The results of a numerical simulation of the generation of runaway electrons in pressurized nitrogen and helium gases are presented. It was shown that runaway electrons generation occurs in two stages. In the first stage, runaway electrons are composed of the electrons emitted by the cathode and produced in gas ionization in the vicinity of the cathode. This stage is terminated with the formation of the virtual cathode, which becomes the primary source of runaway electrons in the second stage. Also, it was shown that runaway electrons current is limited by both the shielding of the field emission by the space charge of the emitted electrons and the formation of a virtual cathode. In addition, the influence of the initial conditions, such as voltage rise time and amplitude, gas pressure, and the type of gas, on the processes that accompany runaway electrons generation is presented.

We conducted experimental studies and computer simulations of standing striations in capacitive coupled plasma in Argon gas. Standing striations were observed at frequencies 3.6, 8.4 and 19.0 MHz, in a pressure range 0.05-10 Torr, tube radius R=1.1 cm, for a certain range of discharge currents (plasma densities). Numerical simulations revealed similar nature of standing striations in CCP and moving striations in DC discharges under similar discharge conditions. Comparison of computer simulations with experimental observations helped clarify the nature of these striations. The non-linear dependence of the ionization rate on electron density is shown to be the main underlying mechanism of the stratification phenomena. DOI:XXXPlasma stratification often occurs in laboratory devices and has been observed in space plasmas [1]. The most studied are striations in direct current (DC) glow discharges [2]. Moving striations are usually observed in noble gases, while standing striations are typical for molecular gases. The nature of moving striations (also called ionization waves) in noble gases is relatively well understood [3,4]. Plasma stratification in molecular gases remains poorly understood.Standing striations have been also observed in radiofrequency (RF) discharges [5]. In Capacitively Coupled Plasma (CCP) created between two wires wrapped around a long dielectric tube, standing striations were observed in Argon over a certain range of frequencies and gas pressures [6,7]. Their wavelength is proportional to the tube radius and decreases with increasing gas pressure. The wavelength varies discontinuously with changing the interelectrode distance in such a way that an integer number of standing waves always forms between the electrodes. The wavelength weakly depends on driving frequency in the range from 3 MHz to 60 MHz. Although discussions of surface-wave and kinetic effects can be found in the literature, no satisfactory explanations of these striations have been proposed so far. Standing striations have been also observed in Inductively Coupled Plasma (ICP) for certain geometries and discharge conditions [8,9].Striations appeared also in Particle-in-Cell (PIC) simulations of plasmas in different gases and gas mixtures [10,11,12]. Plasma stratification, as an example of selforganization at the kinetic level, remains a great challenge for the plasma science and physics of gas discharges [13].Previously, standing striations in argon CCP have been observed in a range of frequencies 3-80 MHz and gas pressure 2 Torr, in a tube with radius R=1.25 cm [7]. By changing the driving frequency and the distance L between the electrodes, the researchers were able to change the number of striations. However, the number of striations per unit length remained constant at different interelectrode distances. For driving frequencies of 70, 29,6, 4.2, and 3.7 MHz, they observed 8,9,10,11,and 12 striations at L=30 cm. Light emission oscillated at the second harmonic of the applied voltage. The nature of the striations remained unclear...

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In this paper, we describe results of self-consistent two-dimensional (x-z) particle-in-cell simulations, with a Monte Carlo collision model, of an orificed micro-hollow cathode operating in a planar diode geometry. The model includes thermionic electron emission with Schottky effect, secondary electron emission due to cathode bombardment by the plasma ions, several different collision processes, and a non-uniform xenon background gas density in the cathode-anode gap. Simulated results showing behavior of the plasma density, potential distribution, and energy flux towards the hollow cathode and orifice walls, are discussed. In addition, results of simulations showing the effect of different Xe gas pressures, orifice size, and cathode voltage, on operation of the micro-hollow cathode are presented. V C 2013 AIP Publishing LLC.

This paper presents the results of a theoretical and experimental study of plasma-assisted reforming of ethanol into molecular hydrogen in a new modification of the "tornado" type electrical discharge.Numerical modeling clarifies the nature of the non-thermal conversion and explains the kinetic mechanism of nonequilibrium plasma-chemical transformations in the gas-liquid system and the evolution of hydrogen during the reforming as a function of discharge parameters and ethanol-to-water ratio in the mixture. We also propose a scheme of chemical reactions for plasma kinetics description. It is shown that some characteristics of the investigated reactor are at least not inferior to characteristics of other plasma chemical reactors. PACS: 52.65.-y, 52.80.-s IntroductionThe interest to alternative fuels research in the last two decades is increased by the depletion of the traditional fossil fuels. Today, ethanol is considered the most perspective fuel for internal-combustion engines [1]. First, it could be produced from renewable sources (biomass, industrial waste, etc). Second, its combustion produces relatively small amount of pollutants. However, the low velocity of the ethanol combustion wave propagation does not allow its use in its pure form as an engine fuel [2]. In order to increase this velocity, one needs to enrich С 2 Н 5 ОН by molecular hydrogen [3], since the latter has higher flame speed than alcohol. Unfortunately, there is another problem of storing H 2 on a vehicle. Recently, several methods were proposed to obtain hydrogen from hydrocarbon fuels before its entering to the engine [1]. The methods are partial oxidation, steam reforming, dry CO 2 reforming, thermal decomposition and plasma-assisted reforming.The use of non-equilibrium plasma looks more attractive as a result of its lower energy consumption. Plasma acts as a catalyst and initiates fast chain reactions that do not progress under normal conditions. Today different plasma chemical reactors are used (for example [1], [4][5][6]) for molecular hydrogen generation from different hydrocarbons (ethanol, methane, etc) in non-equilibrium plasma. In [7][8], a new plasma chemical reactor for ethanol-to-hydrogen conversion was proposed. It was shown