Transport mirrors are critical components in the high-power laser device that connects the main amplification system and the shooting range. Achieving surface cleanliness control is imperative. Particles, particularly those dominated by organic matter, metals, and minerals, are the most important factors influencing the surface cleanliness of transport mirrors. Particles with sizes >30 μm are the primary cause of transport mirrors damage. Thus, they should be removed. Manual wiping and air knife purging are common methods for controlling the surface cleanliness of transport mirrors. The air knife purging method is used in many situations due to its high removal efficiency and stable purging effect, especially in a closed environment that is not easy to operate manually. The removal efficiency of the air knife purging method is highly dependent on various process parameters, including the air knife’s air inlet pressure and the distance between the air knife outlet and the upper surface of the transmission mirror. Consequently, research on particle removal technology on the surface of transport mirrors based on wind knife purging is being conducted to reveal the influence law of working pressure, installation distance, purging time, purging times, and other factors on particle removal efficiency. The trajectory of particles in the purge removal process is studied and experimentally collected to reduce secondary pollution in the purge removal process. Finally, this research is critical for the final formulation of a low-risk and practical on-line particle cleaning control scheme for transport mirror surfaces.
In this study, alumina, stainless steel, and dust particles were employed. Changing the air knife purging process parameters obtained the particle removal efficiency for each process parameter. Finally, the impact of each process parameter on removal efficiency was determined. Simultaneously, the trajectory of particles in the removal process was experimentally captured. The entire procedure was as follows. First, the transmission mirror’s surface was divided into three sections: near, middle, and far ends. Each area had four measurement points that were densely packed with particles. The surface of the transmission mirror was then purged by adjusting process parameters such as air inlet pressure, installation height, outlet gap, and air knife purging times; the number of particles after purging was then measured off-line under various process parameters. Finally, the impact of process parameters on particle removal efficiency was quantified by calculating the particle change rate before and after purging. Video recording and slow-motion pictures were used to obtain the moving trajectory of particles during the removal process.
The closer to the air knife’s outlet side, the greater the particle removal efficiency, and the larger the particle size, the greater the removal efficiency (Fig. 4). This is primarily because the closer to the air knife, the greater the wind speed. The mechanical model of particle removal method states that the smaller the particles, the greater the dynamic pressure required for removal, i.e., the greater the fluid velocity required. When the installation height of the air knife is 4 mm for Al2O3, the removal effect is good. When the inlet pressure is 1.2 MPa, the transport mirror’s overall removal efficiency exceeds 90%. If the inlet pressure is increased further, the removal efficiency improves even more (Fig. 5). Stainless steel particles have a lower removal efficiency than Al2O3 particles (Fig.6), and humidity has a significant impact on dust removal efficiency. When the inlet pressure is less than a certain threshold, increasing the number of purging cycles has no effect on removal efficiency (Fig. 7). When the inlet pressure is 1.5 MPa, the air knife’s two inlet gaps (0.05 and 0.1 mm) achieve the same removal effect (Fig. 10). This is primarily due to the fact that the wind speed obtained at the end of the transmission mirror surface is essentially the same (Fig. 9). Finally, during the removal process, the particles move horizontally and quickly along the air flow direction (Fig. 11).
The effect of process parameters on the particle removal efficiency of various types of particles is investigated, and the particle movement track in the removal process is recorded. The results show that the air knife purging method is an effective way to control the surface cleanliness of transport mirrors. When the air knife outlet width is 0.1 mm, the inlet pressure is 1.5 MPa, and the air knife installation height is 4 mm, a high removal efficiency of Al2O3, stainless steel, and dust particles can be obtained. Increased purging times cannot improve particle removal efficiency at low inlet pressures (≤1.2 MPa); the particles move rapidly and horizontally along the blowing direction of the air flow. Furthermore, the dust samples must be dried and purified to reduce the influence of the surrounding environment, such as humidity, on the experimental results. Obviously, this experiment is being conducted in an open environment. Consequently, more research in a controlled environment is required to fundamentally realize the surface cleanliness control of transport mirrors. For example, active control measures are implemented based on previously obtained good process parameters and particle trajectories.
