BIBECHANA Vol. 21, No. 3, December 2024, 195-212 ISSN 2091-0762 (Print), 2382-5340 (Online) Journal homepage: http://nepjol.info/index.php/BIBECHANA Publisher: Dept. of Phys., Mahendra Morang A. M. Campus (Tribhuvan University) Biratnagar Characterization of atmospheric pressure circular dielectric barrier discharge via electrical and optical methods Roshan Chalise1,2∗, Krishna Regmi2, Sadip Nepal2, Sangat Sharma1, Suresh Basnet1, and Raju Khanal1 1Central Department of Physics, Tribhuvan University, Kirtipur, Kathmandu 44613, Nepal 2Amrit Campus, Department of Physics, Tribhuvan University, Kathmandu 46600, Nepal ∗Corresponding author. Email: plasma.roshan@gmail.com Abstract This research work is concerned with the comprehensive study of the electrical and optical characterization of an atmospheric pressure circular dielectric barrier discharge (APCDBD) in natural air. The effect of airflow and input voltage on the power supply’s behavior has been investigated to keep the constant electrode gap . The energy and power dissipated during the discharge per cycle are calculated by the Lissajous plot and the time average of the voltage and current curve. For the optical characterization of APCDBD, light emission during the plasma discharge is examined and the electron excitation temperature, rotational temperature, vibrational temperature, and plasma density have been computed. The Boltzmann plot method is used to estimate the electron excitation temperature and vibrational temperature. The rotational and also vibrational temperatures are calculated using online MassiveOES software. It is found that multiple filamentary micro discharge is increased while increasing the airflow and input voltage of the power supply. Energy per cycle of the discharge is increased for increasing input voltage and vice-versa in airflow. The discrepancy in the calculation of energy dissipation from discharge between time-averaged and Lissajous’s plot method is estimated. However, the fluctuation is less in the Lissajous’s plot method; hence, the energy calculation from the Lissajous plot method is more suitable for plasma discharge. The power consumption of the plasma reactor is found to be dependent on the input of the power supply. Electron excitation temperature and the rotational temperature of the discharge decrease with increasing the airflow in the discharge and increase with increasing input voltage. Therefore, by using the airflow in dielectric barrier discharge, we can move closer to the room temperature of atmospheric pressure plasma. Keywords Boltzmann plot, electrical parameters, electron excitation temperature, Lissajous curve, rotational temperature, vibrational temperature. Article information Manuscript received: January 19, 2024; Revised: April 19, 2024; Accepted: April 23, 2024 DOI https://doi.org/10.3126/bibechana.v21i3.62034 This work is licensed under the Creative Commons CC BY-NC License. https://creativecommons. org/licenses/by-nc/4.0/ 195 http://nepjol.info/index.php/BIBECHANA plasma.roshan@gmail.com https://doi.org/10.3126/bibechana.v21i3.62034 https://creativecommons.org/licenses/by-nc/4.0/ https://creativecommons.org/licenses/by-nc/4.0/ Roshan Chalise et al./ BIBECHANA 21 (2024) 195-212 196 1 Introduction In recent years, low-temperature atmospheric pres- sure plasma has received a lot of attention as it is widely used in diverse fields of science and technology [1–8]. The dielectric barrier discharge (DBD), one of the various forms of atmospheric low-temperature plasma reactors, has a wide range of industrial uses [9, 10]. At atmospheric pressure, the DBD normally operates in a filamentary dis- charge condition, which restricts the possibilities for its industrial application. However, specific cir- cumstances, homogeneous DBD can be produced, and it has the potential for use in a variety of in- dustrial processes, including plasma sterilization, thin-film deposition, material surface treatment, re- mediation of diesel-contaminated soil, evaluation of bio-compatibility, and many more [11–15]. In the region of non-thermal plasma where thermal non- equilibrium between the electron, ions, and neu- trals exists, the electron temperature can be or- ders of magnitude higher than the temperature of the heavier particles (atoms, molecules, and ions). These plasma do not heat any surfaces as they come in contact with because the ions and the neutrals stay relatively cool [16]. Due to these properties, it is possible to handle heat-sensitive materials, such as polymers and biological tissues, as well as low- temperature plasma chemistry. The DBD plasma typically consists of a large number of microscopic micro-discharges or filamentary that last for only a nanosecond or microsecond. The uniformity of the DBD is highly desired for industrial applica- tions, particularly for operations involving surface treatment [17]. Due to helium gas has low break- down voltage and the long lives of the metastable species, it is simple to create homogeneous DBDs, but the process is expensive and inefficient. Since argon and nitrogen are less expensive gases, it is preferable to construct a homogeneous DBD using them [18,19]. Since argon’s mean free path is short at atmospheric pressure, the breakdown voltage is comparatively larger which may make the glow- to-arc transition easier to occur. Because of this, it is challenging to achieve large-gap homogeneous DBD in argon gas between two plane-parallel elec- trodes [20]. Surface dielectric barrier discharge is considered to be especially promising in aerospace engineering [21,22], biomedical sciences [23], energy conversion, etc [24]. Electrode materials and shapes affect the discharge modes. Meanwhile, the dis- charge influences electrode surface [25]. In the air, at atmospheric pressure, Mahoney et al. created a dielectric barrier discharge plasma. They were calculated that the power consumption for the reac- tor operating without gap separation ranged from a few watts to a maximum of about 14 W using volt- age/charge Lissajous figures. The obtained emis- sion spectrum was mostly within the second pos- itive system of N2 ( C3Πu → B3Πg ) and the first negative system of N+ 2 ( B2 ∑+ u → X2 ∑+ g ) [26]. In atmospheric air, surface dielectric barrier discharge plasma often displays filamentary and diffuse dis- charges [27]. However, Subedi et al. reported that a consistent and uniform DBD discharge was noted between the electrodes, which have a gap of 1 to 3 mm and a 1.5 mm dielectric barrier from argon gas. The gas supply is regulated to a flow rate of 2 liters per minute. High voltage (0 to 20 kV) power sup- ply operating at 10 to 30 kHz frequency produced the discharge, with an electron density of around 1016 cm−3 and an electron temperature of about 1 eV reported [1]. Compared to filamentary-mode discharges with sinusoidal excitation, it is shown that pulsed excitation over a broad voltage range can generate stable and homogeneous DBD with improved energy efficiency. Furthermore, pulsed- excitation DBD uses less discharge power while pro- ducing a higher total transferred charge per volt- age cycle. The critical voltage for creating homoge- neous DBD can be enhanced with water electrodes, and suppressing instabilities with them is desirable for enhancing stability [28]. Fang et al. found that homogeneous discharge exists only under certain conditions. The voltage range for maintaining a stable discharge is found to be wider when the bar- rier thickness is smaller, the gap distance is shorter and the mesh number is greater [29]. Kogelheide applied a damped sinusoidal voltage waveform with oscillation periods in the microsecond time scale to study a volume and a twinning surface DBD created in various nitrogen-oxygen mixtures at atmospheric pressure. It is discovered that the oxygen concen- tration in the working gas mixture has a significant impact on the electron density, the lowered electric field, and the dissipated power [12]. Using an in- situ treatment technique, Dhakal et al. produced spark dielectric barrier discharge plasma and ex- amined the effects on seed germination and water sterilization. Significant alterations were made to the liquids’ physicochemical characteristics by the plasma treatment. Following an 8-minute plasma treatment, the concentrations of H2O2, NO2, and NO3 were raised to 30, 40, and 100 mgL−1, respec- tively. E. Coli and S. aureus were nearly elimi- nated after 8 minutes of plasma treatment. More- over, after five minutes of treatment with plasma, coriander seeds germinated more easily [11]. The most fundamental characteristics in gas discharges are electron density and electron temperature, and knowing these factors is crucial for optimizing dis- charge performance [30]. A plasma’s electron den- sity and temperature may be measured using a variety of techniques. The Langmuir probe, mi- crowave interferometer, Laser Thomson Scattering, optical emission, and absorption spectroscopy are Roshan Chalise et al./ BIBECHANA 21 (2024) 195-212 197 the techniques that are most often utilized. The probe method is cheaper but it disturbs the pro- duced plasma. In atmospheric pressure plasma, dis- charge produced in a small gap between the elec- trodes, if any probe is inserted between this dis- charge the properties of discharge differ or discharge is not formed [31]. Therefore, optical emission spec- troscopy (OES) which is non-intrusive and provides abundant information about the plasma species, is a potent method that is frequently employed for plasma diagnosis as an alternative to probe mea- surement. The temperature, chemical species con- centrations, and ionization state of the plasma may all be determined by examining and interpreting its spectra [30, 32]. Recently, using the Boltzmann plot technique and the assumption that the plasma is in a state of local thermodynamic equilibrium, the electron temperature in plasma in the atmo- spheric pressure range has been measured based on the visible spectrum. In a radio frequency-inductive discharge in the atmospheric pressure range, the existence of the local thermodynamics equilibrium (LTE) condition, which must be met to use the Boltzmann plot technique to determine the electron temperature, has been confirmed [33]. In this work, we have produced the atmospheric pressure circu- lar dielectric barrier discharge (APCDBD) with an airflow system in the stainless electrode character- ized by electrical and optical methods. Dielectric barrier discharges (DBDs) and their behavior in a variety of applications are the main subjects of cur- rent research on the electrical and optical charac- terization of atmospheric circular DBDs. It seeks to improve material treatment procedures, compre- hend plasma dynamics, and optimize reactor de- signs. Through the use of DBD plasma, these inves- tigations seek to improve material treatment proce- dures, comprehend plasma dynamics, and optimize reactor designs.To increase the efficacy and effi- ciency of DBD systems for uses such as surface mod- ification, plasma cleaning, sterilization, and mate- rial treatment, the study examines electron density, ion and current density, mean electron energy, and discharge gap voltage. To explore possible appli- cations, the research also looks at electrical break- down, plasma production, and single filament be- havior in DBDs. literature has been found on using airflow in DBD configuration from natural air to reduce the temperature of discharge and compre- hensive study of plasma parameters of plasma dis- charge like electrical parameters, delivered energy and power, electron excitation temperature, plasma density, and rotational and vibrational temperature of atmospheric pressure discharge for variation air- flow and input voltage of power supply. The vari- ation of these plasma parameters can affect treat- ment process of plasma application, which we have implemented in this work. 2 Methodology The block diagram of the experimental setup em- ployed in this work is shown in figure 1. The APCDBD device consists of two plane-parallel stainless steel circular metal electrodes of diame- ter 57.23±0.08 mm and thickness 4.36±0.07 mm, respectively: at lower electrodes are covered by a dielectric layer 2.00 mm thickness and 90.00 mm diameter. Figure 1: Schematic diagram of APCDBD and its real discharge snapshot. To ensure stable plasma operation, the gap that separates the electrodes is limited to 4.0 mm wide. Plasma discharge flows in the gap which was pro- duced by a high-voltage power supply (Chengdu Chuangyu Xinjie Technology Co. Ltd.) with an output voltage of 3-30 kV. To control the out volt- age of the power supply by changing the input voltage of the power supply (0-24.00 V) DC. The natural airflow is provided on the central hole of the upper electrode by a mini air pump and is regulated by a flow meter. An oscilloscope (Tek- tronix TBS 1052B) and a 1000 × voltage probe (Pintek HPV -40) were used to measure the volt- age across DBD. To monitor the discharge current or transferred charge during plasma formation, a 100 Ω resistor or 10 nf capacitor was connected in series with the ground (10 × voltage probe, Tektronix TPP0201) electrode. The power con- sumption of discharge was calculated using the Lis- sajous curve between the applied voltage and the charge produced during the discharge. The method most commonly used for optical characterization is the extraction of the discharge’s optical emis- sion spectrum (OES). The light that the discharge emits is detected by an optical emission spectrom- eter HR1-high-resolution spectrometer (ASEQ In- strument: pixels 3648, signal-to-noise ratio 300:1, exposure time 2.5-600 ms, CCD reading time 14 ms, wavelength 200-820 nm, 10 µm slit, 0.176 nm resolution), 10.0 mm far from the discharge, with 300 ms exposure time and 30 scan average. The APCDBD plasma is generated in different airflow and input applied voltage at fixed electrode gap Roshan Chalise et al./ BIBECHANA 21 (2024) 195-212 198 and electrode material. Comprehensive electrical parameters such as voltage, current, charge, fre- quency, energy, and power were calculated as fol- lows: discharge current (Idis) and discharge voltage (Vdis) is the corresponding value of current and volt- age of the first micro discharge observed in the I-V curve for one complete cycle. Peak-to-peak volt- age Vpp of the discharge is measured from peak to peak value of the sinusoidal current-voltage wave- form. Vp of the discharge is half of the peak-to-peak value of the Vpp [34]; Vp = Vpp 2 (1) Root mean square (RMS) of sinusoidal voltage in discharge Vrms is calculated as [34]; Vrms = Vp√ 2 (2) The frequency of the discharge is obtained as the reciprocal of the time of one complete cycle of dis- charge by [34] Frequency(f) = 1 T (3) We have implemented two ways of calculating of energy and power of the discharge. Five per cy- cle are carried out to calculate the energy from the voltage-current curve and Lissajous plot, and re- sults are presented per cycle with mean values with standard deviation. The first way is to calculate the time-averaged electrical energy of the plasma discharge using voltage and current curves [34]: Et = ∫ V (t)× I(t)dt (4) where V (t) and I(t) are the voltage and current at the respective time, respectively. The plasma discharge’s time-averaged electrical power consumption can be calculated using [34] Pt = 1 T ∫ V (t)× V (t)dt = f × Et (5) The electrical power calculated by using this method typically fluctuates from cycle to cycle. Af- ter that, several cycles are required to get a con- verged mean value. In addition, to ensure that the current peaks are correctly recorded and provided, the data must be captured using a high sampling rate (usually a few ns) and a high bandwidth os- cilloscope. There is still a problem with this ap- proach, despite its relative simplicity and accuracy: when there are strong current peaks, it can be dif- ficult to resolve the synchronous current accurately because its amplitude is at least one order of mag- nitude smaller than the current peaks [35]. To over- come this issue Manley et al. (1943) developed another simple method [36] used for surface DBD plasma [37]. This method consists of placing a ca- pacitor C between the grounded electrode and the earthing point as shown in figure 1. It is accom- plished by plotting the charge curve of the inserted 10 nf capacitor throughout a full cycle as a func- tion of the applied voltage. Each period’s energy dissipated by the discharge is represented by the area inside the closed Lissajous curve. The shape of the Lissajous curve depends on various factors, including the frequency, amplitude, and phase rela- tionship between the two waveforms. So, to obtain the area of the Lissajous curve, the applied voltage and the charge accumulated curve are plotted. If we take one full cycle of discharge data, the area of this curve indicates the energy that the discharge releases for each cycle [36]. The energy dissipated per cycle is calculated by [36] EL = Area covered by Lissajous (Q-V) plot (6) Using the Lissajous (Q–V plot) approach, the power dissipation across the APCDBD is calculated by multiplying energy with the current-voltage wave- form frequency [36]: PL = (E × f) (7) This approach’s primary benefit is that it is more reproducible from cycle to cycle and does not suf- fer from bias caused by high current peaks in the computation. Using this method, calculating the precise amount of electrical power utilized only re- quires one AC cycle. This makes sense for tests conducted in closed loops where a short loop time is required [35]. The energy and power calculation from every four full cycles of the I-V curve and pre- sented in mean and standard deviation. Optical spectroscopy is a widely used technique to measure the intensity of different wavelengths of light. Because they are noninvasive and have a fast response time, spectroscopic diagnostics are useful for measuring plasma temperatures, density, and several other characteristics in plasma that can be computed with the help of optical spectroscopy. The optical emission spectra are taken ten times and presented as their mean. The plasma temper- atures are categorized into four types, i.e., the elec- tron, vibrational, rotational, and translational tem- peratures [38]. Since there is a high frequency of en- ergy exchange between the rotational and transla- tional temperatures of the plasma, they are thought to be nearly equal [38,39]. However, the rotational temperature Trot is typically referred to as the tem- perature of the kinetic gas [38]. Understanding vi- brational temperature Tvib can help one understand the relative rates of energy exchange processes be- tween vibration and translation [40] and is an im- portant factor in understanding the synthesis of Roshan Chalise et al./ BIBECHANA 21 (2024) 195-212 199 NO2 and the quenching of O3 [41]. Plasma’s vi- brational and rotational temperatures are crucial factors that can impact both the target material’s surface properties and the rate at which the plasma is chemically reacting. Higher vibrational temper- atures have the potential to significantly accelerate chemical reactions in plasma because they indicate the presence of energy in the excited state of molec- ular vibration. Rotational temperature describes the population of rotational levels in molecular or- ganisms [42]. In plasma physics, electron temperature (Te) is one of the most fundamental and instructive character- istics since electrons control the chemical reactions occurring within the plasma in addition to being involved in the excitation, dissociation, and ion- ization of atoms and molecules. Since excitation mechanisms that control the distribution of excited states are primarily driven by free electrons, the ki- netic temperature of the free electrons is typically correlated with the Texc of the bound electrons in an atom or molecule [43]. In high-pressure plas- mas, excitation temperature is commonly used. In particular, because of their close association, deter- mining Te at atmospheric pressure often depends on Texc measurement. Texc is near Te if the sys- tem abides by the local thermodynamic equilib- rium (LTE). Thus, measuring Texc can be a useful substitute diagnostic for plasma discharge [43, 44]. Numerous diagnostic methods are available for de- termining the electron temperature of the plasma; however, the electrostatic Langmuir probe is one of the most widely used and flexible technologies for this purpose [45]. Despite having a straightforward hardware setup, its application may be restricted at times because of issues with analysis, particu- larly when negative ions are present, a potential source of plasma disruption, and difficulties using it with large-volume plasmas. Texc can also be as- certained using optical emission spectroscopy using the proper equilibrium models [44]. The Boltzmann plot method is one of the most widely used techniques for measuring the temper- ature in lab plasmas. The radiative transfer equa- tion for an optically thin, uniform, and isothermal plasma along the line of sight provides the basis for this calculation. The equation is linearized by taking the logarithm of both portions to get the Boltzmann plot. The argument of the logarithm, a dimensionless transcendental function, must also be dimensionless [46]. The Boltzmann plot tech- nique of N(III) species, which uses the data from the NIST database [47] in table 1, is used to compute the electron excitation temperature (Texc). Using this approach, the LTE plasma equation is [48]; ln ( λijIij Aijgj ) = − Ej kBTexc +K (8) The variables in equation (8) are the intensity of the transition from the i to the j state (Iij), wavelength of the transition (λij), transition probability (Aij), statistical weight (gj), higher energy level Ej, elec- tron excitation temperature Texc, and constant K. When we plot the ln ( λijIij Aijgj ) versus Ej, the slope of the obtained graph (Boltzmann plot) is equal to - 1 kBTexc and then the electron excitation temperature is obtained. Table 1: Selected N(III) peaks in optical emission spectroscopy of APCDBD and its corresponding spectroscopic data [47] for plotting of Boltzmann plot. λij (nm) Aij (s−1) Ej (eV) gi-gj 336.734 1.27×108 39.35199 6–6 375.467 3.78×107 38.95798 4–6 377.105 5.59×107 38.95798 4–4 393.852 8.96×107 41.48118 4–6 399.863 1.76×108 42.49548 4–6 400.358 1.88×108 42.49560 6–8 The Saha equation is rooted in the principles of statistical mechanics and considers the balance between ionization and recombination processes in plasma at thermal equilibrium. It provides a way to calculate the relative abundances of different ion- ization states of an element in plasma as a function of temperature and pressure. The plasma density is calculated from the Saha-Boltzmann equation [49]: Ne = 2 I2A1g1λ2 I1A2g2λ1 ( 2πmekBTexc h2 )3/2 exp [ − (E1 − E2 + Ej) kBTexc ] (9) The energy of ionization of a neutral atom is de- noted by Ej; the intensity of the same species line has a larger gap in upper energy, and the wave- lengths are denoted by λ1 and λ2; the transition probabilities are represented by A1 and A2; and the statistical weights are g1, g2. Rotational (Trot) and vibrational (Tvib) tempera- tures are calculated by MassiveOES software [50– 52], in which measured OES spectrum is fitted with the standard spectrum and provide the results with residual error in fitting both spectra. When dealing with nitrogen plasmas or plasmas containing nitro- gen, Boltzmann plots or fits of the band envelopes of various bands belonging to the first negative sys- tem and/or second positive system are commonly employed [53]. So the vibrational temperature is calculated by the Boltzmann plot method also. The line intensity of the vibrational transition’s spectral was provided as [54] Iν′→ν′′ ∝ hc λν′→ν′′ Aν′→ν′′N0 exp(− hcGν′ kBTvib ) (10) Roshan Chalise et al./ BIBECHANA 21 (2024) 195-212 200 where kB, h, c, and Tvib are the Boltzmann con- stant, velocity of light, and vibrational tempera- ture respectively, I, A, G, and λ are transition fre- quency, Einstein coefficient and spectral term of vi- brational transition and wavelength which are re- lated to upper vibrational level (ν′) to lower vibra- tional level (ν”). The spectral factor can be re- placed by the energy difference of the energy lev- els G′ ν = Eν′ − E0. The vibrational energy of the excited molecules on the level of N2 (C3Π) in the quantum harmonic oscillator approximation disre- garding the anharmonicity constant [55]; Eν(eV ) = 1.2398× 10−4(ν + 1 2 )we(cm−1) (11) where ν is the vibrational quantum number, which can be 0, 1, 2, 3,... of ground level, and we is the spacing of vibrational energy on the C3Π (= -2047.17 cm−1). After applying eq. (11) in eq. (10) then we can get the semi-log equation as ln ( Iν′→ν′′λν′→ν′′ Aν′→ν′′ ) = C − Eν′ − E0 kBTvib (12) where Iν′→ν” is the intensity of transition from upper to lower state, λν′→ν′′ is the wavelength of transition, Aν′→ν′′ is the transition probabil- ity, Eν′ is the upper energy level with the vi- brational quantum numbers. When we plot the ln ( I ν ′→ν ′′ λ ν ′→ν ′′ A ν ′→ν ′′ ) versus (Eν′ − E0), the slope of the obtained graph (Boltzmann plot) is equal to - 1 kBTvib and then the vibrational temperature is ob- tained. N2 species have a high vibrational temper- ature, which is shown by the overpopulation of N2 vibrational states. For the Boltzmann plot method, four vibrational bands are taken: ∆ν= +1 (1-0, 2- 1), ∆ν= -1 (0-1, 1-2, 2-3), ∆ν= -2 (0-2, 1-3, 2-4) and ∆ν= +2 (2-0, 3-1) [56]. The National Institute of Standards and Technology reference database is the source of the spectroscopic parameters needed for the Boltzmann plot [47]. The parameters to ob- tain the vibrational temperature is based on the analysis of the N2 (C3Π) or the second positive system emissions of ∆ν = ν ′ -ν ′′ , vibrational transi- tion (ν ′′ -ν ′ ) [56], wavelength λ (nm) [55], transition probability Aν ′ →ν ′′ (106 s −1) [55] and energy Eν′ (eV) calculated according to eq. (11) are listed in table 2. Table 2: Selected wavelengths of the OES spectrum and all other parameters required to Boltzmann plot of Tvib [55]. ∆ν (ν ′′ -ν ′ ) λ (nm) Aν ′→ν ′′ (106 s−1) E ν ′ (eV) -2 2-4 295.32 3.80 1.14 -2 1-3 296.20 4.62 0.88 -2 0-2 297.68 3.34 0.63 -1 2-3 311.67 1.65 0.88 -1 1-2 313.60 5.49 0.63 -1 0-1 315.90 8.88 0.38 0 0-0 337.10 13.90 0.12 +1 2-1 353.60 11.41 0.38 +1 1-0 357.69 13.80 0.13 +2 3-1 375.54 8.68 0.38 +2 2-0 380.49 4.81 0.13 3 Results and Discussion The relationship between the applied voltage and the resulting current is an important aspect of DBD plasma. Initially, at low voltages, there’s lit- tle current flow due to the insulating properties of the dielectric barrier. As the applied voltage in- creases, the voltage across the gap exceeds the di- electric breakdown voltage, leading to the initiation of plasma discharge and a sudden increase in cur- rent [57]. The voltage fault and impulse current occur when the voltage is high enough to ionize the working gas (atmospheric air). When the ap- plied voltage exceeds the breakdown voltage of the atmospheric air, then the micro discharge can be seen [58]. Roshan Chalise et al./ BIBECHANA 21 (2024) 195-212 201 Figure 2: Time variation with voltage and current for the flow variation of (a) 0 LPM, (b) 5 LPM, (c) 10 LPM, and (d) 15 LPM respectively. Figure 3: Time variation with voltage and current for the voltage variation of (a) 12.00 V, (b) 16.00 V, (c) 20.00 V, and (d) 24.00 V respectively. Figure 2 and 3 shows the plot of the discharge current and applied voltage (I-V) waveform at the flow variation at constant input voltage and varia- tion of input voltage at constant air flow rate. The number of current pulses rises and the discharge current intensity falls as flow rates increase from 0 to 15 LPM [59, 60]. The breakdown voltage is also reduced because, when airflow is introduced Roshan Chalise et al./ BIBECHANA 21 (2024) 195-212 202 into the discharge region, the breakdown moment gradually shifts towards the lower time values. The breakdown voltage drops as airflow enters the dis- charge space. Consequently, the number of fila- ments rises while the intensity of the discharge cur- rent falls [61]. Because of the lower breakdown volt- age, more filaments with lower intensities at a fixed input power can be produced in the discharge re- gion [62]. Figure 4 (a) and (b) represent the Lissajous curve of the different LPM at constant voltage and the varying voltages with constant airflow. It is ob- served that with increasing the input voltage and airflow, multiple micro discharges are increased in the I-V plot. So that more and more filamentary discharge is observed during the discharge region. The increase in input voltage has resulted in an in- crease in peak-to-peak voltages and RMS voltage. The energy carried by discharge has also decreased increasing the airflow. The frequency of the dis- charge is independent of applied airflow. It can be seen that the power delivered by the APCDBD also decreased with an increasing the airflow as shown in the table 3. The increase in input voltage has in- creased the frequency. The energy and power of the APCDBD plasma increase with increasing the volt- age that is shown in table 4. There is the fluctuation of energy from time-averaged and the Lissajous plot method. Up to 10 % fluctuation in airflow and 2 % fluctuation in input voltage. Due to the fewer number of cycles measured from our oscilloscope, more fluctuation of energy occurred, these limita- tions result in agreement with previous results [35]. However, the fluctuation of energy per cycle is 4 % in airflow variation and less than 1 % in input volt- age variation, when calculated from Lissajous plot methods. Figure 4: Lissajous plot; charge (Q) concerning the applied voltage (kV) across discharge electrode for different (a) airflow at 24.00 V input voltage, and (b) input voltage at 15 LPM airflow. Table 3: Electrical parameters of APCDBD for various air flows at 24.00 V input voltage. LPM VPP (kV) Vrms (kV) Idis (mA) Vdis (kV) Et (µJ) EL (µJ) f (kHz) Pt (W) PL (W) 0 24.45 8.64 0.41 8.44 116.12±11.98 190.02±1.44 25.60 2.97±0.11 4.86±0.14 5 28.77 10.17 0.75 13.97 158.13±17.01 191.26±7.96 26.10 4.12±0.17 4.98±0.79 10 30.91 10.93 0.87 16.38 160.23±9.68 193.41±6.65 26.70 4.16±0.96 5.02±0.66 15 31.21 11.03 0.88 16.40 179.00±5.35 197.70±4.93 26.90 4.68±0.54 5.15±0.49 Table 4: Electrical parameters of APCDBD at different input voltage at 15 LPM airflow. Vin (V) VPP (kV) Vrms (kV) Idis (mA) Vdis (kV) Et (µJ) EL (µJ) f (kHz) Pt (W) PL (W) 12.00 30.87 10.91 0.63 5.98 172.99±4.98 156.52±2.57 22.17 3.83±0.11 3.47±0.05 16.00 32.58 11.52 0.68 7.60 172.01±3.22 162.48±3.12 22.98 3.95±0.07 3.73±0.07 20.00 33.03 11.67 0.76 8.70 176.17±4.40 174.96±5.78 24.15 4.25±0.11 4.23±0.13 24.00 33.55 11.86 0.91 9.26 165.63±8.35 186.76±9.39 25.77 4.26±0.21 4.99±0.49 Roshan Chalise et al./ BIBECHANA 21 (2024) 195-212 203 When comparing this power estimating process to the time-averaged integration of the voltage and current over five AC cycles, the deviation of value per cycle is far more up to 10 % by calculating the averaged method and nominal standard deviation found in the Lissajous plot method was used. 3.1 Optical Characteristics Optical emission spectroscopy (OES) of plasma in- volves studying the emission of light produced dur- ing the plasma discharge and transferring these emission spectra into their wavelength and inten- sity values. This can provide valuable information about the plasma’s properties, such as its com- position, temperature, and density. The excita- tion temperature, plasma density, rotational, and vibrational temperature of the APCDBD are pre- sented here. Figure 5 (a) and (b) show the OES of APCDBD plasma at different air flows and dif- ferent input voltages. In our research, we have used natural air as a working gas. The intensity of the spectra decreased with increasing the air- flow and increased with increasing applied voltage in the power supply observed in the blowup fig- ure of spectra of figure 5 (b). The population of excited particles, which can be produced by one or more channels depending on the plasma con- ditions, is shown by the emission intensity. The intensity of the electrode field increases in the dis- charge zone as the applied voltage increases. More energetic electrons are produced in the discharge zone as the electrode field strength increases. After that, they accelerate radiation transitions’ higher states, producing more active particles. As a re- sult, when the applied voltage rises, so do the in- tensities of the corresponding emission spectra [42]. In atmospheric air, there is approximately 78 % of nitrogen and 20 % oxygen. Thus, the peak of OES spectra in APCDBD plasma has almost nitro- gen and oxygen species. The major observed spec- tra are the second positive system (310-380 nm) of N2 ( C3Πu → B3Πg ) and the first negative system (390-440 nm) of N+ 2 ( B2 ∑+ u → X2 ∑+ g ) [56]. Ad- ditionally, OH ( A2 ∑+ → X2Π ) radicals is promi- nently seen at 309 nm in the spectrum of the gliding arc discharge which plays a crucial role in plasma chemical reactions such as the oxidation of gas and liquid pollutants. The nitric oxide gamma band NOγ ( A2 ∑+ → X2Π ) is observed in 200 to 280 nm. Reactive oxygen (O) radicals are found at wavelengths 777 nm with electronic transitions 4s(3D)4d→4p(3P) [6, 36]. Figure 5: Optical emission spectroscopy and blow-up spectra of APCDBD for (a) various airflow at 24.00 V input, and (b) various input voltage at 15 LPM airflow. Boltzmann’s plot is used to study the electron excitation temperature of the plasma. The en- ergy level of N (III) species of respective wave- length versus relative intensity distribution was plotted to find the electron excitation temperature of APCDBD. A Boltzmann plot is a graphical repre- sentation used in spectroscopy to analyze the pop- ulations of energy levels within a system. In the context of DBD plasma, which is a type of low- temperature plasma, a Boltzmann plot can be used to understand the energy distribution of excited states of atoms or molecules in the plasma [63]. The slope of Boltzmann’s plot of airflow and input volt- age variation gives the value of electron excitation temperature as shown in figure 6 and 7 respectively. Roshan Chalise et al./ BIBECHANA 21 (2024) 195-212 204 Figure 6: Boltzmann plot for the estimation of Texc using N (III) species for increasing flow variation of (a) 0 LPM, (b) 5 LPM, (c) 10 LPM, and (d) 15 LPM. Figure 7: Boltzmann plot for the determination of Texc using N(III) species for increasing applied voltage (a) 12.00 V, (b) 16.00 V, (c) 20.00 V and (d) 24.00 V. Figure 8 and 9 represent the measured and simulated OES data fitted plot for the determination of Trot and Tvib by using nitrogen second positive system N2 (C3Πu − B3Πg) from MassiveOES software for increasing flow variation for 0 LPM, 5 LPM, 10 LPM, and 15 LPM and increasing applied voltage at 12.00 V, 16.00 V, 20.00 V, and 24.00 V respectively. Roshan Chalise et al./ BIBECHANA 21 (2024) 195-212 205 Figure 8: Measured and simulated OES data fitted plot for the determination of Trot and Tvib by using nitrogen second positive system (SPS) N2 (C3Πu − B3Πg) from MassiveOES software for increasing flow variation for (a) 0 LPM, (b) 5 LPM, (c) 10 LPM and (d) 15 LPM. Figure 9: Measured and simulated OES data fitted plot for the determination of Trot and Tvib by using nitrogen second positive system N2 (C3Πu − B3Πg) from MassiveOES for increasing applied voltage (a) 12.00 V, (b) 16.00 V, (c) 20.00 V and (d) 24.00 V. Roshan Chalise et al./ BIBECHANA 21 (2024) 195-212 206 Trot is typically utilized as an approximation of the gas temperature (Tg) since it is representative of the rotating population of the ground state and provides information on the global temperature of the plasma [64]. The average kinetic energy brought on by the rotating of particles in the plasma is re- ferred to as rotational temperature. The molecu- lar axes of plasma molecules can spin. The energy distribution between these rotating degrees of free- dom is shown by the rotational temperature [65]. On the other hand, electron collisions are the pri- mary source of molecular specie’s vibrational states. The non-equilibrium plasma’s chemical kinetics are significantly influenced by the vibrationally excited species, [66] because vibrational excitation plays the role of an energy reservoir [67]. Figure 10 shows the typical Boltzmann plot of the relative inten- sity distributions versus vibrational energy using nitrogen second positive system N2 (C3Πu−B3Πg) for flow variation of 0 LPM, 5 LPM, 10 LPM, and 15 LPM. The vibrational temperature is correlated with the average kinetic energy of the oscillating motion of the plasma’s particle constituents. Atoms and molecules can oscillate about their equilibrium locations as well as other sorts of vibrations that can occur in plasma. The quantity of energy involved in these oscillations is described by temperature vari- ations Figure 10 and 11 shows a typical Boltzmann plot of the relative intensity distributions flow vari- ation and input voltage variation. After taking into account the scattered data points and fitting errors, the vibrational temperature was obtained Figure 11 shows the Boltzmann plot for the determination of Tvib using nitrogen second positive system N2 (C3Πu − B3Πg) for increasing applied voltage of 12.00 V, 16.00 V, 20.00 V, and 24.00 V. Figure 10: Boltzmann plot for the determination of Tvib using nitrogen second positive system N2 C 3Πu− B3Πg) for flow variation for (a) 0 LPM, (b) 5 LPM, (c) 10 LPM and (d) 15 LPM. Roshan Chalise et al./ BIBECHANA 21 (2024) 195-212 207 Figure 11: Boltzmann plot for the determination of Tvib using nitrogen second positive system N2 C 3Πu− B3Πg) for increasing applied voltage (a) 12.00 V, (b) 16.00 V, (c) 20.00 V and (d) 24.00 V. Table 5 shows the optical parameters of APCDBD for increasing air flows at 24.00 V input voltage. As an increase in the airflow electron ex- citation temperature, and rotational temperature were increased, but density and vibrational tem- perature decreased. The plasma density decreased with increasing air flow rate is supported by pre- viously published results [63]. Table 6 shows the optical parameters of APCDBD at different input voltages at 15 LPM airflow. While increasing the input voltages of the power supply electron excita- tion temperature, electron density, vibrational tem- perature, and rotational temperature are increased. The gas breakdown between the HV electrode and the floating electrode is more likely to occur as the applied voltage increases because of the HV elec- trode’s extremely tiny radius of curvature and the gap, which together produce a strong electric field. Following the disruption in the discharge region, the electron density rises and the plasma resistance value is very low [42]. The plasma power grows with applied voltage, and the energy input into the discharge zone rises as well, increasing the likeli- hood of electron-heavy particle collisions raising the rotational temperature, and highlighting the Joule heating effect [68]. Table 5: Optical parameters of APCDBD for various air flows at 24.00 V input voltage. LPM Texc (K) Ne (× 1023 m−3) Tvib(B.P) (K) Trot(MOES) (K) Tvib(MOES) (K) 0 15272.40 1.39 3260.87 492.51 ± 15.13 2759.99 ± 17.59 5 15226.10 1.45 3284.08 486.17 ± 17.95 2762.64 ± 18.19 10 15248.80 1.49 3307.29 474.78 ± 15.93 2777.78 ± 18.35 15 15240.90 1.54 3349.27 464.84 ± 13.56 2787.04 ± 17.85 Roshan Chalise et al./ BIBECHANA 21 (2024) 195-212 208 Table 6: Optical parameters of APCDBD at different input voltage at 15 LPM airflow . Vin (V) Texc (K) Ne (× 1023 m−3) Tvib(B.P) (K) Trot(MOES) (K) Tvib(MOES) (K) 12.00 15241.50 1.52 3149.27 450.35 ± 16.23 2743.46 ± 18.57 16.00 15279.10 1.51 3214.45 468.44 ± 16.59 2753.47 ± 18.79 20.00 15293.60 1.53 3291.24 486.01 ± 17.75 2754.24 ± 18.99 24.00 15342.80 1.54 3337.66 496.67 ± 16.49 2787.27 ± 17.85 We have calculated all of the plasma parame- ters electrical and optical by available analysis tech- niques in a standard way. We have found our rotational temperature is a little bit more than the room temperature, however, it is decreasing lower but only airflow cannot control the tempera- ture. For this decrement in gas temperature plasma power source is important so we need to develop a power source for reactors for developing heat- sensitive plasma applications. Another major thing of that such types of flat area DBD can treat thin and large surface subtracted but not possible to treat thick substrates. Hence for this application, needed to change such DBD like a plasma jet. 4 Conclusion In this research, comprehensive electrical and op- tical characterization of the atmospheric pressure dielectric barrier discharge plasma is employed by available different methods. To understand these basic parameters calculation is essential for every application of plasma. The DBD is produced in at- mospheric pressure air so that easy to handle and low cost for production. The energy and power cal- culation is more converged by using the Lissajous plot method. However, the time-averaged method has more divergent values per cycle. As the air- flow increases the energy and power of discharge dissipated also goes on decreasing. Similarly, as the voltage increases the power also increases con- cerning the parameter. The optical parameters are increased with increasing the input voltage of the power supply, however, density and rotational tem- perature are decreased while increasing the airflow between the electrodes of APCDBD. It also follows the standard relation of nonthermal plasma temper- ature order Trot < Tvib < Texc. The plasma is in a state known as a non-thermal equilibrium when the temperatures of the electrons, vibrations, and rotations are not equal. The rotational tempera- ture, which is assumed to be equivalent to the gas temperature, rises as the applied voltage increases and falls as the airflow increases, suggesting that the thermal effect is more pronounced at higher in- put voltages and lower airflow. This research work has played a crucial role in understanding the elec- trical and optical characterization of produced at- mospheric discharges. Acknowledgement Roshan Chalise would like to acknowledge the Uni- versity Grants Commission, Nepal for the Ph.D. fellowship award (PhD-78/79-S & T-16) and ex- tend special thanks to Dr. Bhagirath Ghimire for his guidance in the characterization methods. We acknowledge the Research Coordination and Development Council, Tribhuvan University, Kir- tipur, Nepal for the National Priority Research Grant (TU-NPAR-077/78-ERG-12). We also thank Tirtha Raj Acharya, Prajwal Lamichhane, and Oat Bahadur Dhakal of Plasma Bioscience Research Center, Department of Electrical and Biological Physics, Kwangwoon University, Seoul, Republic of Korea for their help in determining the vibra- tional temperature of discharge by Boltzmann plot method. References [1] DP Subedi, RB Tyata, R Shrestha, and CS Wong. An experimental study of atmo- spheric pressure dielectric barrier discharge (DBD) in argon. In AIP Conference Proceed- ings, volume 1588, page 103. American Insti- tute of Physics, 2014. 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DOI: https://doi.org/ 10.1109/27.842901. https://doi.org/10.1063/1.5128455 https://doi.org/10.1063/1.2219392 https://doi.org/10.1063/1.2219392 https://doi.org/10.1109/TPS.2015.2394441 https://doi.org/10.1109/TPS.2015.2394441 https://doi.org/10.1002/ppap.202300102 https://doi.org/10.1002/ppap.202300102 https://doi.org/10.1063/1.555546 https://doi.org/10.1088/1361-6595/aa6426 https://doi.org/10.1088/1361-6595/aa6426 https://doi.org/10.1088/1361-6595/aa674e https://doi.org/10.1088/1361-6595/aa674e https://doi.org/10.1063/1.367051 https://doi.org/10.1063/1.1458684 https://doi.org/10.1063/1.1458684 https://doi.org/10.1063/1.4972095 https://doi.org/10.1063/1.4972095 https://doi.org/10.1109/TPS.2011.2172634 https://doi.org/10.1109/TPS.2011.2172634 https://doi.org/10.1016/j.jwpe.2023.103519 https://doi.org/10.1016/j.jwpe.2023.103519 https://doi.org/10.1063/1.5043182 https://doi.org/10.1088/0963-0252/23/2/023001 https://doi.org/10.1088/0963-0252/23/2/023001 https://doi.org/10.1201/9781482293630 https://doi.org/10.1201/9781482293630 https://doi.org/10.1088/0022-3727/46/34/345201 https://doi.org/10.1088/0022-3727/46/34/345201 https://doi.org/10.1109/27.842901 https://doi.org/10.1109/27.842901 Introduction Methodology Results and Discussion Optical Characteristics Conclusion