PLASMA PHYSICS
TEMPORAL EVOLUTION OF PULSED ATMOSPHERIC PRESSURE DBD IN ASYMMETRIC CONFIGURATION G. BORCIA, C. BORCIA, N. DUMITRASCU Plasma Physics Laboratory, “Al. I. Cuza” University, Bd. Carol I, 11, Iaºi - 700506, Romania, E-mail:
[email protected],
[email protected],
[email protected] Received October 10, 2008
The temporal evolution of DBD is investigated as a function of the voltage pulse parameters, aiming to identify and analyze the factors controlling the various discharge modes. Results point out that the single pulse discharge is generated only under conditions of fast variation of the electric field, allowing effective ionization by collisions with energetic electrons. Key words: DBD, pulsed HV, electrical parameters, homogeneous regime.
1. INTRODUCTION
Dielectric barrier discharges (DBD) are convenient plasma sources for the generation of non-thermal plasmas at atmospheric pressure. These have specific advantages, as short treatment times, room temperature operation, dispensing with the vacuum equipment, excellent flexibility with respect to their geometrical shape, working gas mixture, operation parameters and scaling-up to large dimensions. Barrier discharges exist in several configurations, numerous applications of DBD being currently exploited, based on surface or volume plasma chemistry [1–8]. DBD, sometimes also referred to as silent discharges, are plasmas far from equilibrium, characterized by the presence of at least one insulating layer in contact with the discharge between two planar or cylindrical electrodes (other electrode geometries are possible) connected to an ac power supply. Although a discharge having one or two dielectric boundaries has many similarities with discharges operated between metal electrodes, one fundamental difference is that DBD cannot be operated with dc voltages because the capacitive coupling of the dielectric(s) necessitates an alternating electric field to drive a displacement current. Investigations show that, at atmospheric pressure, electrical breakdown in the DBD electrode configuration most likely occurs in a large number of
Paper presented at the National Conference of Physics, 10–13 September, 2008, Bucharest–Mãgurele, Romania. Rom. Journ. Phys., Vol. 54, Nos. 7– 8 , P. 689–697, Bucharest, 2009
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short-lived current filaments, referred to as microdischarges. If the local electric field strength in the gas gap reaches the ignition level, the breakdown starts at many points followed by the development of filaments, of nanosecond duration. Since a large number of such filaments is induced, randomly distributed in space and time, it results an average uniform distribution of microdischarges over the dielectric surface [9]. The dielectric is the key for the proper functioning of the discharge and it serves two functions: it limits the amount of charge transported by a single microdischarge, with its self-arresting effect (due to localized electrostatic charge build-up), and distributes the microdischarges over the entire electrode area. Nonetheless, a great deal of work has been dedicated to obtain DBD working in the homogenous glow regime, being demonstrated that homogenous diffuse discharges can indeed be obtained in barrier discharge configurations. Such diffuse discharges have obvious advantages over the more common filamentary type, especially for the uniform activation of material surfaces on the entire discharge exposed area [10–12]. Whereby the single current pulse of the filamentary DBD lasts for some ns, the time scale for diffuse DBD is determined by the frequency of feeding voltage. At frequencies of some KHz, the ms–s scale is relevant. However, the generation of stable diffuse DBD at atmospheric pressure requires special operation conditions, allowing only small windows of their existence. Previous studies showed that these conditions are mainly determined by the properties of the feeding gas and the feeding voltage frequency, as one important point seems to be an occurrence of effective pre-ionization, Penning ionization via metastables and primary ionization at low electric field, as compared to the conditions of the filamentary DBD mode [8, 10, 13]. Of course, the diffuse DBD mode is sensitive to impurities, admixtures, metastables and residual ions. The densities of residual species from the previous half period that can initiate the diffuse discharge generation in the next half-cycle, are dependent on the repetition frequency. Therefore, the feeding voltage frequency plays an important role in the transition to the diffuse mode. Some dielectric materials (e.g. electrets) can trap appreciable amounts of charges uniformly on the surface. When the electric field changes its polarity, the charge carriers are expelled from the surface initiating a diffuse discharge development. Although much effort has been put forth to understand the mechanism of the formation of diffuse DBD, there are still many open questions, the basic mechanisms being strongly affected by the properties of the feeding gas [8]. Taking this into account, we investigate here the temporal evolution of a pulsed atmospheric pressure DBD, produced using designed high-voltage (HV) pulses, as a function of the pulse parameters, aiming to identify and analyze the factors controlling the various discharge modes.
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2. EXPERIMENTAL
The experimental set-up is presented in Fig. 1. The discharge is generated in an asymmetrical electrode configuration, with adjustable inter-electrode gap, between 1 mm up to a few cm. The HV electrode is a 2 mm radius disc and the ground electrode is a metallic 50 mm 50 mm plate, covered with a glass plate (dielectric barrier) 1 mm thick [14]. The working gas is introduced, by a gas shower, through the perforated HV electrode. The gas flow generates a discharge beam, about 1 mm wide. It spreads on the dielectric placed on the grounded electrode due to dielectric effect, the discharge covering a surface of about 5 cm2. Under this arrangement the effective height of the surface discharge is 1 mm, whereas the discharge, covering a circular area, has axially symmetrical radial profile.
Fig. 1 – Experimental set-up.
Helium is used here as working gas, at 60 sccm constant flow rate. Helium is selected as being the most convenient gas while working at atmospheric pressure, since it allows stabilizing homogeneous glow discharges much easier compared to other gases, at lower sustaining voltages and interelectrode gaps as high as a few centimeters [10]. Importantly, in the present flow-through system, non-negligible amount of air is entrained in the plasma beam by the gas flow, conducting to occurrence of excited atomic and molecular oxygen species in the discharge. The HV pulses are controlled using a function generator connected to a custom designed HV amplifier. The positive pulses have variable parameters: 2–5 KV amplitude, 500 Hz – 10 KHz repetition frequency, 1 s – 1 ms rising time, 10–100 s pulse width and 1 s – 1 ms falling time. The time-resolved diagnosis is performed by means of electric parameters measurement and emission spectroscopy, the last one to be presented elsewhere.
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The electric parameters, i.e. voltage, current, charge, are recorded with a Lecroy 9304CM digital oscilloscope for each set of experimental conditions, the required calculations of discharge power and energy deposited during one cycle (HV pulse) of the discharge being carried out. The voltage U is monitored using a HV probe, while the voltage via a resistor connected in series to ground yields the current I. The discharge power is calculated as follows [10]:
W1 T
t T
t
I t U t dt
(1)
The resistor can also be replaced with a capacitor, where the voltage across the series capacitor is then proportional to the charge Q stored on the electrodes. The energy deposited during one cycle (pulse) of the DBD is given by the following equation [8]: E
t T
t
Q t U t dt
(2)
This latter measurement method is widely used for obtaining U-Q plots, which form Lissajous figures on a suitable oscilloscope. 3. RESULTS AND DISCUSSION
Selected examples of the DBD voltage and current waveform are presented in Figs. 2 and 3, recorded for fixed HV pulse amplitude (3 KV) and pulse width (100 s), and variable rising time and falling time within the 10–200 s range. The current waveform distinctly shows current pulses due to the primary and the secondary discharge generated by the variation of the voltage. The primary discharge is initiated by the HV pulse rising edge, whereas the secondary discharge occurs during the HV pulse falling edge. Between the primary and the secondary discharge, the current is zero. The maximum peak current is about 2.5 mA for both primary and secondary discharge, although the shape and amplitude of the current pulses are obviously changing, being controlled by the rising time and falling time of the HV pulse. For both discharges, the amplitude of the current pulse decreases with increasing the time of variation of the applied voltage. Increase of the voltage edge also conducts to multiple discharges instead of the single discharge. Importantly, the amplitude of the single discharge peak current is the same for the primary and the secondary discharge when the rising time and falling time are equal (Fig. 3). The width of the HV pulse, as expected for DBD, does not have any influence on the discharge characteristics, as tested for extended range of variation of this parameter.
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Fig. 2 – DBD voltage and current waveform for pulses with variable rising time (3 KV amplitude, 100 s width, 100 s falling time).
It would result thus that the factor controlling the discharge mode is the rate of variation of the voltage, i.e. pulse rising time for the primary discharge and falling time for the secondary discharge, whereas the repetition frequency and the pulse width have reduced or no influence. As observed in Figs. 2 and 3, the current waveform evolution for increasing pulse rising and falling time illustrates the temporal evolution of multiple discharges Townsend mode.
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Fig. 3 – DBD voltage and current waveform for pulses with variable falling time (3 KV amplitude, 100 s width, 10 s rising time).
The first stage of glow discharges is controlled by the Townsend mechanism, which presumes successive generation of avalanches and a discharge self-sustained by secondary cathode emission due to photons, metastables and ions bombardment. The Townsend breakdown mechanism effects in formation of glow diffuse plasma. For DBD, which can work both in filamentary and diffuse regime, the second one is also called homogeneous regime and depends on many parameters.
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Whereas the primary discharge develops by such acknowledged mechanism, the dominant process conducting to the secondary discharge is considered to be the so-called “memory effect”, due to the charge deposited on the dielectric surface during the primary discharge. The characteristics of the secondary discharge would thus depend on those of the primary discharge, because the amount of charge deposited on the dielectric depends on the amplitude of the primary current peak, as well as on the delay of formation of this one. Nonetheless, present results point to the opposite, in that the two discharges seem to work independently. Their respective characteristics are showing dependence only on the edge of the voltage pulse. The multiple discharges Townsend mode, where other current pulses are succeeding the first current pulse, clearly emphasized here in Fig. 2 by the presence of a second current peak during rising time, was reported in the literature and referred to as a ‘pseudoglow’ [15]. It is understood, however, that each pulse of the pseudoglow has the same characteristics, i.e. homogeneous interelectrode glow, as those of the single pulse discharge. Here, this multi-breakdown event phenomenon is observed to depend on the voltage edge, with the delay between the first and the second peak of the primary discharge increasing with the increase of the rising time. The amplitudes of the two peaks are also changing, as both are decreasing. The ratio between their amplitudes I1/I2 decreases strongly with the increase of the rising time up to about 50, then tend to level out, remaining at a value in the range 0.5–0.6, as shown in Fig. 4. The values of the discharge power, calculated by numerical integration using the I–U diagrams and Eq. (1), are presented in Figs. 5 and 6. The results are obtained for fixed HV pulse amplitude (3 KV) and width (100 s), and variable rising time and falling time within 10–200 s range. The maximum DBD power is about 7 W, obtained for the lowest rising time and falling time of the HV pulse, i.e. the fastest variation of the applied voltage.
Fig. 4 – Ratio between amplitudes of first and second current peak I1 / I2 vs. rising time of HV pulses (3 KV amplitude, 100 s width, 100 s falling time).
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Fig. 5 – DBD power for pulses with variable rising time, between 10–200 s (3 KV amplitude, 100 s width, 100 s falling time).
Fig. 6 – DBD power for pulses with variable falling time, between 10–200 s (3 KV amplitude, 100 s width, 10 s rising time).
The trend of modification observed on these representations shows again very rapid evolution versus the applied HV pulse edge in the domain up to about 50 s for the rising time, whereas this range extends up to about 100 s for the falling time. For ranges over these values, the discharge power distinctly levels out. It results that although multiple discharges occur instead of the single discharge, their total “efficiency” diminishes rapidly with decreasing the voltage edge. After some limit range, the discharge maintains its characteristics, without further change, at least in the domain investigated here, i.e. up to 200 s. It results that under fast variation of the electric field (note that 3 KV linear rise in 10 s corresponds to 3108 V/s), the breakdown conditions are favorable to single homogeneous discharge and the power of the discharge is optimized. When the variation of the electric field slows down, the discharge ignites, extinguishes completely and then reignites, whereas the delay between the first discharge and the following one(s) increases with increasing rising or falling
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time. Results suggest that the variation of the electric field should be at least of the order of 108 V/s to allow optimum homogeneous discharge conditions. The overall discharge evolution would point out the main role played by the volume processes in the helium DBD breakdown, as it was previously reported that in helium effective ionization and excitation processes occur in the electric field of the cathode region by direct collisions of atoms with energetic electrons and by three body processes, generating He+ and He2+ ions [8, 10]. Under conditions of fast variation of the electric field, generation of energetic electrons is readily achievable. 4. CONCLUSION
The temporal evolution of helium-DBD at atmospheric pressure in asymmetric configuration is investigated as a function of the voltage pulse parameters, aiming to identify and analyze the factors controlling the various discharge modes. Results point out that the single pulse discharge is generated only under conditions of fast variation of the electric field, allowing effective ionization by collisions with energetic electrons. These results could be further correlated with the time variation of the emission intensity of various atomic and molecular excited species, to be discussed taking into account the respective roles of volume collision processes (electron-heavy particle and heavy particle-heavy particle), and of dielectric barrier by means of stored charge. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
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