Nonequilibrium Plasma Formation by High-Voltage ...

30th EPS Conference on Contr. Fusion and Plasma Phys., St. Petersburg, 7-11 July 2003 ECA Vol. 27A, P-2.37

Nonequilibrium Plasma Formation by High-Voltage Pulsed Nanosecond Gas Discharges S.V.Pancheshnyi, S.M.Starikovskaia, A.Yu.Starikovskii Physics of Nonequilibrium Systems Laboratory Moscow Institute of Physics and Technology, Dolgoprudny, Russia

Experiment Discharge section comprises cube chamber 20x20x20 cm. There is a possibility to pump up to 10-2 Torr and to heat the chamber. Special optical windows made of quartz to register optical emission in a wavelength range of 190-3200 nm. A rotating-interrupter generator was used as a pulsed voltage supply. The high-voltage pulses were fed through a coaxial electric cable to the high-voltage connector of the discharge system. Repetitive frequency of the high-voltage pulses was 1.2 kHz, voltage amplitude in the cable was 11 kV, high voltage pulse on the half-height was 75 ns. Spectral and Electrical Measurements It has been obtained that the character of emission depends significantly upon the degree of closure of the discharge gap. Analysis performed has proved that in the regime of partially closed circuit the temperature of the streamer channel increases (and, consequently, the intensity of

the

continuous

spectrum

rises),

channels

of

part the

an

of

main energy

distribution change essentially and production of atoms and Figure 1. Time-resolved voltage, current, power and energy input. Interelectrode distance is L=3 mm (upper), L=60 mm (lower). Air.

radicals increases. Volt-second characteristics of

the discharges were obtained for gap lengths from 0 to 60 mm, the following parameters were obtained: voltage on the discharge gap; current through the high-voltage electrode; total energy input in the discharge gap during high voltage pulse. These values for two interelectrode gaps are represented in Figure 1.


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Numerical Model We numerically investigated individual cathode-directed streamer discharges for the conditions of our experiments. The numerical model includes the following balance equations for charged particles [1]. It is assumed that heavy particles are immobile throughout the streamer discharge. At the initial time, the gas in the gap is assumed to be quasi-neutral. The onset of the streamer channel is modeled by specifying the electron density at a certain pre-ionization level in the immediate vicinity of the anode. The rate of gas photoionization is described in the same manner as in [2]. Comparison with Experiment Some time after a high-voltage pulse has been applied to the anode (a delayed onset), a narrow channel with a pronounced region of strong electric fields at the leading edge originates from the high-voltage electrode and extends toward the low-voltage electrode. Figure 2. Peak electric field propagation. Comparison of experimental data and calculations. Numerical profiles are represented for time moments τ=28-78 ns with a time step ∆t=5 ns.

The comparison of experimental and calculated peak reduced electric fields along a discharge axis is represented in Figure 2. In the region of relatively weak electric field (up to 600 Td) we obtain a quite good coincidence. At small distances (less than 4-5 mm) difference may be explained

by

the

fact

that

two-term

approximation of the Boltzmann equation is not valid in this region because of strong electric fields near the electrode. To compare with the experimentally studied intensities of emission of 2+ and 1- nitrogen systems the dynamics of population excited states was calculated by calculated electron density, reduced electric field and known radiative lifetime Figure 3. Density of C3Πu (upper) and B2Σu+ (lower) along a discharge axis.

and

collisional

quenching

rate.

Dependencies of production of C3Πu and B2Σu+ per unit length of the channel – states along a


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discharge gap integrated in time is represented in Fig.3. Streamer Development in Long Discharge Gap The main results presented below were obtained in experiments with streamer discharges in an N2-O2 (9:1) mixture at a pressure of 760 torr, the voltage at the anode being held constant, U = 100 kV. According to the experiments carried out in [3], this regime is characterized by a streamer flash consisting of five to ten individual streamer channels, having no considerable tendency to branch. In order to determine the extent to which the modeling of plasmochemical conversion processes in the streamer plasma should be complete, we carried out two series of simulations. In the first series, we took into account only direct electron impact processes, and, in the second series, we tried to simplify the complete kinetic scheme developed in [4] for N2-O2 mixtures to the ultimate possible extent [5]. In the first series of simulations, we ignored the electron loss processes. In the second series, we took into account the processes in which charged particle are lost

but

neglected

the

secondary

processes

in

which charged particles are produced

–

such

as

associative ionization and electron detachment from negative ions. Hence, these Figure 4. Profiles of the electric field and the density of charged particles along the discharge axis at the time 80 ns, at which the streamer head is at a distance of 10 cm from the high-voltage electrode.

two approaches yielded the upper and lower estimates of the electron density in the

streamer channel and of the global discharge parameters. The dynamics of the development shows that on time scales of 30-40 ns and longer, the streamer propagation velocity calculated with allowance for plasma kinetic processes in the streamer channel differs markedly from that calculated without allowance for the plasma kinetics. It was shown that, on time scales of up to 100 ns, electron-ion recombination processes reduce the electron density in the streamer channel by a factor of three (Fig. 4). We have also obtained that, when the electron loss processes are neglected, the discharge


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dynamics changes insignificantly: the electric field in the streamer head changes by less than 1 % and the streamer propagation velocity changes by 7 %, while the anode current and the electric field in the streamer channel change by a factor of about two. In both cases, the numerical results on the rate of development of the discharge and its geometric parameters agree well with those measured experimentally. The calculated anode current is found to coincide with the experimental one only when kinetic processes in the streamer channel are taken into account. Previous analytical and direct numerical modeling studies of a streamer discharge in a short gap [1] showed that the streamer parameters are determined exclusively by the processes in the streamer head and are uniquely related to its electric potential. A decrease in the electron density in the recombination processes leads to an increase in the electric field in the streamer channel, which, in turn, results in a higher potential drop between the anode and the streamer head and, accordingly, in a lower propagation velocity and smaller streamer radius. Conclusions At this work complex study of positive streamer development has been performed. Experimental measurements of streamer discharge dynamics in N2/O2 mixtures at different pressure have been compared with direct numerical simulation within the scope of 2D hydrodynamics approach.

Acqnowlegements The work was supported in part by RFBR (grants 01-02-17785, 02-15-99305, 03-02-06621), grant of president RF for young scientists (MK-313.2003.02), Dept. for Higher Education of Russian Federation ( 02-3.2-98), ISTC 1474 and CRDF MO-011-0.

References [1] Pancheshnyi S.V., Starikovskaia S.M., Starikovskii A.Yu.: J.Phys.D.: Appl.Phys. 34, 105 (2001) [2] M.B. Zheleznyak, A.Kh. Mnatsakanyan, and S.V. Sizykh: Teplofiz. Vys. Temp. 20, 423 (1982) [3] Won J.Yi., Williams P.F.: J.Phys.D.: Appl.Phys. 35, 205 (2002) [4] Kossyi I.A., Kostinsky A.Yu., Matveyev A.A., Silakov V.P.: Plasma Sources Sci. Technol. 1, 207 (1992) [5] S V Pancheshnyi, A Yu Starikovskii: J.Phys.D.: Appl.Phys. In press (2003)