Temperature effect on the performance characteristics of beta raysopen air corona streamer counter

Open air three electrode (point-grid-plane) corona streamer counter with a preamplification gap has been utilized for detecting weakly ionizing beta rays. This is based on the fact that when the primary electrons produced by beta rays are multiplied in the external preamplification gap, they form a trigger electron swarm before entering the counter, produce the same effect as the direct incidence of strongly ionizing particle in the corona counter, and allow the counter to operate efficiently. However, operation of this counter in open air as one of its most attractive features requires a complete analysis of the counter response to the changes of ambient conditions. The effect of temperature on the beta particle corona counter performance is studied. A theoretical model is suggested to calculate the counter operating voltage as well as the beta particles triggered corona pulse characteristics at different temperatures (20°C-50°C). The temperature effects on the counter-energy response is explained. A compensation for the errors due to temperature effects is suggested. All the theoretical findings are confirmed by experiments     

Positive streamer propagation in large oil gaps: experimentalcharacterization of propagation modes

This paper presents an experimental study of positive streamer propagation in mineral oil, in large point-plane gaps under impulse voltage. A systematic investigation was done concerning the influence of gap distance (⩽35 cm), and voltage from streamer inception up to large overvoltages. The measurements presented concern breakdown voltage measurements, time to breakdown, visualization of streamers (streak and still photographs), electrical measurements (transient current, charge), light emission intensity. Streamers are characterized and then classified into different modes (2nd, 3rd and 4th modes) according to their propagation velocities up to more than 100 km/s. According to the applied voltage, transitions between modes are observed, and correlations are established between charge, shape, and streamer velocity. The physical mechanisms as well as practical consequences are then discussed     

Space charge effect for streamer initiation and propagation in water subjected to reciprocal traveling wave voltage pulse

This paper deals with initiation and propagation processes of streamer discharges in water subjected to high-speed repetitive voltage pulse. The 50%-duty repetitive pulses were produced by the reciprocal traveling wave in a pulse forming cable which was connected with a point-plane electrode gap in a water-cell. Laue plot of time lag distributions for the streamer initiation indicated that the initiation probability during the second pulse application became much higher than that during the first pulse application. We concluded that the change in space charges distribution due to internal field during the rest-time after the first pulse enhanced the streamer initiation probability at the second pulse application. Temporal development in the positive streamers during the repetitive pulse application was observed using a gated image intensifies. When the pulse was relatively high, the streamer discharges started the propagation at the first pulse and then the discharge channel became long step by step at the subsequent pulse applications. It was also found that the periodical emission due to excitation of OH radicals was detected for a long period after the development of the streamer discharge in a spectroscopic measurement.     

Influence of molecular structure on propagation of positive streamer discharge in dielectric liquids

In this paper, streamer discharges in dielectric liquids which have various molecular structures, such as a double bond or a triple bond in the molecular formula, are observed in detail for positive point polarity under the application of an impulse voltage to the point electrode. As test liquids, four straight‐chain hydrocarbon liquids—n‐hexane, n‐heptane, n‐octane, and n‐decane—are selected and in addition the influence of a double or triple bond included in their molecular formula on the propagation of positive streamers is investigated in detail by simultaneously using a high‐speed schlieren method and an LED current measurement system. Some differences in the mean propagation velocity and the propagation shape of positive streamers are evident. Their shape is mainly filamentary in test liquids including a double or triple bond. Also, positive streamer propagation is promoted in these liquids. It is thought that this is due to the effect of π bonds which have smaller bond energies than σ bonds. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 149(1): 15–21, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10366     

Positive streamer propagation in large oil gaps: electricalproperties of streamers

This paper presents experimental data and calculations concerning the electrical properties of positive streamers in mineral oil at large gaps and HV. The experiments concern the measurement of charge, electric field, and the determination of the potential drop along streamers, either in the liquid alone, or for streamers guided within insulating tubes. Calculations of charge and field distribution around streamers are carried out by charge simulation. To do this, streamers are represented by objects with simple shapes (spheres or cylinders) equivalent to their macroscopic aspect. These models lead to a correct agreement with measured streamer charge and field on the plane electrode. Qualitative correlations are established between calculated field distributions and streamer behavior such as velocity, transitions between propagation modes. It is also concluded that the potential drop in streamers and branching both act as regulating mechanisms that help to keep the streamer tip field, and hence the velocity, constant over a wide voltage range     

Formulation of the streamer breakdown criterion and its application to strongly electronegative gases and gas mixtures

Contents Following an examination of the definition of the effective coefficient of ionisation , a reformulation of the streamer breakdown criterion is undertaken. The application of this criterion to strongly electronegative gases and gas mixtures is considered, and a figure of merit is introduced to describe dielectric behaviour. By considering both ideal and practical electrode systems, this parameter is shown to be of fundamental importance with respect to the evaluation of the breakdown characteristics of such gases. Its value can be determined from a precision measurement of the linear portion of the Paschen curve.

---

Die Formulierung des Streamer-Durchschlagskriteriums und dessen Anwendung auf stark elektronegative Gase und Gasmischungen

Übersicht Nach einer Diskussion der Definition des effektiven Ionisationskoeffizienten wird eine Neufassung des Streamer-Durchschlagskriteriums aufgestellt. Die Anwendung dieses Kriteriums auf stark elektronegative Gase und Gasmischungen wird besprochen, und ein das dielektrische Verhalten beschreibender Qualitätsparameter wird eingeführt. Es wird gezeigt, daß dieser Parameter für die Berechnung von Durchschlagskennlinien solcher Gase von entscheidender Wichtigkeit ist; dabei betrachtet man sowohl idealisierte als auch praktische Elektrodensysteme. Den Wert des Parameters erhält man durch Präzisionsmessung des linearen Teils der Paschenkurve.

     

Streamer propagation and breakdown under ac voltage in very largeoil gaps

This paper presents a study of prebreakdown and breakdown phenomena under ac voltage in mineral oil in very large gaps (⩽80 cm). The investigations presented concern the measurement of breakdown voltages together with the recording of streamers in rod-plane and sphere-plane electrode systems, at different gap sizes. A breakdown mode specific to ac voltage is found, in which `bursts' composed of streamers initiated at each half cycle appear. This mode leads to the lowest breakdown fields recorded under ac. Values of the average stress required for burst or direct breakdown modes are deduced from the experiments. Positive streamers are responsible for breakdown in large gaps: their propagation is easy and does not depend on the gap geometry and on the presence of particles. On the other hand, the propagation of negative streamers is quenched when not very divergent fields are used     

Surface breakdown phenomena in alumina rf windows

The multipactor and flashover phenomena of alumina rf windows used in a high-power klystron have been investigated. Multipactoring due to the high yield of secondary electron emission takes place during rf operation. A spectrum analysis of luminescence due to multipactoring shows that multipactor electron bombardment causes F centers in alumina, thus leading to surface melting. From the results of a high-power examination of rf windows with several kinds of alumina ceramic, it was found that alumina with a crystallized grain boundary and without any voids between the boundaries, thus having a low loss tangent value, is not liable for F centers, even under multipactoring. Flashover in a tree-like pattern of alumina luminescence occasionally takes place on a TiN-coated surface where multipactoring is suppressed. A sapphire window, whose surface was polished and having pre-existing F centers, shows a lower flashover threshold. The annealing effect of polished window surfaces was also investigated     

Fault arc propagation on cables for space applications in vacuum,oxygen-enriched air and air

Investigations of fault are propagation on spacecraft cables under specific conditions, e.g. vacuum, normal and oxygen enriched atmospheres have been performed. Typical patterns of fault arc extinction have been identified and their consequences with respect to causes of damages are discussed. The results indicate that for a given test current and test voltage the behavior of the arc and the damages caused in a cable bundle depend strongly on the kind of insulation material, cable size and design, as well as on the environmental conditions. For some cables the arc propagation effects are increased at higher oxygen partial pressure and significantly increased under vacuum. In many cases vacuum has turned out to be an important worst-case parameter. It was found that the amount of damage assessed by post-test measurements increases substantially, if the value of the test current grows. More than 250 tests have been performed for test currents in the range of 6 to 30 A at the test voltage of 125 V dc     

The effect of tip curvature on the prebreakdown streamer structurein cyclohexane

In this paper the effect of tip curvature on the generation and propagation of prebreakdown density change streamer in cyclohexane was investigated. The streamer inception voltage increased with tip curvature, which was higher for a positive point than a negative one. The initial structure of the streamer was a single filament for both polarities in cyclohexane. For a negative point, the streamer structure changed from sphere to hemisphere, pagoda and bush-like form with increasing tip curvature and applied voltage. A slim channel connecting the streamer with needle tip was clearly observed for tip radii >5 μm, while it was not for those <3 μm. For positive streamers, on the other hand, it changed from slow bush-like form to fast filamentary with increasing tip curvature and applied voltage. Both magnitude and propagating speed of the streamer at its inception voltage increased with a tip curvature. After the growth stopped, they broke up into small bubbles and contracted with damped oscillation. From the spatial distribution of the field strength between the point and plane calculated by the finite element method, it was found that there exists a specific distance from the tip where the field strength is equal under any tip radii used in this study for both polarities. This distance and field strength may considered to be the minimum length and field for an electron avalanche in liquid     

The impulse breakdown trajectory in air between rod and planeelectrodes

Image processing was used to analyze video recordings of electrical breakdown paths in a rod/plane electrode air gap. The number of segments in the breakdown path was found to vary as the applied over-voltage was changed. The results are discussed in terms of a prebreakdown space charge model     

Studying branching of a cathode-directed streamer in air by means of 3D modeling

A cathode-directed streamer in air is modeled using the hypothesis that branching is initiated by large electron avalanches that develop in a strong electric field in the front of streamer head. A series of streamer discharge simulations are carried out using a three-dimensional numerical model. The possibility of streamer branching is shown as a result of the interaction with two electron avalanches that arise in front of its head. They are directed to the sides from streamer propagation direction. Such a mechanism of branching is brought about by the fact, that at the moment of contact with the streamer, the avalanches have already undergone an avalanche–streamer transition or are close to it. The equality of the number of electrons in these avalanches at the moment of contact is not important. If this equality is violated, branching is asymmetric. If one of the avalanches interacting with the streamer is far from the avalanche–streamer transition, it does not trigger perfect branching, but it may produce an underdeveloped branch that does not evolve further. The streamer trajectory then deviates toward a larger avalanche, interacting with it.     

Estimation of dc breakdown mechanisms in air gaps containingfloating metallic particles

The characteristics of breakdown voltages against the position of floating metallic particles in atmospheric air are investigated experimentally. The breakdown mechanism is estimated on the basis of electric field calculation and visual observation of discharges at different voltages and a set of flow charts for calculation of breakdown voltage is proposed. The studied gaps are needle-to-plane and sphere-to-sphere electrodes of 120 mm in length forming Poissonian and Laplacian fields respectively and the shape of tested floating objects is a needle or sphere. The theoretical breakdown voltages obtained by the flow charts agree reasonably with the experimental ones     

Electrical breakdown and space-charge effect in polyphenylene sulfide films

Electrical breakdown of polyphenylene sulfide (PPS) films, one of the high-temperature engineering plastics, has been investigated. The results show that the dc breakdown strength is lower than the impulse one, and that the difference in electric strength between dc and impulse becomes wider as the sample thickness decreases. The result suggests space-charge accumulation when dc voltage is applied. Further, the impulse breakdown strength in the presence of a dc prestressing voltage is measured in order to elucidate the space-charge effect. The data on the contribution of the prestress voltage to electrical breakdown are used to estimate the amount of space charge accumulated in the specimen during the dc prestressing.     

机加工形成的导体表面粗糙度对SF_6气体击穿影响的研究

本文研究了机加工形成的电极表面粗糙度对SF_6气体绝缘性能的影响;提出了对SF_6气体绝缘电器中导体表面加工要求的看法。文章还将作者的试硷结果与Pedersen模型的理论曲线进行了比较。