Successive Half-cycles Research Articles

Characterisation of surface charge density and net electric field during parallel-plate dielectric barrier discharge generated in atmospheric-pressure air

We measured the surface charge density in a parallel-plate dielectric barrier discharge (DBD) using the Pockels effect, and the axial and lateral components of the net electric field were evaluated for different gap lengths and applied voltages. The DBD was driven by a 5 kHz sinusoidal voltage in gap lengths of 0.5, 0.3, and 0.1 mm at atmospheric pressure with Bi4Ge3O12 crystal as a dielectric. Typical microdischarges (MDs) were observed for every gap length, and the number of MDs increased with the increasing applied voltage. As the gap length decreased, the spot radius of a single MD decreased, whereas the peaks of charge density were almost the same, indicating that the amount of charge transported per unit MD is limited for short gaps. The spatial memory effect, in which MDs occur at the same position in successive cycles, was not always observed under the experimental conditions, and it was evaluated considering the surface charge density distributions. Whether the memory effect works is determined by the surface charge density, and it is likely to occur under a balance between positive and negative charges across successive half-cycles. The net electric field was estimated from surface charge measurements. The axial component of the net electric field strength increased up to 150–170 Td before the MD occurred, and it decreased below 100 Td by the surface charge deposited by MD, being approximately 50%–60% of the external field amplitude. The maximum value of the lateral component of the electric field was approximately 150 Td at the edge of the MD spot with a gap length of 0.5 mm, and it decreased as the gap length decreased. As a negative surface charge generated a weaker lateral field than a positive one, the electric field distribution differed between anodic and cathodic dielectrics.

Square Grid Superlattice Pattern in Dielectric Barrier Discharge System

We report on the observation of the square grid superlattice pattern in dielectric barrier discharge system. Instantaneous images taken by an intensified charge-coupled device show that the pattern is an interleaving of four transient sublattices including three square sublattices and a cross-shaped halo sublattice. Comparing the single-frame instantaneous image with the 30-cycles integration image, it is found that the cross-shaped halo sublattice consists of several sporadic filaments. They discharge at different positions of the discharge-area in successive half-cycles of the applied voltage, which would have little influence on the following discharges. The calculation further verifies the significant influence of the wall charges which are accumulated in the first discharge on the following discharges during each half voltage-cycle.

Static Spiral Patterns in a Dielectric Barrier Discharge

Static spiral patterns are observed in a dielectric barrier discharge system. It bifurcates from the concentric-ring pattern with the increase of the applied voltage. The formation of the static spiral patterns is investigated using an intensified-charge coupled device camera. It is found that the discharge filaments are the basic unit of the spiral patterns and they do not occur at the same location during successive half cycles. The uniform spiral arms, as seen with the naked eyes, are formed by the numerous discharge filaments located in different positions during successive acquisition intervals. The correlation coefficient between the discharges of successive half cycles is relatively low and is independent of the delay time between images.

Fault Development in Wet, Low Voltage, Oil-Impregnated Paper Insulated Cables

Few studies have been carried out on the failure processes of low voltage distribution cables. This is mainly due to their excellent track record of high reliability, and their use of traditional technologies. Growing quantities of aged cable within the network now make management of these assets an important issue. This paper describes measurements of leakage current, partial discharges and arc formation within 400 V oil-impregnated paper insulated cable structures which have been artificially wetted by immersion in bulk water. A generic picture is produced to show the gradual evolution from a leakage current through discharge growth to final catastrophic failure. Rapid changes to the core temperature of a wet cable are shown to produce transient, high levels of leakage current. It is also shown that by increasing the core temperature more slowly these high levels can be reduced. It is suggested that the characteristics of individual arcs within successive half cycles may provide a basis for predicting whether an incipient fault will lead to failure in the short term.

Square Wave Voltcoulometry: A Tool for the Study of Strongly adsorbed Redox Molecules

A new multipotential pulse technique called square wave voltcoulometry (SWVC), based on the analysis of the difference of converted charge signals obtained between two successive half-cycles when a square wave potential is applied, is developed to study charge-transfer processes taking place in electroactive monolayers. The use of SWVC presents the advantage of giving rise to a peak-shaped response, which evolves to a charge plateau at high square wave pulse amplitudes, from which the total surface excess and the formal potential can be immediately measured for quasi-reversible and reversible processes. This characteristic represents its main advantage versus other multipotential step techniques, which lead to a negligible current under reversible conditions. The formal potential of the electroactive systems can be measured from the peak potential of the SWVC curves, even for quasi-reversible behavior. Moreover, the non-faradic effects on the response can be easily evaluated and avoided as is demonstrated in this paper. Experimental verification of the theoretical predictions is given for reversible and quasi-reversible systems.

Phase structure of radar stratigraphic horizons within Antarctic firn

AbstractWe have recorded reflection profiles of firn through large areas of West Antarctica and part of the East Antarctic plateau using 400MHz short-pulse radar. The locations show accumulation rates that vary from well above to well below the vertical radar resolution. Most reflection horizons have extensive lateral continuity, and are composed of distinctive wavelets with a consistent phase polarity sequence within their successive half-cycles. We modeled these waveforms, and conclude that they arise from thin, double layers of ice over hoar, which is consistent with the standard model of firn stratification. In addition, we conclude that ice/hoar layers are extensive throughout West Antarctica and also present (although more sparsely) beneath the Antarctic Plateau.

Damping of the wrist joint during voluntary movement.

Damping characteristics of the musculoskeletal system were investigated during rapid voluntary wrist flexion movements. Oscillations about the final position were induced by introducing a load with the characteristics of negative damping, which artificially reduced the damping of the wrist. Subjects responded to increases in the negatively damped load by stronger cocontraction of wrist flexor and extensor muscles during the stabilization phase of the movement. However, their ability to counteract the effects of the negatively damped load diminished as the negative damping increased. Consequently, the number and frequency of oscillations increased. The oscillations were accompanied by phase-locked muscle activity superimposed on underlying tonic muscle activation. The wrist stiffness and damping coefficient increased with the increased cocontraction that accompanied more negatively damped loads, although changes in the damping coefficient were less systematic than the stiffness. Analysis of successive half-cycles of the oscillation revealed that the wrist stiffness and damping coefficient increased, despite decreasing muscle activation, as oscillation amplitude and velocity declined. This indicates that the inverse dependence of the damping coefficient on oscillation velocity contributes significantly to damping of joint motion. It is suggested that this property helps to offset a negative contribution to damping from the stretch reflex.

Three-dimensional instabilities in wake transition

It is now well-known that the wake transition regime for a circular cylinder involves two modes of small-scale three-dimensional instability, modes “A” and “B”, occurring in different Reynolds number ranges. These modes are quite distinct in spanwise lengthscale and in symmetry, and they are found to scale on different physical features of the flow. Mode A has a large spanwise wavelength of around 3–4 cylinder diameters, and scales on the larger physical structure in the flow, namely the core of the primary Kármán vortices. The feedback from one vortex to the next gives an out-of-phase streamwise vortex pattern for this mode. In contrast, the mode B instability has a distinctly smaller spanwise wevelength (1 diameter) which scales on the smaller physical structure in the flow, namely the braid shear layer. The symmetry of mode B is determined by the reverse flow behind the bluff cylinder, leading to a system of streamwise vortices which are in phase between successive half cycles. The symmetries of both modes are the same as the ones found in the vortex system evolving from perturbed plane wakes studied by Meiburg and Lasheras (1988) and Lasheras and Meiburg (1990). Furthermore, the question of the physical origin of these three-dimensional instabilities is addressed. We present evidence that they are linked to general instability mechanisms found in two-dimensional linear flows. In particular, mode A seems to be a result of an elliptic instability of the near-wake vortex cores; predictions based on elliptic instability theory concerning the initial perturbation shape and the spanwise wevelength are in good agreement with experimental observations. For the mode B instability, it is suggested that it is a manifestation of a hyperbolic instability of the stagnation point flow found in the braid shear layer linking the primary vortices.

Forces on a circular cylinder in elliptical orbital flows at low Keulegan-Carpenter numbers

Flows around a circular cylinder in elliptical orbital flows have been computed using the random vortex method for situations where the flow is attached and laminar; the Keulegan-Carpenter number is less than or equal to 1·5. The forces are related to the flow characteristics and steady streaming is shown to have a significant influence for ellipticity even as low as 1 8 , depending on the Keulegan-Carpenter number. The interdependence of horizontal and vertical inertia coefficients and of horizontal and vertical drag coefficients on ellipticity may be modelled simply for certain ranges of Keulegan-Carpenter number and ellipticity. Force components at frequencies other than the orbital frequency can occur and cause scatter in values of drag coefficient in successive half cycles for the higher Keulegan-Carpenter numbers investigated; this does not occur in the linear oscillatory case (zero ellipticity). Previous experience suggests that there will be qualitatively similar effects at very high, full-scale Reynolds numbers with turbulent flow.

A New Formula for the Analytical Solution of the Radial Dispersion Problem

A new formula is presented for the analytical solution of the radial dispersion problem, which analyzes the dispersive transport of a tracer in radial flow from a recharge well. This formula is simpler than a solution presented by previous investigators, but the two solutions are shown to be equivalent. Because the analytical solution consists of an integral of an oscillatory function, it is evaluated as an alternating series, the terms of which are the integrals over successive half cycles of the integrand. An efficient algorithm is used to accelerate convergence of the series. The accuracies of previous solutions obtained by finite difference, numerical Laplace transform, and perturbation methods are examined by comparison with the analytical solution. Numerical values of the solution are tabulated for readers interested in checking the accuracies of solute transport codes.

Oscillatory flows in wavy-walled tubes

The problem of oscillatory viscous flow in a tube with rigid sinusoidal walls of large amplitude is solved numerically, for Reynolds numbers up to 300 and Strouhal numbers in the range 10−4–1. Flow visualization photographs have confirmed qualitatively many of the predictions. Results analogous to those of Sobey (1980, 1983) for the two-dimensional problem have been obtained for regions of the parameter space studied in detail by that author. However, new flow structures are found in the previously neglected Strouhal number range of 0.02–0.1. These flows are characterized by significant interaction of flow events in successive half-cycles, due to the persistence of strong shed vortices. Bifurcation of the solution structure can then occur for Strouhal numbers between about 0.025 and 0.045, with the development of time-asymmetric flows: thus the velocity field at some instant of a positive-flow half-cycle may not be equal to minus the velocity field at the corresponding instant of a negative-flow half-cycle.

The growth of the high-frequency electrodeless discharge

In this paper we consider only discharges in gases at such low pressure that the mean free path of electrons is greater than the size of the vessel. Part I. The elementary theory of the starting of an electrodeless discharge in a gas at low pressure by a uniform high-frequency electric field has been given by Gill & von Engel (1948). It was shown that a discharge will start when the applied field is large enough to cause multiplication of electrons by secondary electron emission from the end-walls of the vessel; initially, gas ionization is absent. Multiplication occurs if an electron starting at one end-wall with a small energy and in a suitable phase of the applied field crosses the vessel in one half-cycle and hits the opposite wall fast enough to release more than one secondary electron. Secondary electrons usually start in a negative phase of the field, but escape from the wall because of their initial energy, unless the phase is more negative than a certain limiting value; this corresponds to the cut-off wave-length, i.e. the longest one at which a discharge can be started. Here, for the first time, the growth of the discharge is treated in detail, the calculations being based on known atomic data only. When secondary electrons leave an end-wall a positive wall charge is left behind, which retards the electrons. This is important only near the cut-off wavelength. These wall charges cause the phase at one wall to become increasingly negative until, finally, the electrons would fail to escape, and the multiplication would cease, which is contrary to experience. However, the growth can be explained by considering the velocity distribution of the secondary electrons. Then a distribution in phase ensues, which must be repeated in successive half-cycles for an avalanche to develop. During this first stage the current is therefore essentially controlled by secondary emission, and grows exponentially with time. At these low pressures electrons rarely collide with gas molecules. Thus the electrons must make many transits across the vessel to form a large number of positive ions. The ions remain almost stationary in the gas; they are nearly uniformly distributed although slightly concentrated at the centre of the vessel. A second stage in the growth of the discharge begins when the ion space charge first appreciably affects the motion of the electrons. Although electrons are still produced mainly at the end-walls, the rate steadily decreases as the ion space charge grows. The rate of production of ions and electrons in the gas also decreases, and losses of both ions and electrons due to self-repulsion become important. The current thus rises more slowly than it would if space charges did not develop, until it reaches a constant value. It is shown that at very low pressures this second stage may not be reached, because self-repulsion of the electrons stops the development earlier. The final equilibrium state for larger pressures is not included in this treatment. This theory predicts the dependence of the growth on the material of the walls, on the nature of the gas and its pressure, and the effect of a field greater than the starting field. Part II. A new experimental technique has been employed to measure the current actually flowing through the discharge. The large capacitative current flowing across the external electrodes is balanced out by a bridge method, the bridge becoming unbalanced when a current flows through the gas. The unbalanced component is proportional to the discharge current and is amplified, rectified and displayed on an oscilloscope. In order to measure the growth of this current with time, pulses of high-frequency potential are applied across the discharge vessel, and the time-base of the oscilloscope synchronized with the pulses. Oscillograms show how the growth of current depends upon the pressure (2 to 35//) and nature of the gas (hydrogen and helium), the excess voltage applied ( < 20 %) and the frequency of the applied field (10 to 20 Mc/s). The spatial distribution of light from the discharge in the final state is also measured, from which the motion of the electrons can be deduced, and compared with theory. Good agreement is obtained between theoretical predictions derived solely from atomic data, and experimental results. These investigations clearly demonstrate that at low pressure the properties of the wall mainly control the multiplication process in the initial stage and thus the starting field, whereas the properties of the gas become important in the later stages and so essentially determine the total time of growth of the discharge.