Prebreakdown positive corona streamers and the streamer-cathode contact in hydrogen

Current waveforms induced by prebreakdown streamers have been measured in a short positive point-to-plane gap in hydrogen at pressures of (12.5–100) kPa over a wide range of gap voltage values. The current waveforms were measured using a small central cathode probe. The cathode probe signal corresponding to the streamer arrival to the probe is interpreted in terms of recent computer simulation models and consists of an initial sharp current spike due to displacement current followed, some 20 ns later, by a lower current hump due to the ion arrival at the cathode. Dedicated to Prof. Jan Janča on the occasion of his 60th birthday. This work was supported by the Scientific Grant Agency of the Slovak Ministry of Education and Academy of Sciences of Slovak Republic (Project No. 95/5195/136).     

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.     

Zur räumlichen Ausbreitung einer Entladung beim Generationen- und beim Kanalaufbau

The spatial expansion of a discharge was investigated under homogeneous field conditions, using an anode with a small central area separated from the remainder and wide band double beam oscillographic techniques. The discharge currents were initiated by electrons released along traces of single α-particles in the gas between the electrodes. The α-particles enter the gap volume through a small hole in the middle of the separated center of the anode. The measurements have shown that a secondary photon emission from the cathode causes a spreading over the total gap volume within the first few electron generations (N₂, air, CO₂ at lower pressures). If streamer mechanism occurs (O₂, CO₂ at higher pressures), the discharge remains localized to the trace of the α-particle, in which the avalanches yielded by the primary electrons have produced a high density of positive ions. — New details of streamer mechanism are revealed by analysing the rapid increase of the current during streamer formation.     

Über den zeitlichen Verlauf des Stromes einer Gasentladung in Wasserstoff

The transient growth of currents in a Townsend gas discharge system under uniform dc field conditions in Hydrogen is examined. The discharge is started by 10³ to 10⁶ electrons released from the cathode by an UV light pulse within some 10^?7 seconds. Observed oscillations of the current are found to be due to the motion of the electrons through the gap creating new electrons by photoelectron emission at the cathode due to photons generated in the gap. At sparking threshold conditions (Μ ₀=1) the electron current becomes self-sustaining after a few electron transit times. The positive ion current soon exceeds the electron current and grows linearly with time (Μ ₀=1) until the positive ions of the first generation enter the cathode. For times greater than a positive-ion transit time the current becomes self-sustaining. Neglecting space-charge effects one would not expect a spark to occur. The space-charge of the positive ions, however, causes a distortion of the field changing the ionization efficiency of the electrons. It is shown that an observed rapid growth of current leading to breakdown after some positive-ion transit times is in agreement to this conception.     

Streamer propagation mechanism on the basis of plasma oscillations

A mechanism explaining streamer propagation to the anode and to the cathode without photoionization of the gas is proposed. It is shown that the velocity of the anode streamer is determined by the electron mobility in the field of the end of the streamer. The accelerated electrons appearing at the anode end of the streamer excite longitudinal plasma oscillations in the streamer, whereupon streamer propagation to the cathode is explained. An explanation is given of the phenomenon of streamer self-propagation after the disappearance of the external field because of the stored energy of the plasma oscillations. The conditions for the transition of an avalanche into a streamer is defined as the condition for the appearance of accelerated electrons at the anode end of the avalanche.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 34–39, July, 1977.     

Untersuchungen über die Entwicklung des Kanaldurchschlags in Gasen

The temporal development of the current of a discharge leading to breakdown was investigated under static and homogeneous field conditions with wide band oscilloscopic techniques. The discharges were started by 10³ to 10⁵ electrons released within some 10^?9 sec along traces of single α-particles parallel to the electric field. Measurements have shown that streamer mechanism occurs in gases as CH₄ and CO₂ at static breakdown, if a sufficient density of space charge is produced by a great number of overlapping avalanches. The gas amplification of thesingle avalanche only has a value of about 10⁵. Also in O₂ and dry air at highpd-values breakdown develops with streamer mechanism. — In electronegative gases as freon 12 and O₂ a prolongation of the time necessary for streamer formation is caused by the reduction of the effective total space charge by the negative ions. The results of a detailed study of the time necessary for streamer development and the time constant of the increase of the current leading to breakdown confirm the model of streamer mechanism.     

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.     

Experimental Studies on Low-Pressure Plane-Parallel Hollow Cathode Discharges

Hollow cathode discharge (HCD) is widely used in material processing and plasma emission spectroscopy due to several advantages over other plasma sources. Basically, the HCD consists of a cathode with a hollow structure (cavity, hole, or parallel faces) and an anode of arbitrary shape. In this investigation, experimental studies on low-pressure plane-parallel HCD operated at different process conditions are reported. Herein, we investigate the dependence of the discharge current on the product of the gas pressure and inter-cathode distance (pD). In addition, the electron temperature and density were inferred from the current-voltage characteristics of a single cylindrical Langmuir probe positioned between the cathodes, on the discharge axis. The measurements were carried out at different gas pressures, magnetic field intensities, working gases, inter-cathode distances, cathode materials, and discharge voltages. The results showed that, at different gas pressures, the maximum discharge current (I_d,max) is not only a function of the product pD, but also of the pressure itself. Application of a uniform longitudinal magnetic field improved plasma confinement between cathodes, leading to a substantial increase in I_d,max in most of the situations considered in this study. However, for oxygen discharge, a strong discharge current reduction after the application of the magnetic field was observed. In relation to the Langmuir probe studies, it was observed that the uniform longitudinal magnetic field reduced the electron temperature, but this behavior depends strictly on pD. The typical values of electron density and electron temperature in the case of the nitrogen discharge were n_e?=?10¹⁷ m^?3 and T_e?=?2.5 eV, respectively. Finally, our experiments showed that the pD range for hollow cathode effects was between 0.2–5 Pa m.     

Surface condition and electron emission from cold cathodes in vacuum and in noble gas glow discharge

An optogalvanic method is used to measure photoemission coefficient γ_ph in a gas discharge exposed to the resonant radiation of helium atoms. The range of working current j/P _He ² (j is the current density, and P _He is the gas pressure) extends from 2 to 1000 μA/(cm² Torr), and field strength E/N at the cathode varies from 0.45 to 13 kTd. Up to j/P _He ² = 10 μA/(cm²Torr²), photoemission coefficient γ_ph grows and then tends toward saturation at a level of γ_ph = 0.30 ± 0.01. Under the no-discharge conditions, γ_ph = 0.35 ± 0.05. It is concluded that the emissivity of cold cathodes in a gas discharge is governed by adsorption of the working gas on the cathode surface and its implantation into the cathode. With allowance for this factor, the contribution of photoemission to the discharge current is reconsidered. It is shown that, for cathodes with diameter d _c ? l _c (l _c is the length of the cathode layer), a normal or weakly abnormal glow discharge in noble gases is largely of a photoelectron character. In light noble gases, the photoelectron character of the discharge persists even for a strongly abnormal discharge. The energy dependences of coefficients γ of kinetic and potential emission in helium are calculated with allowance for implantation of helium atoms into the cathode and compared with published data. The influence of particle implantation on γ in a vacuum is estimated.     

Negative Corona Current Pulses in SF6 – Air Mixtures

Current waveforms of first negative corona pulses have been measured in dry air + SF₆ mixtures at a pressure of 50 kPa and various overvoltages. Effects of changing cathode secondary electron emission were studied using a copper cathode coated by CuI and graphite. It is concluded that in the mixtures containing less than 10 % of SF₆ the negative corona pulse is associated with the formation of a cathode‐directed streamer in the immediate vicinity of the cathode. In the mixtures containing more than 20 % of SF₆ the streamer is quenched and, consequently, the discharge is governed by the Townsend ionisation mechanism fed by cathode photoemission processes.     

Removal of NOx from NO2∶NO∶N2 mixture by a pulsed and dc streamer corona in a needle-to-plate reactor

The results of laboratory experiments on reduction of NO _x in the oxygen free gas mixture NO₂NON₂ simulating exhaust gas, by means of pulsed and dc streamer corona discharges generated in a needle-to-plate reactor have been presented. The results show that the dc corona discharge is more efficient in De-NO _x process than the pulsed corona discharge. This is in contrast to the results obtained in the wire-to-cylinder reactors where the pulsed corona discharge removes NO _x more efficiently. The results also lead to the conclusion that in the dc streamer corona discharge the short pulses and long interelectrode distances are recommended in order to increase the NO _x conversion rate.Presented at 17^th Symposium Plasma Physics and Technology, Prague, June 13–16, 1995.This work was financially supported by the Polish Academy of Sciences (projects IMP 3.1 and 3.3) and by the Polish Committee for Scientific Research (KBN Grant No. P40103304).     

Probe Measurements of Parameters of Streamers of Nanosecond Frequency Crown Discharge

Investigations of the parameters of single streamers of nanosecond frequency corona discharge, creating a voluminous low-temperature plasma in extended coaxial electrode systems, are performed. Measurements of the parameters of streamers were made by an isolated probe situated on the outer grounded electrode. Streamers were generated under the action of voltage pulses with a front of 50–300 ns, duration of 100–600 ns, and amplitude up to 100 kV at the frequency of 50–1000 Hz. The pulse voltage, the total current of the corona, current per probe, and glow in the discharge gap were recorded in the experiments. It was established that, at these parameters of pulse voltage, streamers propagate at an average strength of the electric field of 4–10 kV/cm. Increasing the pulse amplitude leads to an increase in the number of streamers hitting the probe, an increase in the average charge of the head of a streamer, and, as a consequence, an increase in the total streamer current and the energy introduced into the gas. In the intervals up to 3 cm, streamer breakdown at an average field strength of 5–10 kV/cm is possible. In longer intervals, during the buildup of voltage after generation of the main pulse, RF breakdown is observed at Е_av ≈ 4 kV/cm.     

针-板DBD微流注与微辉光交替生成的机理研究

在介质阻挡放电体系中产生辉光放电可以有效的提高放电体系产生高能电子的性能, 为等离子体化学反应提供更加丰富的活性粒子.本文对针-板介质阻挡放电体系下的放电模式进行了研究,实验发现放电正负半周期表现出不同的放电模式, 激励电压为3 kV时放电正负半周期分别为微流注放电和电晕放电(或者Trichel脉冲放电),激励电压为6 kV时放电正负半周期分别为微流注放电和微辉光放电.微辉光放电形貌具有与典型辉光放电相同的分层次放电结构, 分析了激励电压6 kV时的放电过程,认为足够强的阴极电场强度和裸露针状电极形成的有效的二次电子发射过程是形成微辉光放电的主要因素,绝缘介质层的存在避免了微辉光放电向弧光放电过渡.     

To the role of positive ions in plasma nitridation

The method similar to that of determining the asymmetric componentf ₁ of electron distribution function was used to prove whether the nitrogen ions produced in discharge may be the very particles which directly influence magnesium nitridation in glow discharge. The amounts of created magnesium nitride on surfaces facing the anode and the cathode, and positive ion current to the plane double-probe were measured. It follows from experimental results that positive ions from bulk plasma are not decisive for nitride formation.The authors would like to thank Dr. V.Krejí and Dr. K.Maek for helpful discussions and critical notices.     

Formation stage of a high-voltage pulsed discharge in a long tube

A study was made of discharges in long tubes (d 60–132 cm) in hydrogen through an analysis of the oscillograms of the discharge current and voltage pulses and through a photometric study. As the pressure increases from 0.4 to 5 torr and as the overvoltage decreases from 290 to 90%, the step current I_st decreases, while the times t_d, t_st, and t_f, which characterize the formation stage, increase. It is thus possible that the step forms as a result of the conversion of the avalanche-streamer maximum at the beginning of the formation stage to a cathode-directed streamer. The value pd 300 torr · cm found under the experimental conditions of this study corresponds to a transitional discharge mechanism.Translated from Izvestiya VUZ. Fizika, No. 8, pp. 128–133, August, 1970.The authors thank A. Ya. Sytnik for assistance in the measurements.     

Plasma-volume generation of H<Superscript>−</Superscript> ions in a low-voltage xenon-hydrogen discharge. II

A low-voltage xenon-hydrogen discharge is considered theoretically at an interelectrode distance of L = 1 cm and cathode emission current densities of j _s = 2–20 A/cm². Basic parameters of the discharge plasma, in particular, the total hydrogen and xenon densities, are optimized to attain the maximum possible density of negative hydrogen ions \(N_{H^ - } (L)\) at the plasma-anode boundary. The distributions of the plasma parameters over the discharge gap are calculated for optimized regimes. According to calculations, at intermediate cathode emission current densities (j _s ≈ 5–10 A/cm²) in optimized discharge regimes, the density of negative hydrogen ions in the anode region of the plasma is \(N_{H^ - } (L)\) ≈ (1.5–2.5) × 10¹² cm^?3 and the total plasma pressure is p ₀ = 0.5–0.6 Torr.     

Kinetics of Formation of an Oxide Film on the Surface of the Cathode in an Atmospheric-Pressure Glow Discharge in Helium

The kinetics of formation of an oxide film on the surface of the cathode of an atmospheric-pressure glow discharge in helium has been investigated by the method of laser reflectometry. It has been established that the film is formed at a cathode temperature exceeding 550 K, and in 200 sec from the time of discharge initiation at a current of 1 A it becomes thicker than 1 m. The film consists of several concentric zones. For each of these zones, a change in time of the directional hemispherical coefficient of reflection at a wavelength of 0.63 m was determined. It is shown that the formation of an oxide film on the cathode surface leads to an increase in the cathode drop.     

Zur Ausbreitung der Entladung in selbstlöschenden Zählrohren. IV

Measurements with a Geiger counter with inserted grid between wire and cathode solved the problem whether the discharge process is built up by the photoeffect in the gas or at the cathode of the counter. For negative voltagesU _g of the grid (cathode grounded) only the volume inside of the grid is sensitive to discharges. It was found, that within the limits of error, the threshold did not depend on the masking degree of the cathode. From this it follows that mainly the photoeffect in the gas contributes to the discharge process. But the distributions of time of the discharge spread along the wire atU _g<0,U _g>0 and in a counter with a massive cathode has shown that also the photoeffect at the cathode contributes to the discharge process.     

Cathode Sheath Instability at Frequencies Near the Ion Plasma Frequency

We consider steady-state and nonstationary processes in a near-cathode region. Equations describing the plasma dynamics near a cathode at frequencies close to the ion plasma frequency are derived, and solutions of these equations for various zones of a discharge gap are found. A piecewise-uniform model of a cathode sheath is developed, which points out the possibility of an instability at a frequency slightly less than the near-cathode ion plasma frequency. The gas pressure effect on the instability threshold with respect to the discharge current is considered. The obtained results are in good agreement with the data of experimental studies of the cathode sheath in a hollow-cathode discharge.     

Messung der von Elektronenlawinen erzeugten Raumladungsfelder mittels einer Sondenlawine

The electrical fields due to the space charge of positive ions in a plane-parallel discharge gap are measured by means of an electron avalanche. The measurements were made in a N₂-CH₄ gas mixture (5% CH₄) at a total pressure of 210 Torr (20 °C) in a 3 cm gap, at a voltage UD (static breakdown potential). The positive ions are produced by electron avalanches, which are released from a broad area by an UV flash at the cathode. The ions so produced drift towards the cathode with a drift velocity v₊. After a timeT the probe avalanche is started by a second UV flash in the symmetry axis of the charge distribution. From its temporal growth, which is given by the relation\(i_s (t) \sim \delta v\_\exp \left( {\int\limits_0^t {\alpha v\_dt} } \right)\), one can evaluate the space charge field E(z) of the positive ions. The measured fields are in good agreement with those predicted by calculations.