# Study on the Ablation Behavior of High-Intensity Lasers in Vacuum
## 2 Experimental Setup  
The schematic diagram of the experimental setup is shown in Figure 1. A Nimma-900 laser (Beamtech Optronics Co., Ltd., Beijing, China) was used in the experiment. The maximum single-pulse energy was 900 mJ, with a wavelength of 1064 nm, pulse width of 8 ns, and a repetition rate of 10 Hz. The beam diameter is controlled to 5 mm by an aperture after the laser exits the device. The laser then passes through a 1:9 beam splitter, which reflects 10% of the energy into an energy meter for monitoring, while the remaining 90% is focused on the target using a 300 mm focal length lens; the focal spot of the focusing lens is 300 mm. The beam spot size before the focusing lens is approximately 5 mm in diameter. At the focal point, the beam is focused to a spot size of approximately 300 \(\upmu\)m. The target is mounted on a two-dimensional displacement table with a maximum travel distance of 50 mm. The focusing lens is installed on a one-dimensional displacement table that moves along the laser axis to adjust the distance between the lens and the target material. The displacement tables are housed inside a vacuum chamber, which is connected to external control systems. The movement of the tables is precisely controlled by a computer. The vacuum chamber maintains a minimum stable pressure of 1 \(\times\) 10\({}^{-3}\) Pa environment, and all experiments were conducted at this pressure. Two neodymium magnets and an aperture are installed in front of the target material, with the aperture size carefully selected to allow the laser to pass through. This setup ensures that the focusing lens is not significantly damaged during prolonged ablation. The LEXT OLS5100 laser confocal microscope (Olympus Corporation, Tokyo, Japan) was used as the measurement equipment, which can capture the three-dimensional morphology of ablation holes and measure the depth and volume data. Its measurement accuracy is 0.15 + L/100 \(\upmu\)m (where L is the measurement depth). The target material used in the experiment was a 2 mm thick TC-4 titanium sheet. Before ablation, the material was polished with sandpaper up to 600 grit and then wiped with alcohol-soaked cotton to ensure a consistent surface condition throughout the ablation process. All experiments were performed in triplicate, and the average value was calculated to ensure the reliability of the results. In the experiment, it was observed that when the focusing lens is near the focal position, the titanium alloy target material with a thickness of 2 mm is easily broken down. Therefore, three titanium alloy targets were polished smooth and secured with fixtures to create a target material with a thickness of 6 mm. This setup was designed to use a thicker layered target to prevent full penetration during ablation, enabling accurate measurement of the ablation depth in the unpenetrated section of the material. The defocusing distance primarily affects the laser fluence, and the ability of the laser to pierce the target depends on the location where the laser fluence is lowest. When the laser focus is within the target material, the laser fluence remains high across the upper and lower surfaces of the target. In practical experiments, a defocusing distance range of up to 30 mm was achieved, which exceeds the depth of focus (DOF). Based on this, the experiment was designed with a positive defocusing distance, meaning that the focal point is positioned above the target material. Even if the lens position remains unchanged during the experiment, the diameter of the ablation hole varies due to the non-uniform intensity distribution of the Gaussian beam and the continuous temperature rise during ongoing ablation. The diameter of the ablation hole increases with laser fluence and pulse number [27]. This paper does not address the ablation threshold. The above factors can lead to a discrepancy between the actual ablation hole diameter and the initial laser irradiation area. Using the ablation hole diameter to calculate the energy density may introduce additional errors and complexities.