Nonuniform Currents Research Articles

Radiation Q of electrically large loop antennas with nonuniform currents

AbstractAn analysis of the radiation Q of electrically large loop antennas is presented and its characteristics are discussed. The methodology involves the spherical mode expansion for the fields outside the loop. It is found that higher‐order modes need to be considered for a realistic estimate of the radiation Q of large loops. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 34: 377–380, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10467

Modelling the electric field at the seafloor due to a non-uniform ionospheric current

Electric fields occurring at the seafloor during geomagnetic disturbances drive geomagnetically induced currents (GIC) in submarine cables, possibly leading to problems in the operation of the equipment and systems connected to the cable. To be able to estimate the GIC risk on the cables, it is necessary to know the magnitude of the electric field at the seafloor, which can be conveniently determined from the magnetic data recorded at the surface. In this paper, we consider the theoretical models associated with non-uniform source fields existing at high latitudes. The case of a two-dimensional ionospheric current distribution is summarized together with numerical examples, and formulas for the general three-dimensional ionospheric–magnetospheric current system are derived in detail. The seawater is modelled by a highly conducting surface layer, and the basement is characterized by a surface impedance. In the 2D case, the expression for the seafloor electric field is either an inverse Fourier transform over the wave number or a spatial convolution. Corresponding formulas are also obtained in the 3D case, and both expressions are then double integrals. Three-dimensional numerical computations are therefore much more difficult, and they will be the topic of a future paper. This paper demonstrates the applicability of the method to the estimation of the impact of geomagnetic disturbances on submarine cables.

Trajectory and drift velocity of a sounding instrument in the field of nonuniform currents

We present a method for the determination of the trajectory of a sounding instrument and its drift velocity in the field of currents nonuniform in depth and develop special software for IBM/PC compatible computers. The proposed method and software are checked by using the well-known analytic solution of a similar problem given by Academician A. N. Krylov. We also consider some special examples of application of the developed method to the processing of the data of an OLT profilometer.

Shadow boundary incremental length diffraction coefficients applied to scattering from 3-D bodies

Shadow boundary incremental length diffraction coefficients (SBILDCs) are high-frequency fields designed to correct the physical optics (PO) field of a three-dimensional (3-D) perfectly electrically conducting scatterer. The SBILDCs are integrated along the shadow boundary of the 3-D object to approximate the field radiated by the nonuniform shadow boundary current (the difference between the exact and PO currents near the shadow boundary). This integral is added to the PO field to give an approximation to the exact scattered field that takes into account both PO and nonuniform shadow boundary currents on the scatterer. Like other incremental length diffraction coefficients, any SBILDC is based on the use of a 2-D canonical scatterer to locally approximate the surface of the 3-D scatterer to which it is applied. Circular cylinder SBILDCs are, to date, the only SBILDCs that have been obtained in closed form. In this paper, these closed-form expressions are validated by applying them for the first time to a 3-D scatterer with varying radius of curvature-the prolate spheroid. The results obtained clearly demonstrate that for bistatic scattering the combined PO-SBILDC approximation is considerably more accurate than the PO field approximation alone.

Short gravity waves on steady non-uniform currents obtained through up-/downwelling

Small amplitude water waves propagating in a medium with a steady non-uniform current are investigated. The non-uniform current is obtained by up- or downwelling through the horizontal bed. A new locally valid velocity potential correct to the second order is derived describing the combined wave–current motion. From this solution expressions for the local evolution of the wave amplitude and the wave number are extracted. These expressions are compared with the results found using the principle of wave action conservation and the linear dispersion relation, and good agreement is found at small distances compared to the wavelength. Unlike earlier works there is no restriction to deep water. The results valid for deep water are found as a special case of the general solution and agree with the solution found by Longuet-Higgins, M.S. and Stewart, R.W. (1961) The changes in amplitude of short gravity waves on steady non-uniform currents. Journal of Fluid Mechanics, 10(4), 529–549. Furthermore, it is shown that the principle of wave action conservation in fact holds for waves propagating in a medium with a steady non-uniform current maintained by up-/downwelling also on finite depth.

Microwave radiation from zero field singularities in stacks of two long Josephson junctions

We present experimental results on the radiation emitted from two-stacks of long Josephson junctions with double overlap geometry. Microwave radiation was received from the voltage-locked Zero Field Steps of the c/sup -/ family, corresponding to fluxon-antifluxon pairs oscillating in the stack. It was also received from other current singularities, interpreted as resonances excited by the magnetic field generated by the nonuniform bias currents. In the former case, there is experimental evidence for an appreciable signal, while, theoretically, the net signal produced is expected to be vanishingly small. This event can he explained in terms of a possible delay between the component signals, as the junctions in the stack do not have identical length.

Three-dimensional surface waves propagating over long internal waves

A non-uniform current, such as may be generated by long internal waves, interacts with short surface waves and causes patterns on the sea surface that are of interest. In particular, regions of steep breaking waves may be relevant to specular radar scattering. A simple approach to modelling this problem is to take a set of short, surface waves of uniform wavenumber on the sea surface, as may be caused by a gust of wind. The direction of propagation of the surface waves is firstly taken to be the same as that of the current, and surface tension and viscous effects are neglected. We have a number of methods of solution at our disposal: linear (one-dimensional) ray theory is simple to apply to the problem, a nonlinear Schrödinger equation for the modulated wave amplitude, modified to include to effect of the current, can be used and solutions can be found using a fully nonlinear irrotational flow solver. Comparisons between the ‘exact’ nonlinear calculations for two dimensions (which are too complicated/ computationally intensive to be extended to three dimensions) compare well with the two approximate methods of solution, both of which can be extended, within their limitations, to model the full three-dimensional problem; here we present three-dimensional results from the linear ray theory. By choosing such a simple (although we consider physically realistic) initial state of uniform wavenumber short waves and assuming a sinusoidal surface current, we can reduce the two-dimensional problem to dependence on three non-dimensional parameters. In three-dimensions, we consider an initial condition with a uniform wavetrain at an angle α say, to the propagating current, thus introducing a fourth parameter into the problem. Extension of the linear ray theory from one space to two space dimensions is numerically quite simple since we maintain uniformity in the direction perpendicular to the current, and the only difficulty lies with the presentation of results, due to the large number of variables now present in the problem such as initial wavenumber, angle of propagation, position in ( x, y, t) space etc. In this paper we present just one solution in detail where waves are strongly refracted and form two distinct foci in space-time. There is a collimation of the short waves with the direction of the propagating current.

Method-of-moments analysis of electrically large circular-loop antennas: Nonuniform currents

A method-of-moments analysis is carried out so as to obtain, in closed form, the nonuniform current distributions, and their resulted radiation patterns in both near and far zones, of circular loop antennas with electrically larger circumferences. An oblique incident field in its general form is considered in the formulation of the nonuniform current distributions. In Galerkin's method-of-moments analysis, the Fourier cosine series is considered as the full-domain basis-function series. As a result, the current distributions along the circular loops are expressed analytically in terms of the azimuth angle for various diameters of large loops. Finally, the radiated electromagnetic (EM) fields and their power pattern in both near and far zones are determined by applying the dyadic Green's function (DGF) in spherical co-ordinates and plotted using a commercial software package, respectively.

ROUND BUOYANT JETS INTO NONUNIFORM FLOWING ENVIRONMENT

This paper presents a mathematical model for submerged turbulent round buoyant jets into a nonuniformly flowing stratified environment. The model derived herein is based on the conservation of mass flux, momentum flux and densityd eficiency.A new entrainment function which can be decided by the model and a constanto f drag coefficient are also presented here. The model is validated by comparing prediction results and large number of experimental data publishedb y other investigators. The mathematica mlodel is then used to analyze behavior of some buoyant jets into a nonuniform current. The present model should be capable of predicting not only the three-dimensional buoyant jets into nonuniformly flowing ambient fluids, but also nonbuoyant jets and simple plumes, something an existing model can not cover.

Design parameters influencing non-uniform current distributions in superconducting multi-stage stranded cables

We have suggested that non-uniform currents occur due to certain design parameters, for example a combination of cabling pitches, in a previous study. In the present study, we evaluate the occurrences of non-uniform current distributions in terms of design parameters, i.e., the combination of cabling pitches, winding methods (solenoidal and pancake winding) and cross-sectional shape of the cable by numerical analyses. Finally, some basic design concepts to solve this problem are proposed.

The effect of paleotopography on lithic distribution and facies associations of small volume ignimbrites: the WTT Cupa (Roccamonfina volcano, Italy)

The distribution of lithic clasts within two trachytic, small volume, pumiceous ignimbrites are described from the Quaternary `White Trachytic Tuff Cupa' formation of Roccamonfina volcano, Italy. The ignimbrites show a downslope grading of lithics, with a maximum size where there is a major break in the volcano's slope, rather than at proximal locations. This is also the location where ignimbrites are thickest and most massive. The break in slope is interpreted to have reduced flow capacity and velocity, increasing the sedimentation rate, so that massive ignimbrite formed by hindered settling sedimentation. Ignimbrite Cc, exhibits no vertical grading of lithics, though it does show downslope grading with maximum size at the major break in slope and a rapid decrease further downslope. Ignimbrite Cc thins away from the break in slope, and shows an upward fining of the grain size within the topmost few decimeters of the unit. The ignimbrite is stratified proximally, and grades to massive facies at the break in slope, and distally to stratified facies with numerous inverse-graded beds. The simplest mechanism accounting for these downslope variations is progressive aggradation from a quasi-steady, nonuniform pyroclastic density current. The changes in deposit thickness and facies are interpreted to record downcurrent changes in sedimentation rate. The upward fining reflects waning flow. Inversely graded, bedded depositional facies in distal areas is interpreted to reflect flow unsteadiness and a decrease in suspended sediment load. Ignimbrite Cd shows vertical, as well as downslope grading of lithics. This characteristic, coupled with the widespread massive facies of the deposit and the tabular unit geometry are features that can be reconciled with both the debris flow/plug analogy for pyroclastic flows ( Sparks, 1976) and the progressive aggradation model ( Branney and Kokelaar, 1992). However, none of them appears to satisfy completely the field evidences, implying that when dealing with massive ignimbrites, other evidence than lithic grading needs to be presented to better understand the related transport and depositional processes.

Fast Convergent Integrals for Nonuniform Currents on Wedge Faces

This paper fills in some gaps in the Physical Theory of Diffraction (PTD). Possible extensions of the notion of the uniform and nonuniform currents are discussed for scattering problems involving objects with complex shapes and complex material properties. In the case of a perfectly conducting wedge, exact expressions for the nonuniform (fringe) current are presented in terms of fast convergent integrals. Their analytical properties are investigated in details. These integrals together with asymptotic expressions provide an efficient tool for numerical calculations of the nonuniform current. The results can be used for example in the analysis of scattering and antenna problems for large polyhedrons.

Instabilities driven by electron velocity and temperature gradients in tokamak scrape-off layers with non-uniform parallel currents

A study of instabilities driven by gradients in the parallel electron velocity and electron temperature in magnetized plasmas bounded by a conducting surface is presented. The parameters and general conditions considered are relevant for the scrape-off layer of magnetically confined plasmas. The inclusion, for the first time, of a finite current and a current gradient results in unstable modes which are quite different from the modes driven solely by the electron temperature gradient. In the parameter regime corresponding to high power discharges with attached plasmas, where the typical scale length of the velocity gradient is much shorter than that of the temperature gradient, the velocity gradient is the dominant effect. The anomalous thermal diffusivity, from mixing length theory, obtained including the velocity gradient is larger than that obtained including only the temperature gradient.

The NEURON simulation environment.

The moment-to-moment processing of information by the nervous system involves the propagation and interaction of electrical and chemical signals that are distributed in space and time. Biologically realistic modeling is needed to test hypotheses about the mechanisms that govern these signals and how nervous system function emerges from the operation of these mechanisms. The NEURON simulation program provides a powerful and flexible environment for implementing such models of individual neurons and small networks of neurons. It is particularly useful when membrane potential is nonuniform and membrane currents are complex. We present the basic ideas that would help informed users make the most efficient use of NEURON.

Correction to "Application Of Incremental Length Diffraction Coefficients To Calculate The Pattern Effects Of The Rim And Surface Cracks Of A Reflector Antenna"

Incremental length diffraction coefficients (ILDCs) for the half-plane are integrated around the rim of a paraboloid reflector antenna to obtain well-behaved far fields of the nonuniform current for all angles of observation. These far fields, when added to the physical optics far field, produce a more accurate total far field of the reflector. Excellent agreement with the far fields obtained from a method-of-moments solution to the electric field integral equation applied to a 20-wavelength-diameter reflector shows that the cross polarization, farther-out sidelobes, and fields near nulls of reflector antennas can be appreciably modified by the fields of the nonuniform currents. ILDCs are also used to investigate the effect of cracks on the surface of reflectors that can result from the imperfect fitting together of panels to form large reflectors. Three models of cracks are studied. Significant pattern effects are found, depending on the model and orientation of the cracks. >

Analysis of integrated quantum-well infrared photodetector and light-emitting diode for implementing pixelless imaging devices

The conversion of images into nonuniform distribution of the output current density in a multiple-quantum-well structure for an infrared pixelless imaging system is considered using an analytical device model. The nonuniform current, which reproduces the incident image, is converted back to a light-emitting diode emission image by the device considered here. The developed model takes into account transport processes responsible for the device operation. An explicit expression for the contrast transfer characteristic is derived as a function of the number of quantum wells (QWs) and the electron capture parameter. It is shown that the quality of the up-converted images (contrast and resolution) is improved with increasing number of QWs. The pixelless imaging devices under consideration can effectively convert long-wavelength infrared images into short-wavelength infrared or visible images with contrast transfer ratio close to unity.

Benthic hydroacoustic antennas in three-dimensional nonuniform currents: Computation and optimization

A method is proposed for computing the configuration, tensile stress and directivity pattern of a linear hydroacoustic antenna, situated in a 3-dimensional nonuniform current. Consideration is given to the possibility of optimizing the hydromechanical parameters of the antenna, in order to improve their spatial stability under actual ocean conditions.

Quenches in a high-temperature superconducting tape and pancake coil

Stability and normal-zone propagation experiments have been performed on singlelength and double-pancake coils made of Bi-2223/Ag high-temperature superconducting tape. Experiments were conducted with liquid nitrogen and gaseous helium cooling in temperatures from 5 to 77 K. No distinctive normal-zone propagation was observed in the single-length conductor tests. Non-uniform critical currents were, however, observed over the length of the conductor. To dramatize the effect on the stability of a magnet wound with this kind of non-uniform conductor, a pancake coil was wound with conductors composed of a two-tape bundle on the inner turns and a single-tape on the outer turns. Quenches of the coil were observed when currents higher than the critical current of the single-tape section were applied and held for more than a few minutes. The coil quenched without a distinctive ‘normal’ front propagation or a large temperature gradient.

A Novel Gas-Dynamic Levitation Scheme for Noncontact Coating of Spherical ICF Targets

A novel gas-dynamic levitation technique has been developed to facilitate noncontact coating of spherical ICF targets. Using this technique three metal balls 450 {mu}m, 650 {mu}m and 950 {mu}m in diameter were levitated very stably for several hours, with the balls rotating continuously. Unlike the conventional gas-dynamic levitation scheme in which a single gas-emitting fixture, placed below an object, lifts it up and contains it in a confined volume, the present scheme relies on two fixtures, one placed under and the other above the object. The bottom fixture, as is with the conventional scheme, is a gas emitter; however, the top one is a gas collector shaping the flow field around the object so as to confine the object near the axis of symmetry of the levitation system. As a result, the present system exhibits excellent stability and robustness, and is immune to such external disturbances as nonuniform temperature fields and air currents, and small changes in the levitation gas pressure. The apparatus is inexpensive to fabricate and simple to operate. The details of the apparatus and the preliminary data demonstrating the capability of the levitation scheme are presented. A target coating method, compatible with the present target levitation schememore » and suitable for uniform coating of ICF targets, is indicated. 6 refs., 5 figs.« less

Effect of alongshore nonuniformities on longshore current predictions

Longshore currents have been primarily modeled by assuming alongshore uniformity. In this paper we revisit the problem of nonuniform longshore currents to refocus attention on the importance of accounting for alongshore nonuniformity in longshore current predictions and to provide an analytical estimate of the importance of this nonuniformity. We derive a semianalytical solution for the longshore currents that allows for weak alongshore variations in the bottom topography. The solution shows that the alongshore pressure gradient induced by alongshore variations in the bottom topography could contribute substantially to the forcing for the longshore currents. An example calculation shows that the longshore current could deviate by up to ±30% from the mean for a ±10% deviation of the bottom topography. Thus we suggest that in many practical cases it is important to include the alongshore pressure gradient to accurately model longshore currents.