N. L. Aleksandrov

Mechanism of ultra-fast heating in a non-equilibrium weakly ionized air discharge plasma in high electric fields

Observations of a shock wave propagating through a decaying plasma in the afterglow of an impulse high-voltage nanosecond discharge and of a surface dielectric barrier discharge in the nanosecond range were analysed to determine the electron power transferred into heat in air plasmas in high electric fields. It was shown that approximately half of the discharge power can go to heat for a short (~1 µs at atmospheric pressure) period of time when reduced electric fields are present at approximately 103 Td. A kinetic model was developed to describe the processes that contribute towards the fast transfer of electron energy into thermal energy under the conditions considered. This model takes into account previously suggested mechanisms to describe observations of fast heating in moderate (~102 Td) reduced electric fields and also considers the processes that become important in the presence of high electric fields. Calculations based on the developed model agree qualitatively with analyses of high-voltage nanosecond discharge observations.

The effect of coronae on leader initiation and development under thunderstorm conditions and in long air gaps

The initiation and development of a leader is theoretically studied by considering an electrode which is embedded in a cloud of space charge injected by a corona discharge. The focus is on the initiation of upward lightning from a stationary grounded object in a thundercloud electric field. The main results are also applicable to the leader process in long laboratory air gaps at direct voltage. Simple physical models of non-stationary coronae developing in free space near a solitary stressed sphere and of a leader propagating in the space charge cloud of coronae are suggested. It is shown that the electric field redistribution due to the space charge released by the long corona discharge near the top of a high object hinders the initiation and development of an upward leader from the object in a thundercloud electric field. The conditions for the formation of corona streamers that are required to initiate a leader are derived. The criteria are obtained for a leader to be initiated and propagate in the space charge cloud. A hypothesis is proposed that the streamers are never initiated near the top of a high object under thunderstorm conditions if at ground level there is only a slowly-varying electric field of the thundercloud. The streamers may be induced by the fast-rising electric field of distant downward leaders or intracloud discharges.

Simulation of long-streamer propagation in air at atmospheric pressure

The propagation of the long cathode-directed streamer in air is simulated in the approximation of a fixed radius of the streamer channel. The results of the simulation turn out to be sensitive to the channel radius and to a model of charged particle kinetics. A model including ionization expansion of the streamer channel and yielding results which depend only slightly on the initial channel radius is developed. For the streamers propagation in a non-uniform field, our calculation agrees well with the available experimental values of the total anode current, the streamer length and the average electric field in the streamer channel. Simulation of the streamers evolution in a uniform electric field shows a steady propagation if the field exceeds .

Kinetic mechanism of plasma-assisted ignition of hydrocarbons

Ignition of hydrocarbon-containing gaseous mixtures has been studied experimentally and numerically under the action of a high-voltage nanosecond discharge at elevated temperatures. Ignition delay times were measured behind a reflected shock wave in stoichiometric CnH2n+2 :O2 mixtures (10%) diluted with Ar (90%) for n = 1‐5. It was shown that the application of the gas discharge leads to more than an order of magnitude decrease in ignition delay time for all hydrocarbons under consideration. The measured values of ignition delay time agree well with the results of a numerical simulation of the ignition based on the calculation of atom and radical production during the discharge and in its afterglow. The analysis of simulation results showed that a non-equilibrium plasma favours the ignition mainly due to O atoms produced in the active phase of the discharge. (Some figures in this article are in colour only in the electronic version)

The ionization kinetics and electric field in the leader channel in long air gaps

A kinetic model for the ionization processes in high-temperature air in a strong electric field is developed. The model is used to study numerically the problem of the relaxation of the plasma properties in atmospheric-pressure air to a new steady state after an instantaneous change in the gas temperature between 300 and 1000 - 6000 K. Our simulation shows that the electrical properties of the plasma under the conditions of the leader being a long spark nearly follow the local value of the gas temperature. The reduced field E/N in the leader channel is found to decrease considerably as the gas temperature T increases from 1000 to 6000 K. The decrease in E/N is caused by the formation of NO molecules with low ionization energy at T 4500 K. As a result, the value of E/N falls to 10 Td at T = 5000 K and to 1.5 Td at T = 6000 K for a current of 1 A and a channel radius of 0.1 cm. This agrees well with the experimental evidence showing that the leader bridges very long air gaps (about 200 m) at a moderate applied voltage (< 5 MV).

Simulation of the ignition of a methane-air mixture by a high-voltage nanosecond discharge

The ignition dynamics of a CH4: O2: N2: Ar = 1: 4: 15: 80 mixture by a high-voltage nanosecond discharge is simulated numerically with allowance for experimental data on the dynamics of the discharge current and discharge electric field. The calculated induction time agrees well with experimental data. It is shown that active particles produced in the discharge at a relatively low deposited energy can reduce the induction time by two orders of magnitude. Comparison of simulation results for mixtures with and without nitrogen shows that addition of nitrogen to the mixture leads to a decrease in the average electron energy in the discharge and gives rise to new mechanisms for accumulation of oxygen atoms due to the excitation of nitrogen electronic states and their subsequent quenching in collisions with oxygen molecules. Acceleration of the discharge-initiated ignition is caused by a faster initiation of chain reactions due to the production of active particles, first of all oxygen atoms, in the discharge.

Ionization processes in spark discharge plasmas

The dominant mechanisms of electron generation and loss during different phases of spark discharge are considered. The focus is on spark formation in long non-uniform-field air gaps through leader and streamer processes. The streamer development in dry air and the effects of humidity, temperature and density on the properties of a long positive streamer in air are discussed. The ionization kinetics in the leader channel in long air gaps is described. The ionization mechanisms which lead to the re-breakdown within a post-arc air channel and to the streamer breakdown in undisturbed air and in Ar are presented.

Corona initiated from grounded objects under thunderstorm conditions and its influence on lightning attachment

Lightning attachment to grounded structures due to the initiation of an upward connecting leader from them is considered taking into account the effect of corona space charge near the structures. It is shown that the corona space charge strongly affects the initiation and development of the connecting leader. Specific features of a non-stationary corona are analysed analytically and numerically for one-dimensional electrode geometries and for a grounded rod coronating in a slowly varying thundercloud electric field that can be enhanced by the charge of an approaching downward lightning leader. Initiation and development of an upward connecting leader or upward lightning from high ground objects are investigated. Prospects of using the effect of coronae to control downward lightning discharges are discussed.

Dependence of lightning rod efficacy on its geometric dimensions?a computer simulation

A numerical simulation is used to investigate the effect of rod dimensions on lightning attachment to the lightning rod. The effect is studied by considering a sequence of discharge processes, from a corona ignited in a slowly rising thundercloud electric field to the development of an upward leader in the electric field of an approaching downward leader. It is concluded that the efficacy of a lightning rod is almost independent of the rod radius in the range 0.05?5?cm. This is in agreement with measurements of the breakdown voltage in long laboratory rod-to-plane air gaps for various rod tip radii but is at variance with the conclusions reached by Moore et al (2000a Geophys. Res. Lett. 27 1487, 2000b J. Appl. Meteorol. 39 593, 2003 J. Appl. Meteorol. 42 984) from their observations under thunderstorm conditions.

Corona discharge at the tip of a tall object in the electric field of a thundercloud

Characteristics of a positive transient corona discharge near the tip of a tall solitary grounded object in the electric field of a thundercloud are studied analytically and numerically. The time evolution of the discharge current and the space distribution of the total electric field are simulated for different growth rates of the external field and the dimensions and geometry of the stressed electrode. The effect of aerosol ions is shown to be negligible at a short duration of the corona. The developed simplified analytical approach agrees with numerical simulations.