Parallel Wire Grid Research Articles

Reflection characteristics of a parallel wire grid embedded in a general bi-isotropic medium.

A periodic wire grid composed of infinitely long thin parallel conducting wires with a bi-isotropic background is considered. The reflection properties of this wire grid have been investigated using numerical calculations. The analytic expressions for the surface impedance and reflection coefficient have been derived using the wave field decomposition approach and with the application of impedance boundary conditions for a wire grid. The influences of various background media upon the reflection magnitude and phase have been studied. The considered background media are taken to be bi-isotropic, Tellegen, and chiral. It is investigated if the zero reflection for a wire grid with a free space background under normal illumination vanishes for bi-isotropic, Tellegen, and chiral background media. It is found that, for a specific type of realistic bi-isotropic background medium, an equivalent wire grid of ideally conducting wires becomes nearly a perfect reflector. It is also studied if this type of perfect reflector is almost independent of incident angles.

Horizontal two-phase flow pattern recognition

In the present work a Wire Mesh Sensor (WMS) has been adopted to characterize the air–water two-phase flow in a test section consisting of a horizontal Plexiglas pipe of internal diameter 19.5mm and total length of about 6m. The flow quality ranges from 0 to 0.73 and the superficial velocity ranges from 0.145 to 31.94m/s for air and from 0.019 to 2.62m/s for water. The observed flow patterns are stratified–bubble–slug/plug–annular. The WMS consists of two planes of parallel wire grids (16×16) that are placed across the channel at 1.5mm and span over the measuring cross section. The wires of both planes cross under an angle of 90°, with a diameter Dwire of 70μm and a pitch equal to 1.3mm. The void fraction profiles are derived from the sensor data and their evolution in time and space is analyzed and discussed. The dependence of the signals on the measured fluid dynamic quantities is discussed too. The main task is to identify the flow pattern under any set of operating conditions as well as to establish the value of the characteristic flow parameters.

Ambiguities in the grid-inefficiency correction for Frisch-Grid Ionization Chambers

Ionization chambers with Frisch grids have been very successfully applied to neutron-induced fission-fragment studies during the past 20 years. They are radiation resistant and can be easily adapted to the experimental conditions. The use of Frisch grids has the advantage to remove the angular dependency from the charge induced on the anode plate. However, due to the Grid Inefficiency (GI) in shielding the charges, the anode signal remains slightly angular dependent. The correction for the GI is, however, essential to determine the correct energy of the ionizing particles. GI corrections can amount to a few percent of the anode signal. Presently, two contradicting correction methods are considered in literature. The first method adding the angular-dependent part of the signal to the signal pulse height; the second method subtracting the former from the latter. Both additive and subtractive approaches were investigated in an experiment where a Twin Frisch-Grid Ionization Chamber (TFGIC) was employed to detect the spontaneous fission fragments (FF) emitted by a 252Cf source. Two parallel-wire grids with different wire spacing (1 and 2mm, respectively), were used individually, in the same chamber side. All the other experimental conditions were unchanged. The 2mm grid featured more than double the GI of the 1mm grid. The induced charge on the anode in both measurements was compared, before and after GI correction. Before GI correction, the 2mm grid resulted in a lower pulse-height distribution than the 1mm grid. After applying both GI corrections to both measurements only the additive approach led to consistent grid independent pulse-height distributions. The application of the subtractive correction on the contrary led to inconsistent, grid-dependent results. It is also shown that the impact of either of the correction methods is small on the FF mass distributions of 235U(nth, f).

Ion dispersion near parallel wire grids in orthogonal acceleration time-of-flight mass spectrometry: predicting the effect of the approach angle on resolution

Ion dispersion near parallel wire grids in orthogonal acceleration time-of-flight mass spectrometry: predicting the effect of the approach angle on resolution

Ion dispersion near parallel wire grids in orthogonal acceleration time‐of‐flight mass spectrometry: predicting the effect of the approach angle on resolution

Ions experience small deflections in the vicinity of grids in accelerators and ion mirrors in time-of-flight (TOF) mass spectrometers. Recent experiments with an orthogonal acceleration (oa) TOF instrument have verified that the effect can significantly degrade resolution when ions approach grids at an angle deviating from 90 degrees. The phenomenon becomes significant only when ions have components of velocity at right angles to the wires of the grids. A model is presented in this study to predict this phenomenon for parallel wire grids. The fractional energy spread of ions (calculated in the static TOF-spectrometer frame of reference) scales directly with the approach angle of ions to the grid (as measured from normal approach). The energy spread also scales with the range of angles that is a consequence of the focusing effect in each gap between the wires of the grid. The equations imply that closely spaced parallel wire grids are best for deployment in oa-TOF systems where non-zero approach angles are unavoidable. Such grids are relatively impractical to manufacture and support but rectangular repeat cell grids with relatively few wires at right angles to the source axis are shown experimentally to introduce minimal energy spread. When these grids are rotated by 90 degrees, the resolution measured in a Q-TOF spectrometer is degraded in approximate agreement with the parallel wire model. A practical implication of this work is that grid transmissions in oa-TOF systems may be significantly increased without loss of resolution. Improvements of approximately 200% (V-mode) and approximately 400% (W-mode) in ion transmission were obtained in this study without compromising resolution. This was achieved with approximately 73% transmission grids and greater potential improvements in transmission are being realised since this study with approximately 89% transmission grids having similar geometry.

Reducing grid dispersion of ions in orthogonal acceleration time-of-flight mass spectrometry: advantage of grids with rectangular repeat cells

In orthogonal acceleration time-of-flight mass spectrometers (oa-TOFMSs), where ion velocity in the axis of the ion source is preserved, the ions approach grids at an angle not equal to 90°. In this situation ions are expected to be dispersed more than in the case of the normal approach. This may be attributed primarily to the effect of grid wires that are not parallel with the source axis. The dispersion leads to a broadening of the flight-time distribution for ions of a given mass-to-charge ratio and hence it degrades mass resolving power. A novel 20 kV matrix-assisted laser desorption/ionisation (MALDI) oa-TOFMS instrument has been used in this study to investigate grid dispersion. The results show that the dispersions that ions experience as they pass through grids/meshes that divide regions of different electric field strength have a significant effect on the resolving power when the majority of the wires are not aligned with the source. Numerical simulations of ion motion point to an advantage in using ideal parallel wire grids with line densities in excess of one hundred lines per centimeter, orientated with the ion source axis. The orientation of parallel wire grids has previously been predicted to significantly affect resolving power in oa-TOF instruments. This article presents the first experimental data to demonstrate the effect. Parallel wire grids of such high line densities are impractical to construct and support so a grid design that approximates parallel wires has been lithographically mastered for electroforming 70% transmission grids with 120 lines per centimeter in one direction and 12 lines per centimeter at right angles to this direction. A resolution loss of ∼40% (from a resolution of m/Δ m = ∼8000 at full width at half maximum) was observed when a three-grid orthogonal accelerator constructed with this material was rotated through 90° from the predicted ideal orientation. The observed linewidths were in reasonable agreement with those predicted by ion trajectory calculations.

FIR integrated optics: Scattering of guided waves due to single parallel wires and wire grid couplers

Des fils paralleles a la surface d'un guide d'onde dielectrique ouvert peuvent servir comme elements optiques, reflecteurs ou coupleurs. Les reseaux de fils sont employes dans les coupleurs. On etudie ici la diffusion par un fil puis par un reseau periodique de fils

Diagnostics of a plasma jet with grid electrodes

We describe the diagnostics of a plasma jet ejected from the anode hole of a pulsed arc hydrogen plasma source [1-3]. The front part of this source and the grid electrodes are shown in Fig. i. Grid 1 is at the potential of the plasma, and grid 2 is grounded. Therefore, for a positive plasma potential such a grid diode forms a proton beam [4]. If the diameter of the grids is much larger than the distance between them, the effect of space charge between the grids on the divergence of the beam can be neglected. In a number of cases the effect of the space charge field after the grids can also be neglected if a charge-exchange cell (CC), into which gas is admitted to neutralize the space charge of the beam, is located right after the grids [2, 5]. Therefore, such parallel wire grid electrodes, when combined with a charge-exchange cell, form a beam whose divergence along the wires is determined mainly by the velocity distribution of the protons in the plasma jet [i, 2]. From the phase characteristic of this proton beam it is possible to recover the velocity distribution of protons in the plasma in the direction of the grid wires. It is particularly simple to recover this distribution by analyzing the fast hydrogen atoms formed from these protons in the charge-exchange cell rather than the protons. In this case the particle motion can be considered as free streaming, since the effect of space charge has been eliminated over the whole beam path.

Electrical transparency of a corona triode

An analytical expression is derived for the electrical transparency of a corona triode with a biased parallel wire grid. This analysis is extended to a square grid by modelling it with an equivalent parallel grid having the same optical transparency. The experimental results obtained with a small corona triode, agree with the theoretical model for the parallel wire grids and the square grid with a large optical transparency. For a square grid with transparency less than 60%, good agreement is obtained by allowing one of the terms in the mathematical model to become an experimentally adjusted constant.

Theoretical studies of the frequency response of some far-infrared interferometers with wire-grid beam dividers

A method developed recently by the authors for calculating the scattering of electromagnetic waves by parallel-wire grids is used to compute some frequency response curves of three polarising far-infrared interferometers with wire-grid polarisers and beam dividers. Simple diffraction theory indicates that the highest critical frequencies are obtained by laying the wires 'as far as possible parallel' to the radiation. The best ratio of the diameter to the period is about one to three. Modulation by rotating the polariser is very good at low frequencies, but attempts to use this system for wideband spectroscopy may lead to undesirable background levels on the wings of the interferogram.

Theory of the scattering of electromagnetic waves by a regular grid of parallel cylindrical wires with circular cross section

A method is described for calculating the scattering of electromagnetic waves by a grid of equally spaced parallel cylindrical wires with finite conductivity. It is an adaptation of the 'Green function' method used in solid-state band-structure theory, as described by Ham and Segall (see Phys. Rev., vol.124, p.1786, 1961). The theory is restricted to the case when the direction of incidence is in a plane perpendicular to the wires. Curves are shown for the transmission at normal incidence in situations typical of infrared spectroscopy.

Multi-conductor coaxial cables operated in the dipole-mode and their possible applications

High-frequency transmission in the dipole-mode along a cable whose inner structure consists of a cylindrical grid of parallel wires running lengthwise and electrically insulated from one another, has been investigated. It has been shown that, in comparison with the corresponding TEM coaxial cable, reduced losses are possible especially in the dielectric used.

New grids for improved polarization diplexing of microwaves in reflector antennas

Grids of thin wires are mounted between a paraboloid and a microwave feed at its focus, so as to obtain linear polarization everywhere in the far-field of the paraboloid. Three types of grids are proposed: 1) a family of hyperbolae on an arbitrary plane <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</tex> , 2) projections of these hyperbolae on an arbitrary surface, and 3) a set of straight lines that converge on a certain point <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F'</tex> on a plane <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</tex> . If in cases 1) and 2) a point source is placed at the focus, the resulting plane wave reflected by the paraboloid will be polarized in one direction. In ease 3), there will also he a small cross-polarized component reflected by the paraboloid; the optimum choice of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F'</tex> that minimizes this undesirable component is determined. Only for certain orientations of S will the optimum choice of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F'</tex> be at infinity (in which case the grid is an ordinary parallel wire grid). A simple technique for fabricating grids on curved surfaces is also described. Using the grids discussed here, perfect (or nearly perfect, in case 3)) polarization diplexing at microwave frequencies can be achieved.

On the theory of scattering from a periodically loaded wire grid

A formulation for electromagnetic scattering from a regular grid of thin parallel wires is developed. The wires are characterized by a series impedance per unit length that itself is a periodic function. The final result is a formula for the effective surface transfer impedance that relates the averaged fields at the grid. Under certain limiting situations, earlier known results are obtained.

Low-loss propagation in the dipole-mode along a coaxial waveguide

A theoretical investigation has been made as a step towards defining more precisely the operating conditions of a fully screened dipole-mode waveguide, comprising a coaxial structure whose inner consists of a cylindrical grid of parallel longitudinal wires separated by air and whose outer is represented by a concentric tube of continuous metal.

Measurement of the Mean Pressure Distribution About a Double-wedge Strut in Turbulent Shear Flow

SummarySurface pressure distributions have been measured on a cylinder with a double-wedge cross section in a two-dimensional channel flow. The flow is sheared in the direction of the cylinder axis, using a grid of parallel wires with variable spacing, in order to simulate the type of geometry and velocity variation which occur for annulus support struts in turbomachines. The strut had a 38° included angle at the leading and trailing edges and an aspect ratio of 1.33 based on the maximum thickness. This was fixed between the parallel walls of a rectangular wind tunnel with end leakage eliminated. Surface pressure distributions were measured over a Reynolds number range of 0.5 × 105 to 1.5 × 105 for three different shear flow conditions. Despite the presence of sharp edges at the mid-chord position, delayed boundary-layer separation beyond the plane of maximum thickness has been found for sufficiently high spanwise shear in the upstream flow. Surprisingly, therefore, some similarity with previous results for struts of circular cross section appears to exist. The influence of the shear flow conditions on the pressure drag force has been determined by numerical integration of the data. Only a slight dependence on the upstream flow conditions is found, although the effect of secondary velocity components may still be observed if the spanwise variations in the local drag force are examined.

Errors in ion and electron temperature measurements due to grid plane potential nonuniformities in retarding potential analyzers

The behavior of a retarding potential analyzer in a laboratory plasma has been studied to investigate the effects of potential nonuniformities in the grid planes. Significant effects are found for the kind of grids used in the Ogo 1, 3, and 4 traps. It is found that in order to explain the laboratory results the retarding grid potential in the usual planar approximation must be replaced by an effective potential, which may be obtained from the laboratory data. Since the potential distribution about a grid of parallel wires is available analytically, this model is used to calculate the ion current through such a retarding grid. When ion trajectories are taken into account, these calculations show that the effective grid retarding potential is very closely related to the saddle point of the potential near the retarding grid. Applying these corrections to the Ogo 4 O+ temperatures results in a downward revision of about 32%, bringing them into much better agreement with radar backscatter results.

A position sensitive surface barrier array type detector

A position sensitive array type surface barrier detector system has been constructed and mounted in the focal surface of the Argonne double focussing magnetic alpha particle spectrometer. The array is 9″ long, and consists of 224 detector elements, each 0.040″ wide by 1″ high. The elements are made in blocks of 16 on single silicon crystals 0.640″ wide and 1″ long by evaporating the gold through a grid of parallel wires. The energy of the detected alpha particles is uniquely determined by the position of the detecting element in the array, and background rejection is accomplished by pulse height selection. The immediate availability of the data pulse makes possible coincidence counting experiments, thus enhancing the versatility of the instrument. The details of the fabrication and the analysis of the performance of the detector system are discussed.

Tunable Submillimeter Interferometers of the Fabry-Perot Type

Fabry-Perot Interferometers (FPI's) have been constructed for the far infrared and submillimeter wave region with metal grids as reflectors. Two-dimensional grids with square holes (metallic mesh) can be used for unpolarized radiation, too, and they are more convenient for the construction than one-dimensional (parallel wire) grids. The performance of several FPI's has been measured in the 100-600 /spl mu/ wavelength region. Q values in first-order range from 5 to 30, and peak transmissions up to 0.9 have been reached. The experimental results are in qualitative agreement with the theory of thin parallel wire gratings. The influence of the unevenness of the reflectors is studied theoretically. Applications of a submillimeter FPI include its use as the dispersion element in a spectrometer, as a narrow-band filter to check the radiation purity of a grating spectrometer, and as a separator of the harmonics from a crystal harmonic generator.

Effective impedance of a wire grid parallel to the earth's surface

The reflection of electromagnetic waves from a parallel wire grid, located near the plane interface of two homogeneous media, is considered. The incident wave is polarized such that the magnetic vector is perpendicular to the grid wires. It is shown that, for this particular case, the grid may be represented by a pure shunt element in the equivalent transmission line circuit. The application of the results to the design radial wire ground systems for vertical monopole systems is mentioned.