Fractal dimension of long electrical discharges

The fractal dimension of 500 mm long electrical discharges is presented by analyzing a set of photographic images. Three popular fractal dimension estimation techniques, box counting, sandbox and correlation function methods were used to estimate the fractal dimension of the discharge channels. To remove the apparent thickness due to varying magnitudes of current in the discharge channels, edge detection algorithms were utilized. The estimated fractal dimensions for box counting, sandbox and correlation function for long laboratory sparks were 1.20 ± 0.06, 1.66 ± 0.05 and 1.52 ± 0.12 respectively. Within statistical uncertainties, the estimated fractal dimensions of positive and negative polarities agreed very well.     

Features of electrical discharges in air triggered by laser

A numerical study of laser-triggered discharges in air at atmospheric pressure is presented for an ultraviolet laser in small gaps. Two models, one for the ionization of the air by the laser pulse and the second for the streamer evolution have been computed. From results of numerical simulations the influence of the laser parameters such as energy, pulse duration and beam radius is analyzed and electron distributions are obtained for different small gaps. Electric field, streamer velocity and evolution of the ionized volume are calculated by means of streamer simulations. This paper shows the main features of the laser-triggered discharges and also the importance of using numerical simulations in a laser-triggered experiment.     

Macroscopic models of internal dynamics of electrical discharges

The existence of thresholds for electrical discharge onset suggests a functional relation between macroscopic resistivity and current. At low current, the resistivity should be inversely proportional to the magnitude of the current. Macroscopic models which employ this scaling predict many empirically observed properties of transient electrical discharges such as: (i) thresholds for the onset of current, (ii) the abrupt termination of current in active regions of a current channel, (iii) current restart in passive regions of current channels, (iv) leaders, and (v) residual charge, both in channels and at sources when current terminates. An overview of research with these models is presented and examples are used to illustrate the results that have been obtained. These models are shown to predict current channel formation and describe results of efforts to benchmark theory with experimental data     

Production of ball-lightning-like luminous balls by electrical discharges in silicon

We performed electric arc discharges in pure Si to generate luminous balls with lifetime in the order of seconds and several properties usually reported for natural ball lightning. This simple experiment does not rely on energy sources and excitation mechanisms that are improbable in the natural phenomenon and clearly demonstrates the role of vaporization and oxidation of Si, as proposed by the Abrahamson-Dinniss theory for ball-lightning formation.     

Nanosecond electrical discharges between semiconducting sulfide mineral particles in water

A model for developing electric discharges between sulfide mineral (pyrite) particles under high-voltage nanosecond pulses in a liquid medium (water) is considered. A possibility of electrical breakdowns of liquid gaps between particles under nanosecond pulses is shown. This probability and the energy released in discharge channels depend strongly on the sulfide conductivity.     

Optical fiber sensor-based detection of partial discharges in power transformers

In this paper, a fiber optic acoustic sensor system is designed and tested for on-line detection of the partial discharges inside high voltage power transformers. The fiber optic sensor uses a silica diaphragm and a single mode optical fiber encapsulated in a silica glass tube to form an extrinsic Fabry–Perot interferometer. Test results indicate that the developed fiber optic sensors are capable of detecting the acoustic signals propagating inside the transformer oil with high resolution and high frequency.     

Leader channel models for long air positive electrical discharges

The models proposed for the positive long air gap electrical discharge can be considered to be either engineering or physical in their approach. In this work, we make a general review of the available models and use two of them for a comparison with experimental data. Common underlying assumptions were found in most of the models analyzed. The comparison with the experimental data revealed that the results obtained from the models were a good representation of the physical situation when the leader potential distribution and the leader-corona region evolution were described with certain physical assumptions.     

Parametric study of Xe2 * dimer in high-pressure electrical discharges

Optical and electrical properties of a high-pressure discharge of pure xenon and xenon-helium and xenon-neon mixtures are studied experimentally. Uniform discharge at total gas pressure as high as 10 atm is achieved in xenon-lean mixtures. Vacuum ultraviolet emissions due to the first and second continua of Xe₂ ^* are examined spectroscopically. The vibrational relaxation rate constant k_He of Xe₂ ^* by helium is determined to be of the order of 10^-11 cm³ s^-1 from spectroscopic data. Laser oscillation for the 172-nm band was attempted without success. Discharge instability at high gas pressures is considered to the cause of the unsuccessful laser experiments. Some issues related to discharge instability in a high-pressure rare-gas discharge are discussed. Received: 1 October 2001 / Revised version: 11 June 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +852-2603/5204, E-mail: dlo@phy.mhk.edu.hk     

Light emission during cathode sheath formation in preionizedhigh-pressure glow discharges

The cathode sheath formation in a transient spatially small X-ray preionized high-pressure glow discharge with parameters typical for neon buffered XeCl lasers is investigated experimentally. Pictures of the light emission are taken with high temporal (≈1 ns) and spatial (≈25 μm) resolution using a gated CCD-camera as well as a streak camera. Quantitative data on the spatial and temporal development of the sheath structure are given. The most prominent feature is an extremely fast formation of a thin and very bright cathode sheath at a time which could be time correlated with an accuracy of ±0.2 ns with the discharge current. The results can be used to check numerical model calculations of the sheath formation     

Some objects formed in the interaction of electrical discharges with metals and polymers

An experimental approach is developed for producing autonomous objects reminiscent of ball lightning by the charging of an activated base made of materials such as polymers and metals with an energetic metastable plasma. Methods of obtaining these autonomous objects and some of their properties are described. It is found that as the energy-deposition parameters are increased, these objects increasingly resemble ball lightning. An attempt is made to indicate the specific forms of the charging and to give a brief phenomenological treatment of the processes responsible for the observed effects. Zh. Tekh. Fiz. 67, 19–28 (March 1997)     

Investigation of the interaction between electrical discharges and low resistivity silicon substrates

In this work the impact of single discharge pulses in air on single-crystalline, p-type silicon having a low bulk resistivity of 0.009-0.012 Ω cm is investigated. Compared to platinum specimens, the craters in silicon have lateral dimensions which are about one order of magnitude larger despite comparable values for the melting point and the melting energy. This finding is attributed to the substantially higher bulk resistivity of silicon leading a higher energy input into the substrate when spark loaded. The energy generated by joule heating is, however, distributed across a larger area due to a current spreading effect. To study the impact of different surface properties on the sparking behaviour, the crater formation on the silicon substrate is investigated applying coatings with different material properties, such as sputter-deposited aluminium layers and thermally-grown silicon dioxide. In general, the crater characteristics formed on unmodified silicon is not influenced when a thin aluminium layer of 24 nm is deposited. At higher film thickness above 170 nm, the sparking energy is almost completely absorbed in the top layer with low influence on the underlying silicon substrate. In the case of a dielectric top layer with a thickness of 155 nm, the formation of many small distinct craters is supported in contrast to a 500 nm-thick SiO₂ film layer where the generation of a single crater with a large area is energetically favoured. A surface roughness of several nm on the silicon probes has no measurable effect on crater formation when compared to an original surface characteristic with values in the sub-nm range.     

Gas formation in liquid hydrocarbon dielectrics under the action of particle discharges

A study is performed of destruction of the hydrocarbon liquid n-nonane under the action of particle discharges in a test cell with needle-plane electrode system. The process of gas formation is studied, and the products of electrical aging are identified.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 24–27, April, 1986.     

The influences of gas and electron temperatures on electron attachment in gas electrical discharges

Various electron attachment processes are reviewed, emphasising the way in which the rates and products of some selected reactions vary with the attaching gas temperatureT _g, the temperature,T _e, and the energy of the attaching electrons. The examples illustrating the variety of reactions are the efficient dissociative attachment reaction to CCl₄, attachment to SF₆ which involves both dissociative and non-dissociative attachment, attachment to CHCl₃ which requires activation energy, and attachment to CCl₃Br which results in both Cl- and Br- product ions. A model has been presented which is able to quantitatively explain the difference influences ofT _g andT _e on the rates of some of these reactions. Also described are the unusually efficient attachment properties of the fullerene molecules C₆₀ and C₇₀ as revealed by our FALP experiments, noting that these molecules have potential importance as efficient suppressers of electrical breakdown through gases such as those used to insulate high voltage devices. We emphasise throughout this paper the importance of an understanding of the separate influences of gas and electron temperature on attachment reactions for the modelling of practical gas discharge media such as etchant plasmas. We dedicate this paper to Professor Jan Janča on the occasion of his sixtieth birthday in recognition of his major contributions to gas discharge physics.     

Dynamic modeling of stepwise internal transport barrier formation in DIII-D negative-central-shear discharges

The GLF23 transport model is used to dynamically follow bifurcations in the energy and toroidal momentum confinement in DIII-D discharges with an internal transport barrier. The temperatures and toroidal velocity profiles are evolved while self-consistently computing the effects of E x B shear stabilization during the formation and expansion of internal transport barriers. The barrier is predicted to form in a stepwise fashion through a series of sudden jumps in the core-electron and ion temperatures and toroidal rotation velocity. These results are consistent with experimental observations. In the simulations, the step transitions are a direct result of local E x B driven transport bifurcations.     

碳纤维复合材料用于光学镜面

介绍了从 2 0世纪 80年代初开始 ,国际上将碳纤维复合材料用于光学镜面中的现有的技术水平及国内的现状 ,提出了碳纤维复合材料用于光学镜面将有可能达到的精度以及所开展的工作。     

Spectroscopic measurements and electrical diagnostics of microhollow cathode discharges in argon flow at atmospheric pressure

This paper is dedicated to the study of the electrical and optical characteristics of direct current microhollow cathode discharges (MHCD) in argon flow. Experiments have been carried out in order to determine the so-called Paschen’s curves in a static open MHCD. Current-voltage characteristics were obtained as a function of the pressure and hole diameter. MHCD enable stable direct current discharge operation, which could be ignited for pressures ranging from 12 to 800 Torr, in a very wide range of current densities and electrodes materials. Optical emission spectroscopy and analysis of the spectral line broadening of plasma line emissions were performed in order to measure parameters such as electron number density (2–4 × 10¹⁴ cm^-3)^{-3}), gas temperature (460–640 K), excitation temperature (~ 7000 K) and electron temperature (~ 8500 K), for current ranging from 7 to 15 mA. Lower gas temperature was measured compared to the static MHCD ones.