3.1. Electrical properties of the plasma jets
The multiple gas discharges were driven by an AC voltage pulse, as shown in Fig. 2(a). Each voltage phase has 3 major pluses with a total pulse width of 7 µs. The first pulse had high intensity, and immediately voltage rose by a pulse width of 1.75 µs; the second pulse maintained a pulse width of 1.75 µs, but the voltage intensity gradually decreased. In contrast with the third pulse, both intensity and rising rate of voltage decreased (pulse width of 3.5 µs and voltage intensity decreased to zero). In response to varying voltage, the current had 5 major pulses with a total pulse width of 3.5 µs for each voltage phase, and the current intensity decreased correspondingly to the decreasing intensity of the voltage pulse. As a result, the maximum intensity of the current pulse was around 100 mA level.
The effect of applied voltage and Ar flow rate on the generation of plasma discharge was observed, as shown in Fig. 3. It can be observed that with increased applied voltage, the discharge power increases slightly, whereas the discharge power holds the opposite trend with increased Ar flow rate (reduced discharge power). The increased applied voltage raises the energy/density of electrons and excited/metastable species in the plasma zone and increases the amount of charge transfer owing to the enhanced electric field across the electrodes, which is one of the key factors in advancing the plasma jet. However, the discharge power decreases ~ 17% on average when the Ar flow rate increases from 1 to 3 L/min, which can be attributed to reduced retention time in the discharge zone due to increased gas velocity from 5.3 to 15.9 m/s respectively.
Since an AC voltage pulse drives the plasma jet, the current discharge has high intensity, suggesting high energetic electrons that the plasma jet source can provide. Moreover, with a short width pulses time, the discharge power deposited into the discharge zone will be less; consequently, the gas temperature, due to heating during plasma discharge, after plasma discharge is not high. Indeed, the discharge power under varying applied voltage and Ar flow rate was in the range of 0.77 to 1.05 W. The specific heat capacity of Ar in the atmospheric pressure is around Cp = 20.8 J/mol K [34]; consequently, the highest increase of gas temperature during plasma is not over 73 K (\({\Delta }\text{T} \le \frac{P*60}{{C}_{p}*\frac{F}{24.4}}=\frac{1.05*60}{20.8*\frac{1}{24.4}}=73 K\) ; P (W): discharge power; F(L/min): Ar flow rate, volume of 1 mol gas at 25°C is 24.4 L). The plasma jet was performed under atmospheric conditions (room temperature is around 25°C). Thus, the Ar gas temperature through the orifices of the quartz tube will be estimated not over 98°C (25+73°C), suggesting low-temperature plasma can be obtained by the 4-bore DBD plasma jet.
3.2. Plasma jet performance under varied voltage and flow rate
Even though the Ar flow rate had a negative effect on the discharge power, the flow rate and applied voltage both positively influenced the plasma jet length, as presented in Fig. 4(a). In Fig. 4, the above and below surface points represent the experimental data higher and lower than that of anticipated data by the quadratic or linear model, respectively. The minimum plasma jet length was observed to be 7 mm at [flow rate (L/min), applied voltage(kV)] = [1, 8], and the maximum jet length was acquired to be 17 mm at [flow rate (L/min), applied voltage(kV)] = [3, 10]. Although increased applied voltage contributes to the enhancement of jet length, a part of the input energy is spent on heating the discharge gas as well as the reactor, resulting in variation of jet temperature with respect to input parameters. Herein, with increased applied voltage, more energy is input to the system, as evident by increased discharge power in Fig. 3, which results in the jet temperature elevation, as presented in Fig. 4(b). However, with an increment in flow rate, the jet temperature displayed a reduced tendency. That is, the increased flow rate, meaning increased gas volume (amount), significantly reduced the changed temperature (DT = \(\frac{E \left(\frac{J}{min} \right)}{{C}_{p}\left(\frac{J}{mol.K}\right)*\frac{F\left(\frac{L}{min}\right)}{\frac{24.4L}{mol}}}\); E is part of the input energy for heating gas), resulting in the reduced jet temperature. Furthermore, at high velocity, it works like a functional cooling system. Particularly, a minimum jet temperature of 26 oC was observed at a combination of 3 L/min Ar flow rate and 8 kV applied voltage, whereas a maximum jet temperature of 40.6 oC was seen at a combination of 1 L/min Ar flow rate and 10 kV applied voltage, which is way below the highest possible overall temperature of 98 oC during plasma generation with Ar as the discharge gas. This can be explained by input energy (discharge power) being used not only to heat gas but also to create energetic electrons, ions, radicals, excited species, photons, etc., heating other reactor parts [35, 36]. While the plasma plume moves from the discharge zone to reach the glass slide, there is an energy transfer from the Ar flow to the reactor body and ambient air, which is also a reason for the low-temperature plasma jet.
A comparison between plasma jet generation by 1-bore and 4-bore plasma jet was performed and shown in Fig. 5. The comparison was based on the same total cross-section of bores and applied voltage. The data of the 1-bore plasma jet was taken from the previous report [37]; herein, the plasma jet was generated in a quartz tube with an inner diameter of 2 mm and outer diameter of 4 mm. Consequently, 1-bore plasma also had a cross-section bore area of π mm2, which equals to total cross-section bores area in the 4-bore plasma jet. The same applied voltage of 8 kV at 30 kHz was supplied to these reactors for the generation of the plasma jet. The figure demonstrated a significant decrease in jet temperature and Ar consumption for the 4-bore plasma jet case. Particularly, Ar consumption by 4-bore plasma is from 1 to 3 L/min for obtaining jet temperature below 40°C, whereas it required up to 5 L/min Ar flow rate for 1-bore plasma jet to obtain plasma jet temperature ≤ 40°C; a plausible reason is the low discharge powers in 4-bore plasma jet (1 W level). However, under the same conditions, the jet length of the 1-bore plasma jet is longer than that of the 4-bore plasma jet; although a shorter jet length of the 4-bore plasma jet, its length is still suitable for the applications.
To practically apply the plasma jet as a potential plasma source in the bio application field, low to no hazardous chemicals [such as O3, NOx (NO and NO2)] emissions are desired when the plasma jet interacts with ambient air. The emission of O3 and NOx, when the plasma jet was exposed to ambient air, was examined at a maintained flow rate of 2 L/min by varying the applied voltage from 8 to 12 kV. The threshold limit of O3, NO, and NO2 that would not adversely affect humans upon exposure is 0.1, 25, and 0.2 ppm, respectively [38]. Figure 6 shows the gas emitted and its concentration; herein, no NOx, either NO or NO2, was detected under the experimental conditions (EcomⓇ EN2, ecom GmbH, Germany, 1.5 L/min gas sample). However, around 0.3 ppm of O3 was observed at an applied voltage of 12 kV, which does not significantly surpass the limits, considering the possibility of dilution and decomposition to O2 [39]. Notably, O3 was under the threshold limit, and no NOx was detected at an applied voltage \(\le\) 10 kV.
3.3. Exhibited plasma jet when it interacts with human skin
A test of the plasma jet interacting with human skin was conducted and illustrated in Fig. 7. It can be seen that the plasma jet length was poor without a finger at an applied voltage of 8 kV and flow rate of 2 L/min. Under these conditions, no hazardous gas was emitted, and the plasma jet temperature was about 30.7 oC, less than the body temperature, which is ideal for bio-applications such as skin treatment, cancer treatment, sterilization, and wound healing. With the introduction of a human finger in the scenario at a proximity of 25 mm, a visible improvement in the jet length and intensity was observed, which was further enhanced when the finger was placed closer and closer to the reactor outlet. This phenomenon can be attributed to the finger acting as a floating ground that helps channel the plasma jet, improving the jet length and intensity [20]. The phenomenon also proposed that the visible optical jet length strongly depended on the surrounding environment of the plasma jet, especially when the plasma jet interacted with an object that can function as a floating electrode.
3.4. Optical emission spectra
The optical emission spectra of multiple plasma jet was plotted in Fig. 8. The spectra indicated a strong emission of Ar plasma was obtained by the 4-bore DBD plasma jet. Indeed, the intensities of Ar lines in this work are much higher than in the previous report [37] with the same typical applied voltage, i.e., these intensities increased around 1.5 times. Futhermore, by the method of the plasma jet interacting with the CaF2 window and the tip of the optical fiber located at another the CaF2 window side, it is more accessible to carry out optical emission spectra recording than the usual way [20]. The spectra of the 4-Ar plasma jet consisted of main Ar lines ranging from 690 nm to 900 nm and merge lines of OH and N2 at 308 with high intensity, overall higher than 104 a.u. intensity. During the propagation of the plasma jet into the atmosphere, Ar plasma jet interacted with ambient air, resulting in several excited active species; they are represented through several intense peaks in the spectra, e.g., N2, N2+, O, and OH. Strong lines of N2 in the second positive system (C3Πu → B3Πg) were observed in the range from 300 to 500 nm, while the emission of N2+ in the first negative system (B 2Ʃu+ → X 2Ʃg+) was also detected in the range wavelength from 380 to 450 nm, as shown in the inset figure. The atomic oxygen was indicated by a peak at 777 nm. Interestingly, no clear NO lines were detected in the A2Ʃ+ → X2Π system with a wavelength from 200 to 300 nm, which agreed with no NOx detected by the NOx analyzer. To summarize, analysis of the 4-bore DBD plasma jet spectra demonstrated high intensities of emission of excited species. In other words, the plasma provides a source of reactivated chemicals at close room temperatures (~ 40°C), proposing potential applications in bio-applications and material treatment when materials are sensitive to high temperatures or chemical agents.