# Study on the Ablation Behavior of High-Intensity Lasers in Vacuum
## 3 Results and Discussion
### Ablation Plume  
Several factors contribute to the nonlinear increase in laser ablation depth. First, when the laser interacts with the target material, the temperature of the material rises. For metals, the laser absorption rate typically increases with temperature due to changes in the behavior of free electrons and surface properties, which enhance the efficiency of laser energy absorption [28]. Second, the vacuum environment minimizes atmospheric interactions, enabling more efficient energy transfer to the material surface [31]. Third, as the depth of the ablation hole increases, laser reflection within the irradiation area leads to multiple reflections on the walls of the ablation hole, eventually refocusing at the bottom,  
Figure 6: (**a**) Effect of single-pulse energy on average single pulse-ablation volume; (**b**) effect of single-pulse energy on average volume ablation efficiency. The different curves correspond to different single-pulse energies as indicated. which results in an increased absorption of laser energy [32]. Additionally, in atmospheric environments, heat dissipation during nanosecond laser drilling occurs primarily through convection. In contrast, in vacuum environments, convection is absent, and heat dissipation occurs solely through thermal radiation and thermal diffusion. To investigate the underlying causes of the observed phenomenon, a series of experiments were conducted to capture ablation plumes under varying conditions, as shown in Figure 7. The integration time, aperture, and distance used for plume image capture were set to 0.5 s, 35 cm, and f6.4, respectively. A laser energy of 100 mJ was used throughout the ablation plume experiments, and the target material was positioned at the focal point. Figure 7 presents the initial plasma image generated upon the laser's first irradiation of the target. Next, a hole was drilled into the target, allowed to cool, and then irradiated with the plasma inside the hole, as depicted in Figure 7. A comparison between these two images reveals that when the laser is focused inside the hole, a slightly higher amount of plasma is generated, indicating that the hole enhances the laser absorption rate. After approximately 210 pulses of repeated irradiation with a 1 Hz laser, the plasma image, shown in Figure 7, displayed a significantly larger amount of plasma, suggesting an increased laser absorption rate during this phase. Furthermore, when a 10 Hz laser was used for continuous ablation of the target, as shown in Figure 7, bright linear streaks were observed, which may indicate the presence of ejected molten material. These features resemble patterns typically associated with molten particles during laser ablation. Given that the vaporization latent heat of titanium alloy greatly exceeds its melting latent heat, the emergence of molten splashing material is undoubtedly the primary factor contributing to the sharp increase in ablation efficiency. As seen in Figure 7, due to the excessively high intensity of the plasma, it was not possible to clearly distinguish the ejected material. Therefore, a neutral density filter was added in the areas with higher plasma intensity, which reduces the light intensity by 100 times. This effectively prevented CMOS saturation, allowing the ejected material to be distinguished. Figure 7: Ablation plume. (**a**) The plasma image generated upon the laser’s initial irradiation on the target; (**b**) the plume image produced by repeated ablation of about 210 times with a 1 Hz laser; (**c**) the ablation plume produced by irradiating a laser through a small hole; (**d**) the plume image produced by repeated ablation with a 10 Hz laser. Since the experiment was conducted in a vacuum environment, heat dissipation primarily occurred through radiation and conduction, rather than convection. As a result, localized heat accumulation may have occurred in the laser-irradiated area.