## 3 Results and Discussion
### Experimental Analysis of LIBS Signal Changes in Flow Fields  
In the LIBS experiment, the locations of the plasma generator, detector, and inlet and outlet ports for gas flow are fixed. However, it is difficult to accurately predict the flow direction that most affects the plasma, due to the presence of chamber walls, the jig for the target, and the laser delivery optics where the flow exists. Therefore, it is necessary to determine the azimuth of an inlet port capable of obtaining a meaningful LIBS signal assuming a fixed detector. If the direction (azimuth) of a specific flow is determined experimentally, the relationship between the flow rate and the signal can be predicted. Figure 12 shows the change in LIBS signal amplitude according to the flow direction (azimuth) of the flowing gas. The changes in intensity of the LIBS signal were measured while the flowing gas was rotated around the plasma generated by the laser. The flowing gases used in the experiment were Ar, N\({}_{2}\), and He, and the signal changes of the Al element (wavelength = 396.152 nm) were measured in the counterclockwise direction relative to the reference point described above. The gas flow rate confirmed the change in plasma signal in the range of 300-420 cm\({}^{3}\)/s. Combined with the measurement results, the correlation between the flow rate and signal was relatively clear with a gas outflow direction of 240\({}^{\circ}\) and specific azimuth angle for Ar, N\({}_{2}\), and He. At this angle, as the flow rate increased, the LIBS signal strength tended to increase. On the other hand, the flow rate change and LIBS signal strength changed irregularly at 60\({}^{\circ}\), 120\({}^{\circ}\), and 180\({}^{\circ}\). Therefore, 240\({}^{\circ}\) coincided with the gas flow and detector directions, indicating that it is a suitable angle for gas flow measurement in this experimental setup. Figure 11: Normalized electric field intensity according to the relative positions of the plasma and detector calculated in the simulation. When the direction of gas flow is properly aligned in this setup, the LIBS signal strength changes according to gas flow in atmospheres of Ar, N\({}_{2}\), and He (Figure 13). To allow comparison, the experimentally obtained LIBS signal was normalized, based on the maximum signal strength. The results suggested that the plasma signal increases up to a certain flow rate as the gas flow rate increases. However, for our flow device, the association of LIBS signal intensity and flow rate was weakened at a flow rate \(\geq\) 370 cm\({}^{3}\)/s. There are several possible reasons for this observation, but the main cause is that the value of the signal at the detector decreases due to the mutual geometry between the plasma and detector above a certain zenith angle, as shown by the simulation results described above. The occurrence of fluid turbulence due to an increase in flow rate can be considered as another cause. In this case, the flow direction of the plasma appears inconsistent, and the signal is irregular. Therefore, the flow rate affecting the plasma is limited. The graphs in the bottom row of Figure 13 show the similar trends of the simulation and experimental results. The solid lines are theoretical values that non-dimensionalize the intensity of the electric field signal when the plasma affected by the flow is tilted from 0\({}^{\circ}\) to 35\({}^{\circ}\) during modeling. In the simulation, it was assumed that the plasma would be deformed in the flow direction. According to this assumption, a larger zenith angle would be associated with a greater influence of the flow. In the numerical prediction, the effect of the type of gas was not taken into account. In the simulation, the slope of the plasma can be analyzed at a zenith angle of 0\({}^{\circ}\)-35\({}^{\circ}\), synchronous with a gas flow rate of 300-370 cm\({}^{3}\)/s in the gas flow experiment. These results show the possibility of predicting the flow rate according to changes in the plasma signal in a limited system.