Verification of positive streamer mechanism for negative corona Trichel pulses in O2/H2 mixtures

Current waveforms of the first negative corona pulses have been measured in O₂ + H₂ mixtures at a pressure range from 27 kPa to 50 kPa. It was observed that the hydrogen admixtures less than 4% do not change significantly the pulse current waveforms. Effects of changing cathode secondary electron emission were studied using a copper cathode coated by CuI and graphite. The results obtained support the theory of the cathode-directed streamer formation during the negative corona pulse rise.     

Impulse breakdown characteristics of the plane-to-plane electrode system with a needle-shaped protrusion in SF6

This paper presents the impulse pre-breakdown and breakdown characteristics of the plane-to-plane electrode system with a needle-shaped protrusion in SF₆ gas. The breakdown voltage–time (V–t) characteristics and the breakdown voltage–gas pressure (V–p) characteristics of a highly non-uniform SF₆ gas gap under positive and negative lightning impulse voltages are investigated in the pressure range between 0.1 and 0.5 MPa. The pre-breakdown developments are examined by the corona current and light emission measurements with high time resolution. As a result, the dielectric strengths versus time-to-breakdown of SF₆ gas gap under positive lightning impulse voltages were nearly independent of the gas pressure. The first streamer corona was initiated at the tip of the needle electrode, and the streamer corona pulses developed with a stepwise propagation. The discharge paths were zigzag, and the branches of the discharge channel for positive polarity were created. On the other hand, the leader channel in the negative polarity was thicker and brighter than that in the positive polarity.     

Comparison of the first negative corona current pulses in N2 + SF6 and CO2 + SF6 mixtures

The current waveforms of the first negative corona pulses in a small point-to-plane gap have been measured with a nanosecond time resolution in N₂ + SF₆ and CO₂ + SF₆ mixtures at a pressure 50 kPa for various contents of SF₆ as a function of applied gap voltages. The physical mechanism for the pulses in these mixtures with low concentration of SF₆ has been described using the streamer-based theory. The influence of changing admixtures of SF₆ in N₂ and CO₂ has been compared. Differences in the pulse waveforms observed in N₂- and CO₂-based gas mixtures are explained by differences in the first and second Townsend ionization coefficients. This work was supported by the Grant Agency VEGA from the Ministry of Education of Slovak Republic under contracts 1/1011/04 and 1/2017/05.     

Features of gas discharge in narrow-gap micropattern gas detectors at a high level of alpha-particle background

Preliminary results of research into electron multiplication in micropattern gas detectors (MPGDs) at a high level of alpha-particle background are presented. This work has expanded the borders of understanding of the streamer mode nature. It is seen as a complex of electrostatic and electromagnetic interactions which begin the appearance of the precursor in plasma state. In an inter-electrode gap, plasma oscillations occur which are accompanied by longitudinal elastic waves of ionization, which can reach the cathode surface with an induced negative charge. With the release of this charge, due to the previously established conducting channel, there is a strong pulse accompanied by emission due to the recombination of positive and negative ions and a thin cord or streamer. With the aim of protecting the MPGDs from spark breakdown at a high-level alpha-particle background, the following gas composition of buffer, cooling, and electronegative components is offered: 70% He + 28% CF₄ + 2% SF₆.     

The characteristics of a negative corona discharge in He/Xe/SF6 mixture

The results of investigation of the electrical and optical characteristics of a negative corona discharge in the electrode system of the type needle-grid lit in a He/Xe/SF₆ mixture under the pressure of 100–300 kPa are presented in this paper. The given medium is working for a pulse-periodic excimer lamp on = 351 nm XeF(B–X). Distributed on the length of a lamp active media a bipolar corona discharge is intended for using in the electric circulation of the XeF-lamp working medium. It is shown that the given discharge is ignited steadily in the He/Xe/SF₆ mixture, containing 0, 5kPa SF₆ molecules and is characterised by a low power consumption; the current of the discharge is limited to the development of instability by increasing the voltage on the needles; in hot zones of corona discharge SF₆ molecules cause the effective population 6s of XeI state.     

The first Trichel pulse of negative corona discharge in N2 with a small admixture of SF6

Current growth waveforms of transient negative point-to-plane discharge in N₂ and in N₂ with 0·025-0·1% admixture of SF₆ at a pressure of 40 kPa have been measured and compared. The transient glow discharge regime in pure N₂ was found to be preceded by a peaked current signal of conspicuous similarity to the first Trichel pulse rise and its initial decay in N₂+SF₆ mixture. The results are in a basic agreement with a hypothesis on streamer mechanism for Trichel pulse and indicate occurrence of field emission during the rise of Trichel pulse current.     

The velocity of streamer tips in impulse point-to-plane corona in air,using Lichtenberg figure techniques

Positive and negative streamer lengths were measured in point-to-plane impulse corona in air at atmospheric pressure using Lichtenberg figure technique. Applying short time pulses from 9 to 40 nsec duration time-distance plots were obtained, which allowed one to determine the streamer tip velocity. For a 4 cm gap, a 1 mm diameter point, and a 30 kV pulse an average tip velocity of 1.8×10⁸ cm/sec was found. Comparing these data withHudson's photomultiplier measurement, we may identify the Lichtenberg figure with the “primary” streamer ofHudson. The negative streamer tip velocity in the cathode part of the gap was approximately an order of magnitude smaller than the positive streamer velocity.     

β-SiC薄膜在SF6和SF6+O2中的等离子体刻蚀研究

以SF₆和SF₆+O₂为刻蚀气体,采用等离子体刻蚀工艺成功地对化学气相淀积工艺制备的β-SiC单晶薄膜进行了有效的刻蚀去除.实验指出当气体混合比约为40%时,刻蚀速率达到最大值.俄歇能谱分析表明,在SF₆和SF₆+O₂气体中被刻蚀后的样品没有形成富C表面的SiC层.研究结果为各种SiC器件的研制奠定了必要的实验基础. 关键词：     

Abnormal polarity effect in nanosecond-pulse breakdown of SF6 and nitrogen

The breakdown of gas gaps in an inhomogeneous electric field at subnanosecond and nanosecond voltage pulse rise times are studied, and the famous polarity effect in point-to-plane gaps is investigated. It is shown that at a voltage pulse rise time of ∼0.5 ns, the inversion of polarity effect takes place not only in electronegative gases such as SF₆, but also occurs in electropositive nitrogen. The inversion of polarity effect is related to a delay of electron emission from the plane cathode on arrival of the ionization wave front anode to the cathode. It is found that with a voltage pulse rise time of ∼0.5 ns, the inversion of polarity effect occurs at SF₆ and SF₆–N₂ pressures of 0.25 MPa and lower, and with a voltage pulse rise time of 15 ns, at a SF₆ pressure lower than 0.12 MPa.     

The production of excimer molecules and excited atoms in a positive corona discharge in He/Xe(Kr)/SF6/CCl4 mixtures

Electrical and optical characteristics of a positive corona discharge in He/Xe(Kr)/SF₆/CCl₄ mixtures, which are of interest for the use in multiwavelength excimer radiation sources, are studied in the needle-grid electrode configuration. The length of the discharge, which is usually used to pump repetitive high-pressure multiwavelength radiation sources, is equal to the length of the electrodes of an excimer laser or lamp pumped by a transverse electric discharge. The discharge current-voltage and frequency characteristics, panoramic emission spectra, and the dependences of the relative emission intensity from the halogenides and excited noble gas atoms on the corona discharge current are investigated. The main processes resulting in the production of halogenides, as well as xenon and krypton excited atoms, in the generation regions of a corona discharge are studied.     

Vorstadium des Funkens,untersucht mit dem Bildverstärker

Streamer development (near cathode, midgap, and near anode streamers) was investigated by means of image intensification and image deflection as to conditions for streamer onset and streamer velocities during the various stages of the avalanche-to-plasma channel transition in several gases, e.g. N₂, H₂, A, CH₄ as well as air, and some more mixtures of gases, in the pressure range 100 to 500 Torr. Quantitative results were obtained, and the influence of the discharge parameters upon streamer development is discussed. — The enhanced and accelerated streamer stage (II) was now observed in either direction; the significantly fast and steep ionization waves (stage III) were found to start at the cathode as well as at the anode on arrival of the respective streamers. Velocities up to 10⁹ cm/sec were measured.     

Dielectric properties of SF6 mixtures containing oxygen and other gases

Conclusion This paper has explored the influence of an oxygen addition on the dielectric strength of the SF₆ by numerically solving the Boltzmann equation with given cross section data. The linear variation of the dielectric strength with a mole fraction of oxygen was obtained. Similar calculations were performed for SF₆ + air for which only approximative estimates had existed in the literature. To check the results and for other practical reasons the method of the Boltzmann equation was applied to the SF₆ + He and SF₆ + N₂ mixtures, the dielectric strength of which had already been explored by other authors. In comparison with other mixtures currently studied the SF₆ + O₂ mixture reveals rather poor dielectric properties. In spite of the fact that the oxygen is an attaching gas, its addition to SF₆ and its mixtures spoils the dielectric strength. This numerical analysis exactly qualified this behaviour and it is in agreement with the few available experiments.     

Some Aspects of Current Interruption Physics in High Voltage Circuit Breakers

Recent results of experimental and theoretical investigations on current interruption processes are presented with a focus on SF₆ high voltage circuit breakers. Various aspects of thermal interruption at the zero crossing of the current are shown, including the scatter and the distribution of arc voltage shortly before CZ and the role of turbulence. The thermal interruption capability of air and CO₂ are compared to that of SF₆. Investigations on the dielectric recovery are shown for SF₆ and CO₂. The breakdown voltage during the dielectric recovery can be described by simple streamer and leader inception models. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stepped form of negative corona current pulses in hydrogen

Current wave forms of initial stages of discharge formation in a short negative point-to-plane gap have been measured with a nanosecond time resolution in hydrogen at pressures (12.5–76) kPa and for various overvoltages and cathode point radii. The measurements revealed the existence of a stepped form of negative corona current pulses in hydrogen. To test existing models for negative corona pulse formation, effects of changing cathode secondary electron emission were studied using copper and brass cathodes coated by CuI and graphite. It is concluded that a negative corona pulse is associated with the ignition of a cathode-directed streamer in the vicinity of the cathode. We report what we believe are the first experimental observations of non-Trichel oscillations of negative corona current with a frequency of (1–10) MHz. This work was supported by the Scientific Grant Agency of the Ministry of Education and Academy of Sciences of Slovak Republic (Project No. 1/5190/98).     

Importance of pre-ionisation for the non-chain discharge-pumped HF laser

The importance of pre-ionisation for the non-chain discharge-pumped HF laser is studied through experiments on an X-ray photo-triggered laser using mixtures of Ne, SF₆, and ethane. The discharge dynamic in Ne/SF₆ mixtures or pure SF₆, as well as the stabilisation effect induced by C₂H₆ and consequences for the laser performance, are investigated for pre-ionisation electron density values, n_eo, ranging from 10⁶ cm^-3 up to 10⁹ cm^-3, as well as for the so-called discharge self-breakdown mode. Without ethane, the minimum n_eo value which is needed to complete 100% homogeneous charge deposition in the plasma is a very sharply increasing function of the SF₆ pressure. This hinders performance optimisation when the molecule used to react with F-atoms, for instance H₂, has no effect on the discharge dynamic. The minimum ethane partial pressure that is needed to stabilise the discharge depends on n_eo, the pumping pulse duration, the deposited electric charge, and the SF₆ pressure. Discharges in Ne/SF₆ can be much more efficiently stabilised by addition of a small amount of ethane than by an increase of n_eo. A pre-ionisation density as low as 10⁶ cm^-3 is sufficient to achieve the maximum laser energy value, but total suppression of the pre-ionisation has a detrimental effect on the active medium homogeneity. Received: 30 May 2000 / Revised version: 9 October 2000 / Published online: 9 February 2001     

Current analysis of DC negative corona discharge in a wire-cylinder configuration at high ambient temperatures

To study the characteristics of DC negative corona discharge in a wire-cylinder configuration at an ambient temperature range of 350–850 °C, the I–V characteristics and the current composition are analyzed under different conditions. A simple method is proposed to determine the DC corona onset threshold voltage. At high ambient temperatures, in the DC negative corona discharge gap, some electrons are not attached to the electronegative gas molecules and move to the anode tube. Thus, these electrons form an electron current, which may account for most of the total discharging current. The ratio of the electron current to the total discharging current increases with increasing temperature. In a mixture of O₂ and N₂ and a mixture of CO₂ and N₂, the ratio of electron current increases with increasing N₂ content in the mixtures. The cathode material has little influence on the corona discharge characteristics at high ambient temperatures.     

Photon echo and stimulated photon echo study of various collisional-relaxation channels

Collisional relaxation in SF₆ gas and its mixtures with He and Xe is studied by photon echo and stimulated photon echo methods from the standpoint of the possibility of identifying the contributions of different types of collisions. The nonexponential nature of the kinetic curve of the photon echo is clearly observed for pure SF₆, it is weaker in the mixture SF₆+Xe, and it is virtually completely absent for high degrees of dilution of SF₆ with helium. These features can be explained on the basis of estimates, made from experimental data, of the critical delay between the exciting pulses (for which the nonexponential behavior should be most strongly manifested). In pure SF₆ it is possible to distinguish the contribution of the inelastic channel (rotational relaxation) and the contribution of weak collisions. To distinguish successfully the relaxation channels in mixtures with buffer gases a heavier buffer gas and a much better time resolution must be used. It is shown that data obtained on the orientation and alignment relaxation rates by the stimulated photon echo method can serve as an upper limit for the rates of inelastic processes which cannot be measured by the photon echo method. The combined use of photon echo and stimulated photon echo methods made it possible to obtain data on the cross sections for elastic and inelastic scattering of the collisional pairs SF₆–SF₆, SF₆–Xe, and SF₆–He. Zh. éksp. Teor. Fiz. 116, 47–56 (July 1999)     

A KrF fast discharge laser in mixtures containing NF3, N2F4 or SF6

A fast discharge KrF laser system (λ = 248.5 nm) has been operated at 25 mJ/pulse, 3.0 MW peak power in high pressure He: Kr: fluoride mixtures containing low concentrations of both krypton and the fluorine donors N₂F₄, NF₃ and SF₆. Lasing action is reported for the first time in N₂F₄ and SF₆ with optimum energy output at 750 and 160 mJ/l respectively.     

Decomposition of NO2 in Oxygen-free NO2: N2 Gas Mixture by Back-Corona Generated Plasma

The back-corona discharge has been successfully applied as a plasma source for decomposition of NO₂ in the oxygen-free gas mixture of N₂:NO₂. The paper reports a first attempt to use back-corona discharge for noxious gas conversion. The preliminary results of laboratory experiments in a needle-to-plate reactor show that the De-NO_x processes in back-corona discharge are similar to the dc streamer corona discharges generated in the same geometry. Both types of discharges convert NO₂ to nitrogen, oxygen and also to N₂O and NO. However, back-corona discharge produces less NO, and is more efficient energetically in NO_x decomposition than the dc streamer corona discharge.