Direction of motion of striations in an inert gas-hydrogen mixture

Measurement of the direction of motion of striations in a He-H₂ and Ne-H₂ mixture proved the additivity of the mechanism of motion of striations in an inert gas and hydrogen. In a mixture of roughly 40% Ne and 60% H₂ at a pressure of 2 mm Hg the authors found a mixed wave of stratification with the phase velocity of the striations equal to zero. The striations produced in the immediate neighbourhood of the perturbation have the same direction of motion as in Ne but those at a greater distance are standing and at a still greater distance have the direction of motion as in H₂. The cause of the opposite direction of motion of the striations in hydrogen must be sought in the displacement of the ionization maximum towards the anode from the place of maximum concentration of the ions.Reported on at the 2nd Czechoslovak Conference of Electronics, Prague, April 1962.     

A study of the propagation of moving striations in inert gases by means of artificial feedback

A study is made of the propagation of moving striations in inert gases. The method elaborated in [6] combined with the optical method of photo-multipliers was used. The results of the measurements show that despite the fact that the striations themselves move from anode to cathode, their propagation takes place in the opposite direction. Measurements with artificial feedback further confirmed the conclusions reached in [1] and [2] on the mechanism of self-excitation of the striations by means of feedback over the external discharge circuit.In conclusion the authors thank Prof. V. Kunzl and M. Novák for valuable remarks and K. Jakoubková for help in the experiments.     

A theory of the successive production of moving striations in the plasma of inert gases

On the basis of results already published of experiments carried out by the method of excitation of transient processes with small perturbation, a theory is elaborated on the successive production of moving striations (waves of stratification) in an inert gas. The stratification of the plasma is interpreted as the successive production of regions with an alternately positive and negative space charge, i. e. as the producti n of a characteristic macroscopic periodic polarization of the plasma. The basic assumption of the theory is the relative independence of the chain of processes in each dark or light region of the striations, so that interaction between the regions occurs as a result of the electric field of the space charge in the neighbouring region.Equations are derived expressing the chronological order of the processes leading to the production of a space charge in each individual region and the interaction of the regions. The solution of the equations leads to functions, some of which are in agreement with experimental data while others cannot be verified on the basis of the experimental material at present at our disposal and require the carrying out of new experiments.The main results contained in this paper were reported on at the Conference on the Physics of Plasma in Leipzig on October 8th, 1956.     

氮气中空心阴极放电条纹的发光特性

利用发射光谱法,在氮气环境下研究了圆柱型空心阴极放电条纹的特性。测量得到了气压为20 Pa,放电电流为1.3 mA时条纹区的发射光谱,结果表明发射光谱主要为氮分子的第一正带系(B3П_g →A3П_u)和 第二正带系(C3П_u→B3П_g )。利用双原子光谱发射理论,计算得到了氮分子振动温度的空间分布特性。结果表明光谱线强度呈周期性分布,明纹中心处的谱线强度高于暗纹中心处的谱线强度。明纹中心处的N₂分子振动温度为3 500～4 400 K,并且从阴极到阳极,明纹中心处光谱线强度和分子振动温度逐渐下降。同时测量得到了放电电流为1.0和1.5 mA时的发光条纹特性,研究了放电电流对条纹特性的影响。随着放电电流的增加,明纹中心处的分子振动温度升高,条纹间距增加。另外,利用测量得到的发光条纹,计算得到了条纹区的平均约化电场强度为44～49 m-1·Pa-1,并且由阴极向阳极逐渐降低。对于揭示气体放电中发光条纹的形成机理和促进空心阴极放电的稳定性有重要的参考价值。     

空心阴极放电条纹特性的光谱诊断

利用发射光谱法,研究了圆柱型空心阴极放电条纹的特性.测量了条纹区的发射光谱,在此基础E计算得到r电子激发温度、相对电子密度和电子平均能量的空间分布特性.结果表明条纹区的光强、电子激发温度和电子密度均呈非等幅的周期性变化.与暗纹中心处相比,明纹中心具有较高的电子激发温度和较低的电子密度.由阴极向阳极,明纹中心处的电子激发温度幅值逐渐减小.此外,条纹区的电子激发温度随着电流的增加而增加.     

Growth of buckytubes on the anode at are discharge

It is shown that for an arc discharge between carbon electrodes in an inert gas atmosphere the temperatures at the electrode surfaces play a key role in determining the structure and the electrode on which a deposit can grow. The heat balance equations determine that the anode temperature is higher due to the energy carried by the electrons. This leads to anode sublimation and deposition on the cathode. It is shown that by cathode heating, by anode cooling or by a combination of these, a deposit may be obtained on the anode due to cathode erosion. The deposit grown by the “inverse” method is compared with a deposit obtained on the cathode under the same conditions but at reverse supply voltage polarity. The material from both deposits, studied by TEM, shows that there are graphite crystals within the anode deposit, and that the carbon forms within have a relatively small number of structural defects while the buckytubes are greater in length than those within the cathode deposit. The reasons for these differences are discussed. In the “inverse” method, the constant decrease in cooling of the anode surface leads to an equalization of the anode and cathode temperatures. This creates conditions that favor buckytube growth.     

Growth of buckytubes on the anode at are discharge

It is shown that for an arc discharge between carbon electrodes in an inert gas atmosphere the temperatures at the electrode surfaces play a key role in determining the structure and the electrode on which a deposit can grow. The heat balance equations determine that the anode temperature is higher due to the energy carried by the electrons. This leads to anode sublimation and deposition on the cathode. It is shown that by cathode heating, by anode cooling or by a combination of these, a deposit may be obtained on the anode due to cathode erosion. The deposit grown by the “inverse” method is compared with a deposit obtained on the cathode under the same conditions but at reverse supply voltage polarity. The material from both deposits, studied by TEM, shows that there are graphite crystals within the anode deposit, and that the carbon forms within have a relatively small number of structural defects while the buckytubes are greater in length than those within the cathode deposit. The reasons for these differences are discussed. In the “inverse” method, the constant decrease in cooling of the anode surface leads to an equalization of the anode and cathode temperatures. This creates conditions that favor buckytube growth.     

Breakdown formation in a transient hollow cathode discharge-astatistical study

Discharge formation at low pressure is found to be greatly influenced in the presence of a suitable hollow cathode region. The formation of a moving virtual anode which extends the anode potential to within the hollow cathode region is thought to be responsible for the enhanced ionization growth which subsequently leads to gas breakdown. In this paper, the spatial evolution of the local potential in the discharge region of a pulsed hollow cathode discharge has been measured in a range of pressures with two different cathode apertures. An extensive data set has been collected and analyzed using a statistical technique. From the characteristic of the statistical distribution of the data, unique features associated with the role of hollow cathode at the different stages of discharge formation have been identified. It was found that the influence of the hollow cathode region is strongest in the start of ionization growth and in the final change over to high current breakdown     

Behavior of the spatial phase of modulated ionization waves in a glow discharge

Work is described in which it was shown experimentally that a modulated glow discharge with striations acquires an additional characteristic which can be called spatial phase of the oscillations. In fact, this characteristic reflects the spatial distribution of the maxima of the time-averaged integrated plasma luminescence on the tube axis which arises due to the influence of the current modulation on traveling striations. It was discovered that for fixed discharge conditions such a distribution remains constant when the discharge is switched off and on, i.e., it is not random. An experimental investigation was made of the influence of a magnetic field on the spatial phase of the modulated ionization waves. It was found that in a low-pressure glow discharge with small currents the striations are initiated in the cathode region. The anode region does not directly initiate the striations but it influences their spatial phase characteristics through a feedback mechanism. State University, St. Petersburg. Ukhtinskii Industrial Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 108–113, January, 1997.     

Operational characteristics of a plasma cathode with a grid stabilization in a two-stage ion source

The discharge characteristics and the parameters of the cathode plasma in a two-stage ion source with a grid plasma cathode and a magnetic trap in the anode region are investigated. It is shown that an increase in the gas pressure and the accompanying increase in the reverse ion current in the bipolar diode between the cathode and anode plasmas lead to an increase in the cathode plasma potential and a transition of the cathode into the regime of electron emission from the open plasma boundary. The dependence of the ion current extracted from the anode plasma on the area of the exit aperture of the hollow cathode and the mesh size of the grid plasma cathode is explained. The conditions at which the ion emission current from the anode plasma is maximum are determined. The potential difference at the bipolar diode is measured by using the probe method. It is shown that, when the gas pressures reaches a critical value determined by the mesh size of the grid plasma cathode, the discharge passes into a contracted operating mode, in which the ion current extracted from the anode plasma decreases severalfold.     

Cold-hollow-cathode arc discharge in crossed electric and magnetic fields

A crossed-field cold-hollow-cathode arc is stable at low working gas pressures of 10⁻²–10⁻¹ Pa, magnetic-field-and gas-dependent arcing voltages of 20–50 V, and discharge currents of 20–200 A. This is because electrons come from a cathode spot produced on the inner cathode surface by a discharge over the dielectric surface. The magnetic field influences the arcing voltage and discharge current most significantly. When the plasma conductivity in the cathode region decreases in the electric field direction, the magnetic field increases, causing the discharge current to decline and the discharge voltage to rise. The discharge is quenched when a critical magnetic field depending on the type of gas is reached. Because of the absence of heated elements, the hollow cathode remains efficient for long when an arc is initiated in both inert and chemically active gases.     

Factors affecting the self-excitation of low-frequency oscillations in an electric discharge

The influence of the individual factors affecting the production of low-frequency oscillations connected with moving striations in a discharge is analysed and experimentally verified. From the conception of two mechanisms of feedback, which leads to self-excitation of these oscillations, it follows that the following influence the production of oscillations: 1) the tendency of the plasma to stratification, 2) the length of the positive column, 3) processes in regions at the electrodes, 4) the external electric circuit.The measurements confirm the influence of all these factors. It is shown that the main factor conditioning the production of oscillations is the property of the plasma of the positive column — its tendency to stratification. The plasma of a glow discharge in neon — in the region of low pressures and currents investigated — has a very strong tendency to stratification with the exception of a small interval in the neighbourhood of the pressure of 4 mm Hg and current 3 . Low-frequency oscillations and moving striations are therefore very often present in a discharge in neon.The influence of the external circuit on the oscillations investigated was also experimentally proved, but it is very weak.     

Mikrowellendiagnostik von Instabilitäten des Säulenplasmas Elektronendichte, -temperatur und Helligkeitsverlauf in selbsterregten laufenden Schichten einer Wasserstoffentladung

Microwave methods are used to obtain the periodic variations in time and space of some plasma parameters. The electron density is obtained using a time-display of the reflection coefficient due to a plasma-perturbed cavity, and the electron temperature from the noise signal of a Dicke radiometer gated periodically. Assuming simple one-dimensional sinusoidal variations of temperature and density, the fundamental convolution integrals relating the appropriate cavity parameters to the plasma properties are solved and formulas are obtained to calculate the real fluctuating quantities. Also in that case where the lenghth of the cavity field region equals the dimension of the variations the correct phase relation between electron temperature and density is obtained. The method is used for diagnostics of self exited moving striations in hydrogen. The striations (frequency 24 kHz, striation lenghts ～1 cm) move from cathode to anode and the sequence of the different maxima for an observer in rest is electron density, temperature and light-emission. The phase shift between density and temperature is about 90 degrees, the appropriate density variation is 10:1 and the temperature variation >2:1.     

Excessive hydrogen and deuterium Balmer lines broadening in a hollow cathode glow discharges

Results of a Doppler spectroscopy study of hydrogen and deuterium Balmer lines in the stainless steel and copper hollow cathode glow discharge, operated in pure hydrogen, deuterium and mixtures of inert gases with hydrogen, are reported. For all gases and gas mixtures plasma observations perpendicular to electric field revealed the excessively large Doppler broadening. By changing mode of glow discharge operation, the Doppler broadened line profiles in helium-hydrogen mixture are recorded parallel to the discharge electric field as well. The excessively broadened part of the H line profile is shifted towards blue or red wavelength by changing the direction of electric field vector. The presence of large excessive Balmer lines broadening in pure hydrogen and in its gas mixture with neon as well as shifting of the excessively broadened part of line profile by means of electric field is in contradiction with the resonance transfer model proposed by Mills et al. in several publications, see e.g. [IEEE Trans. Plasma Sci. 31, 338 (2003)].     

Intense emission from a grid-stabilized plasma cathode

Intense emission from a grid-stabilized plasma cathode based on a glow discharge with an expanded anode area is studied. In the electrode system of the ion source, the potential difference between a large-mesh grid electrode (a hole diameter of 4–6 mm) and cathode and anode plasma reaches 200 V and the glow discharge current is up to 1 A. The current distribution over the electrodes of the plasma cathode is taken, and the dependences of the electron extraction efficiency and electron-emitting-plasma potential on the gas pressure and discharge parameters are obtained. A relationship between the geometric parameters of the grid, cathode plasma potential, and efficiency of electron extraction from the plasma is derived. It is shown that stable intense emission from the plasma cathode can be provided in wide ranges of gas pressure and discharge current by varying the geometry and mesh size of the plasma cathode grid. Discharge contraction in the grid plane at elevated gas pressures is explained. It is assumed that the emitting plasma becomes inhomogeneous due to variation in the thickness of near-electrode layers in the holes of the grid, which makes the distribution of the emission current from the plasma more nonuniform.     

Electromagnetic radiation of a nanosecond discharge in an open gas-filled diode

The properties of the discharge in and radiation from an open gas-filled diode to which high-voltage nanosecond pulses are applied from the RADAN-220 generator are studied. Electromagnetic radiation in the X-ray, UV, visible, and near-IR ranges of the spectrum, as well as high-power subnanosecond (0.5-to 0.7-ns-long) pulses of ultra-wide-band (UWB) electromagnetic radiation, are recorded when a diffuse discharge is initiated in atmospheric pressure air. For the coaxial cathode and anode, the open gas diode emits radially polarized UWB pulses, whereas for the cathode in the form of a segment, the UWB radiation is linearly polarized. The effective potential for both designs of the diode is ER = 6 kV. It is shown that the plasma in the discharge gap serves as a source of soft X rays and the metallic anode generates hard X rays.     

低气压长间隙介质阻挡放电的光谱诊断

设计了一种电极间隔为10 cm的介质阻挡放电装置,以氩气为工作气体,在低气压下产生等离子体。采用发射光谱法,研究了放电空腔内等离子体电子温度和电子密度随空间位置的变化规律。等离子体电子温度的变化通过使用Corona模型计算获得,等离子体电子密度的变化通过分析Ar原子750.4 nm谱线强度变化得到。实验发现空腔内不同位置的等离子体电子温度和电子密度是不同的。当测量位置从阴极向阳极移动时,电子温度先略上升而后迅速下降,再缓慢上升;电子密度先缓慢而后迅速地增大。     

Mechanism of Striation in Dielectric Barrier Discharge

The mechanism of striations in dielectric barrier discharge in pure neon is studied by a two-dimensional particle- in-cell/Monte Carlo collision （PIC-MCC） model. It is shown that the striations appear in the plasma background, and non-uniform electrical field resulting from ionization and the negative wall charge appear on the dielectric layer above the anode. The sustainment of striations is a non-local kinetic effect of electrons in a stratified field controlled by non-elastic impact with neutral gases. The striations in the transient dielectric barrier discharge are similar to those in dc positive column discharge.     

Runaway of electrons in dense gases and mechanism of generation of high-power subnanosecond beams

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.     

Characterization of CO tolerance of PEMFC by ac impedance spectroscopy

The CO tolerance of a proton exchange membrane fuel cell (PEMFC) was investigated by ac impedance spectroscopy. The impedance of the fuel cell could be obtained by feeding oxygen into the cathode side and simulated gas into anode side. Furthermore, the anode impedance could be obtained by feeding hydrogen into the cathode side and simulated gas into anode side. The CO gas had a greater effect on the charge transfer reaction (high frequency arc) and hydrogen dissociative chemisorption (medium frequency arc) but little effect on the low frequency arc. Although the cathode impedance is a main part at high temperature, irrespective of CO concentration (≤100 ppm), the impedance of the full cell depends on anode impedance at low temperature and high CO concentration. It was found that CO gas has little effect on cathode impedance.