Lorentz Center - The multiscale nature of spark precursors and high altitude lightning from 9 May 2005 through 13 May 2005
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    The multiscale nature of spark precursors and high altitude lightning
    from 9 May 2005 through 13 May 2005

 
Posters (with available abstracts) in alphabetic order

Posters (with available abstracts) in alphabetic order

Electric shielding factor and geometrical diffusion in negative ionization fronts

M. Arrayas, M.A. Fontelos and J.L. Trueba

 

We study the properties and structure of anode-directed ionization fronts with zero diffusion coefficient for curved geometries. An electric shielding factor can be introduced which determines the behaviour of the electric field and the particle densities. From the minimal streamer model, a Burgers type equation which govern the dynamics of the electric shielding factor is deduced. This allows us to consider a Lagrangian formulation of the problem simplifying the analytical and numerical study of the fronts. We apply this new formulation to planar as well as curved geometries (typical in experimental set-ups). Power laws for the velocity and the amplitude of streamer fronts are observed numerically. Theses laws are also calculated analytically by using the shielding factor formulation. The geometrical diffusion phenomenon is explained and clarified, and a universal self-similar asymptotic behaviour is derived.

 

 

Field assisted emission in negative corona discharges at atmospheric pressure

D. Bessičres, J. Paillol, LGE, Université de Pau, France

N. Soulem, LEGP, Université de Pau, France

A. Bourdon, CNRS EM2C, Ecole Centrale Paris, France

A. Michau, K. Hassouni, CNRS LIMHP, Université Paris 13, France

E. Marode, CNRS LPGP, Université Paris Sud, France

P. Segur, CNRS CPAT, Université Paul Sabatier Toulouse, France

 

The role of positive ions and field emission in the negative corona triggering is discussed, in room air, at atmospheric pressure. First, an artificial positive space charge is created in a point-to-plane gap by focusing a pulsed ultraviolet laser beam at a determined distance from the point. The triggered corona current pulse is compared with the natural one. Numerical simulations which are presented take into account the insulating layers charging by positive ions on the cathode surface. The theory of electron emission is based upon a conduction mechanism in the bulk of insulating layers. Second, the role of field emission is enhanced by covering the point with a graphite coating. Field emission is modelled by introducing a field enhancement factor and current instabilities are attributed to the switch-off of emission sites on the cathode surface.

 

 

Enhanced radio emission of cosmic rays in thunderstorms

Buitink, S. & lopes collaboration

 

lofar (Low Frequency Array) will be the first radio telescope using an array of omni-directional antennas. The electronic signals from the antennas are combined with software to emulate a conventional antenna. In the full design lofar will consist of 25.000 antennas spread out over an area of 350 km in diameter, sensitive to frequencies below 250 MHz.

lofar offers a unique possibility for studying high-energy cosmic rays. When a primary cosmic ray hits the atmosphere it produces an air shower. In the geomagnetic field air showers emit coherent radio emission below 200 MHz (Huege and Falcke 2003).

lopes (lofar Prototype Station) is a phased array of dipole antennas and is co-located with the kascade (Karlsruhe Shower Core and Array Detector) experiment, a scintillator array that detects electrons, photons and muons. kascade provides triggers for lopes and well-calibrated information about air shower properties.

To investigate the effect of thunderstorm activity on the radio emission associated to cosmic ray air showers, we selected 6 thunderstorms by comparing lightning stroke maps with the daily dynamic spectra from lopes.

We found 5 thunderstorm events with a relatively high signal-to-noise ratio and compared them to similar fair weather events. In all cases the thunderstorm events show an enhanced radio signal as predicted by (Gurevich et al. 2002).

References:

Huege, T. & Falcke, H. "Radio emission from cosmic ray air showers. Coherent geo-synchrotron radiation." Astron. & Astrophys. 412, 19-34 (2003)

Gurevich, A. V. et al., "Radio emission due to simultaneous effect of runaway breakdown and extensive atmospheric showers" Phys. Lett. A 301, 320-326 (2002)

 

 

Global lightning acquisition system

M. Füllekrug

 

Lightning flashes are natural transmitters, which radiate radio waves into the atmosphere. Extremely sensitive magnetometers can record these signals around the entire globe in the frequency range from 5-90 Hz. Particularly intense lightning discharges can excite some kind of upward lightning flashes, denoted sprites, which reach from the top of thunderclouds up to the ionosphere in 100 km height. The associated radio signals are similar to those of intense lightning discharges. With a global network of magnetometers, it is possible to determine individual lightning flash locations around the globe by triangulation. This method makes it possible to monitor the spatial and temporal evolution of the global lightning activity.

 

 

Sprite thunder: automated sprite detection with infrasound recordings

M. Ignaccolo, T. Farges, E. Blanc, M. Füllekrug

 

Recent work by T. Farges et al. [1] shows how sprites generate an infrasound signature. This signature is a chirp in the frequency range 1-10 Hz, which has been observed for 70% of the sprite events, as confirmed with optical observations from the "Observatoire du Pic du Midi" in the Pyrenees (southern France). This promising high detection efficiency makes infrasound recordings highly relevant for sprite detection. In this work, we compare the traditional tools of Fourier analysis with the wavelet analysis [2] and a complex system approach [3] to investigate the properties of the sprite induced chirp, and try to determine the false alarm rate. We then discuss how the results can be used to successfully detect sprites with infrasound recordings.

[1] "Identification of Infrasound produced by sprites during the Sprite 2003 campaign", T. Farges, E. Blanc, A. Le Pichon, T. Neubert and T. H. Allin, Geophys. Res. Lett., 32, L01813, doi:10.1029/2004GL021212.

[2] "A Wavelet Tour of Signal Processing", S. Mallat, 2nd Edition 1999, Academic Press.

[3] "Facing non-stationarity conditions with a new indicator of Entropy increase: the CASSANDRA algorithm" P.Allegrini, P. Grigolini, L. Palatella, G.Raffaelli and M. Virgilio. in: Novak M.N. (ed.): Emergent Nature. World Scientific, Singapore (2002).

 

 

Formation of chemically active species in streamer discharges in products of biomass gasification

G.V. Naidis

 

Pulsed positive corona discharges are considered as a perspective tool for removal of heavy hydrocarbons from flue gas produced at biomass gasification [1]. Such discharges have a structure of a number of streamers propagating in discharge gap. Chemically active species taking part in removal of harmful components are produced in the regions of high electric field - in streamer heads. These primary active species participate in a number of kinetic processes resulting in formation of final products. This work is aimed at calculation of G-values (numbers of particles produced per 100 eV of input electric energy) for production of primary active species. To this end, two-dimensional simulation of positive streamers in non-uniform gaps has been performed. Obtained streamer parameters are close to those calculated for flue gas produced at combustion of hydrocarbons [2]. G-values are presented for production of O, H, N and C atoms, excited nitrogen molecules and other active species.

[1] S.A. Nair, A.J.M. Pemen, K. Yan, E.J.M. van Heesch, K.J. Ptasinski and A.A.H. Drinkenburg, Plasma Chem. Plasma Process. 23, 665 (2003).

[2] N.Yu. Babaeva and G.V. Naidis, IEEE Trans. Plasma Sci. 26, 41 (1998).

 

 

Spatio-temporal pattern formation in a semiconductor-gas-discharge system

D.D. Sijacic, I. Rafatov, U. Ebert,

 

A short gas discharge layer is sandwiched with a high-ohmic semiconductor layer between planar electrodes to which a DC voltage is applied. This initially homogeneous and stationary system can exhibit a wealth of spatio-temporal structures in different parameter regimes.

We analyze the predictions of a classical continuum model for this system where the discharge operates between Townsend and glow regime and the semiconductor is approximated linearly. Patterns form spontaneously due to space charge effects. Major statements are:

1) The stationary one-dimensional states of the minimal discharge model are investigated in full parameter space in the regime between Townsend and glow discharge. For any value of the secondary emission from the cathode, a system with large pd shows the textbook subcritical behavior, while for small pd, the transition is supercritical, i.e., no regime of negative differential conductivity exists in the current-voltage characteristics.

2) Negative differential conductivity is considered vital for the spontaneous emergence of patterns, but we show by an explicit counterexample, that spontaneous oscillations can also occur for positive differential conductivity. We discuss where the common argument fails.

3) We calculate explicit phase diagrams for the transition to spontaneous oscillations, that are in semi-quantitative agreement with experimental results of [Strumpel, Purwins et al.].

4) We also calculate the onset of spontaneous pattern formation in the transversal direction.

This poster summarizes the recent Ph.D. thesis of Danijela Sijacic. For references, see http://homepages.cwi.nl/~ebert/Glow.html

 

 

Filamentary Structures in a DBD -- Pattern Formation in a Drift-Diffusion-Model

L. Stollenwerk, J.-P. Boeuf, H.-G. Purwins

 

We investigate a lateral extended dielectric barrier discharge system with a small discharge gap. The first ignition of the system is nearly uniform, but after a few further ignitions the discharge becomes filamentray. Numerical simulations of a drift diffusion model were performed in two and three dimensions to investigate the genesis of these self organised patterns. Especially the influence of the experimental boundary conditions on the pattern forming process is taken into account. Finally the dependence of pattern properties on the gas preassure are examined. The numerical findings become compared to experimental results. A qualitative and even a rathergood quantitative agreement is found.

 

 

Ionization wave in a Lattice-Boltzmann model based on microscopic cross sections.

Wim Vanroose, Pieter Van Leemput, Giovanni Samaey, Dirk Roose

 

We’ve build a lattice Boltzmann model where the microscopic collision rules are inspired on the cross sections of the impact-ionization reaction, that is: fast particles react with a certain rate and produce two slow particles. This Boltzmann equation is coupled to a PDE for evolution of the electrical field, which again, appears as an external force in the Boltzmann equation. We then look at the emergent macroscopic wavefronts of this system. We present numerical bifurcation diagrams for the emergent waves for varying parameters of the micro rules and compare with earlier PDE models.

 

 

Influence of streamer properties on the chemical efficiency of pulsed corona applications,
G.J.J. Winands, Z. Liu, A.J.M. Pemen, E.J.M. van Heesch and K. Yan

abstract in pdf.

 

 

 

 

 

Talks (with available abstracts) in alphabetic order

Upward Lightning and Counter-Leaders from High Structures and Attraction of Downward Lightning

E. Bazelyan, Yu.P. Raizer

 

Streamer breakdown, leader mechanism of a long spark and lightning are considered, accenting what we understand today and what we do not. It is considered what problems of lightning protection are solved by means of lightning rod technology and what do not.

 

 

A surprising free boundary problem: void electromigration

M. Ben Amar

 

 

Filaments, streamers, and homogeneous plasma in dielectric barrier discharges

J.-P. Boeuf

 

 

Numerical modeling of filamentary dielectric barrier discharges in nitrogen taking metastable states into account

A. Bourdon and P. Ségur

 

This work presents a two-dimensional model of a filamentary discharge in atmospheric pressure nitrogen between two parallel-plate electrodes covered by dielectrics. A detailed kinetic scheme for nitrogen is taken into account with different nitrogen ions and molecular excited states (radiative and metastable states). Excited species play a significant role in glow discharges but are generally not considered in streamer models. In this work, different secondary electron emission processes have been taken into account : photoelectric effect, secondary emissions due to ion and metastable species bombardments.

The different phases of the discharge propagation have been studied: growth of an initial avalanche, positive streamer propagation, cathode sheath formation and post-discharge.

Spatio-temporal evolutions of charged species show that N2+ ions, produced by direct electron impact ionization of N2 molecules, are rapidly converted to N4+ ions, which are major ions in the discharge. This study shows that nitrogen metastable states are efficiently created in the gap and in particular close to dielectric surfaces. Secondary emission due to metastable species is small during the three first stages of the discharge but becomes significant in the post-discharge. As this mechanism allows ionization at weak electric field it seems to be of great interest to explain the filamentary/homogeneous discharge transition.

 

 

Experimental investigation and nanosecond imaging of streamers

T. Briels, E.M. van Veldhuizen, U. Ebert

 

Positive streamers in air in a point-plate electrode geometry (i.e., in an inhomogeneous electric field) are investigated experimentally using voltage pulses with nanosecond risetime. The streamer propagation pattern is studied with a fast intensified charged coupled device (iCCD) camera. Time-resolved photographs of the streamer pattern show that only the streamer head emits light and not the complete streamer channel. Time-integrated pictures show differences in channel diameter, and in length and number of branches when a parameter in the experimental setup is changed (e.g. the power supply, pressure, gas and voltage). Measurements show that streamer propagation is determined by the local electric fields rather than by the averaged applied electric field over the complete electrode gap - as is to be expected.

Scaling with pressure is the basis for the interpretation of sprite discharges as up-scaled streamers. This scaling is based on the fact that fast two particle processes are the dominant processes in streamer propagation. Therefore, for fixed electric potential, all lengths should scale inversely with pressure, including the streamer diameter. Preliminary data in the range of 1 to 0.1 bar show that the streamer diameter approximately follows this behaviour, even when the radius and surface of the emitting electrode needle is unchanged.

The total energy per pulse is determined from current-voltage measurements. The energy per streamer is estimated to check the empirical observation that the number of discharge branches might be proportional to the energy input.

 

 

Simulation of streamer propagation using a PIC-MCC code: Application to sprite discharges

O. Chanrion

 

 

Stability of propagating ionization fronts

G. Derks

 

 

Fingering, fronts, and patterns in superconductors

A. Dorsey

 

When magnetic flux is expelled from a superconductor the interface between the region with flux and the flux-free region may undergo a fingering instability (akin to the Mullins-Sekerka instability in dendritic growth of a solid into a supercooled liquid) and form highly ramified patterns. I will touch on some of the simple physics of this process, show lots of experimental pictures, and if possible, draw connections to the main topic of this conference.

 

 

A few remarks on the scales in streamers

U. Ebert

 

I will briefly discuss the different mechanisms in streamer propagation with their associated length and time scales. If these scales are sufficiently separated, a sequence of model reductions can be performed. I will discuss a road map to a hierarchy of models, from particle models up to multiply branched structures.

 

 

Remote sensing of lightning and sprites with radio waves

M. Füllekrug

 

Extremely-low frequency electromagnetic waves are used to explore the atmospheric electromagnetic environment of the Earth. Three networks of magnetometers record the properties of natural electromagnetic fields on the global, regional, and on the local scale.

The global magnetometer network detects locations of lightning discharges around the globe and monitors the temporal and spatial evolution of particularly intense thunderstorms. Satellite based cloud cover recordings help to determine the effective charge density of thunderclouds and reveal the electrical nature of severe weather.

The regional magnetometer network detects mesospheric electrical breakdown between the troposphere and the ionosphere, optically imaged with an intensified video camera as a transient optical emission, denoted sprite. About 20% of the sprites produce electromagnetic signals which are similar to intense lightning discharges and the global detection efficiency of those signals is on the order of 80% with a false alarm rate of 20%.

The local magnetometer network is operated as an interferometer to measure the electromagnetic wave propagation speed, which is determined by the mesospheric conductivity. This variable conductivity is controlled by solar short wave radiation and energetic particle precipitation into the atmosphere, and can be monitored from the diurnal to the decadal time scale and this variability is likely to modulate the remote sensing of intense lightning discharges and prites.

 

 

Observations of streamer and diffuse glow processes in sprites using a telescopic imager

E. Gerken

 

 

Questions about the structure and the chemistry of the streamer induced filamentary plasmas in atmospheric air

E. Marode

 

As already mentioned in the text defining the objective of this workshop, a great variety of non thermal atmospheric plasma reactors have been developed, directed towards applications such as ozone production, pollution control, surface processing as well as electrical energy transport and high-voltage pulse devices. Generally, these reactors show filamentary structures recognized today as being built by ionisation streamer waves. A considerable amount of knowledge has been gained during the last decade both from computational and experimental methods about the physics of such channels. A great deal of information has been obtained using the discharge produced by a point-to-plane configuration in DC mode. Indeed, in such configuration, repetitive and reproducible successive streamer channels appear at a quite well defined spatial position. It allowed data acquisition by stroboscopic techniques. As usual, new knowledge generates new questions. And the aim of this talk will be to raise some old and new issues about streamers, streamer channels and chemistry. Remarks will be made, first, on what makes a positive streamer discharge take a filamentary structure. Then a basic question can be addressed AS to whether all these dc obtained streamer channels are representative of all types of filamentary discharges? In all cases, we all agree that hydrodynamic phenomena and chemical activities within the discharge core strongly depend on the energy density E.j gained by the charged species. So, what about the current density j, which for a given total current, is linked to the cross section of the filamentary discharge? What can be said about this channel cross section radius? The discharge radius, is, in turn connected to the streamer wave, and the old question of the photo-ionisation will be addressed again. Then how much of this energy density E.j is released into processes which can increase the discharge gas temperature? Since, generally, each discharge is a time dependent process, the answer will depend on the various stages of the discharge development. Namely, if during the spark formation, the discharge current is externally controlled, a discharge stage known as prevented spark can be obtained. How similar is this `prevented spark stage' to the 'leader' stage obtained in long air gaps? On the chemical side, the filamentary discharge can be thought of as an injection pipe of chemical radicals and excited species. Can an excited species chemistry be identified?

 

 

Streamer ionization fronts as moving boundaries: analytical results with conformal mapping methods

B. Meulenbroek

 

Modeling streamers by purely numerical means leaves many questions open, that are vital for further upscaling of the model. Furthermore, numerical solutions show features that require further analytical insight to distinguish numerical from physical features. This was, e.g., the case for streamer branching where analytical solutions do show that the somewhat counterintuitive tip splitting instability of streamer channels indeed is physical [Meulenbroek, Rocco, Ebert, PRE 2004].

This talk deals with the derivation of moving boundary approximations from the partial differential equations describing ionization fronts, and with solutions of this moving boundary approximation. These solutions relie on conformal mapping techniques. In the simplest approximation, however, they lead to cusp formation within finite time, and an important question is to identify a regularization mechanism from the underlying p.d.e.'s that prevents these unphysical cusps. Such a mechanism has now been identified, and first analytical calculations of the linear stability of simple exact shapes for specific parameters do show that these shapes are linearly convectively stable.

Moving boundary problems of this type appear similarly, e.g., in viscous fingering in two-fluid-flow, and in the growth of bacterial colonies or corals; they are notoriously difficult to treat. Our new boundary condition of mixed Dirichlet-Neumann-type allows us to proceed much further with analytical solutions than was possible in the classical visous fingering problem.

 

 

Runaway breakdown and its implications

G. Milikh

 

Mechanism of runaway breakdown was first introduced about a decade ago [Gurevich, Milikh and Roussel-Dupre, 1992], and it was inspired by X-ray observations in thunderclouds. The basic idea is that the fast seed electrons often present in the atmosphere as secondaries generated by cosmic rays, ionize gas molecules producing a number of free electrons. Some of these free electrons have energy higher than the critical energy of runaway. These electrons are accelerated by the electric field and in turn are able to generate fast electrons. The avalanche-like reproduction of fast electrons is accompanied by an exponential increase in the number of thermal secondary electrons, i.e. the electrical breakdown of gas occurs.

The objective of this paper is to describe runaway breakdown in detail and discuss then its implications. The paper begins with the basics of runaway breakdown and introduces its three main features, namely critical or threshold field of runaway breakdown, the avalanche length, and minimum energy of fast electron needed to trigger the breakdown. The attempts to observe runaway breakdown in laboratory are described then, followed by the discussion regarding manifestations of runaway breakdown in the atmosphere. The latter discussion covers intracloud X-ray pulses and charge transfer observed by balloons, gamma-ray bursts and narrow bipolar pulses observed by means of ground-based technique, and terrestrial gamma-ray flashes observed by satellites. It is followed by a discussion on theoretical models related to the above observations.

 

 

PDE simulations with adaptive grid refinement for negative streamers in nitrogen

C.Montijn, U. Ebert and W. Hundsdorfer

 

We present adaptive grid simulations of anode directed streamers in a non-attaching gas like nitrogen. The simulations are based on a fluid model for the electrons and positive ions, that includes drift and diffusion of the electrons, and a local field dependent ionization term. The local field is coupled to the particle densities through the Poisson equation for the electric potential. One of the difficulties in simulating this model comes from the multiscale character of the phenomenon: the ionized channel may become orders of magnitude larger than the small active region where electrons collide with neutrals, and it propagates through a much larger, non-ionized background.

Up to now streamer simulations have been performed on uniform grids in both low [1,2,3] or high fields [4,5], and on moving grids with a fixed number of gridpoints [6]. We have implemented a static regridding strategy for both the transport and the Poisson equation, with refinement criteria based on local error monitors. The spatial discretizations are based on finite-volume methods, and a flux limiting scheme is used to cope with the large spatial density gradients [7]. The time integration is performed with an explicit scheme, the time steps being adjusted to the Courant criterium for stability. This approach enables us to gain a large amount of computational memory and time. Moreover, since the required grid sizes becomes smaller when going to higher fields because gradients then become steeper, this strategy makes it possible to investigate the propagation in large systems at low fields, as well as in relatively small systems but with strong fields. In both cases, the charged layer eventually becomes flatter and flatter, approaching the limit of planar streamer fronts that are known to be unstable [8], and reaches a branching state.

[1] S.K. Dhali and P.F. Williams, J. Appl. Phys. 62 (1987) 4696-4707

[2] M.C. Wang and E.E. Kunhardt, Phys. Rev. A 42 (1990) 2366-2373

[3] P.A. Vitello, B.M Penetrante and J.N. Bardsley, Phys. Rev. E 49 (1994) 5574-5600

[4] A. Rocco, U. Ebert and W. Hundsdorfer, Phys. Rev. E 66 (2002) 035102(R)

[5] N. Liu and V.P. Pasko, J. Geophys. Res. 109 (2004) A04301/1-17

[6] S.V. Pancheshnyi and A.Yu. Starikovskii, J. Phys. D 36 (2003) 2683-2691

[7] W. Hundsdorfer and J.C.G. Verwer, Numerical Solution of Time-Dependent Advection-Diffusion-Reaction Equations, p.217, Springer, New York (2003)

[8] M. Arrayás and U. Ebert, Phys. Rev. E 69 (2004) 036214

 

 

Modeling of streamer breakdown of short non-uniform air gaps

G. Naidis

 

The initial stage of discharge development at sufficiently high pressures is the formation and propagation of ionization waves - streamers. Crossing the gap by the primary streamer does not necessary lead to breakdown. The next stage of discharge development - evolution of the streamer channel is governed by two major factors: 1) lowering of the gas density inside the channel due to expansion of the heated plasma resulting in the growth of the mean reduced electric field, and 2) accumulation of active particles (radicals and excited molecules) changing the balance between the rates of generation and loss of electrons due to acceleration of the detachment, stepwise and associative ionization, etc. The results are presented of simulation of spark breakdown with account of both mentioned factors. The time delay Tbr between the bridging of the gap and spark formation is calculated versus the applied voltage. Obtained values of Tbr are compared with experimental data [1,2].

[1] M. Cernak, E.M. van Veldhuizen, I. Morva, and W.R. Rutgers, J. Phys. D: Appl. Phys. 28, 1126 (1995).

[2] A. Larsson, J. Phys. D: Appl. Phys. 31, 1100 (1998).

 

 

Discharges observed in the high-altitude atmosphere over Europe

T. Neubert

 

 

Experimental and numerical investigations of streamer discharge development

M.M. Nudnova, A.V. Krasnochub, A.Yu. Starikovskii

Physics of Nonequilibrium Systems Laboratory

Moscow Institute of Physics and Technology

masha@neq.mipt.ru

 

The comparison of results of previously developed numerical 2D model with experimental data is shown. The model allows to describe adequately the development of a single streamer and its main characteristics.

Experimental investigations of the development both cathode-directed and anode-directed streamers within the wide range of pressures (350-1300 torr) and voltages (24-48 kV) were performed. It is shown that anode-directed streamer doesn’t branching under these conditions. For cathode-directed streamer the area of streamer branching is obtained as well as dependence of branching intensity form the discharge parameters.

An experimental technique for electrodynamic radius measurement by streamer’s head emission profile using inverse Abel transformation is suggested.

With the help of this technique a fine structure of streamer’s head was experimentally restored. It was shown that our model adequately describes the structure of streamer’s head.

 

 

Numerical schemes for streamer discharges at atmospheric pressure

J.Paillol, D.Bessieres - LGE University of Pau

A.Bourdon - CNRS EM2C Ecole Centrale Paris

P.Segur - CNRS CPAT University of Toulouse

A.Michau, K.Hassouni - CNRS LIMHP Paris XIII

E.Marode - CNRS LPGP - Supelec - Paris XI

 

A streamer discharge model is based on plasma continuity equations coupled to Poisson equation. One of the challenge of the modelling is to find a numerical scheme for continuity equations able to cope with shock front propagation. In this field, different numerical schemes may be used : upwind scheme, second order schemes associated with TVD limiters, Quickest and ultimate schemes (3rd and 5th order), FCT and Weno scheme (5th order). In this work, different numerical experiments are presented including propagation of several shapes and positive streamer head propagation. 5th order quickest scheme and high order FCT scheme are found to be the most efficient schemes with respect to accuracy and run time.

 

 

The role of electronegative gas admixtures in streamer start, propagation and branching phenomena

S. Pancheshnyi

 

A critical analysis of photoionization as the major process for seed electron production ahead of the cathode-directed streamer has been made. The accumulation of O2- ions between pulses and fast electron detachment in an electric field is an effective source of seed electrons for repetitively pulsed discharges in air. Measurements and 2D calculations in the hydrodynamic approximation of streamer parameters (anode current, propagation velocity and radiative channel diameter) can only agree if one assumes uniform background pre-ionization, and not by taking into account the photoionization process generally adopted for pulsed discharges in air. Results of analytical estimates and full 3D simulations confirm that the seed charge distribution in the undisturbed gap has an effect on the streamer head formation and leads to the streamer branching phenomena.

 

 

Physical mechanisms of transient luminous events between thunderstorm tops and the lower ionosphere

V.P. Pasko

 

Transient luminous events (TLEs) are large-scale optical events occurring at stratospheric and mesospheric/lower ionospheric altitudes, which are directly related to the electrical activity in underlying thunderstorms. Several different types of TLEs have been documented and classified. These include relatively slow-moving fountains of blue light, known as 'blue jets', that emanate from the top of thunderclouds up to an altitude of 40 km; 'sprites' that develop at the base of the ionosphere and move rapidly downwards at speeds up to 10,000 km/s; 'elves', which are lightning induced flashes that can spread over 300 km laterally, and upward moving 'gigantic jets', which establish a direct path of electrical contact between thundercloud tops and the lower ionosphere [e.g., Pasko, Nature, 423, 927, 2003, and references therein]. The goal of this talk is to provide a limited overview of some of the recent modeling efforts directed on interpretation of observed features of TLEs termed sprites and jets [e.g., Pasko and George, J. Geophys. Res., 107 (A12), 1458, 2002; Liu and Pasko, J. Geophys. Res., 109, A04301, 2004; Liu and Pasko, Geophys. Res. Lett., 32, L05104, 2005]. We will start with a discussion of a physical mechanism proposed for explanation of sprites, which is build on original ideas advanced many decades ago by C.T.R. Wilson [Wilson, Proc. Phys. Soc. Lond., 37, 32D, 1925]. We will discuss similarity properties of electrical discharges as a function of gas pressure in the context of a selected set of results from the recent laboratory studies of streamers [e.g., van Veldhuizen et al., IEEE Trans. Plasma Sci., 30, 162, 2002; Yi and Williams, J. Phys. D. Appl. Phys., 35, 205, 2002; Pancheshnyi et al., Phys. Rev. E, 71, 016407, 2005; Briels et al., IEEE Trans. Plasma Sci., to appear, 2005], which are directly applicable for understanding of high spatial resolution imagery of sprites revealing many internal filamentary features with transverse spatial scales ranging from tens to a few hundreds of meters [Gerken and Inan, J. Atmos. Solar. Terr. Phys., 65, 567, 2003]. Some of the currently unsolved problems in theory of TLEs will also be discussed.

 

 

Leaders, Lightning, and Lightning Protection: Solved and Unsolved Problems

Yu.P. Raizer, E.M. Bazelyan

 

Development of counter-leader in the field of downward lightning and upward lightning in the field of thunderstorm cloud are considered. Condition of upward leader viability  is presented. Mechanism of attraction of downward lightning to high structures is discussed as well as perspectives to control counter-leaders and lightnings.

 

 

Los Alamos Kinetic Boltzmann Calculations of Air Breakdown and its Application to the Lightning Discharge

Robert Roussel-Dupré, Jonah Colman, Eugene Symbalisty, Bryan Travis, and Laurie Triplett

 

The lightning discharge is a nonlinear process that involves an applied thunderstorm electric field, the details of electron transport in air, initiation of an avalanche, and the self-consistent evolution of the net electric field in the time-dependent, conducting medium created by the breakdown process itself. In the past, conventional breakdown initiated by seed thermal electrons that accelerate in the presence of a strong external electric field has been invoked to describe all aspects of lightning. More recently, a new breakdown mechanism (runaway breakdown) seeded by energetic (keV-MeV) electrons has been proposed as the potential initiator of lightning discharges. The precise role of either or both mechanisms remains an open question even as increased evidence for the occurrence of energetic processes in association with lightning is obtained.  In this paper we present the electron distribution function derived from detailed kinetic Boltzmann calculations relevant to both breakdown mechanisms under various atmospheric conditions and for a range of applied fields. Fluid simulations, rooted in the kinetic results, of an intra-cloud discharge initiated by a cosmic ray shower are also described.

 

 

The Diverse Phenomenology of Transient Luminous Events in the Upper Atmosphere

D. Sentman

 

Following the first observations in 1989 of brief optical flashes in the upper atmosphere above thunderstorms, research rapidly uncovered a wide variety of related transient optical effects excited by lightning. Loosely categorized by a whimsical naming scheme that reflects their essentially transient (ms to 100s-of ms) time scales, red sprites, sprite halos, elves, blue jets and gigantic blue jets, pixies and gnomes, collectively known as Transient Luminous Events (TLEs), provide an optical window into the effects of lightning on the middle and upper atmosphere that is considerably more complex than is described by the traditional quasi-DC linear picture of Atmospheric Electricity. TLEs exhibit a wide variety of distinct forms and dynamical behavior, and in their totality span the full distance between the troposphere and the lower ionosphere. In this talk, a quasi-historical tour is given of the various forms of TLEs that have been recorded to date and numerous examples are shown illustrating their principal morphologies and dynamical characteristics. In addition to providing a window into nonlinear electrical effects in the upper atmosphere, the optical emissions from TLEs provide a new, remote-sensing means to study large scale discharge processes in general, using the upper atmosphere as a natural laboratory without walls, on time scales that are difficult to achieve in ground based facilities. Finally, consideration of the general microphysical problem of electrical breakdown and streamer formation in arbitrary neutral gases permits one to speculate on the occurrence of analogous processes in other planetary atmospheres. Some preliminary results are presented concerning possible extraterrestrial analogues to TLEs that might be expected to accompany lightning in the atmospheres of other bodies within the Solar System.

 

 

Plasma Supported Combustion and Aerodynamics

N.B. Anikin, I.N. Kosarev, A.V. Krasnochub, E.I.Mintoussov, S.M. Starikovskaia, D.V.Roupassov, I.N.Zavialov, A.Yu. Starikovskii

Physics of Nonequilibrium Systems Laboratory

Moscow Institute of Physics and Technology

astar@neq.mipt.ru

 

The problem of the uniform ignition of combustible mixtures is of crucial importance from both scientific and technological standpoints. The reaction of fuel oxidization proceeds via a chain mechanism. It is well known that the chain reactions of fuel combustion are fast and the combustion rate is limited by the rate at which active particles are produced. The easiest way to produce free radicals is to decompose the weakest bond of a molecule. Even relatively small amount of atoms and radicals (10-5-10-3) of the total number of the gas particles) can shift equilibrium in the system and initiate a chain reaction. The goal of the work is the method development for the plasma flow control and weak shock wave structures in transonic regimes.

Boundary layer separation zone change under the conditions of additional energy deposition by electric discharge was investigated. When such an interaction occurs in the boundary layer, we can change the effective gas velocity, temperature and pressure and control boundary layer separation. Thus, we change the flow pattern near the body, which is equivalent to the change of the body shape. In transonic regimes this small change may lead to the sufficient flow reorganization with change the weak shock wave position along to the wing chord and pressure re-distribution. Thus, the mechanism under investigation can provide the ultra-fast plasma control of the wing lift force and the entire airplane control.

 

 

Cross-correlation spectroscopy applied to the diagnostics of filamentary plasmas

H.-E. Wagner (1), R. Brandenburg (2), K.V. Kozlov (3), A.M. Morozov (3)

(1) Ernst-Moritz-Arndt University of Greifswald, 17489 Greifswald, Germany

(2) Institute of Low Temperature Plasma Physics, 17489 Greifswald, Germany

(3) Moscow State University, 119899 Moscow, Russia

 

The cross correlation emission spectroscopy (CCS) has been applied to the spatio-temporal diagnostics of non-equilibrium plasmas. The main idea of this method is to replace a direct measurement of the single pulse luminosity of a repetitive light pulse emitter by a statistically averaged determination of the correlation function between two optical signals, both originating from the same source. If the repetitive light pulses reproduce each other sufficiently exactly, and if the synchronizing signal detection is adjusted in such a way as to occur always at the same moment of a single light pulse evolution, then the recorded probability density function is proportional to the light intensity I(t) of the source under consideration. A detailed description of the theoretical foundations of CCS technique is given in the review papers [1,2]. Extremely high sensitivity of the CCS combined with the temporal resolution in a sub-nanosecond range is the main advantage of this experimental technique, as compared to other methods of emission spectroscopy.

The CCS will be demonstrated to be a powerful tool to analyze the spatio-temporal structure of cold non-equilibrium plasmas in air and N2/O2 mixtures at atmospheric pressure. Namely, filamentary discharges, as single microdischarges of the dielectric-barrier discharge (DBD) [2-7], and the "point-to plane" coronas of both polarities [8-11] have been investigated by this technique. An overview on these research activities and selected examples will be given. In DBDs, the velocities of the cathode-directed ionising waves and the effective lifetimes of selected excited nitrogen states were calculated from the CCS data [3,6]. The results were sufficient for the quantitative estimation of the electric field strength and relative electron density in air at atmospheric pressure [3]. Special attention was devoted to the investigation of the transition between the filamentary and the diffuse modes of barrier discharges, being caused by the variation of the oxygen content within the range 500-1000 ppm in nitrogen [6]. Under selected experimental conditions it was possible, to study in very detail the axial and radial development of isolated single microdischarges [7].

[1] Ware W.R., Technique of pulse fluorometry, In: Time-Resolved Fluorescence Spectroscopy in Biochemistry and Biology, NATO ASI Series A: Life Sciences, ed. by Cundall R.B. and Dale R.E., 69, Plenum Press, New York, London, 1983, p.23-57.

[2] Kozlov K.V., Dobryakov V.V., Monyakin A.P., Samoilovich V.G., Shepeliuk O.S., and Wagner H.-E., Brandenburg R., Michel P., In: Selected Research Papers on Spectroscopy of Nonequilibrium Plasma at elevated Pressures, ed. by Ochkin V.N., Proceedings of SPIE, Washington 2002, vol. 4460, p.165.

[3] Kozlov K.V., Wagner H.-E., Brandenburg R., and Michel P., J. Phys. D: Appl. Phys. 34, 3164-76 (2001).

[4] H.-E. Wagner, R. Brandenburg, K.V. Kozlov, A. Sonnenfeld, P. Michel, J.F. Behnke, Vacuum 71 (2003) 417.

[5]  H.-E. Wagner, R. Brandenburg, K.V. Kozlov, J. Adv. Oxid. Technology 7 (2004) 11-19 (Review).

[6]  K.V Kozlov., R. Brandenburg, H.-E.Wagner, A.M. Morozov, P.Michel, J. Phys. D: Appl. Phys. 38 (2005) 518, (Special cluster on atmospheric pressure non-thermal plasmas for processing and other applications).

[7]  R. Brandenburg, H.-E.Wagner, A.M. Morozov, K.V. Kozlov, J. Phys. D: Appl. Phys. 38 (2005) (Special cluster on microplasmas, to be published online in June 2005).

[8] Teich T.H., In: Proc. 3rd Int. Symp. on High Pressure Low Temperature Plasma Chemistry, ed. by M. Lecuiller, Strasbourg, France, 1991, p. 77-83.

[9] Teich T.H., In: NATO ASI Series, Non-Thermal Plasma Techniques for Pollution Control, ed. by Penetrante B.M. and Schultheis S.E., Vol. G34, Part A, Springer-Verlag, Berlin Heidelberg, 1993, p.230-247.

[10] Ikuta N., Kondo K., In: Proc.4th Int. Conf. on Gas Discharges and Their Applications, Swansea, UK, 1976, p.227-230.

[11] Kondo K., Ikuta N., J. Phys.D: Appl. Phys. 13 (1980) L33-38.

 

 

Polarity Asymmetry in Sprite Lightning: A Paradox

E. Williams, Massachusetts Institute of Technology, Cambridge, MA 02139, USA, earlew@ll.mit.edu

 

The mechanism for sprite initiation in the mesosphere, formulated by C.T.R. Wilson and supported by contemporary theoretical modeling (Pasko et al, 1995), is polarity-independent. Conventional dielectric breakdown in the mesosphere is as easily initiated by an upward as a downward-directed electric field. Yet observations show that 99% of sprites are caused by ground flashes with positive polarity. (Only two well-documented cases of `negative' sprites populate the literature (Barrington-Leigh et al, 1999). The following hypotheses are explored toward resolving this paradox: (1) the vertical charge moments for positive ground flashes are overwhelmingly larger than those for negative flashes. (This test fails.) (2) The negative ground flashes with large charge moments occur systematically in daylight hours when sprites cannot be observed. (This test explains away part of the paradox.) (3) The negative flashes with large charge moments occur more preferentially over oceans, where sprites are not observed. (These results do not remove the paradox.) (4) The negative flashes with large charge moments are more impulsive in nature, and less-easily-detected haloes rather than sprites are more likely to develop in response. (Independent observations (Bering et al, 2004) support this picture and help relieve the paradox, but do not resolve it.) (5) Sprite initiation is dependent on the electron runaway process that favors negative charge (electrons) moving upward to produce bremstrahlung. (Recent observations with the RHESSI satellite do not show upwardly moving gamma rays from sprite-producing lightning (S. Cummer, personal communication, 2005)). The paradox remains.

 



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