# Study on the Ablation Behavior of High-Intensity Lasers in Vacuum
## 3 Results and Discussion
### Ablation Volume and Efficiency  
The ablation depth can reflect the longitudinal development process of the ablation holes, but it does not accurately represent the true ablative efficiency. Therefore, we further measured the volume ablation parameters of the ablation holes. As shown in Figure 5, when the single pulse energy is 660 mJ, the total ablation volume at various defocusing distances is obtained by adjusting the position of the lens. Figure 4: (**a**) Effect of different single-pulse energies on ablation depth; (**b**) effect of different single-pulse energies on average removed per pulse. The different curves correspond to different single-pulse energy as indicated. It can be observed that when the pulse energy is less than 500 pulses, the ablation volume remains almost unchanged with the defocusing amount. As the defocusing distance decreases, the ablation area becomes smaller, but the power density increases, resulting in an increase in ablation depth. This factor compensates for the effect of the reduced ablation area. Therefore, we speculate that the primary material removal mechanism in this case is gasification. We believe that the gasification observed during the laser ablation of the metal surface is evaporation rather than boiling. This is because, in the laser ablation process, laser energy is rapidly focused onto a small area of the metal surface, causing a sharp rise in temperature. Due to the extremely rapid heating, atoms on the metal surface gain sufficient energy to transition directly from solid or liquid to gas, which is characteristic of evaporation. This differs from conventional boiling, which typically occurs when the entire liquid reaches its boiling point, accompanied by bubble formation and growth. Thus, the high energy density and rapid heating in laser ablation result in a direct phase change, more consistent with evaporation. In this case, the ablation volume is primarily determined by the single-pulse energy. When the number of laser pulses reaches 1000, the ablation volumes at different defocusing distances begin to diverge. We speculate that this is due to the more concentrated heat deposition during the laser ablation process as the ablation area decreases, leading to higher temperatures and an increase in laser absorption [28]. Subsequently, the lens was adjusted to a position near the focal point, and the effect of single-pulse energy on the average removal volume was measured, as shown in Figure 6a. The \"average removal volume\" is defined as the ablation volume divided by the number of pulses, while the \"average removal efficiency\" is calculated by dividing the average single-pulse ablation volume by the single-pulse energy. Due to limitations of the measurement equipment, ablation holes with a depth-to-diameter ratio greater than 4 could not be measured. Therefore, the number of pulses was reduced, and the average single-pulse ablation volume was measured as a function of energy density and pulse number, ranging from 50 to 250 pulses. Figure 5: Relationship between the ablation volume and the number of pulses; the laser energy remains constant at 660 mJ. The different curves correspond to different distances of lens–target defocusing as indicated. The average volume ablation efficiency of the target material was calculated as a function of single-pulse energy, as shown in Figure 6b. When the single-pulse energy ranged from 30 to 150 mJ, the average volume ablation efficiency increased slowly. With an increasing number of pulses, the ablation data exceed the volume ablation efficiency observed in the nanosecond laser ablation of titanium alloys under atmospheric conditions [29]. In Figure 6, it can be observed that both the average removal volume and average removal efficiency continue to rise with the number of pulses. This trend may initially seem inconsistent with the results shown in Figure 3a, where the ablation depth starts to plateau at higher pulse numbers.