AC BARRIER PIN-PLANE CORONA: SIMILARITIES AND DISTINCTIONS TO DC POSITIVE AND NEGATIVE CORONAS AND DIELECTRIC BARRIER DISCHARGE

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1 AC BARRIER PIN-PLANE CORONA: SIMILARITIES AND DISTINCTIONS TO DC POSITIVE AND NEGATIVE CORONAS AND DIELECTRIC BARRIER DISCHARGE Yu. S. Akishev, A.V. Demyanov, V. B Karal nik, A. E. Monich, N. I. Trushkin SRC RF TRINITI, Troitsk, Moscow region, , Russia Abstract: Alternative current (AC) atmospheric pressure corona between the sharpened pin and metallic plane covered with a thin dielectric material is widely used in applications. This kind of discharge can be classified the AC barrier corona (ACBC), because one would expect that ACBC exhibits the properties, which are close to those of both AC dielectric barrier discharge (DBD) and DC (direct current) positive and negative pin-plane coronas. Experimental and numerical study on properties of ACBC in air, nitrogen, Ar, He at atmospheric pressure was carried out. A detailed comparison between ACBC and DC positive and negative pin-plane coronas was made. 1. INTRODUCTION AC atmospheric pressure corona between the sharpened pin (or blade) and metallic plane covered with some thin dielectric material is widely used in applications. It is clearly that presence of dielectric barrier on the metallic plane provides the similarity this discharge to a classical DBD [1] excited in narrow gaps. On the other side, an existence of the sharpened metallic pin will ensure an appearance of a corona discharge in the gap with a barrier. Indeed, as a rule, the length of inter-electrode gap and frequency of the applied voltage do not exceed several millimeters and tenths of kilohertz respectively. Under such experimental conditions, the characteristic time for ion drift across inter-electrode gap is small in comparison with a half-period of the applied sine voltage. It means the charge generated in the vicinity of a sharpened electrode fills quickly the bulk, and corona discharge has a time to be formed in a pin-plane gap during each positive and negative half-periods of alternative voltage. Such type of discharge can be named the AC barrier corona (ACBC), because one would expect that ACBC exhibits the properties, which are close to those of both AC DBD and DC positive and negative pin-plane coronas. In such case, a spatial-temporal evolution of AC barrier pin-plane discharge at positive and negative half-periods will be controlled by evolution of quasi-stationary positive and negative coronas. From physical point of view, it is interesting to consider more particularly the similarities and distinctions of ACBC to DBD and DC coronas. We have carried out extensive experimental and numerical studies on physical properties of a pin-plane ACBC, and a detailed comparison between properties of this kind of discharge and both DBD and DC steady-state positive and negative coronas was done. 2. EXPERIMENTAL SETUP Experimental study of ACBC was carried out with use of an acrylic plastic gas discharge chamber of 150x150x150 mm 3 in size (Fig. 1). Pin-plane electrodes were built-in the chamber. The inter-electrode gap was varied from 1.5 to 30 mm. The radius of a pin s tip equals 0.1 mm. The plane metallic electrode was fabricated in a form of a disk of 100 mm in diameter. The edges of this disk were smoothly rounded. The different polymer films like polyethylene, polypropylene, Teflon, etc. were placed on the plane electrode. A thickness of films lies in the range of µm. Ambient air, nitrogen (99.999% of N 2 ), argon (99.99% of Ar) and helium (99.99% of He) were used in experiments as process gases. To maintain the initial purity of process gases within the discharge chamber, we used a weak flow of Ar, He and N 2 through chamber (gas velocity less than or equal to

2 0.1 m/s), and sustained this quasi-static gas at a pressure slightly in excess of ambient. An additional series of experiments was performed with air at lower pressure (P=380 Torr). We used two power supplies for the experiments; the first has a fixed sine frequency, 50 Hz, and voltage amplitude up to 35 kv. This power supply was used for excitation of AC barrier corona in ambient air. The amplitude U a and frequency f of the second power supply could be varied up to 4.5 kv, and from 50 to 10 5 Hz, respectively. The latter power supply was used for excitation of ACBC in Ar, He and N 2. The waveforms of current and voltage were recorded with an oscilloscope having a bandwidth of 10 MHz. The both a charge transferred through inter-electrode gap during one halfperiod and active discharge power were determined from the Volt-Coulomb Characteristics (VCC). Remind, VCC is displayed on the oscilloscope screen in a form of Lissajous figure; the area outlined with Lissajous figure corresponds to active energy dissipated by ACBC over one period. 3. EXPERIMENTAL RESULTS ACBC in all gases investigated appears in two states like to those in common DBD and DC coronas, which are named the diffusive (i.e. homogeneous) and streamer (i.e. non-homogeneous or filamentary) regimes. These different regimes can exist under the same U a but at various (positive and negative) half-periods. In the beginning, we present therefore some discharge characteristics averaged over period of ACBC. The most outstanding distinction of ACBC from DBD and DC coronas is an ability to expand readily over the surface of a dielectric barrier. It is well known [2], the current distribution over metallic plate for DC pin-plane corona is characterized by Warburg low. According to this low, the effective radius R w of a corona cross-section at the plane equals nearly 0.7d (d is the length of inter-electrode gap). To a first approximation, the radius R W depends only slightly on both the type of gas and corona current regime (diffusive or streamer). In bulk of the gap, the radius of DC corona current channel R c increases with distance from the pin approximately like this manner: R c R w G) 1/2 (R w 0.7d is the :DUEXUJUDGLXV LVWKHFRRUGLQDWHRULJLQDWHGIURPWKHWLSRIDSLQ,WPHDQV'&FRURQDFXUUHQWWXEH has a convex shape. An existence of a dielectric barrier on the metallic plate results in both a strong spreading the pinplane discharge over surface and modifying the shape of a corona current tube from a convex form to the concave one. The spreading of ACBC over dielectric surface is most pronounced in Ar. In this gas, the effective corona radius R c characterizing the area occupied with surface streamers ranges up to several tens (!) of d. Helium and nitrogen are next gases in this series. The spreading of ACBC in air appears only slightly. The effective radius of ACBC in air does not practically depend on U a, and equals almost Warburg radius R w. It means the properties of ACBC in air at positive and negative halfperiods must be close to those of DC positive and negative coronas. The dependence of the effective corona radius R c in Ar and He on amplitude of the applied voltage U a is presented in Fig. 2. In the case of electropositive gases, the effective corona radius R c has a different behavior at low and high currents (we mean an average discharge current during a single half-period of ACBC). At low average currents (i.e. under homogeneous regime), radius R c, as in air, does not practically depend on U a. One can state therefore that diffusive ACBC in Ar, He and N 2 exhibits itself like DC positive and negative coronas. Once streamers occur at higher average currents, ACBC in these gases expands extremely over barrier in a form of long and branching surface streamers. In contrast to DC coronas, the effective radius R c in this regime does not practically depend on the inter-electrode distance d. The sharpened electrode possesses a strong electric field around itself and provokes the formation of streamers under lower voltage across the inter-electrode gap than that in a classical DBD. It follows that streamers in ACBC are weaker than those are in a barrier discharge between plane electrodes. Therefore, ACBC with streamers in electropositive gases has nothing in common with DBD and DC coronas. Typical VCCs of ACBC at lower (without streamers) and higher (with streamers) average currents are presented in Fig. 3. Once streamers occur, the charge Q transferred through gas gap during a single

3 half-period increases dramatically with the amplitude U a and frequency f (Fig. 4). At the same frequencies and high amplitudes U a, the magnitudes of Q for ACBC in Ar and He are close to each other despite the fact that effective radii of these discharges differ greatly in size (see Fig. 2). It seems, this correlates with the most density of the surface streamers observed in He in comparison with those for ACBC in Ar and other gases. It is well known, the current waveform of a classical DBD has the same shape for the positive and negative half-periods. In the case of ACBC, this symmetry for the current waveforms is absent (Figs. 5, 6), which is to say that ACBC has different properties at positive and negative half-periods. In this connection, it is reasonably to give a detailed description of ACBC for positive and negative halfperiods separately. At positive half-period, ACBC in all gases appears in two states that are diffusive and streamer regimes. The diffusive regime is characterized by absence of sharp current splashes in oscillograms (Fig. 5a), and it exists within a narrow region of the initial corona currents no greater than tens of µa. The streamer regime is a basic one of ACBC at positive half-period. At this regime, ACBC is nonuniform. It exhibits itself in a form of numerous non-stationary streamers (thin current filaments) spreading chaotically over the barrier surface. This regime is characterized by sharp current splashes in oscillograms. The amplitudes of these splashes range from tens to hundreds of ma (Fig. 5b). A quantity of splashes in a single half-period increases with amplitude of the applied voltage U a. At the fixed magnitude of U a, a number of splashes depends on a sort of the process gas. ACBC in He shows the most number of splashes (Fig. 6a). It correlates with the most density of the surface streamers in He as noted above. At negative half-period, ACBC in all gases appears solely in a diffusive regime at low and high currents. This property is identical to that for DC negative corona, which does not show the streamer regime up to spark. The current splash of small amplitude at the beginning of a negative half-period (Figs. 5, 6) is associated with a diffusive breakdown of the gas gap (or ignition of a negative corona). After this, the diffusive quasi-stationary stage like that in common DC negative corona has been established. In the case of air, the trains of regular current pulsations (Trichel pulses) occur at the beginning and the end of a negative half-period (Fig. 6b). At small average currents, the train of current pulsations was observed also in Ar. A frequency of self-running current pulsations for DC negative corona in N 2 and He is too low, that results in absence of such pulsations for ACBC in these gases at frequencies of the applied voltage f 1 khz. So, the diffusive regime of ACBC at negative half-period is similar to DC negative corona rather than to diffusive DBD, because homogeneous DBD in narrow gap is sustained only at very low current densities corresponding to subnormal glow discharge [3]. 4. NUMERICAL RESULTS The numerical calculations of ACBC using the electric scheme drown in Fig. 7 were performed. The Volt-Ampere Characteristics of ACBC for diffusive regimes in <+> and < > half-periods were taken in a parabolic form like that for common DC positive and negative coronas: I ± = k ± U (U - U 0 ± ), where k is the dimensional coefficient; in the case of electropositive gases k + << k, in the case of air k + k. A value U 0 + coincides with the inception voltage U + of a positive corona. In the negative corona, the accordance of U 0 with an inception voltage U depends on sort of the process gas. In the case of electropositive gases, the ignition voltage of a negative corona exceeds its operating voltage: U > U 0 ; in the case of air at atmospheric pressure U U 0. The circumstances pointed were taken into account at the modeling of ACBC under negative half-period. Besides, the model for ACBC under positive half-period incorporates the transition from diffusive regime to streamer one, if the voltage imposed across the gap reaches the critical magnitude U s > U +. After this transition, the discharge voltage drops down drastically. The magnitudes of U 0 ±, U ±, U s were taken from the experiment. The calculations (Fig. 8) revealed one more property, which far differs ACBC from a classical DBD. In the event, a great negative charge on a barrier exists during most part of both negative and positive

4 half-periods. A positive charge on the surface equals almost zero at the rest of a positive half-period. An existence of different (or non-symmetrical) current regimes -diffusive and streamer- at negative and positive half-periods is physical reason of preferential accumulation of the negative charge on treated surface in ACBC. If ACBC is used in applications like surface modification, it is necessary to take this interesting property of AC pin-plane barrier corona into account. ACKNOWLEDGMENTS This work was supported in part by the RFBR (Project # ). 8W 4W,W a8 D FRVωW PP 5D $U +H 8 D N9 FIGURE 1. Scheme of experimental setup for ACBC investigations. 1 is a pin; 2 is the plane electrode; 3 is a dielectric film; 4 is the current shunt with a small inductance; 5 is the capacitor to measure a charge transferred by ACBC; 6 is the voltage divider; 7 is a ballast resistor. FIGURE 2. The effective visual radius R c of ACBC occupied by this discharge on the surface of barrier vs the applied voltage amplitude U a. Inter-electrode gap d=1.5 mm; powering frequency f=0.4 khz for ACBC in He; f=1 khz for ACBC in Ar. 4Q& $U ) +] +H ) +] a) b) FIGURE 3. Volt-Charge Characteristics of ACBC in Ar at different current regimes. f=0.4 khz. a) diffusive regimes at both negative and positive halfperiods; the voltage amplitude U a =3 kv; d=6 mm; vertical scale is 1 kv/div; horizontal scale is 1 nc/div. b) diffusive regime at negative half-period and streamer regime at positive one; U a =3 kv; d=1.5 mm; vertical scale is 1 kv/div; horizontal scale is 200 nc/div. 8 D N9 FIGURE 4. Charge Q transferred by ACBC through gas gap during single half-period vs the applied voltage amplitude U a. Inter-electrode gap d=1.5 mm; all other experimental parameters are shown in figure.

5 a) a) FIGURE 5. Representative experimental oscillograms of sine voltage and current (lower curves in photos) for ACBC under different current regimes. a) diffusive regime at both half-periods; Ar; d=6 mm; f=400hz; [I]=0.1 ma/div; [U]=4kV/div; [t]=0.5 ms/div. b) diffusive regime at negative half-period and streamer regime at positive one; N 2 ; d=1.5 mm; f=3 khz; [I]=2 ma/div; [U]=4kV/div; [t]=0.1 ms/div. b) FIGURE 6. Representative experimental oscillograms of sine voltage and current (lower curves in photos) for ACBC under diffusive regime at negative half-period and streamer regime at positive one. a) He; d=1.5 mm; f=1 khz; [I]=4 ma/div; [U]=2 kv/div; [t]=0.2 ms/div. b) air, P=380 Torr; d=5 mm; f=3 khz; [I]=0.4 ma/div; [U]=4 kv/div; photo I: [t]=0.1 ms/div; photo II: [t]=20 µs/div. b) U, V; U p, V U U p 0,006 I Q b 0,004 0,002 0,000-0,002 I, A; Q*50, C -0, ,006 2,5 2,6 2,7 2,8 2,9 3,0 3,1 3,2 Time, ms FIGURE 7. Scheme of equivalent electrical circuit that was used for modeling of ACBC. R 1 is the source resistance; R 2 is the ballast resistor; R 3 is the effective resistance of ACBC determined by its volt-ampere characteristics; C 1 is a stray capacitance of an external circuit; C 2 is capacitance of gas gap; C 3 is capacitance of barrier. FIGURE 8. Numerical calculations of a voltage drop U p across gas gap and current I of ACBC in N 2 under applied sine voltage U. Q b is a charge accumulated on the surface of a barrier having a capacitance of 25 pf. Pin-plane gap is 1.5 mm. Power frequency is 3 khz. The amplitudes of two current splashes at positive halfperiod equal about 70 ma. REFERENCES [1] B.Eliasson and U. Kogelschatz, IEEE Trans. on Plasma Sci., 19, (1991) [2] E. Warburg, Ueber die Spitzenentladung, -Wied. Ann., 67, (1899); -Charakteristik des Spitzenstormes. In: Handbuch der Physik, Vol. 4, Springer Verlag, Berlin, 1927, pp [3] F. Massines, R. Messaourdi and C. Mayoux, Plasmas and Polymers, 3, (1998)