electromagnetic field in aperture-coupled cavities which are excited by electron beams, requires an accurate representation of the field. Furthermore, a fast tool for simulation of beam-field interaction in electron tubes is desirable. Application of the modal expansion method, which utilizes both the solenoidal and the irrotational eigenfunctions of the equivalent short-circuited cavity, is generally rigorous but numerically inefficient. In this contribution, three main steps towards a more accurate and simultaneously more efficient analysis are presented. First, it is shown how the irrotational magnetic eigenfunctions can be eliminated from the analysis. Furthermore, some poorly convergent series in the frequency domain analysis as well as in the time-domain analysis are replaced by analytic expressions. Finally, the modal analysis is directly formulated in time domain using rigorous boundary conditions. Numerical results are presented for idealized structures with impressed current density and for self-consistent calculations which are compared to analytical or to numerical results, respectively. Thus, excellent accuracy of the developed method is proved and significant simplifications are justified. For weakly inhomogeneous cavities, the influence of mode conversion on field profile and on numerical aspects is also discussed.

Hydrogen Uptake and Steel Properties Due to Electrochemical Chloride Extraction from Reinforced Concrete

The uptake of hydrogen by steel during electrochemical chloride extraction (ECE) from reinforced concrete is monitored using novel electrochemical permeation technique. Concrete blocks were cast using two chloride (Cl - ) ion concentrations of 1.7% and 3.0% by mass of cement (batches #1 & #2) respectively, and reinforced axially with a central 15 mm plain steel square bar. Two cathodic (DC) current densities of 1.0 A/m 2 and 3.0 A/m 2 of concrete surface, were used for the ECE, with 0.1M sodium borate electrolyte solution and inert titanium anode mesh. Axial tension tests as per ASTM E8M-91 were carried out after treatment to investigate the differences in fracture loads, yield and strain to fracture of treated steel. In the ECE treatment, the hydrogen uptake of steel in the batch #1 concrete was substantially greater than those of the higher contamination (batch #2). The hydrogen permeation of batch #1 specimens with an impressed current density of 3.0 A/m 2 was about four and half times that observed at a current density of 1.0 A/m 2 . At the higher chloride content, the hydrogen uptake of the steel at 3.0 A/m 2 was about twice the level at 1.0 A/m 2 . Irrespective of the impressed current density and level of concrete contamination, the ECE specimens consistently showed about 50% reductions in tensile parameters. ECE treated specimens showed significant reductions in yield and ultimate loads, with the hydrogen effects being dependent on the applied cathodic current density and initial chloride contamination. It is thus likely that the hydrogen induced failure of steel in a candidate ECE structure will be more pronounced where there is small chloride contamination coupled with the use of high strength steel.

Pull-out and bond degradation of steel rebars in ECE concrete

It is demonstrated in this study that application of electrochemical chloride extraction (ECE) to reinforced concrete alters the pull-out strength and bond between the embedded high strength steel (HSS) and surrounding concrete. Concrete cubes were cast using two chloride (Cl −) ion concentrations of 1.7% and 3.0% by weight of cement respectively, and reinforced axially with a central ASTM #6 plain steel bar. Two cathodic (d.c.) current densities, 1.0 A/m 2 and 3.0 A/m 2 of concrete surface, were used for the ECE studies, with 0.1M sodium borate electrolyte solution and inert titanium anode mesh. Axial pull-out tests as per ASTM Standard Test Method C234-91a, were carried out within 24 hours of terminating treatment to investigate the differences in bond stress and bar slip behaviour. ECE treated specimens showed significant reductions in pull-out strength, with the degradation in bond being dependent on the applied cathodic current density and initial chloride contamination. At an impressed current density of 3.0 A/m 2, the 1.7% Cl − ions specimens showed about 58% loss in bond strength compared with about 44% for specimens premixed with 3.0% Cl − ion. The bond strength reduction at an impressed current density of 1.0 A/m 2 was about half the reduction at current density of 3.0 A/m 2. The loss in bond strength were based on (untreated) control specimens of each type. The alkali ion accumulations around the steel rebars were observed to follow similar trend as bond degradation. It is predicted the softening effect of the alkali (sodium and potassium) ions on the cement silicate hydrates around the steel-concrete interface will have severe effect on concrete structures reinforced with HSS irrespective of their stress state. Also, concrete structures containing steel rebars with substantial corrosion product coverage are likely to experience pronounced cathodic disbondment during ECE application and within a few days of terminating the treatment.

Time-dependent scalar Beltrami-Hertz potentials in free space

We have solved the Beltrami-Maxwell equations for free space in terms of time-dependent scalar functions, the so-called scalar Beltrami-Hertz potentials. The two Beltrami fields have been represented in terms of scalar Beltrami-Hertz potentials. While the method is formulated for general sources, it is at its most powerful when the impressed source current densities are unidirectional: each Beltrami field, a complex-valued vector, can then be derived from a single scalar Beltrami-Hertz potential. We have calculated the corresponding scalar Green function explicity and given closed-form solutions for dipolar sources. Finally, the connection between the Beltrami-Maxwell formalism and conventional electromagnetic theory has been re-affirmed.

Separation of sources of neuromagnetic examinations

Phantom experiments and computer simulations in combination with the Wiener-Helstrom filter reconstruction technique demonstrate the ability of the improvement of source separation in distributed current arrangements. A spatial region of interest (ROI), in which the current distribution is expected, is defined and discretized into three-dimensional voxel elements. Inside the ROI the number and spatial extension of the current sources are determined by reconstructing the impressed current density distribution separately from volume conductor effects. The volume currents can be determined depending on the actual head shape and the location of the ROI using finite difference methods before the reconstruction of the impressed current density. As a result the reconstruction procedure is very fast. Without a priori information about the number of active generators and the spatial extension inside the ROI, the method improves the separation of simultaneously active sources and distributed sources in comparison with the reconstruction of the total current density.

Integrated imaging of neuromagnetic reconstructions and morphological magnetic resonance data

New neuromagnetic imaging methods provide spatial information about the functional electrical properties of complex current distributions in the human brain. For practical use in medical diagnosis a combination of the abstract neuromagnetic imaging results with magnetic resonance (MR) or computed tomography (CT) images of the morphology is required. The biomagnetic images can be overlayed onto three-dimensional morphological images with spatially arbitrary selectable slices, calculated from conventional 2D data. For the current reconstruction the 3D images furthermore provide a priori information about the conductor geometry. A combination of current source density calculations and linear estimation methods for handling the inverse magnetic problem allows quick imaging of impressed current source density in arbitrary volume conductors.

Field representation in gyrotropic media by one scalar superpotential

The electromagnetic field of an arbitrary current density distribution parallel to the distinguished axis of a gyrotropic medium is represented in terms of one scalar superpotential. The fourth-order partial differential equation for the superpotential is solved for some special forms of the impressed current density distribution.

The conditions for incorporation of electrolyte ions into anodic oxides

A model is developed to account for the variation in pick-up, in the oxide, of anions from the solution during anodic oxidation. According to this model, during constant current anodizing, the pick-up depends upon the [anion]/[OH −] ratio at the electrode surface. This in turn is a critical function not only of the pH of the bulk electrolyte but also of the impressed current density which can alter the local concentration of ions, particularly at the electrode—electrolyte interface. The model explains the abrupt change of pick-up observed experimentally when in certain ranges, the pH of the bulk solution or the impressed current density are changed.

Propagation of magnetic domain walls by a self-induced current distribution

Calculations are made for the motion of a domain wall surrounded by a perturbed region of current density resulting from the action of the magnetic field of the domain on a uniform impressed current density. The perturbed region follows the motion of the domain wall and produces a magnetic field which causes the domain to propagate. In particular, a ferromagnetic layer containing a cylindrical domain, with a Hall-effect overlayer, is considered.