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## Research Paper

Applied Science and Convergence Technology 2020; 29(6): 170-175

Published online November 30, 2020

https://doi.org/10.5757/ASCT.2020.29.6.170

## Experimental Study of Argon and Helium Dielectric Barrier Discharge with Coplanar Electrodes at Intermediate Pressure for Reducing Radar Cross Section

Jang Jae Leea , Si Jun Kima , b , Young Seok Leea , Chul Hee Choa , Min Su Choia , In Ho Seonga , Sang Ho Leea , Won Nyoung Jeonga , and Shin Jae Youa , c , *

aDepartment of Physics, Chungnam National University, Daejeon 34134, Republic of Korea
bNanotech Optoelectronics Research Center, Yongin 16882, Republic of Korea
cInstitute of Quantum Systems (IQS), Chungnam National University, Daejeon 34134, Republic of Korea

Correspondence to:E-mail: sjyou@cnu.ac.kr

Received: October 12, 2020; Revised: November 18, 2020; Accepted: November 19, 2020

### Abstract

In this study, we experimentally investigated the discharge characteristics of argon and helium dielectric barrier discharges with coplanar electrodes at intermediate pressure to reduce the radar cross section for plasma-based stealth applications. The discharge patterns of argon and helium were investigated according to the pressure, driving frequency, and gas ratio with a fixed input voltage, and the discharge voltages and currents were measured. The power dissipated by the device in one cycle was calculated using the measured values. From the discharge pattern and measured values, we confirmed that a bright glow discharge occurred at a few Torr of argon and tens of Torr of helium, and the discharge was brighter and more uniform in the driving frequency range 5–6 kHz. When a mixture of the two gases was used at tens of Torr, the brightness of the discharge pattern and the number of current pulses increased with the proportion of helium, while the intensity of the current pulses decreased. We confirmed that an efficient and stable discharge can be generated at frequencies 5–6 kHz using several Torr of argon or tens of Torr of helium.

Keywords: Coplanar dielectric barrier discharge, Intermediate pressure, Plasma, Radar cross section

### 1. Introduction

Dielectric barrier discharges (DBDs) are self-sustaining electrical discharges at electrodes that include dielectric materials in their discharge path. They are structurally simple, consume less power than other types of devices, and can be designed using diverse patterns. There are various configurations of DBDs depending on the dielectric and electrode structure, discharge path, etc. [1]. Owing to these characteristics, DBDs are used in processes such as powder surface treatment [2], surface cleaning and activation [3,4], and atomic layer deposition [5] as well as in various applications such as plasma display panels [6] and flat light sources [7,8].

In addition, DBDs have been used in electromagnetic stealth technology [9,10] because they have a simple structure compared with other plasma sources and their size can be easily adjusted. Wolf et al. [9] measured the radar cross section (RCS) as an indicator of stealth capabilities by generating a DBD at atmospheric pressure and calculated the attenuation effect of plasma using the Lorentz model to compare the measured RCS. Ha et al. [10] also measured the RCS using a DBD fabricated with different configurations and compared the measured values with electromagnetic simulation results. However, most of the plasma parameters used in the abovementioned calculations were based on the results of other research groups, and the plasma properties were not measured experimentally. In addition, other studies have confirmed that the RCS can be reduced more significantly at low pressures [11] and when an inert gas such as argon or helium is discharged [12]. However, experimental studies on the characteristics of DBDs with gases below atmospheric pressure, especially a few Torr or tens of Torr of pressure, have not been reported thus far.

In this study, we experimentally investigated the discharge characteristics of argon and helium DBDs with coplanar electrodes at intermediate pressure. Coplanar DBD devices are a configuration in which both electrodes are situated on the same side of the dielectric material, and plasma is generated on the dielectric surface. Coplanar DBDs have a higher power density than volume DBDs and their plasma is more uniform [1]; hence, they are promising structures for stealth applications. Although Cech et al. studied coplanar DBDs using nitrogen gas for a wide range of pressure [13], no coplanar DBD has been studied using other gases at sub-atmospheric pressure. For the coplanar argon and helium DBD, we measured the discharge pattern, voltage, and current at various frequencies and pressures with a fixed voltage. In addition, when a mixture of argon and helium was used, we investigated the discharge characteristics according to the gas ratio.

### 2. Experimental details

Figure 1 shows the structure of the coplanar DBD employed in our study. Two tempered glass plates with a thickness of 5 mm and a surface area of 200 × 200 m2 were used. Each plate had a hole near one edge for connection with the gas injection and pumping system. A space for generating plasma was provided by inserting a 3-mm-thick rubber gasket between the two glass plates. Copper electrodes with a width of 10 mm, thickness of 70 μm, and spacing of 5 mm were attached to the lower glass plate of the plasma generator, and polyimide tape with a thickness of 65 μm was attached as a dielectric material thereon. The above-mentioned materials were combined to form the DBD using polyetheretherketone (PEEK) as a jig.

Figure 1. Structure of DBD generator.

Figure 2 shows a schematic of the experimental setup for evaluating the characteristics of the coplanar DBD plasma source used in this study. A square-wave voltage source with an output voltage of 0–1.25 kV and a frequency of 0–10 kHz was used to generate plasma. The electrical characteristics of the DBD plasma depends on the input voltage waveform. The square waveform has a higher voltage change rate over time than other input waveforms; hence, the density of the current peak is high, and the discharge area is wide, resulting in a broad distribution of the excited species [14]. A rotary pump was employed to maintain the background gas pressure in the DBD below 10-2 Torr. The pressure was measured using a capacitive diaphragm gauge (Baratron, MKS Instruments Inc.) and adjusted using an angle valve connected between the pump and the DBD. Argon and helium were supplied at a fixed flow rate of 0.031 slm via a mass flow controller (M3030VA, Line Tech Inc.), which is the minimum value that can be injected in this experiment. An oscilloscope (TDS3052A, Tektronix Inc.) connected with a voltage probe (TekP5100, Tektronix Inc.) and current probe (TCP202, Tektronix Inc.) was used to measure the voltage and current waveforms.

Figure 2. Schematic of the experimental setup.

### 3. Results and discussion

Figure 3 shows the discharge pattern of argon and helium in the coplanar DBD according to the pressure. In this case, the driving frequency was fixed at 10 kHz so that we could investigate the discharge pattern in a wide range of pressure. As shown in Fig. 3(a), when using argon, the discharge area darkened and local filamentary discharges occurred as the pressure increased. In contrast, when using helium [Fig. 3(b)], the discharge area increased as the pressure increased up to around 50 Torr; subsequently, the discharge area became darker, and arcing occurred at the edge of the electrode as the pressure increased above 50 Torr.

Figure 3. Discharge patterns of the (a) argon and (b) helium discharges at different pressures under an input voltage of 1.25 kV and frequency of 10.0 kHz.

To understand the tendency of the discharge pattern according to the pressure change described above, we measured the discharge voltage and current. Figure 4 shows the measured voltage and current waveforms according to the pressure in the case of the argon discharge. As the pressure increased, the number of current pulses per cycle decreased, while the intensity of the current pulses increased. There was no significant difference in the case of the discharge voltage. At higher pressures, it was impossible to measure the discharge voltage supply itself owing to excessive current. Meanwhile, in the case of the helium discharge, the voltage and current tendencies varied with the pressure range. Below 10 Torr, when the applied voltage was greater than the breakdown voltage, a current spike was formed per halfperiod. Figure 5(a) shows that discharge occurred when the voltage was 0.4 V at 3.6 Torr. Above 10 Torr, the magnitude of the voltage did not change significantly; however, the number and intensity of the current pulses increased with the pressure, similar to the argon discharge [see Figs. 5(b)–(d)]. However, at hundreds of Torr, as the pressure increased, the voltage increased while the intensity of the current pulses decreased (Fig. 6).

Figure 4. Applied voltage and discharge current waveforms in the case of the argon discharge with a fixed input voltage of 1.25 kV and frequency of 10 kHz. (a) 4.2, (b) 10.1, (c) 20.3, and (d) 30.3 Torr.

Figure 5. Applied voltage and discharge current waveforms in the case of the helium discharge with a fixed input voltage of 1.25 kV and frequency of 10 kHz. (a) 3.6, (b) 10.7, (c) 20.2, and (d) 31.1 Torr.

Figure 6. Applied voltage and discharge current waveforms in the case of the helium discharge with a fixed input voltage of 1.25 kV and frequency of 10 kHz. (a) 53.6, (b) 156.7, (c) 256.3, and (d) 352.6 Torr.

As mentioned above, the argon and helium discharges change from a glow discharge to a filamentary discharge as the pressure increases. This is interpreted as an increase in the pressure at a constant electrode spacing, resulting in more non-ionizing collisions between the electrons and the gases and restricting the discharge path of the charge. Accordingly, the voltage required for discharge increases, and the amount of charge moving per unit area, i.e., the intensity of the current pulses, increases. When the pressure was low, the discharge characteristics of the two gases were different because of their different ionization energies and cross sections. As argon has a higher electron bombardment ionization cross section and lower ionization energy than helium [15], it has a higher ionization probability than helium at low pressure in the case of our coplanar DBD source. As helium has a larger mean free path than argon, its discharge area was limited to the edge of the electrode at a few Torr, whereas the discharge area was evenly distributed at tens of Torr. The Paschen curve makes it easy to account for the tendency of the argon and helium discharges according to the pressure. When the gap between the electrodes is constant, the breakdown voltage of the gas is a function of the pressure. In the case of our coplanar DBD source with an electrode spacing of 5 mm, according to the Paschen curve, the argon and helium discharges have minimum breakdown voltages at approximately 2 and 20 Torr, respectively [16]. As can be seen in Fig. 3, from our experimental results, we confirmed that a stable discharge occurred near these values.

Figure 7 shows the discharge pattern of argon and helium in the coplanar DBD according to the frequency. In this case, the pressure was fixed at 4 Torr—a condition close to glow discharge when argon was used. Both the argon discharge and the helium discharge were the darkest at a driving frequency of 1 kHz, and they became brighter as the frequency increased. The change in the discharge pattern according to the frequency was more pronounced in the case of the helium discharge than in the case of the argon discharge. Under the conditions described above, in the case of the argon discharge [Fig. 7(a)], the plasma was spread over the electrode, whereas in the case of the helium discharge [Fig. 7(b)], the plasma was maintained mainly between the electrodes, and there was a dark area where no plasma was generated. As the frequency increased, the dark area gradually became wider.

Figure 7. Discharge patterns of (a) argon and (b) helium discharges with different frequencies and input voltage of 1.25 kV at a pressure of 4.0 Torr.

Figures 8 and 9 show the measured values of the voltage and current waveforms at frequencies of 1, 2, 5, and 10 kHz in the cases of the argon and helium discharges, respectively. As can be seen in Figs. 8(a) and 9(a), there are two periods at 1 kHz. One is a period in which the discharge was maintained while the current was generated as a pulse and the voltage was changed over time, and the other is a period in which the lowest current flowed while the full voltage was applied to the electrodes. When the voltage of the electrodes reached the maximum, the surface charges accumulated on the dielectric surface were continuously neutralized, and the spatial electric field was insufficient for the gas to break down before the polarity of the voltage changed [17]. Therefore, at low frequencies, there was a period in which no current flowed, and the discharge pattern was relatively dark (see Fig. 7). As the frequency increased, the period became shorter and the discharge pattern became brighter. In addition, the time for changing the polarity of the electrode became shorter than the time for the voltage to reach the maximum value, and the magnitude of the voltage decreased. In the case of the helium discharge, the tendency of the voltage according to the frequency was similar to that in the case of the argon discharge (Fig. 8). However, at high frequencies, the polarity of the voltage changed before a sufficient voltage was applied; thus, the charges were supplied insufficiently, and inefficient discharge could occur. As shown Fig. 7(b), the dark area in the center discharge area increases with the frequency.

Figure 8. Applied voltage and discharge current waveforms in the case of the argon discharge with a fixed input voltage of 1.25 kV at a pressure of 4 Torr. (a) f = 1.0, (b) 2.0, (c) 5.0, and (d) 10.0 kHz.

Figure 9. Applied voltage and discharge current waveforms in the case of the helium discharge with a fixed input voltage of 1.25 kV at a pressure of 4 Torr. (a) f = 1.0, (b) 2.0, (c) 5.0, and (d) 10.0 kHz.

Figure 10 shows the discharge pattern according to the gas ratio when a mixture of argon and helium was used. When only the gas ratio was changed at a fixed pressure of 12.1 Torr, the larger the helium ratio, the brighter the discharge area. However, when only helium was used, dark areas appeared again. As can be seen in Fig. 11, the trend of the gas ratio also appears in the measured voltage and current. In particular, the change in the measured current with the gas ratio is remarkable. The number of current pulses increased while the intensity decreased when helium was mixed with argon compared with when only argon was used. The addition of helium contributed to the relative change in the ionization energy and electron impact ionization cross section of the argon/helium mixture, thereby lowering the breakdown voltage of the mixture [15]. Accordingly, new current pulses were generated in a region adjacent to the existing current pulse, the amount of charge moving per unit area was reduced, and the intensity of the pulses was interpreted to be weakened.

Figure 10. Discharge patterns of the argon and helium mixture discharge at different gas ratios with a total gas flow rate of 0.20 slm, input voltage of 1.25 kV, frequency of 10.0 kHz, and pressure of 12.1 Torr [argon:helium = (a) 10:0, (b) 8:2, (c) 6:4, (d) 4:6, (e) 2:8, and (f) 0:10].

Figure 11. Applied voltage and discharge current waveforms in the case of the argon and helium mixture discharge at different gas ratios with a total gas flow rate of 0.20 slm, input voltage of 1.25 kV, frequency of 10.0 kHz, and pressure of 12.1 Torr [argon:helium = (a) 10:0, (b) 8:2, (c) 6:4, (d) 4:6, (e) 2:8, and (f) 0:10].

Figure 12 shows the calculation of the power dissipated by the argon and helium discharges as a function of the frequency, pressure, and gas ratio. The dissipated power was calculated as follows:

Figure 12. Calculated dissipated power with respect to (a) frequency, (b) pressure, and (c) gas ratio of the discharges with an input voltage of 1.25 kV.

$P = 1 T ∫ 0 T V t ⋅ I t d t$

where P is the calculated consumed power in watts, T is the period of the voltage in seconds, V(t) is the voltage in volts, and I(t) is the discharge current in amperes. All the values were calculated as average values over four cycles. Figure 12(a) shows that the dissipated power decreased as the frequency increased. As mentioned above, as the driving frequency increases, the insufficiently accumulated charges in the dielectric decrease and the voltage across the electrodes decreases; however, the current, which is the amount of charge transfer over time, increases (see Figs. 8 and 9). When the frequency is low, the amount of accumulated charge decreases and the dissipated power also reduces considerably; however, at frequencies above 5 kHz, the change in the dissipated power is not significant. Figure 12(b) shows that the dissipated power increases with the pressure at low pressure; however, in the case of helium, the dissipated power decreases above 200 Torr of pressure. As mentioned above, as the pressure increases, the ionization of the gas increases occurs while the electrical resistance decreases. Accordingly, the plasma current increases, resulting in an increase in dissipated power. However, at higher pressures, as the non-ionizing collisions increase, the plasma current decreases because ionization becomes difficult. Thus, under this condition, the dissipated power decreases as the pressure increases. Figure 12(c) shows that the greater the amount of helium, the lower the dissipated power. As can be seen in Fig. 11, the voltage is nearly similar for all the gas ratio conditions; however, the intensity and number of current pulses decrease. This is interpreted under the given conditions as follows: argon has a lower ionization threshold energy and a smaller cross section than helium; hence, it undergoes frequent ionization, and accordingly, the plasma current is large and the dissipated power increases.

DBD plasma can reduce the RCS by absorbing or scattering electromagnetic waves by controlling the plasma parameters related to the dielectric constants; such as the plasma frequency and collision frequency [9]. However, these parameters can be effectively controlled only when the generated plasma is uniformly distributed. The thickness and area of the plasma also affect the RCS reduction. The thicker and wider the generated plasma, the more efficient the RCS reduction [9,10]. From our experimental results, we conclude that more efficient conditions for RCS reduction can be established according to the DBD plasma pattern rather than the plasma parameters. The DBD plasma is more efficient in reducing the RCS in the case of glow discharge rather than filament discharge.

### 4. Conclusions

We experimentally studied the discharge characteristics of an argon and helium coplanar DBD source as a plasma generator for RCS reduction over a wide range of pressure. In the case of argon discharge, the lower the pressure (several Torr), the brighter was the discharge. From the measured voltage and current, we found that as the pressure decreased, a lower breakdown voltage was required, and numerous current pulses were generated. In the case of helium discharge, uniform glow discharge occurred at 20–50 Torr higher than the optimum pressure condition for argon discharge. The discharge characteristics for the driving frequencies of argon and helium were also investigated. In the case of argon, a relatively dark discharge occurred at 1 kHz, and this is considered a characteristic due to the existence of a period in which the current does not flow in time. As the frequency increased, the discharge pattern gradually became brighter. In terms of the electrical characteristics, as the frequency increased up to 5 kHz, the discharge voltage decreased and the number of current pulses increased; above 5 kHz, the opposite tendency was observed. Therefore, in the case of argon, we confirmed that a frequency of 5 kHz at a pressure of 4 Torr was the optimal discharge condition. The helium discharge under the same pressure conditions clearly showed the effect of the frequency. As the drive frequency increased to 6 kHz, the discharge pattern became brighter and the maximum value of the discharge voltage decreased. In the case of a driving frequency of 6 kHz or higher, there was a dark central area, and a high breakdown voltage was required. When a mixture of the two gases was used, at several tens of Torr, the higher the proportion of helium, the brighter was the discharge pattern; further, the number of current pulses increased while the intensity decreased. However, when only helium was used, plasma with relatively low uniformity was generated. Therefore, for efficient and uniform glow discharge in terms of electron density and uniformity, we recommend using a mixture of argon and helium with a high proportion of helium during coplanar DBD discharge. The results of this study are expected to be applied to RCS reduction with coplanar DBD devices in the future.

### Acknowledgements

This research was supported by the Aerospace Low Observable Technology Laboratory Program of the Defense Acquisition Program Administration and the Agency for Defense Development of the Republic of Korea.

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