IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS J. Phys. D: Appl. Phys. 42 (2009) 045213 (8pp) doi:10.1088/0022-3727/42/4/045213 Electrical modelling approach for discharge analysis of a coaxial DBD tube filled with argon U N Pal1 , A K Sharma1 , J S Soni1 , Sonu Kr1 , H Khatun1 , M Kumar1 , B L Meena1 , M S Tyagi1 , B-J Lee2 , M Iberler2 , J Jacoby2 and K Frank3 1 Microwave Tubes Area, Central Electronics Engineering Research Institute (Council of Scientific and Industrial Research), Pilani, Rajasthan-333031, India 2 Institute of Applied Physics, Johann Wolfgang Goethe University, 60438 Frankfurt am Main, Germany 3 University of Erlangen-Nuremberg, Erwin-Rommel-Str. 1, D-91058 Erlangen, Germany E-mail: [email protected] Received 25 July 2008, in final form 13 November 2008 Published 30 January 2009 Online at stacks.iop.org/JPhysD/42/045213 Abstract Dielectric barrier discharges (DBDs) occur in the presence of at least one insulating layer in contact with the discharge between two planar or cylindrical electrodes connected to a high voltage supply. A quartz coaxial DBD tube, filled with argon, has been studied and an electrical model characterizing the discharges has been proposed. The proposed model considers the geometry of the DBD tube, gas gap spacing and the properties of the dielectric barrier material. A sinusoidal voltage up to 2.4 kV peak with frequencies from 20 to 100 kHz has been applied to the discharge electrodes for the generation of microdischarges. By comparisons of visual images and electrical waveforms, the filamentary discharges have been confirmed. The simulated discharge characteristics have been validated by the experimental results. A good correlation between the experimental and simulated results was found, which is used to deduce the circuit impedance and other electrical parameters, in particular, the conduction current, charge accumulation, energy deposited and consumed power during discharges. (Some figures in this article are in colour only in the electronic version) on experimental conditions such as discharge gas, gas pressure, gas gap, dielectric surface properties and applied voltage waveform [5–11]. In the recent literature, many researchers have explored new efficient VUV and UV sources based on the radiation of excimer or exciplexes excited by DBDs. DBD modelling has been of interest in the literature. DBD configuration and dielectric phenomenon are represented by analogous electrical components. Kogelschatz [12], Chiper et al [13] and Naud´e et al [14] included a variable resistor, which represents the microdischarge channels. Bhosle et al [15, 16] included a resistor ignited by a TRIAC device in order to implement the single filamentary discharge and the highly nonlinear variable impedance to represent the discharges. Liu and Neiger [17] proposed an electrical model of a DBD under an arbitrary excitation voltage. Valdivia-Barrentos et al [18] have used the 1. Introduction Dielectric barrier discharges (DBDs), also referred to as silent discharges, are generated in discharge configuration with at least one dielectric barrier between two planar or cylindrical electrodes connected to an ac or pulse power supply. These dielectric layers act as a current limiter and prevent the formation of a spark or an arc discharge. DBDs are the easy way to a generate non-thermal and non-equilibrium plasma at atmospheric pressure [1]. DBDs are considered as promising alternatives to conventional mercury based discharge plasmas producing highly efficient vacuum ultraviolet (VUV) and ultraviolet (UV) radiations and have found a number of industrial applications ranging from plasma display panels to surface treatment [2–4]. The discharge appearance of DBDs can be either filamentary or homogeneous, depending 0022-3727/09/045213+08$30.00 1 © 2009 IOP Publishing Ltd Printed in the UK J. Phys. D: Appl. Phys. 42 (2009) 045213 U N Pal et al and the inner electrode of Cusil foil has been inserted into the coaxial tube in close proximity to the inner wall. A high voltage signal is applied on the inner electrode, while the outer mesh electrode is grounded. The inner radius of the outer quartz tube is 18.5 mm and the thickness is 1.5 mm, while the inner radius of the inner quartz tube is 15 mm and the thickness is 2 mm. The gas gap is 1.5 mm. The total length of the DBD tube is 30 cm while the outer mesh and the inner foil electrode wrap 9 cm of the tube. 2.2. Set-up Figure 2 shows the experimental set-up. A sinusoidal high voltage supply (Huttinger HF Generator TIG 10/100 PSC) up to 2.4 kV peak with frequencies from 20 to 100 kHz has been applied to the discharge electrodes for the generation of microdischarges. The variable oscillator circuit capacitances of the generator (parallel connection of individual capacitors within a block) make it possible to change the operating frequencies and matching to the output load. The DBD cell has been mounted on an ultra-high vacuum pump station. It has been evacuated to 8 × 10−9 mbar pressure and baked up to 450 ◦ C to achieve an ultimate pressure of 5 × 10−9 mbar. At room temperature, argon gas of 99.99% purity (BOC Gases) has been filled in the DBD cell. The gas flow has been controlled by a mass flow controller (Matheson: 82720453). The pressure has been measured by pressure gauges (Leybold: 16040, Pfeiffer: PKR251) and the pressure has been maintained by vacuum valves (Matheson: 316L, Varian: 9515091). The outer wire mesh electrode acts as a cathode while the inner foil electrode as an anode. The wire mesh electrode allows the radiation to come out of the tube for spectroscopic analysis. An external capacitor Cext (500 pF) has been used to measure transferred charges. A 1 : 1000 high voltage probe (Tektronix P6015A) measures the voltage across the DBD tube and the Rogowski-type Pearson current monitor, Model 110 (0.1 V A−1 , 1 Hz–20 MHz, 20 ns usable rise time) measures the total current flowing through the DBD tube. The total current and applied voltage waveforms are visualized by means of a four-channel Tektronics TDS 3034B digital oscilloscope. The oscilloscope is interfaced with the personal computer for real time analysis and recording the voltage and current waveforms. Figure 1. (a) Fabricated coaxial DBD tube and (b) schematic description of the coaxial tube. concept given in [17] to simulate the model by superimposing microdischarges on the displacement current waveform for different operating frequencies. We have proposed a novel simulation model of DBD based on the equivalent electrical circuit for an argon filled DBD cell. In our approach, we have compared the experimental and simulated DBD discharge characteristics explicitly, which includes the total current, applied voltage, gas gap voltage, discharge current, supplied power and consumed power. We have also deduced the change in the plasma impedance based on the correlation of the experimental and simulated results of all discharge characteristics, which differs from earlier works [15–18]. A quartz coaxial DBD cell filled with argon gas has been used for the experiments and the discharges are characterized under different operating conditions. Experimental data have been used for numerically solving the circuit equations to determine the discharge characteristics such as the gap voltage, discharge current, charge accumulation and the power consumed during discharge. This model is implemented using MATLAB Simulink in which the observed discharge conditions have been included. The results obtained from the simulated model have been compared with the experimental results. With good correlation between the experimental discharge characteristics and the simulated results the dynamic nature of discharge impedance has been deduced. 3. Electrical analysis and modelling 3.1. Equivalent electrical circuit An electrical analogous circuit of the DBD tube is shown in figure 3. The equivalent electrical model of the DBD tube consists of three capacitors in series connection. The inner and outer quartz tubes form the dielectric barrier capacitance Cd1 and, Cd2 whereas the gas gap forms the capacitance Cg . Since Cd1 and Cd2 are in series combination, it can be represented by a single capacitance Cd . The equivalent capacitance has been calculated theoretically using coaxial topology of a two quartz barrier. With the experimental values of voltage and current, the effect of the additional capacitance Cs in parallel to 2. Experimental set-up 2.1. DBD cell design Figure 1(a) shows the picture of a coaxial DBD cell filled with argon. Figure 1(b) depicts the schematic of the coaxial DBD lamp consisting of two fused quartz tubes. The outer surface of the quartz tube is wrapped by a copper wire mesh electrode 2 J. Phys. D: Appl. Phys. 42 (2009) 045213 U N Pal et al DBD Tube Pressure Gauge HV Probe HF Generator TIG 10/100 PSC (20-100 kHz) Valve Rogowski coil Gas Cylinder Cext Computer Vacuum Pump Oscilloscope Figure 2. Experimental set-up. Itc(t) where Vd = Vd1 + Vd2 is the voltage across the dielectric and Cd = Cd1 Cd2 /(Cd1 + Cd2 ) is the total capacitance of the dielectric: dVg (t) Idg (t) = Cg . (5) dt Differentiating equation (1) with respect to time and substituting (4) and (5) in (1), Idbd(t) Cd1 Isc(t) Vd1(t) Sw Va(t) Cs Cg Vg(t) Zd Idis(t) dVa (t) Idbd (t) 1 . (Idbd (t) − Idis (t)) + = dt Cg Cd (t) Idg(t) Cd2 Vd2(t) Rearranging equation (6), Cg dVa (t) Idbd (t) − Cg Idis (t) = 1 + Cd dt Figure 3. Equivalent electrical circuit of the DBD tube. the DBD cell has been noticed. It represents the parasitic capacitance along with the cable capacitance of the RF supply and additional electrical circuit components. The impedance of microdischarges is represented by Zd , which is in parallel to Cg . Two discharges are taking place for a particular time interval during one complete cycle of the applied voltage waveform [18]. The switch Sw is used to act as a virtual circuit component to represent this phenomenon. The discharge plasma is represented by a voltage controlled current source Idis (t). This conductive discharge current depends on the gas gap voltage Vg (t). In this model Va (t), Vd1 (t), Vd2 (t), Idg (t), Isc (t) and Itc (t) are the externally excited voltage, the voltage across the inner and outer dielectric barriers, the displacement current through the gap, the displacement current through the Cs and the external current, respectively. Using Kirchoff’s theorem for the circuit given in figure 3, we obtained the following equations: Va (t) = Vd (t) + Vg (t), (1) Itc (t) = Idbd (t) + Isc (t), (2) Idbd (t) = Idis (t) + Idg (t). (3) dVd (t) , dt (7) The dielectric barrier voltage, Vd (t), and the gas gap voltage, Vg (t), are 1 Itc (t) dt + Vm0 , (8) Vd (t) = Cd 1 Vg (t) = Va (t) − Itc (t) dt − Vm0 , (9) Cd and the memory voltage for ac voltage excitation is given by Vm0 = − 1 2Cd T /2 Idbd (t) dt. (10) 0 In equations (8) and (9), Vm0 corresponds to the memory voltage, induced by charge accumulation on the dielectric barriers during the previous half period. In other words, Vm0 ‘memorizes’ the previous discharge events. The memory voltage has a different value corresponding to the applied voltage waveform [17]. The numbers of data points for Va (t) and Itc (t) curves have been obtained by connecting the oscilloscope to the computer. These experimental values are used to solve the above equations for evaluating all the discharge characteristics. Instead of directly using the capacitance value of the DBD cell from the numerical calculation or the LCR meter, we have deduced the values from the experimental data. The effect of stray capacitance is observed when an analytical study has been done in the no discharge activity region of the Itc (t) curves. In The total external current through DBD, Idbd (t), and the displacement current through the gap, Idg (t), can be written as Idbd (t) = Cd . (6) (4) 3 J. Phys. D: Appl. Phys. 42 (2009) 045213 U N Pal et al However, parameters governing ignition and extinction of microdischarges are taken into account. The effect of discharge phenomenon on the gas gap capacitance is considered, which has also been proved as an efficient way of analysing the plasma impedance during discharge. The capacitance values used in this simulation model have been deduced from figure 4. The pulse generator which derives the switch Sw is programmed according to ignition and extinction discharge timings. The timing is deduced from the observed breakdown voltage and the frequency of the experimental results. The switch Sw , which is actually a virtual circuit component for numerical analysis of experimental data, is effectively used in this simulation to investigate the effect of impedance in the discharge phenomenon. It reduces the whole circuit to a purely capacitive circuit when no discharge is taking place. During the discharge period, when the switch Sw is on, the plasma impedance is introduced into the circuit. The plasma impedance comprises a capacitor Cdis in series with resistance Rdis , which represents the resistance of filamentary microdischarges. The capacitor Cdis is different from the gas capacitance Cg as it varies due to a change in the relative permittivity of gas during ionization [20]. Since it is not possible to determine the values of Cdis and Rdis experimentally, simulation has been done on multiple values of impedance until a good correlation between experimental and simulated results has been achieved. The controlled current source block has been modelled cautiously so that it cannot deviate from equation (7). Rc represents the resistance of the wires and connectors in the circuit, which is of the order of tens of ohms. Different values of voltage, current and power are directly measured by providing measuring components at various nodes of the circuit. Thus, this modelling proves faster methods of analysing discharge characteristics as compared with the long numerical calculations. 1000 750 Ceq (pF) 500 250 0 -250 -500 -750 -1000 0 5 10 Time (µs) 15 20 Figure 4. Equivalent capacitance waveform of the DBD cell for one period. 4. Results and discussion 4.1. Discharge mode Figure 5. Simulation model in Simulink. In the experiment the voltage applied to the metal foil electrode and mesh electrode has been manually increased very slowly. When the applied voltage rose to a certain value, Vbd (breakdown voltage), the discharge began with some filaments distributed on the dielectric wall, but the intensity of the visible light emitted from the discharge gap was very low. If the applied voltage is increased further, the numbers of filaments increases and finally gets diffuse. Figure 6(a) shows the average image of discharges taken with a digital camera (SONY DSC-P100, exposure time: 25 ms). The image makes sure that the diffuse discharge covers the entire surface of the electrodes. Figure 6(b) shows the total current trace together with the applied voltage, where the discharge current waveform having a number of current pulses with nanosecond order, which are superimposed on the total current, confirms filamentary discharges [12]. this region the total DBD can be considered as a pure capacitive circuit. During this duration, the total displacement current through the DBD cell can be written as Itc (t) = Ceq Va (t)/dt, which is given in figure 4. The average value of the curve is determined without considering the effect of peaks, which arises due to numerical singularities occurring during the zero crossing of the denominator [19]. This value of the average capacitance is carefully used to nullify the effect of the stray capacitance and other unwanted circuit noise so that it cannot influence the actual discharge characteristics. 3.2. Equivalent Simulink model The simulation model made in Simulink is shown in figure 5. This simulation model is based on the equivalent electrical circuit proposed in figure 3, in which gas properties are not considered. Thus, emphasis has been mainly laid on the electrical operating conditions of the circuit. 4.2. Discharge characteristics Figure 7(a) represents the applied voltage Va (t) and the total current Itc (t) waveforms measured at 1000 mbar argon 4 J. Phys. D: Appl. Phys. 42 (2009) 045213 U N Pal et al Figure 6. DBD in argon atmosphere (pressure: 1000 mbar, f = 45.7 kHz): (a) picture of the diffused discharge and (b) applied voltage and total current waveforms. atmosphere for 45.7 kHz frequency. Figure 7(b) represents the simulated results for the same operating conditions. The amplitudes of total current for both experimental and simulated curves are same but the phases after the occurrence of discharges are different. This difference is owing to discrete time modelling of the simulation model, in which the plasma impedance is affected for a particular time only and the decay time of this effect is nearly zero. Furthermore, the inclusion of the plasma impedance is a one time phenomenon which is evident from the single discharge peak in the simulation result. This simulation model is a basic step towards a novel approach to analyse dynamic behaviour of the plasma impedance in nano time scale. The emphasis is mainly on the overall performance of simulation in terms of the amplitude and nature of discharge peaks. The dynamic behaviour of different voltages for the DBD cell has been calculated from theoretical equations and given in figure 8. The dielectric barrier voltage Vd (t) and memory voltage Vm0 are calculated using equations (8) and (10), respectively. Discharge occurs when the applied voltage reaches the breakdown voltage and results in significant electron production. After that, the produced electrons move towards the momentary anode driven by gap voltage and reverse the polarity of the initial memory voltage, increasing its magnitude in the direction opposite to the applied voltage Va (t). It is clearly evident that the gap voltage attains a positive value prior to the applied voltage which confirms the effect of the memory voltage (figure 8). The gas gap voltage has been calculated according to equation (9). Figure 9 shows that the gap voltage increases with the external applied voltage nearly at the same rate till the external voltage reaches the breakdown value. The small hump in the gap voltage marks the ignition condition. The discharge hump corresponds to the weakening of the internal electric field Figure 7. Applied voltage and total current waveform (gas: argon at 1000 mbar, f = 45.7 kHz): (a) experimental and (b) simulated results. Va (t) Vg (t) Vm0 (t) Vd (t) 1000 Voltage (V) 500 0 -500 -1000 0 5 10 15 Time (µs) 20 25 Figure 8. Experimental values of different voltages (gas: argon at 1000 mbar, f = 45.7 kHz) for the DBD cell. in the gas gap due to the momentary flow of charges during discharge. It is observed that when the breakdown occurs, the gas gap voltage and the external applied voltage are 310 V and 392 V, respectively. During the second discharge, the gas gap voltage increases to −561 V while the external voltage to 5 J. Phys. D: Appl. Phys. 42 (2009) 045213 1000 U N Pal et al (a) 60 Va(t) Vg(t) (a) Idis(t) Itc(t) 40 Current (mA) Voltage (V) 500 0 -500 20 0 -20 -40 -1000 -60 0 5 10 15 20 25 0 Time (µs) (b) 10 15 80 25 (b) Itc(t) Idis(t) 60 500 40 Current (mA) Voltage (V) 20 Time (µs) Va(t) Vg(t) 1000 5 0 -500 20 0 -20 -40 -60 -1000 -80 0 5 10 15 20 25 0 Time (µs) Pdis (t) = Vg (t)Idis (t). (12) 15 20 25 Figure 10. Comparison of the total current Itc (t) and the discharge current Idis (t) (gas: argon at 1000 mbar, f = 45.7 kHz): (a) experimental and (b) simulated results. −380 V. From the total current Itc (t), the discharge current is extracted according to the relation given in equation (7). The time evolution of the discharge current in figure 10 shows two current pulses, one in the rising half cycle and other in the falling half cycle. The width of the first current peak is larger than that of the second current peak. The discharge current Idis (t) has its peak values only during the discharge period and a very low value or nearly zero for the remaining time. The instantaneous input power delivered by the electric supply, Psup (t), and the power consumed, Pdis (t), during the discharge are, respectively, (11) 10 Time (µs) Figure 9. Comparison of experimental and simulated values of the gap voltage (gas: argon at 1000 mbar, f = 45.7 kHz): (a) experimental and (b) simulated results. Psup (t) = Va (t)Itc (t), 5 The electrical energy deposited in one discharge is Wdep = Pdis . 2f (15) The instantaneous values of the input power and the consumed power during the discharge have been obtained using equations (11) and (12) and the comparison is shown in figure 11. The real power input occurs during the discharge phase. Figure 11 shows that the external circuit provides the same power during both discharges. However, the consumed power in the second discharge is higher, as the memory charges from the previous discharge deposited on the dielectric surface contribute to the energy consumed [17]. The average power supplied and consumed in the DBD cell are calculated using equations (13) and (14) and are found to be 15.09 W and 4.85 W, respectively. The energy deposited by a single discharge, Wdep , has been calculated by equation (15) and is 53 µJ. This deposited energy can also be calculated by the enclosed area of the Lissajous figure. From figure 12, the enclosed area is 44.8 µJ. The theoretical calculated energy and the measured energy obtained from Lissajous figure are nearly equal. With all these The mean value of the supplied power Psup and the consumed power Pdis are as follows: 1 T Psup (t) dt, (13) Psup = T 0 1 T Pdis (t) dt. (14) Pdis = T 0 6 J. Phys. D: Appl. Phys. 42 (2009) 045213 30 U N Pal et al (a) the experiment it is not possible to determine the plasma impedance. In this model, the plasma impedance during the discharge is taken into account, which has been brought into effect in the simulation model as discussed in section 3.2. The good correlation between the experimental and simulated discharge characteristics helps in inferring the value of this plasma impedance. It is observed that during the discharge, the gap capacitance reduces to 42% of its original value. The resistive part of the impedance is nearly 4.8% of the total discharge impedance. Thus, with this simulation model we can infer the dynamic nature of the plasma impedance. The ongoing research includes the effect of the applied frequency on the plasma impedance which will help us in achieving the maximum efficiency condition for the DBD cell and also in designing an efficient power supply for the system. Psup(t) Pdis(t) 25 20 Power (W) 15 10 5 0 -5 -10 -15 0 5 10 15 20 25 Time (µs) 30 (b) 5. Conclusion Psup(t) Pdis(t) 25 150 The discharges are analysed in the coaxial DBD cell and are found to be the filamentary. An electrical circuit has been proposed for the discharge analysis of the DBD cell. For this, equations based on the equivalent electrical circuit have been formulated. The dynamic behaviour of the discharge parameters (gas gap voltage, discharge current, supplied power, consumed power and the energy stored, etc) has been studied. The behaviour of memory charges gave logical reasons for the observed dynamic changes in the gap and dielectric barrier voltages. An analogous model for the proposed electrical circuit has been implemented in the MATLAB Simulink. A good correlation has been achieved between the dynamic behaviour of the discharge characteristics evaluated with the simulation model and the experimental values, which validates the functionality of the model. The model allows us to estimate the plasma impedance, which cannot be directly measured during the experiment. During discharge, the gap capacitance value reduces to 42% of its original value and the resistive part is 4.8% of the total plasma impedance. 100 Acknowledgments Power (W) 20 15 10 5 0 -5 0 5 10 15 20 25 Time (µs) Figure 11. Instantaneous Psup (t) and Pdis (t): (a) experimental and (b) simulated results. Qdbd (nC) 200 The work has been carried out under the CSIR Network Programme and under the CSIR-DLR collaboration Science Programme. 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