Magnetic control of breakdown : toward energy-efficient hollow-cathode magnetron discharges

Characteristics of electrical breakdown of a planar magnetron enhanced with an electromagnet and a hollow-cathode structure, are studied experimentally and numerically. At lower pressures the breakdown voltage shows a dependence on the applied magnetic field, and the voltage necessary to achieve the self-sustained discharge regime can be significantly reduced. At higher pressures, the dependence is less sensitive to the magnetic field magnitude and shows a tendency of increased breakdown voltage at the stronger magnetic fields. A model of the magnetron discharge breakdown is developed with the background gas pressure and the magnetic field used as parameters. The model describes the motion of electrons, which gain energy by passing the electric field across the magnetic field and undergo collisions with neutrals, thus generating new bulk electrons. The electrons are in turn accelerated in the electric field and effectively ionize a sufficient amount of neutrals to enable the discharge self-sustainment regime. The model is based on the assumption about the combined classical and near-wall mechanisms of electron conductivity across the magnetic field, and is consistent with the experimental results. The obtained results represent a significant advance toward energy-efficient multipurpose magnetron discharges.

Effect of ambipolar fluxes on nanoparticle charging in low-pressure glow discharges

The effect of ambipolar fluxes on nanoparticle charging in a typical low-pressure parallel-plate glow discharge is considered. It is shown that the equilibrium values of the nanoparticle charge in the plasma bulk and near-electrode areas are strongly affected by the ratio S ath i of the ambipolar flux and the ion thermal velocities. Under typical experimental conditions the above ratio is neither S ath i≪ 1 nor S ath i≫1, which often renders the commonly used approximations of the purely thermal or "ion wind" ion charging currents inaccurate. By using the general approximation for the ambipolar drift-affected ion flux on the nanoparticle surface, it appears possible to obtain more accurate values of the nanoparticle charge that usually deviate within 10-25 % from the values obtained without a proper accounting for the ambipolar ion fluxes. The implications of the results obtained for glow discharge modeling and nanoparticle manipulation in low-pressure plasmas are discussed.

Dispersion of corona ions downwind from a high-voltage power line

Corona discharge is responsible for the small ions found near overhead power lines, and these are capable of modifying the ambient electrical environment such as the dc electric field at ground level (Fews, Wilding et al. 2002). Once produced, small ions quickly attach to aerosol particles in the air, producing ‘large ions’ which are roughly 1 nm to 1 µm in diameter. However, very few studies have reported measurements of ions produced by power lines and its impact on particle charge concentrations. In this present study, the measurements were conducted as a function of normal downwind distance from a 275kV power line for investigating the effect of corona ions on air ions, aerosol particle charge concentration and dc e-filed.

Charged particles produced by corona emission from an overhead high voltage powerline

High voltage powerlines may give rise to corona breakdown, resulting in the release of large concentrations of charged ions into the surrounding environment. These ions quickly attach to aerosols and the resulting charged particles are carried by prevalent winds. This paper describes a study carried out at a site near an overhead double circuit ac transmission voltage powerline to investigate factors that control the rate at which charged particles are produced, and to determine the total particle number concentrations, total particle charge concentrations and vertical dc electric fields in the proximity of the line. Measured mean values of these three parameters at a perpendicular distance of 50m from the line were 1.8 x 103 particle cm-3, 518 ions cm3 and 520 V m-1 respectively. The net electric charge was positive and the electric field was directed downwards. These parameters were correlated with each other and monitored at four different distances from the line. Effects of meteorological parameters such as wind speed and wind direction were also investigated.

The influence of small amounts of N2 in SF6 on prebreakdown discharges

Pre-breakdown discharge activity in SF6 and SF6 +2% N2 at 1 bar absolute has been investigated to assess the feasibility of adding small amounts of N2 to SF6 to improve the luminosity of the pre-breakdown discharges. It has been shown that small amounts of N2 does not significantly alter the breakdown strength of the mixture but will have a profound influence on the pre-breakdown discharge activity under 50 Hz voltages and lightning impulse voltages. This influence is attributed to the following three mechanisms: 1. The increased availability of free electrons due to the lower ionisation potential of the N2 compared to SF6 2. The removal of high energy electrons from the discharge by electron scattering due to negative ion resonance in N2, thus effectively increasing the attachment of SF6. 3. The creation of longer lived SF6- ions by the attachment of the scattered low energy electrons from N2 thus effectively enhancing the negative ion space charge.

Preliminary work on the detection of partial discharges in SF6 under impulse voltages

This paper presents preliminary results of an investigation into the detection of partial discharges on the rise of impulse voltages from a point-to-plane gap in SF6. A parallel RC detection impedance is placed in the earth path of a point. Computer simulations are done to determine the values of R and C that will result in the smallest impulse voltage signal and the largest discharge signal across the detection impedance. These simulations and the experimental work show that the impulse voltage signal can not be sufficiently attenuated during the rise time of the applied voltage impulse using the RC detection impedance alone. An alternative discharge detection method is proposed in which a resonant partial discharge coupler is used. Elimination of noise and the impulse voltage signal can be achieved by shorting the coupler plate to the ground plane in the middle of the disk. However, due to the bandwidth of the measuring equipment and noise from the impulse generator it was not possible to detect discharges on the rising edge of a 1.5s voltage impulse using a coupler shorted in the middle. It was found that for this particular coupler, with no shorting points, and if the rising edge of the voltage impulse is longer than 5us, (10us) PD activity can be detected on the rising edge.

Buoyant Jet Discharges into Finite Ambient Waters

The paper presents the results of an experimental study regarding the effect of the lateral dimension of the receiving water on the spreading, mixing, and temperature decay of a horizontal buoyant surface jet. The widths of the ambient water in the experiments have been 240, 120, 90 and 60 times the diameter of the jet nozzle. Based on the experimental data, correlations are carried out and empirical equations for prediction of jet width, thickness in vertical direction and longitudinal temperature decay are obtained. The available data of earlier investigators are included to obtain generalized equations for the spreading and temperature decay. Similarity of temperature profiles in the lateral and vertical directions is observed. The longitudinal temperature decay is found to vary inversely with distance in the flow direction and ¼th power of the densimetric Froude number.

Partial discharges at low pressures in a dielectric interface between uniform field electrodes

Partial discharges in a gaseous interface due to the presence of a dielectric between two uniform field electrodes in air at different pressures from 0.5 to 685 mm Hg have been studied and measurements of inception and extinction voltages, number of pulses and their charge magnitudes at inception are reported. It has been observed that the extinction voltage can be as low as 70% of the inception voltage suggesting that the working voltage in such cases should be about 30% lower than the observed inception voltage. Small magnitude pulses are found to be more in number than large magnitude pulses. The charge is found to be pressure dependent. The results have been explained on the basis of an equivalent circuit consisting of resistance and capacitance in which the discharge gap functions as a switch.

Numerical simulation of conductive particle behaviour at the surface of a plate electrode affected by a DC corona field

The electric field in certain electrostatic devices can be modeled by a grounded plate electrode affected by a corona discharge generated by a series of parallel wires connected to a DC high-voltage supply. The system of differential equations that describe the behaviour (i.e., charging and motion) of the conductive particle in such an electric field has been numerically solved, using several simplifying assumptions. Thus, it was possible to investigate the effect of various electrical and mechanical factors on the trajectories of conductive particles. This model has been employed to study the behaviour of coalparticles in fly-ash corona separators.

Various Methods of Designing Spillway Capacity of Dams and Estimating Flood Discharges

Abstract is not available.

NOx Reduction and Desorption Studies Under Electric Discharge Plasma Using a Simulated Gas Mixture: A Case Study on the Effect of Corona Electrodes

In this study, reduction and desorption of oxides of nitrogen (NOx) were conducted using an electrical discharge plasma technique. The study was carried out using a simulated gas mixture to explore the possibility of re-generation of used adsorbents by a nonthermal plasma desorption technique. Three different types of corona electrodes, namely, pipe, helical wire, and straight wire, were used for analyzing their effectiveness in NOx reduction/desorption. The pipe-type corona electrode exhibited a nitric oxide (NO) conversion of 50%, which is 1.5 times that of the straight-wire-type electrode at an energy density of 175J/L. The helical-wire-type corona electrode exhibited a NOx desorption efficiency almost 4 times that of the pipe-type electrode,indicating the possibility that corona-generated species play a crucial role in desorption.

Measurements of partial discharges-corrections to erroneous counts occurring in a cumulative pulse-counting system

The discharge pulse rates at different magnitude levels are often used as criteria for monitoring the partial-discharge aging of insulation systems. Use of suggested corrections for errors in cumulative probability counting leads to better use of available counters.

Significance of Branching Angles in Streamer Discharges

The phenomenon of branching at specific angles in streamer breakdown studies is found to be more universal than was ever thought. The angles measured in the breakdown of gases show that the coefficient of field distortion, K, lies in the range from 1 to less than 0.1. The values of K, so obtained agree well with those envisaged in the criterion of the streamer mechanism. It is hoped that branching angles observed in various types of breakdown may, possibly, be explained similarly.

Environmental effects of thermal and radioactive discharges from nuclear power plants in the boreal brackish-water conditions of the northern Baltic Sea

During recent decades, thermal and radioactive discharges from nuclear power plants into the aquatic environment have become the subject of lively debate as an ecological concern. The target of this thesis was to summarize the large quantity of results obtained in extensive monitoring programmes and studies carried out in recipient sea areas off the Finnish nuclear power plants at Loviisa and Olkiluoto during more than four decades. The Loviisa NPP is located on the coast of the Gulf of Finland and Olkiluoto NPP on that of the Bothnian Sea. The state of the Gulf of Finland is clearly more eutrophic; the nutrient concentrations in the surface water are about 1½ 2 times higher at Loviisa than at Olkiluoto, and the total phosphorus concentrations still increased in both areas (even doubled at Loviisa) between the early 1970s and 2000. Thus, it is a challenge to distinguish the local effects of thermal discharges from the general eutrophication process of the Gulf of Finland. The salinity is generally low in the brackish-water conditions of the northern Baltic Sea, being however about 1 higher at Olkiluoto than at Loviisa (the salinity of surface water varying at the latter from near to 0 in early spring to 4 6 in late autumn). Thus, many marine and fresh-water organisms live in the Loviisa area close to their limit of existence, which makes the biota sensitive to any additional stress. The characteristics of the discharge areas of the two sites differ from each other in many respects: the discharge area at Loviisa is a semi-enclosed bay in the inner archipelago, where the exchange of water is limited, while the discharge area at Olkiluoto is more open, and the exchange of water with the open Bothnian Sea is more effective. The effects of the cooling water discharged from the power plants on the temperatures in the sea were most obvious in winter. The formation of a permanent ice cover in the discharge areas has been delayed in early winter, and the break-up of the ice occurs earlier in spring. The prolonging of the growing season and the disturbance of the overwintering time, in conditions where the biota has adjusted to a distinct rest period in winter, have been the most significant biological effects of the thermal pollution. The soft-bottom macrofauna at Loviisa has deteriorated to the point of almost total extinction at many sampling stations during the past 40 years. A similar decline has been reported for the whole eastern Gulf of Finland. However, the local eutrophication process seems to have contributed into the decline of the zoobenthos in the discharge area at Loviisa. Thermal discharges have increased the production of organic matter, which again has led to more organic bottom deposits. These have in turn increased the tendency of the isolated deeps to a depletion of oxygen, and this has further caused strong remobilization of phosphorus from the bottom sediments. Phytoplankton primary production and primary production capacity doubled in the whole area between the late 1960s and the late 1990s, but started to decrease a little at the beginning of this century. The focus of the production shifted from spring to mid- and late summer. The general rise in the level of primary production was mainly due to the increase in nutrient concentrations over the whole Gulf of Finland, but the thermal discharge contributed to a stronger increase of production in the discharge area compared to that in the intake area. The eutrophication of littoral vegetation in the discharge area has been the most obvious, unambiguous and significant biological effect of the heated water. Myriophyllum spicatum, Potamogeton perfoliatus and Potamogeton pectinatus, and vigorous growths of numerous filamentous algae as their epiphytes have strongly increased in the vicinity of the cooling water outlet, where they have formed dense populations in the littoral zone in late summer. However, the strongest increase of phytobenthos has extended only to a distance of about 1 km from the outlet, i.e., the changes in vegetation have been largest in those areas that remain ice-free in winter. Similar trends were also discernible at Olkiluoto, but to a clearly smaller extent, which was due to the definitely weaker level of background eutrophy and nutrient concentrations in the Bothnian Sea, and the differing local hydrographical and biological factors prevailing in the Olkiluoto area. The level of primary production has also increased at Olkiluoto, but has remained at a clearly lower level than at Loviisa. In spite of the analogous changes observed in the macrozoobenthos, the benthic fauna has remained strong and diversified in the Olkiluoto area. Small amounts of local discharge nuclides were regularly detected in environmental samples taken from the discharge areas: tritium in seawater samples, and activation products, such as 60Co, 58Co, 54Mn, 110mAg, 51Cr, in suspended particulate matter, bottom sediments and in several indicator organisms (e.g., periphyton and Fucus vesiculosus) that effectively accumulate radioactive substances from the medium. The tritium discharges and the consequent detection frequency and concentrations of tritium in seawater were higher at Loviisa, but the concentrations of the activation products were higher at Olkiluoto, where traces of local discharge nuclides were also observed over a clearly wider area, due to the better exchange of water than at Loviisa, where local discharge nuclides were only detected outside Hästholmsfjärden Bay quite rarely and in smaller amounts. At the farthest, an insignificant trace amount (0.2 Bq kg-1 d.w.) of 60Co originating from Olkiluoto was detected in Fucus at a distance of 137 km from the power plant. Discharge nuclides from the local nuclear power plants were almost exclusively detected at the lower trophic levels of the ecosystems. Traces of local discharge nuclides were very seldom detected in fish, and even then only in very low quantities. As a consequence of the reduced discharges, the concentrations of local discharge nuclides in the environment have decreased noticeably in recent years at both Loviisa and Olkiluoto. Although the concentrations in environmental samples, and above all, the discharge data, are presented as seemingly large numbers, the radiation doses caused by them to the population and to the biota are very low, practically insignificant. The effects of the thermal discharges have been more significant, at least to the wildlife in the discharge areas of the cooling water, although the area of impact has been relatively small. The results show that the nutrient level and the exchange of water in the discharge area of a nuclear power plant are of crucial importance.

Intensity Oscillation in the Corona as Observed during the Total Solar Eclipse of 29 March 2006

We obtained the images of the eastern part of the solar corona in the Fe xiv 530.3 nm (green) and Fe x 637.4 nm (red) coronal emission lines during the total solar eclipse of 29 March 2006 at Manavgat, Antalya, Turkey. The images were obtained using a 35 cm Meade telescope equipped with a Peltier-cooled 2k x 2k CCD and 0.3 nm pass-band interference filters at the rates of 2.95 s (exposure times of 100 ms) and 2.0 s (exposure times of 300 ms) in the Fe xiv and Fe x emission lines,respectively. The analysis of the data indicates intensity variations at some locations with period of strongest power around 27 s for the green line and 20 s for the red line. These results confirm earlier findings of variations in the continuum intensity with periods in the range of 5 to 56 s by Singh et al. (Solar Phys. 170, 235, 1997). The wavelet analysis has been used to identify significant intensity oscillations at all pixels within our field of view. Significant oscillations with high probability estimates were detected for some locations only. These locations seem to follow the boundary of an active region and in the neighborhood, rather than within the loops themselves. These intensity oscillations may be caused by fast magneto-sonic waves in the solar corona and partly account for heating of the plasma in the corona.