Yu. S. Akishev

Negative corona, glow and spark discharges in ambient air and transitions between them

Obtaining new information about different forms of self-sustained dc discharges that can be realized in pin-to-plane electrode geometry in ambient air is the goal of this paper. Experimental and numerical calculation data uncovering the physics of the temporal and spatial evolution of the negative corona and glow discharge (GD), with increase in current up to the transition to the spark, are presented. Special attention is paid to the properties of diffusive GD at atmospheric pressure, which is a necessary stage (steady-state or transient) preceding the spark and determining the threshold conditions of sparking.

Pulsed Regime of the Diffusive Mode of a Barrier Discharge in Helium

Periodic pulsations of the active current component are revealed experimentally in transversely homogeneous barrier discharges in helium at small values of the parameter Pd (below 500 torr mm) and moderate frequencies of the applied voltage (f < 100 kHz). The frequency of the current pulsations is higher than the frequency of the well-studied pulsations in a transversely inhomogeneous streamer barrier discharge in air by a factor of approximately 100. Numerical calculations show that the physical nature of the observed pulsations can be explained in terms of the negative differential resistance of the cathode fall region, which occupies essentially the entire interelectrode gap in each half-period of the applied voltage.

Transition of a Multipin Negative Corona in Atmospheric Air to a Glow Discharge

It is commonly accepted that, as the current increases, a diffuse negative corona inevitably goes over to a strongly nonuniform and nonsteady spark discharge. In this paper, a new effect—the transition of a negative corona to a diffuse glow discharge at atmospheric pressure—is studied experimentally and numerically. The evolution of the corona parameters during the transition to the regime of a glow discharge is traced.

Development of a spark sustained by charging the stray capacitance of the external circuit in atmospheric-pressure nitrogen

Results are presented from experimental studies and numerical simulations of a spark discharge excited in a short point-plane gap filled with atmospheric-pressure nitrogen. The discharge was powered from a high-voltage source connected to the discharge gap through a large ballast resistance. In this case, a short-term spark develops only due to the charging of the stray capacitance of the external circuit; therefore, the energy released by the spark and its intensity are both low. Rapid current growth in a weak spark is accompanied by the contraction of the current channel rather than by its gasdynamic expansion, as it occurs in long-duration kiloampere sparks. Simulations show that, because of the very short spark lifetime, the plasma in a weak spark is substantially nonequilibrium and the gas temperature is fairly high.

Comparison of the AC barrier corona with DC positive and negative coronas and barrier discharge

Results are presented from experimental studies of ac corona discharges between a point electrode and a dielectric-coated plate in nitrogen, argon, helium, and air in the voltage frequency range f=50 Hz–50 kHz. The characteristic features of this type of discharge are compared with the well-known features of dc positive and negative coronas and a barrier discharge between plane electrodes. It is shown that the presence of a dielectric barrier on the plane electrode significantly changes the electric characteristics and spatial structure of the corona, whereas the main phases of the discharge evolution remain unchanged as the voltage increases. With a point electrode, the breakdown voltage of the barrier corona decreases substantially as compared to the breakdown voltage of a barrier discharge with plane electrodes. This leads to softer conditions for the streamer formation in a barrier corona, which becomes more stable against spark generation.

Pulsed mode of a negative corona in nitrogen: I. Experiment

The pulsed mode of a negative corona discharge in air has long been known; however, in electropositive gases, this mode has not been previously observed. This paper presents the results from a systematic study of a newly discovered pulsed mode of a negative corona in nitrogen over a wide range of experimental parameters. The conditions under which the pulsed mode is realized are described in detail. The dynamic characteristics of current pulses are determined. The shapes and parameters of current pulses in nitrogen and air are compared.

Decomposition of toluene in a steady-state atmospheric-pressure glow discharge

Results are presented from experimental studies of decomposition of toluene (C6H5CH3) in a polluted air flow by means of a steady-state atmospheric pressure glow discharge at different water vapor contents in the working gas. The experimental results on the degree of C6H5CH3 removal are compared with the results of computer simulations conducted in the framework of the developed kinetic model of plasma chemical decomposition of toluene in the N2: O2: H2O gas mixture. A substantial influence of the gas flow humidity on toluene decomposition in the atmospheric pressure glow discharge is demonstrated. The main mechanisms of the influence of humidity on C6H5CH3 decomposition are determined. The existence of two stages in the process of toluene removal, which differ in their duration and the intensity of plasma chemical decomposition of C6H5CH3 is established. Based on the results of computer simulations, the composition of the products of plasma chemical reactions at the output of the reactor is analyzed as a function of the specific energy deposition and gas flow humidity. The existence of a catalytic cycle in which hydroxyl radical OH acts a catalyst and which substantially accelerates the recombination of oxygen atoms and suppression of ozone generation when the plasma-forming gas contains water vapor is established.

Generation of a nonequlibrium plasma in heterophase atmospheric-pressure gas-liquid media and demonstration of its sterilization ability

Results are presented from experiments on the generation of a low-temperature nonequilibrium plasma in atmospheric-pressure heterophase gas-liquid media of different compositions: (i) a liquid with air bubbles and (ii) air with liquid aerosol. To illustrate possible application of a low-temperature plasma in a heterophase medium, experiments on the inactivation of some microorganisms by a low-temperature plasma have been performed.

Alternating Nonsteady Gas-Discharge Modes in an Atmospheric-Pressure Air Flow Blown through a Point-Plane Gap

The electric and spectral characteristics of a nonsteady discharge in an atmospheric air flow blown through a point-plane interelectrode gap were investigated experimentally. The discharge was produced by applying a constant positive voltage to the point electrode, the amplitude of the applied voltage being much higher than the corona ignition voltage. The nonsteady character of the discharge is due to the spontaneously repeating streamer-spark breakdown, followed by the formation of either a diffuse ultracorona or a filamentary glow discharge. In the latter case, the length of the plasma column increases progressively, being blown off by the gas flow from the discharge gap. The extinction of a filamentary discharge is unrelated to the break of the current channel: the discharge decays abruptly when the filament length reaches its critical value. The distribution of active particles (O, OH, and N*2) carried out from the discharge gap is determined from the data of spectral measurements.

Numerical simulations of Trichel pulses in a negative corona in air

Numerical simulations of a negative corona in air demonstrate that the experimentally observed regime of self-oscillations, known as Trichel pulses, is well described by a three-dimensional axisymmetric model that is based on the standard transport equations and in which only the ion-induced secondary electron emission at the cathode is taken into account. The quantitative difference between the measured and calculated values of the mean current and the pulse repetition rate most likely stems from the insufficiently large dimensions of the computation region and from the fact that the point shape adopted in simulations somewhat inexactly conforms to that used in experiments. It was found that the transverse discharge structure near the cathode radically changes during the pulse. Specifically, as the current grows, a cathode sheath forms at the discharge axis and expands over the cathode surface. When the current falls off, the cathode sheath is rapidly destroyed; as a result, the characteristic field structure is well defined only near the discharge axis and becomes virtually indistinguishable as the current decreases by an order of magnitude.