## 2 Methodology and Experimental Setup  
The experimental setup is shown in Figure 3. To examine the effect of gas flow on plasma, a flow device using a chamber was designed. In a rectangular acrylic chamber with a width of 50 mm, length of 50 mm, and height of 30 mm, three gases (Ar, N\({}_{2}\), and He) flowed over the specimen in one direction. Plasma was generated by irradiating the specimen with a laser beam, and the spectral signal from the plasma was measured. The flow direction of the gas and incident path of the laser were both set up vertically, with the left side of the chamber set to 0\({}^{\circ}\). The inlet flow direction of each gas flowing in the chamber was changed to 360\({}^{\circ}\) by rotating 30\({}^{\circ}\) counterclockwise from this reference point. The direction of gaseous flow induced by the influence of the outer wall of the chamber and the interference of the specimen setup for LIBS measurement did not match the direction of the surface on which the specimen was placed. Therefore, by changing the direction of the inlet flow by 360\({}^{\circ}\) and analyzing the resulting LIBS spectrum, meaningful results regarding the correlation between the gas flow and LIBS signal could be derived. This step was included in order to determine the inlet and outlet directions of the flowing gas, which showed a similar tendency to the modeling results. Through this process, it was possible to proceed with analysis of the change in signal of the LIBS according to the characteristics of the flow. The flowing gas was maintained at a volumetric flow rate of 300-420 cm\({}^{3}\)/s inside the chamber; in this state, plasma was generated on the specimen through the laser. Generating plasma by focusing a laser beam directly into the flow stream without any solid target is ideal for verifying the relationship, but this requires a high-quality, high-energy laser beam source. In addition, it is relatively difficult to maintain consistency in terms of the generation of plasma in the flowing gas during the experiment. A magnesium (Mg) alloy with the elemental composition shown in Table 1 was used as a target medium for generating plasma by laser ablation. The signal detector that acquired the LIBS plasma signal was fixed in a specific direction, as shown in Figure 3. In this setup, the change in plasma signal induced by gas flow could be expressed as a function of the angle of the detector and inlet direction of the flowing gas. In the LIBS setup used in the experiment (RT100; Applied Spectra), an Nd:YAG pulsed laser with a wavelength of 1064 nm was used as the laser beam source (Table 2). This laser source irradiated the material with an energy of 16 mJ per pulse. The temporal pulse width of the laser was 5 ns, and the size of the spot focused by a lens on the surface of the laser beam was around 100 \(\upmu\)m. Plasma generated by the laser beam was collected in the form of a spectral line using a signal detector. The gate delay time of the signal detector was set to 0.1 \(\upmu\)s. Quantitative analysis of specific elements in the sample was performed using the data of the signal detected in the plasma to confirm the flow characteristics. The surface of the Mg alloy specimen was irradiated with the laser a total of 30 times, and the analysis was performed using an average of 29 spectral signals (excluding the first signal on the top surface). As the surface of the target was irradiated with the laser, ablation at a depth of about 2 \(\upmu\)m was observed each time the sample was irradiated. Simultaneous with the laser ablation, a blind hole was created on the surface of the specimen. There may be differences in the plasma signal measurement depending on the microscopic shape of the sample surface. However, as the size of the generated micro-holes was thought to be small compared to the size of the plasma, the effect on gas flow of the holes created on the surface was negligible.