## 3 Results and Discussion
### Temperature Control Performance of Multi-Gas Temperature-Controllable Plasma Jet  
Figure 5 shows the relationship between the TCF temperature, the plasma jet body temperature, and the gas temperature before and after the plasma generation. The results of the CFD analysis are also plotted in the N\({}_{2}\) result. The gas and plasma gas temperatures of carbon dioxide, argon, nitrogen, and oxygen were varied in the ranges of \(-\)17-77 and 29-108, \(-\)18-74.5 and 3-85, \(-\)20-78 and 28-108, and \(-\)21-78 and 17-106 \({}^{\circ}\)C, respectively, by varying the plasma jet body temperature from \(-\)30 \({}^{\circ}\)C to 90 \({}^{\circ}\)C. Assuming that the plasma jet body temperature affects the plasma gas temperature, a single regression analysis was performed. The coefficients of determination for each approximation line are listed in Table 1. In the case of gas irradiation, the coefficients of determination for the R\({}^{2}\) values were greater than 0.99 for all gas species. The gas temperatures of the plasmas had a  
Figure 4: Metal 3D-printed multi-gas temperature-controllable plasma jet: (**a**) outside view; (**b**) plasma generation for various gas species. coefficient of determination for the R\({}^{2}\) values greater than 0.98 for all gas species. These results show that the approximation line fits effectively and that the plasma jet body temperature, gas temperature, and plasma gas temperature vary proportionally. The gas temperature of the plasma was indirectly controlled by controlling the temperature of the plasma jet body. The instability of the plasma gas temperature was observed when the plasma jet body temperature was between \(-\)10 \({}^{\circ}\)C and 20 \({}^{\circ}\)C because of the water droplets that condensed on the plasma jet body near the plasma outlet. These water droplets were caught in the gas flow of the plasma, causing instability in the plasma temperature. When the measured nitrogen gas temperature was compared with the CFD temperature control performance, a decrease in the temperature control performance of approximately 3 \({}^{\circ}\)C and 5 \({}^{\circ}\)C was observed at a plasma jet body temperature of \(-\)30 and 90 \({}^{\circ}\)C, respectively. The CFD results and actual measurements were not in perfect agreement. This may be due to the fact that the CFD does not reproduce external disturbances such as the air flow around the plasma jet. For all gas species, a temperature increase was observed when the plasma was generated. The ranges of the temperature increase for carbon dioxide, argon, nitrogen, and oxygen at the plasma jet body temperature of \(-\)30 \({}^{\circ}\)C were 46, 22, 49, and 37 \({}^{\circ}\)C, respectively. The range of temperature increase for carbon dioxide, argon, nitrogen, and oxygen at the plasma jet body temperature of 20 \({}^{\circ}\)C, which was close to the room temperature, was 31, 15, 41, and 34 \({}^{\circ}\)C, respectively. The temperature increase for carbon dioxide, argon, nitrogen, and oxygen at the plasma jet body temperature of 90 \({}^{\circ}\)C was 31, 10, 31, and 28 \({}^{\circ}\)C, respectively. For all gas species of plasma, the range of temperature increase due to plasma generation decreased as the gas temperature increased. One of the reasons for this phenomenon may be the influence of the room temperature through mixing with the \begin{table}
\begin{tabular}{c c c c c} \hline  & **CO2** & **Ar** & **N2** & **O2** \\ \hline Gas & 0.9975 & 0.9967 & 0.9981 & 0.9996 \\ \hline Plasma & 0.9828 & 0.9898 & 0.9861 & 0.9893 \\ \hline \end{tabular}
\end{table} 
Table 1: Coefficient of determination between plasma jet body temperature and gas temperature for each gas species. Figure 5: Relationship between plasma gas temperature and plasma jet body temperature. ambient air. To achieve lower or higher plasma gas temperatures, it is necessary to vary the temperature of the TCF significantly.