# Water Modification by Cold Plasma Jet with Respect to Physical and Chemical Properties
## 3 Results and Discussion  
### Plasma Jet Operational Window  
The present DBD-based plasma jet setup has three adjustable operating parameters, namely: the applied voltage amplitude, the applied voltage frequency, and the gas flow rate. These parameters are here varied over the range 8-12 kV peak-to-peak, 5-20 kHz, and 0.5-3 slm, respectively, and the generating species emitting in the UV-NIR region are probed. Figure 3a,b unveil the principal emissive species detected downstream of the plasma jet. The species identification with the corresponding transitions are summarized in Table 1[38; 39; 40]. Excited molecular nitrogen, hydroxyl, and abundant argon excited atoms are seen. Figure 3c presents both the experimentally recorded and the numerically reproduced rotational distribution of the hydroxyl molecule, OH(A), which has been accepted as a reliable molecule for rotational temperature measurements [41]. Hence, following the curve fitting shown in Figure 3c, an average gas temperature of around \(350\pm 40\) K is evaluated along the plasma jet. The data of Figure 3 refer to the optimal operational window, in terms of reactivity, which is determined as follows. Figures 4-6 provide the relative spectral intensity of the abovementioned species, as a function of the voltage amplitude, voltage frequency, and gas flow rate, respectively. In the case of amplitude variation, i.e., Figure 4, species population increases for higher externally applied electric field. This is due to intense ionizing waves, propagating along the argon channel, which outplace and penetrate the atmospheric air [29; 42]. Thus, considering the power supply limit of 12 kV peak-to-peak and a rational safety factor, the voltage of 11 kV peak-to-peak is used hereafter. Figure 4: Optical emission intensity of the main emissive species detected as a function of the applied voltage amplitude. (**a**) N\({}_{2}\) and OH species. (**b**) Ar species. Operational conditions: 17.5 kHz and 1.5 slm. Mean values and standard deviations are calculated from two series of experiments carried out on different dates. Figure 5: Optical emission intensity of the main emissive species detected as a function of the applied voltage frequency. (**a**) N\({}_{2}\) and OH species. (**b**) Ar species. Operational conditions: 11 kV\({}_{\text{PP}}\) and 1.5 slm. Mean values and standard deviations are calculated from two series of experiments carried out on different dates. In the case of frequency variation, i.e., Figure 5, the OES intensity peaks around 17.5 kHz for most of the species. In other words, while an increasing frequency promotes species production initially, possibly due to the higher power injected into the system, elevated frequency leads to adverse results eventually. This fact may be attributed to the transition from laminar to turbulent gas flow profile (see discussion on the frequency role in reference [42]), which provokes increased argon-air mixing and disturbs the ionization wave propagation. Thus, the value of 17.5 kHz is applied hereafter. Regarding the gas flow rate, i.e., Figure 6, the intermediate value of 1.5 slm appears favorable to produce most species. The existence of an optimal gas flow is directly related with the breakdown of the laminar flow into turbulent flow for increased gas flow rates. This electro-hydrodynamic interplay between the plasma species and the gas jet has been studied extensively by our group [32,43,44]. Consequently, the value of 1.5 slm is considered henceforth. Two comments should be made on the above spectroscopic analysis: (i) The emission which is related to the carrier gas (i.e., argon atomic lines) has consistent peaks and patterns against all the operating parameters. Similar patterns and peak locations are observed for the hydroxyl molecule too. However, the emission intensity which is related to the surrounding air (i.e., nitrogen molecule bands) deviates from the above patterns when either the frequency or the gas flow rate is being swept.