## 1 Introduction  
Laser-induced breakdown spectroscopy (LIBS) is a method for analyzing elements on the surface of a material by measuring plasma generated by the interaction of a sample surface with a focused laser beam as a discontinuous spectrum [1, 2]. LIBS is based on the principle of analyzing the plasma signal generated by laser ablation on the material surface in the form of a spectrum. As laser ablation is used on the sample surface, it can be classified as a three-dimensional non-contact measurement technique [1]. In the plasma state, the elements of a material exist as atoms and ions in excited states. As time elapses and the plasma cools, the excited atoms and ions return to the ground state, releasing radiation energy [3]. Figure 1 shows the changes in intensity of plasma energy and types of energy emitted from the plasma over time, from the moment of laser irradiation [4]. Theoretically, plasma generation is initiated by laser ablation from femtoseconds to picoseconds immediately after the material is irradiated with the laser. At the nanosecond point, the plasma expands and emits emission lines from ions with strong and continuous emission lines. After a few hundred nanoseconds to microseconds, the plasma starts to cool and emits emission lines from atoms and spectral lines from molecules. After milliseconds have elapsed, the plasma condenses [4, 5]. In this process, the elements have their own LIBS spectra, and information on the LIBS spectral line is used to calculate the concentration of major or minor elements in the test material through the LIBS database [6]. Figure 2 shows a schematic diagram of a LIBS setup and data collection method for plasma samples. The LIBS method does not require pre- or post-treatment of the sample, or a vacuum environment, and thus analysis is possible at normal pressure and room temperature. It is also possible to measure a wide range of elements regardless of the state of matter, and it can be seen that measurement is largely non-destructive because it is possible to analyze even minor elements in the range of several nanograms to micrograms [6]. Recently, in addition to elemental analysis of materials using LIBS, several studies have analyzed various physical properties of samples through LIBS [7; 8]. Plasma generated through laser ablation of the material surface may include, not only chemical information of the material, but also physical information of the material surface. For example, the possibility of analyzing the physical properties of the surface of the material has been reported based on the characteristic that the spectral signal of plasma appears different depending on the stress, hardness, and roughness of the material [9; 10; 11; 12]. In elemental measurement using LIBS, plasma induced by laser ablation plume can change shape depending on the ambient atmospheric pressure [13]. In addition, the plasma itself is also affected by the type of atmospheric gas [14; 15]. In a He gas atmosphere, the plasma is cooled so that the signals of the induced plasma atomic and ion emission lines are limited. In addition, in an Ar gas atmosphere, the temperature of the plasma increases, thereby amplifying the signal. Thus, laser-induced plasma interacts with the surrounding gas [14; 16]. The energy propagation of plasma, which is the basis of signal measurement in LIBS analysis, is closely related to radiation [17]. Emission energy radiated from the plasma is directional with respect to a specific irradiated surface. This direction can be expressed as the zenith angle (\(\theta\)) and azimuthal angle (\(\varnothing\)) of the spherical coordinate system. Therefore, the energy propagation of plasma tends to be proportional to the area projected on the observation plane. Gas flow around the plasma will affect the observation area thereof [4]. Figure 1: Schematic of laser pulse irradiation and plasma behavior over time. Figure 2: Schematic of LIBS setup.