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Gas Electronegativity Influence οn Electrical Breakdown Mechanisms

100%
Lončar B., Radosavljević R., Vujisić M., Stanković K., Osmokrović P.
Acta Physica Polonica A
|
2011
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vol. 119
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issue 3
364-368
EN
This paper discusses the role of different gas breakdown mechanisms depending on electronegativity of the gas, at low values of pressure and inter-electrode gap. Static (dc) electrical breakdown of electropositive, electronegative and noble gases has been investigated theoretically, experimentally and numerically. In the case when the electron mean free path is comparable with characteristic dimension of the electrode system (inter-electrode distance d), then the breakdown occurs through the Townsend mechanism. In the case when the electron mean free path is much shorter than the characteristic dimension of the electrode system, the breakdown is said to occur through the streamer mechanism. But, between the regions where the breakdown occurs solely by either the Townsend or the streamer mechanism, there is a region where breakdown occurs by the combination of these two mechanisms. The width of this region expressed by the quantity of pd product (product of pressure, p, and inter-electrode distance, d) depends on observed gas tendency to form negative or positive ions (electronegative and noble gases, respectively). The level of the anomalous Paschen effect expression is dependent on whether the observed gas is electronegative or noble. The combined mechanism effects of breakdown and anomalous Paschen effect are quantitatively determined and theoretically explained within this paper.
2
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Electrical Breakdown Mechanisms in Vacuum Diodes

100%
Osmokrović P., Ilić D., Stanković K., Vujisić M., Lončar B.
Acta Physica Polonica A
|
2010
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vol. 118
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issue 4
585-588
EN
This paper investigates the roles of different mechanisms constituting the process of electrical breakdown in vacuum. Random variable "vacuum breakdown voltage" was measured for pulse breakdowns in five types of vacuum diodes, with different values of vacuum pressure and inter-electrode gap. Conclusions about the influence of vacuum pressure and inter-electrode distance on the parameters of theoretical statistical distributions have been drawn. Based on the distribution to which the "vacuum breakdown voltage" random variable adheres, the dominance of certain breakdown mechanisms has been established for specific ranges of vacuum pressure. The observed absence of anode vacuum breakdown at small inter-electrode gaps has been given a theoretical interpretation.
3
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Synchronization Study and Introductory Experiment οf Laser-Triggered Surface Flashover in Voltage Pulse

63%
Pan R., Wang J., Sun Y., Sun G., Yan P., Zhang D.
Acta Physica Polonica A
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2009
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vol. 115
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issue 6
1167-1169
EN
With the aim of studying the characteristics of laser-triggered surface flashover in voltage pulse, synchronization problem of laser pulse and voltage pulse should be solved. A single/double harmonic, with wavelength λ of 1064/532 nm, Q-switched Nd:YAG laser is used to trigger the surface flashover. The synchronization problem is solved using a self-made digital delay/pulse generator. The delay time and jitter of Marx's trigger input and output, Marx's trigger output and Marx's output, laser input and output are respectively measured. Based on the result of the delay time and the timing sequence of the laser triggering system, the synchronization of laser pulse and voltage pulse is obtained through adjusting the channel delay time of digital delay/pulse generator. In addition, introductory experiment of laser-triggered surface flashover is carried out using the flat electrodes and columned insulators.
4
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Runaway of electrons in dense gases and mechanism of generation of high-power subnanosecond beams

51%
Tkachev A., Yakovlenko S.
Open Physics
|
2004
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vol. 2
|
issue 4
579-635
EN
New understanding of mechanism of the runaway electrons beam generation in gases is presented. It is shown that the Townsend mechanism of the avalanche electron multiplication is valid even for the strong electric fields when the electron ionization friction on gas may be neglected. A non-local criterion for a runaway electron generation is proposed. This criterion results in the universal two-valued dependence of critical voltage U cr on pd for a certain gas (p is a pressure, d is an interelectrode distance). This dependence subdivides a plane (U cr, pd) onto the area of the efficient electron multiplication and the area where the electrons leave the gas gap without multiplication. On the basis of this dependence analogs of Paschen’s curves are constructed, which contain an additional new upper branch. This brunch demarcates the area of discharge and the area of e-beam. The mechanism of the formation of the recently created atomospheric pressure subnanosecond e-beams is discussed. It is shown that the beam of the runaway electrons is formed at an instant when the plasma of the discharge gap approaches to the runaway electrons is formed at an instant when the plasma of the discharge gap approaches to the anode. In this case a basic pulse of the electron beam is formed according to the non-local criterion of the runaway electrons generation. The role of the discharge gap preionization by the fast electrons, emitted from the plasma non-uniformities on the cathode, as well as a propagation of an electron multiplication wave from cathode to anode in a dense gas are considered.
5
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Impact of Volume Breakdown on Surface Flashover in High Pressure SF_{6}

51%
Neuber A., Krile J., Rogers G., Krompholz H.
Acta Physica Polonica A
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2009
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vol. 115
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issue 6
995-997
EN
We investigate the pulsed flashover voltage of dielectric samples at up to 4 bar SF_{6} in the simultaneous presence of a high current (>10 kA, ∼20 microsecond pulse) volume discharge nearby. The chosen distance, ∼7 cm, between surface and volume breakdown is consistent with conditions found in the Sandia-Z-machine type rimfire switch. For a flashover gap distance of 24 mm and a simultaneous excitation within ∼ 5 microseconds, we observe an average reduction in the flashover voltage from 164 kV to 142 kV at 3.7 bar when the volume discharge is turned on. The test setup utilizing a magnetic switching scheme operating at 320 kV and 10 kA is briefly discussed along with the breakdown properties and the spectral characterization of the volume/surface flashover discharge plasma. In general, UV light propagates relatively unattenuated for wavelengths >160 nm in the high pressure SF_{6} from the volume discharge to the dielectric surface, setting up conditions which are conducive to photoelectron emission from the dielectric.
6
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Measuring Picosecond Flashover in Closing Coaxial Spark Gap Switch

51%
Klüss J., Hällström J.
Acta Physica Polonica A
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2009
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vol. 115
|
issue 6
1004-1006
EN
Flashover voltage and picosecond risetime of breakdown in pressurized SF_{6} gas is studied on a coaxial line. Gas pressure is varied from 1 to 19 bar. The homogeneous field gap has an opening from ca. 0.2 to 0.9 mm, and flashover voltages reach 120 kV. Measurements are performed using a D-dot probe with 16 to 20 GHz real-time oscilloscopes and a 50 GHz sampling oscilloscope. Measured risetimes are down to ca. 50 ps, and the dependence of voltage collapse on gas density and electric field between the electrodes is reported. Integrity of data is analyzed and experimental results are compared with present theory found in literature and previous results measured with alternative real-time systems.
7
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Electric Field Modelling in Gas-Insulated Substation for Analysis of Conditions for Partial Discharge Phenomena

51%
Florkowska B., Zydroń P., Jackowicz-Korczyński A.
Acta Physica Polonica A
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2015
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vol. 128
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issue 3
319-325
EN
The paper refers to diagnostics problems connected with the presence of partial discharges in gas-insulated substation. The basic stimulus for partial discharges generation is local, high value electric field, greater than the inception one. The numerical simulations of the electric field distribution in the part of the gas insulating system and in the vicinity of the metallic protrusion located on high voltage conductor are presented. Such defects are often present in SF₆ insulated constructions and are the cause of a local increase of the electric field. Theoretical analysis of physical mechanism of partial discharges initiation and development in the case of micro-needle type defects is described.
8
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On the Role of Radicals in Kinetics of Plasma Etchers in Ar/CF_4 Mixtures

51%
Nikitović Ž., Stojanović V., Petrović Z.
Acta Physica Polonica A
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2009
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vol. 115
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issue 4
765-767
EN
In this paper we present the transport coefficients in Ar/CF_4 mixtures with realistic abundances of CF_x radicals, F atoms, and F_2 molecules that are standard products of plasma chemistry in plasma etching systems and are present in large abundances of the order of few percent in realistic plasma etching devices. It was found that, although radicals make a minimum impact on distribution function and mean energy, the effect on drift velocity is significant and the effect on rates of attachment is large and may change the mode of operation of plasmas.
9
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On the mechanism of the runaway of electrons in a gas: The upper branch of the paschen curve

45%
Tkachev A., Yakovlenko S.
Open Physics
|
2004
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vol. 2
|
issue 1
132-146
EN
Basing on the simulation results, it is shown that the Townsend mechanism of electron multiplication in a gas at sufficiently large interelectrode distances is valid at least up to such large values of E/p at which relativistic electrons are generated. Correspondingly, the runaway electron producing in a gas is determined not by the local criteria accepted presently, but by the ratio of interelectrode distance and the characteristic electron multiplication length. It is shown that the critical discharge voltage U, at which the runaway electrons appear in a given gas, is a function of the product of the interelectrode distance by the gas pressure. This function (U-pd dependence) defines not only well-known Paschen curve but also an additional branch, which describes the absence of a self-sustained discharge at a high voltages sufficiently rapidly supplied across the electrodes. Critical discharge voltage dependence for helium and xenon are presented.
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