Initiation Of Breakdown Research Articles

Properties of ZnO varistor blocks under multiple lightning impulse voltages

Abstract This paper presents the effect of multiple impulse voltages on the electrical and physical properties of a zinc oxide (ZnO) varistor. To investigate the dynamic characteristics of a ZnO varistor subjected to multiple impulse voltages, we designed and fabricated the multiple combination wave generator. The multiple combination wave generator can produce quintuple 1.2/50 μs impulse voltages of 100 kV and quintuple 8/20 μs impulse currents of 12 kA. Applying these voltages and currents, the electrical and physical properties of the ZnO varistor were evaluated. The multiple impulse failures of a ZnO varistor are mainly caused by the surface flashover. When multiple impulse voltages are injected, the charge enhancement near the joined edges of a ZnO varistor and metal electrodes plays a decisive role on the surface flashover or breakdown initiation and developments. Also, after the injection of the quintuple 8/20 μs impulse currents of 10 kA, the residual voltage and quiescent current of the ZnO varistor were changed markedly.

The HIV‐Tat protein induces chromosome number aberrations by affecting mitosis

To analyze the effects of the HIV-Tat-tubulin interaction, we microinjected HIV-Tat purified protein into Drosophila syncytial embryos. Following the Tat injection, altered timing of the cortical nuclear cycles was observed; specifically, the period between the nuclear envelope breakdown and anaphase initiation was lengthened as was the period between anaphase initiation and the formation of the next nuclear envelope. These two periods correspond to kinetochore alignment at metaphase and to mitosis exit, respectively. We also demonstrated that these two delays are the consequence of damage specifically induced by Tat on kinetochore alignment and on the timing of sister chromatid segregation at anaphase. Furthermore, we show that the expression of Tat in Drosophila larvae brain cells produces a significant percentage of polyploid and aneuploid cells. The results reported here indicate that Tat impairs the mitotic process and that Tat-tubulin interaction appears to be responsible for the observed defects. The presence of polyploid and aneuploid cells is consistent with a delay or arrest in the M phase of a substantial fraction of the cells expressing Tat, suggesting that mitotic spindle checkpoints are overridden following Tat expression.

Calculation of the Threshold Power of Optical Breakdown during Interaction between a Laser Pulse and Droplets of Dielectric Liquid

A mathematical model is developed of optical breakdown on droplets of dielectric liquid under the effect of pulsed laser radiation. The heating and evaporation of a particle, and the generation of a vapor halo and its ionization, as well as the propagation of shock waves in the space surrounding the particle, are treated as separate stages of the process. The threshold characteristics of the laser beam parameters are determined, which are sufficient for the initiation of optical breakdown on droplets of dielectric liquid. The dependence of the threshold power of optical breakdown on the size of laser spot is calculated.

Method and structure for local emission regulation and arc prevention in field emitter arrays

Field emission from air-exposed silicon and other surfaces typically occurs at energies up to several volts below the Fermi level, and the energies can change substantially during emission. We conclude this behavior results from emission from initial states in a dielectric (oxide) surface coating, and that the energies of these states change relative to the emitter Fermi level due to changing oxide state density and charge density. When emission occurs below the bulk Fermi level, energy is deposited into the emitter. In some cases this energy can be coupled to chemical bonds, creating states in the oxide and improving electron transport. If the power density deposited this way becomes excessive, oxide breakdown and arc initiation can result. Thus preventing excessive emission current at potentials more than 1–2V positive relative to the bulk Fermi level may prevent arc initiation. We propose that might be done by building a second (filter) aperture above each gate aperture in an array, and placing the aperture at a low positive voltage relative to the emitter contact. That structure creates a potential barrier for emitted electrons, which will cause those electrons emitted at excessively positive potentials to be returned to the gate electrode. A circuit element in series with the gate electrode will reduce the gate voltage, thereby limiting the current emitted at low energy. To limit or regulate the total emission current, a small resistor or other circuit element can be placed in series with the emitter, forcing the emission energy to be reduced in response to a given current.

Mechanism of degradation and breakdown in PET films under high intensity AC fields

The mechanism of electrical degradation and breakdown of polymeric insulating films is examined by this paper as a two-stage process. Electron injection by Fowler-Nordheim tunnelling, provides the energy for the creation of the first cavity near to the injection contact (the beginning of the formation of the so-called low-density region) where impact ionisation of molecules and hence electron avalanche, can occur. This stage is assisted by intrinsic tunnelling of electrons through local potential barriers. The second stage, the macroion bond scission and the creation of another macroion and a free radical is a thermofluctuational process which involves the action of the stretching force by the local electric field. From the other hand breakdown initiation starts when an electron following a Poole-Frenkel hopping mechanism, is accelerated in a hole with sufficiently large dimensions. The role of the applied electric field F a and its relation to the local electric field F l in both stages is examined. Experiments were executed for measuring the breakdown AC voltage V b and dielectric strength F b , with voltage rising rate 3 KV/s, in order to examine their relation with specimen thickness d and hence to derive a steady state breakdown criterion. Measurements of PET films life-time were also taken in order to examine the degradation mechanism and the polymer's lifetime.

Efficiency of Pulsed Electrode Conditioning in a Vacuum

It is shown that the efficiency of pulsed electrode conditioning in a vacuum, which is estimated from the state of the electrode surface, rises as conditioning pulses get shorter and may exceed the efficiency of conditioning by dc breakdowns by two orders of magnitude or more. A criterion with which one can judge the cathodic mechanism of breakdown initiation in the steady-state regime is suggested. Under optimal electrode processing conditions (pulse width tp 108 V/m), the ultimate dielectric strength corresponding to the cathodic mechanism of breakdown initiation in a vacuum can be reached.

Dynamics of formation of the pulsed electrical breakdown of highly overvoltaged gas gaps in the subnanosecond range

The mechanisms of the initiation of pulsed electrical breakdown of gas gaps in the subnanosecond range are of great interest for gas-discharge physics. Recently developed voltage-pulse generators make it possible to form high-voltage pulses with 200-ps-wide fronts and to apply voltages much higher than the static-breakdown voltage to a gas gap. Under these conditions, field emission from a cathode with the subsequent explosion of microinhomogeneities on its surface begins to make an increasing contribution to the initiation of breakdown [1]. Moreover, for high ratios of the electric field to pressure at the stage of breakdown delay and the initial stage of commutation, the energy gained by some free electrons per unit path can be higher than the energy lost in inelastic collisions. This regime is called the regime of the continuous acceleration of electrons, and these electrons are named fast or runaway electrons [1]. Fast electrons intensely ionize the gas medium in the interelectrode gap, and the breakdown formation time can decrease sharply in comparison to the classical streamer mechanism [2]. When fast electrons are decelerated on an anode, x rays are emitted [3‐6] and initiate secondary electrons and new avalanches. Since these processes are short-term and overlap in time, it is very difficult to analyze the dynamics of the initiation of subnanosecond gas breakdown. It is also worth noting that the regime of the continuous acceleration of electrons was analyzed primarily theoretically, because the corresponding experiments are complicated. The existence of fast electrons has been experimentally proved in only a few works [3‐6]. Experimental data related to the effect of continuously accelerated electrons on the E p

Diagnostics of Solitary Waves of Electric Breakdown in Gases

The characteristics of solitary waves of electric breakdown in gases are considered for the purpose of their use in experimental diagnostics. From an analysis of the experimental results on breakdown initiation by a long laser spark, it is concluded that these waves are the breathers of the sine-Gordon equation. The qualitative model for the description of other waves of electric breakdown is outlined on the basis of this conclusion. Further experiments are proposed for verification of this model.

Efficiency of Pulsed Regimes for Enhancing the Breakdown Strength of Vacuum Insulation

Processing the experimental data on breakdown delay time in a vacuum was used to obtain the function Kβ(tp) of the relative change in the field gain factor β at microscopic inhomogeneities of the cathode surface resulting from the realization of optimum pulse conditioning regimes. It is shown that over the range of pulse durations 10–8 ≤ tp ≤ 10–6 s, the relative change in the state of the cathode surface corresponds to the relative changes in the breakdown strength of all-metal electrodes and to the voltage of the appearance of local flashes in a system of evaporated electrodes—a microchannel plate and the screen of an image intensifier. Applying optimum regimes for conditioning the surface with pulses of durations tp < 10–8 s makes it possible to achieve the limiting breakdown strength determined by the cathode mechanism of breakdown initiation.

Effects of aerosols and laser cavity seeding on spectral and temporal stability of laser-induced plasmas: applications to LIBS

The spectral and temporal effects of laser cavity seeding and aerosol presence in the laser-induced plasma volume are investigated with the goal of examining factors affecting the precision of laser-induced breakdown spectroscopy. Improvements in the temporal stability of laser-induced breakdown initiation were observed with laser cavity seeding. However, laser cavity seeding produced no significant improvement in analyte precision for a range of gas-phase atomic emission lines, including nitrogen, hydrogen, and carbon. In contrast, greater shot-to-shot analyte precision, as measured by a nearly 60% reduction in relative standard deviation, was realized with the elimination of concomitant aerosols from the analyte sample stream. Such findings are consistent with the expected interactions of ambient aerosols and the laser-induced breakdown process.

Kinetics of ions and neutral active states in the afterglow and their influence on the memory effect in nitrogen at low pressures

The influence of positive ions and neutral active states on the memory effect in nitrogen-filled discharge tubes at 1.3 and 4.0 mbar pressures has been investigated. Analysing the thermal velocity of these particles as well as drift velocity of positive ions, it is shown that the positive ions have a dominant role in the breakdown initiation for afterglow periods lower than 30 ms, while the role of the neutral active states can be ignored because of their electrical neutrality. The estimated value of the positive ion recombination time in the afterglow for the pressures considered is about 30 ms. The atoms in ground states have a dominant role in breakdown initiation for afterglow periods higher than 30 ms. Our supposition is based on the earlier results that the long-lived Lewis–Rayleigh glow lasts up to several hours as a consequence of the recombination of these atoms on the wall. A detailed analysis of relaxation processes has shown that a contribution of vibrationally excited molecules as well as atomic and molecular metastable states created during the previous discharge cannot contribute to the memory effect significantly. Also, the contribution of cosmic rays to memory effect is analysed for the afterglow periods when the concentration of atoms in the ground state decreases to such a low value that their role in the secondary electron emission from the cathode is ignored.

Electronic conduction and breakdown in liquid helium and liquid neon

New data of the electronic properties of liquid helium and liquid neon are discussed with respect to their relevance for the initiation of electric breakdown. It is concluded that under ideal conditions the cathode plays a decisive role in the initiation of the breakdown process. It is speculated that a Townsend type breakdown mechanism is absent in the liquid phase of these elements.

Breakdown initiating mechanisms at electrode interfaces in liquids

Two hitherto neglected mechanisms, which occur at the interface between insulating liquids and metal electrodes under high electrical fields, are considered and shown to be significant for the initiation of breakdown. One involves the Lippmann effect in which the electrical fields of the double layers at the electrodes reduce the interfacial tension and lead to the generation of low-density microcavities on the electrode surfaces. The other is the Auger effect in which non-radiative recombination of electrons and positive holes across the large energy gap between these states leads to secondary electrons of high energy. The coupling between these two mechanisms is expected to be highly conducive to streamer initiation at the electrodes.

Dielectric strength of a vacuum interrupter contact gap after making current operations

Experimental results concerning the breakdown voltage and prebreakdown current of a vacuum interrupter contact gap, after making current operations, are presented. The dielectric strength of the contact gap is lowest for a switching sequence during which a making of high current is followed by a no-load contact opening. The breakdown voltage is much lower than after making operation, followed by interruption of a sufficiently high current. The basic process of breakdown initiation appears to be field electron emission from a specific structure on the contact surface, created by weld rupture.

Analytical approach to SF/sub 6/ breakdown under transient conditions

Under many circumstances, the initiation of breakdown can be viewed as conversion of energy stored in the electric field (capacitive energy) to thermal energy which converts matter in a thin channel from a nonconducting state to a conducting plasma. This paper analyzes this process in the context of breakdown in SF/sub 6/ as a prelude to future papers which treat the subject of breakdown under highly inhomogeneous thermal field, quasihomogeneous electric field conditions.

Oblique Shock‐Vortex Interaction

AbstractBreakdown of a slender vortex caused by an oblique (OSVI) shock is studied using numerical solutions of the Euler equations for unsteady three‐dimensional flow. A Burgers vortex with a given circulation and axial velocity distribution is prescribed at the inflow boundary. The calculations show that like in incompressible flows supersonic breakdown is primarily controlled by pressure forces. The shock is deformed into an ‘s’‐shaped part near the vortex core where the shock becomes normal. The results indicate that initiation of breakdown is more sensitive to variations in the axial velocity than in the circulation, and that the .ow structure is clearly time‐dependent.

Initiation of microwave-induced electrical breakdown of high-pressure gases

Measurements of microwave-induced electrical breakdown at 2.45 GHz in Ar, Kr, and Xe have been made in a tunable microwave cavity. The influence of UV illumination on gas breakdown at pressures up to 300 torr, and data on fiber initiator-induced breakdown above atmospheric pressure are presented. A marked decrease in the statistical spread in breakdown was observed with UV illumination, but the maximum pressures at which breakdown occurred with available microwave power, with and without UV illumination, were the same. To initiate breakdown in gases above 1 atm, a conducting fiber was used to enhance the applied macroscopic electric field at the fiber tip. Using 8-/spl mu/m diameter SiC fibers coated with 0.2-/spl mu/m-thick Pt, breakdown was obtained with pulses as short as 0.25 ms in the pressure range 1-3 atm for Ar, Kr, and Xe. The required microwave electric field at 2280 torr with one fiber was less than that at 200 torr without a fiber. The increase of the breakdown field with pressure was much slower than linear. The effect of fiber length, diameter, orientation, and conductivity, and of the number of fibers, on the required breakdown electric field, and fiber initiation in Cl/sub 2/ mixture gases, were also investigated.

Analytical Approach of SF6 Breakdown under Transient Conditions

Under many circumstances, the initiation of breakdown can be viewed as conversion of energy stored in the electric field (capacitive energy) to thermal energy which converts matter in a thin channel from a nonconducting state to a conducting plasma. This paper analyzes this process in the context of breakdown in SF6 as a prelude to a second paper that treats the subject of breakdown under highly inhomogeneous thermal field, quasi-homogeneous electric field conditions.

Stochastic model of breakdown initiation in dielectric liquids

An electric strength of perfluorodibutyl ether and transformer oil in small gaps between the hemispherical electrodes was investigated experimentally under AC voltage of linearly increasing amplitude. The macroscopic stochastic approach was proposed that describes breakdown initiation by certain function μ(E). This function is the probability density of streamer inception on a small element of electrode area in a short interval of time and depends on local electric field. This macroscopic approach allows one to obtain the analytical expressions for breakdown initiation probability on various parameters of experiment for flat, cylindrical and hemispherical electrodes under constant DC test-voltage, DC and AC ramp test-voltages. Several methods were developed to reconstruct the function μ(E) using experimental data on series of breakdown voltages for case of breakdowns in small gaps between electrodes. The values of function μ(E) for transformer oil and perfluorodibutyl ether were determined in the range of electric field from 0.3 to 0.9 MV cm−1. It was shown that perfluorodibutyl ether is a prospective dielectric liquid, especially for use in environmentally appropriate technologies. Stochastic computer simulations of breakdown inception were carried out. The series of breakdown voltages and pitting on surface of hemispherical electrode were obtained using stochastic model.

Visualization of late breakdown in vacuum by ICCD camera

Experimental observations of late breakdowns in a vacuum circuit breaker were made using an intensified charge coupled device camera (ICCD). These late restrikes can cause arc reignition and interruption failure. This type of failure is rare and can be initiated by a number of mechanisms. This paper illustrates the method used to observe this phenomenon. Some key image examples of the phenomenon occurring during breakdown initiation are shown.