Positive point-to-plane corona discharge in air...

28th ICPIG, July 15-20, 2007, Prague, Czech Republic

Positive point-to-plane corona discharge in air: electrical and optical analysis

N. Merbahi(1), O. Eichwald, D. Dubois, A. Abahazem, M.Yousfi

Université de Toulouse, LAPLACE UMR du CNRS 5213, UPS, 118 rte de Narbonne, Bat 3R2, 31062 Toulouse Cedex 9, France

(1) [email protected]

This paper is devoted to electrical and optical experimental analysis of positive point-to-plane corona discharges in the dry air and different N2/O2/CO2 mixtures. The aim is a better understanding of these corona discharge features and to use them in a future work for the validation of the discharge modeling tools in the case of non thermal pollution control device. The paper details more particularly the micro-discharge evolution and morphology under several operating conditions such as the applied DC voltage magnitude and the gas composition. For this purpose, two optical techniques (CCD and streak camera) and electrical diagnostics are used. The primary and secondary streamer developments are more particularly emphasized. The velocity of the primary streamer is deduced from the analysis of both instantaneous discharge currents and streak pictures.

1. Introduction Atmospheric corona discharge is an alternative

device to the traditional pollution control processes of exhaust fumes especially when pollutants (such as NOx, SOx, COV and soot) are in weak concentration in the flue gas. The corona process goes through two main phases: during the discharge phase, which last some tens of nanoseconds, charged, excited and radical species are sowed in the corona reactor by positive streamers. Then, during the post-discharge phase, the previously created species induce a chemical reactivity leading to the pollutants transformation into harmless species or in other pollutants easier to remove. However, further experimental and theoretical investigations are needed to enhance the removal efficiency of the atmospheric non thermal plasma device used for pollution control.

The experimental results given in this paper contribute to highlight some discharge characteristics of a DC positive point to plane corona discharge reactor filled with gas mixtures at atmospheric pressure and involving different proportions of N2, O2 and CO2. The micro-discharge evolution and morphology are analysed as a function of several experimental parameters like DC applied voltage magnitude, interelectrode distance and gas composition. Several optical (streak camera) and electrical diagnostics are used in order to better understand the streamers development and characteristics under the previously listed operating conditions. 2. Experimental setup

Figure 1 shows a schematic diagram of our experimental setup. The positive corona discharges are ignited under DC voltage condition between a point to plane electrode configuration in a closed

cell made of stainless steel (1). The point is machined starting from a stem of 1mm of diameter and 5mm length. The tip radius of the point, ρ, is around 20µm. The plane electrode is a disc of copper with 4cm diameter and 2cm thickness. The inter-electrode distance, d, can be easily regulated through a screw from 0 to 15mm. It is fixed in this work at 7mm. The chamber is connected to a filling (4) and pumping (5) systems. The pressure inside the cell is given by a capacitive gauge. The Vacuum system involves a primary dry pump recorded to a turbo molecular pump. In fact, in order to limit the gas impurities, the cell is pumped from atmospheric pressure down to a secondary vacuum before each experimental investigation.

Figure 1: Experimental setup

An adjustable DC high voltage source (with a maximum magnitude of 15kV) is connected to the point electrode through a load resistance R = 25MΩ. The gap voltage, U0(t), is measured using a high voltage probe (PPE20kV) and the instantaneous discharge current i(t) is obtained through the 50Ω resistance r. The relevant voltages are recorded using a 5GHz digital sampling 500MHz oscilloscope.


For the optical measurement a high-speed streak camera (Hamamatsu TSU C7700) coupled to optical intensifier and high resolution cooled camera (C4742) give the streak picture (6). 3. Experimental results 3.1 General overview

The electrical measurements show pulsed current lasting some tens of nanoseconds and which periodically appears with a frequency of about 10kHz. The corona discharge visually appears as a single luminous channel between the two electrodes. Figure 2 gives a typical instantaneous corona current in our operating condition in the case of the dry air. ICCD pictures, integrated over current pulse duration (i.e. during the discharge phase), confirm that no branching are created (see picture inserted in figure 1).

Figure 2. Instantaneous current for dry air corona discharge (d = 7mm, Uo = 8.2kV, ρ = 20µm). Snapshot of corona discharge (time camera shutter = 10µs) The streak camera allows detailing the spatio temporal evolution of a discharge phase. Figures 3 show pictures obtained with the streak camera in dry air for different applied voltages. The brightness of figures 3 is normalised starting from the luminous intensity of picture 3c. The horizontal-axis is the time axis and the vertical-one is the interelectrode distance. The electrode location is shown in the drawing at the left side of figures 3. For a given time, the light emission of the discharge filament at each position between the electrodes is focused along the corresponding vertical-axis coordinate. Looking the results when 8.2kV is applied on the point electrode (see the discharge current shape in figure 2 and the streak picture 3d) two main phases can be distinguished during the discharge development. The first one corresponds to the primary streamer propagation from the anode point to the cathode plane (see region 1 in figure 2). The primary streamer propagates a luminous spot (streamer head) which leaves the first narrow luminous trail shown in the streak picture 3d. The

current curve in figure 2 shows that the primary streamer starts its propagation just after 5ns (corresponding to the current increase) and reaches the cathode plane (7mm underneath the point) at around 25ns according to the first maximum current peak. The sharp current rise in region 2 of figure 2 up to the first pick (from 25 to 27ns) corresponds to the high local current density rise when the streamer head reaches the cathode plane.

Following the 8.2kV results, we observed in picture 3d, the development of a secondary streamer starting from the point just after the arrival of the primary streamer on the cathode plane. The associate light emission is more diffuse thus indicating that the radiative excitation activity is distributed along the secondary preionized channel and not only in the front of them as for the primary streamer. The development and propagation of the secondary streamer induce a second current peak referenced by region 3 in figure 2. 3.2 Influence of the gap voltage

Starting from 8.2kV and decreasing the voltage magnitude down to 6.2kV, the discharge characteristics change remarkably as shown for example by the instantaneous corona current shape and magnitude in figure 4. At lower applied voltage, the second current peak, associated to the development of the secondary streamer, vanishes. Nevertheless, at 7.2kV, the current relaxation is slowed just after the current peak due to the secondary streamer development. The streak pictures of figure 3 clearly show that the light emission of the secondary streamer decreases when the DC applied voltage decreases. The secondary streamer disappears at 6.2kV.

The velocity and light emission of the primary streamer are also affected by the applied voltage variation. The mean primary streamer velocity can be estimated by using either the current curves or the streak camera pictures [1]. The results given from the two methods are compared in figure 5. Other authors [2-5] have also studied the primary streamer velocity with one of these methods. Figure 5 shows that the mean streamer velocity increases with the applied voltage. Both methods used to calculate the mean streamer velocity give the same variation tendency.

Looking at the streak picture 3a associated to the 6.2kV gap voltage, the streamer head velocity is quite constant. In fact, the trail left by the light emission of the streamer head during its propagation is a linear time-space function. At a voltage higher than 6.2kV, the streamer head accelerates when approaching the cathode plane and its brightness increases too (see pictures 3b, c and d).


Figures 3. Streak camera pictures for different DC voltages magnitudes in dry air at atmospheric pressure and

ambient temperature. (a) U0 = 6.2 kV, (b) U0 = 6.8 kV, (c) U0 = 7.2 kV, (d) U0 = 8.2 kV

Figure 4: Current peak for different gap voltages. The current are aligned in order to make easier the comparison.

These results mean that the dynamics of the charged space ionization wave becomes higher so that the electron energy gained in the streamer head increases during the discharge propagation. On can notice that for an applied voltage of 8.2kV, the secondary streamer propagates towards the cathode up to the mid-gap (see picture 3d). However, if it propagates above the mid-gap, by increasing the gap voltage, the spark discharge is obtained. This description is similar to those given by different authors [6-8]. 3.3 Influence of the gas composition

The effect of gas composition on the discharge evolution is analysed by increasing the proportion of carbon dioxide in a mixture of N2, O2 and CO2, where the O2 proportion is kept constant at 20%. The DC applied voltage U0 is fixed at 7.2kV.

Figure 5: Mean velocity of the primary streamer for different gap voltages

Figures 6, 7 and 8 show respectively the current curves, the streak camera pictures and the mean primary streamer velocity for different proportions of CO2 (0% corresponding to the previous result in dry air, 5%, 10% and 15%) in the N2, O2, CO2 gas mixture.

The effects of increasing the carbon dioxide proportion in the gas mixture are not linear. In fact, the results clearly indicate that the discharge dynamics is higher when only 5% of CO2 is added. Indeed, the current peak maximum is higher (see current curve 75/20/05 in figure 6) as well as the mean streamer velocity (see figure 8). Furthermore, the secondary streamer is more luminous (see picture 7b) in comparison to the other gas mixture. The associated secondary streamer current can be distinguished in the current curve of figure 6 because of the slowing down of the current decrease just after the peak.


Figure 7. Streak camera pictures for different N2/O2/CO2 gas mixture (a) N2 = 80 %, O2 = 20 %, CO2 = 0 %, (b)

N2 = 75 %, O2 = 20 %, CO2 = 5 %, (c) N2 = 70 %, O2 = 20 %, CO2 = 10 %, (d) N2 = 65 %, O2 = 20 %, CO2 = 15 %

Figure 6. Current pulse shapes for different gas mixtures of N2/O2/CO2 under DC voltage conditions

However, increasing the CO2 proportion above 5% limits the streamer dynamics. Indeed, with 10% of CO2, the discharge behavior is quite similar to the one obtained in dry air. The maximum current intensity is around 14mA (compare the current curves in the gas composition cases 80/20/00 and 70/20/10 shown in figure 6). The mean primary streamer velocity is roughly around 2x107cm s-1 (depending on used methods) and the corresponding streak pictures 7a and 7c are also quite similar. The adjunction of 20% of CO2 still limits the primary and secondary streamer dynamics.

These observed tendencies can be explained as follow. Adding a small proportion of carbon dioxide in dry synthetic air makes the gas reactivity more complex. The results show that a small proportion of CO2 (less than 5%) enhances the ionization process efficiency in the streamer head thus leading to a higher discharge dynamics. However, as already shown, increasing the CO2 proportion beyond 5% (for a fixed O2 percentage of 20%) limits the discharge dynamics. In fact, O2 and CO2 are able to create negative ions (O2

-, CO2- and O-) by either

three body, or direct or dissociative attachment (see eg [9,10]). The two last reactions are more efficient for high reduced electric field while the first one is more efficient at low reduced electric field.

Figure 8. Velocity of the primary streamer for different gas mixtures N2/O2/CO2 under DC voltage conditions

Nevertheless, the results indicate that when the CO2 proportion increases, the attachment processes are more efficient. This therefore consumes the available electrons in the electrodes gap. As the electronic current density is the main contribution to the discharge current, the electronic density diminution in the ionized channel limits the current intensity and the primary and the secondary streamer development. References [1] E. Marode. (1975) J. App. Phys, 4, 2005-15 [2] R. Ono, T. Oda. J. Phys. D: Appl. Phys, 36, (2003) 1952-58. [3] P. Tardiveau, E. Marode, A. Agneray. J. Phys. D: Appl. Phys, 35, (2002) 2823-29. [4] E.M. van Veldhuizen, W.R. Rutgers. J. Phys. D: Appl. Phys, 35, (2002) 2169-79. [5] E.M. van Veldhuizen, W.R. Rutgers. J. Phys. D: Appl. Phys, 36, (2003) 2692-96. [6] L. B. Loeb. Univ. of California press. (1991) [7] E. Nasser. Wiley intersciences (1971). [8] D. Bessière, J. Paillol, A. Gibert, L. Pécastaing, T. Reess. XV GD, Toulouse (2004). [9] J. de Urquijo, J.L. Henandez-Avila And S. Rodrigez, 27th ICPIG Eindhoven 18-22 July (2005) [10] D. Dubois, N. Merbahi, O. Eichwald, M. Yousfi, M. Benhenni, (2007), J. Appl. Phys. 101

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