N Yu Babaeva

Two-dimensional modelling of positive streamer dynamics in non-uniform electric fields in air

The results of two-dimensional (2D) numerical simulation of single positive streamer development in atmospheric air in sphere - plane electrode configuration are presented. Correlation of streamer parameters with sphere radius and applied voltage U is revealed. The values of the electric field at the streamer head and in the channel are shown to be almost independent of and U and equal to 150 - 180 and correspondingly. Streamer velocity and radial dimensions linearly increase with U, the coefficient in these linear relations being independent of . Comparison of the results obtained by 2D modelling and in the frame of the commonly used 1D (or quasi-2D) models is made.

SIMULATION OF POSITIVE STREAMERS IN AIR IN WEAK UNIFORM ELECTRIC FIELDS

Abstract The results of two-dimensional numerical simulations of single positive streamer development in atmospheric air in a uniform electric field superimposed on the nonuniform field of a spherical anode are presented. It is shown that the form of the streamer channel and the rate of growth of the streamer velocity with its length are determined by the value of the applied field.

Non-stationary charging of a dust grain in decaying streamer-channel plasma

The process of a dust grain charging in non-stationary (decaying) air plasma in external electric fields typical of a streamer channel at atmospheric air pressure is investigated numerically using two-dimensional spherical coordinates. The characteristic timescale on which the grain acquires maximal electric charge is found to be determined by the rate of electron attachment. For zero external field, the process of charging in decaying air plasma is compared with the analogous process in nitrogen (non-attaching gas). For non-zero external electric fields in air plasma, the correlation between the charging time and the field dependence of the attachment rate is revealed. For high values of external electric fields, a streamer discharge starting from the dust grain is observed, analogous to Trichel pulses in a negative corona.

Simulation of subnanosecond streamers in atmospheric-pressure air: Effects of polarity of applied voltage pulse

Results of simulation of subnanosecond streamer propagation in corona gap configuration, obtained in the framework of 2D fluid model, are presented. Effects related with the polarity of a voltage pulse applied to the stressed electrode are discussed. It is argued that these effects (dependence of the discharge current and propagation velocity on the polarity of applied voltage) observed in experiments can be attributed to the difference in initial (preceding the streamer formation) distributions of charged species inside the gap. This difference can be caused by preionization (at negative polarity) of the gas inside the discharge gap by runaway electrons. Calculated streamers have large widths (up to 1 cm) and move with velocities in the range of 109–1010 cm s−1, similar to experimental data.

Capacitively coupled plasma source operating in Xe/Ar mixtures

The parameters of a capacitively coupled plasma source operating in Xe/Ar mixtures have been studied using particle-in-cell/Monte Carlo methods. The plasma density, ion energy distributions (IEDs) for different sorts of ions, ion current densities, energy delivered by ions to the electrodes and other parameters of a Xe/Ar plasma are systematically investigated as a function of the Xe/(Ar + Xe) mixing ratio. It is shown that the addition of xenon to an argon discharge increases the plasma density, changes the electron energy distribution and lowers the electron temperature. Both single and dual frequency plasma issues are addressed. The dependence of the IED on the observation position in the sheath is also investigated. Computational results are compared with available experimental data.

Kinetic and electrical phenomena in gas–liquid systems

Disperse systems consisting of a liquid and gas bubbles located in it are considered. Two possible versions of evolution of bubbles under the conditions studied are assessed. In simple liquids, contact between two bubbles causes them to merge, as the separating film breaks. In the case of complex organic liquids, amphiphilic film is formed on the surface of bubbles, and the lifetime of bubbles in contact increases with their size. Under an external electric field, chains of bubbles are formed, lined up along the electric field potential lines. The presence of bubbles in liquid greatly lowers the breakdown threshold, as the critical parameters of the breakdown field in liquids are two to three orders of magnitude higher than those in gases at atmospheric pressure. Various breakdown mechanisms in liquids are discussed from the viewpoint of formation of the gas phase during the passage of an electric current through a liquid medium. The character of propagating a streamer in separate bubbles is studied with their random distribution in liquid and in the case of formation of some structures of bubbles; the critical parameters of disperse systems, that can lead to their electrical breakdown, are presented. Along with the general concepts of electrical breakdown in dispersed systems, experimental studies of these processes are considered, and the nature of electrical breakdown in liquid dielectrics, including transformer oil, is discussed.

Simulation of stepped propagation of positive streamers in SF6

A model of streamer propagation is developed, which accounts for heating and radial expansion of gas in the streamer channel. The model describes a stepped pattern of streamer propagation at applied voltage constant in time. The results of numerical simulation of positive streamer dynamics in a sphere-plane gap in SF6 at 2 bar are presented.

Modeling of positive streamers in liquid argon

A numerical simulation is used to investigate the dynamics of positive streamers in liquid argon. It is shown that as in gases, the nature of the streamer propagation depends on the field strength in the discharge gap. It is established that electron-ion recombination in the streamer channel plays a significant role compared with streamers in gases.

On the physical nature of apokampic discharge

Experimental and theoretical investigations of a diffuse jet (streamer) of apokampic discharge are carried out for various values of air pressure. It is established experimentally that this regime of pulse-periodic discharge is formed stage by stage. At the first stage, in a microsecond discharge of a voltage pulse of positive polarity, a potential spark channel formed during the first pulses between two needle electrodes is transformed into a diffuse channel. At the second stage, a weakly glowing halo is formed near the discharge channel, and a bright offshoot arises near the bending point. Finally, at the third stage of discharge in the steadystate mode, for frequencies of a few to tens of kilohertz in each pulse, the offshoot becomes a source of plasma bullets (streamers) moving with a velocity of up to 200 km/s. As a result of simulation of a streamer in atmospheric pressure air under conditions corresponding to the experimental data, a propagation velocity of up to 400 km/s is obtained for the streamer. It is shown that the formation of a jet significantly depends on the air temperature.

Formation of Wide Streamers during a Subnanosecond Discharge in Atmospheric-Pressure Air

Results are presented from experimental and computational studies of a subnanosecond breakdown of atmospheric-pressure air in a nonuniform electric field. It is shown that the ionization waves (streamers) formed in the prebreakdown stage have a nearly spherical or conical shape. The diameter of the streamer in its widest part is found to increase with increasing voltage and discharge gap length. For a rise time of the voltage pulse of ≈0.5 ns and its amplitude of ≈250 kV, streamers about 8 cm in diameter were observed in a 7-cm-long gap.