# Aerodynamic Window Sealing of a Large-Aperture Channel for High-Power Laser Transmission
## 1 Introduction  
High-power laser emission equipment can be carried on cars, ships, and aircraft. Transmitting in a large-aperture channel makes the laser free to change the direction of the beam for better tracking a target, which is extensively used in military and manufacturing applications [1; 2; 3]. In the process of high-power laser transmission, the aerosol particles in the external environment absorb part of the laser energy, which increases the local temperature in the large-aperture channel, producing thermal blooming [4; 5; 6]. When aerosol particles are attached to the surface of optical elements, irradiation by a high-power laser will cause irreversible effects, such as component damage [7; 8]. Therefore, it is crucial to ensure that the gas in the channel is in a dry and clean state [9]. The aerodynamic window sealing method was adopted to continuously transport purge gas to the large-aperture channel and utilize the incompressibility of the gas to maintain the pressure difference between the inside and outside of the channel, realizing the blocking of ambient air and dust particles into the interior of the channel. With the increasing applications of lasers, there have been many studies on the window sealing of laser transmitting channels. The literature covers the use of crystal lenses for window sealing and the analysis of mechanical, thermal, and optical properties of crystalline materials as optical windows by considering the absorption dissipation and thermo-optical coefficients of different crystalline materials [10; 11; 12]. Mineral acid leaching and HF-based etching technology have been employed to mitigate the laser damage precursor in the subsurface of crystal lenses to enhance the laser-induced damage resistance [13]. The reflection loss for spinel was reduced from 7% per surface to 0.9% by applying anti-reflection surface structures (ARSS) to the crystalline material, and for fused silica with ARSS, the reflection loss was reduced to 0.02% near 1 \(\upmu\)m, effectively increasing the transmission and laser damage thresholds of the crystal [14]. With the increase in laser power and channel aperture, the structure size of the crystal lens window becomes larger, which is more difficult to manufacture. The crystal material will absorb part of the laser energy and produce thermal deformation, thereby losing the beam quality, which limits the use of traditional solid windows. The researchers introduced a transverse stream of airflow to separate the environment, which uses high-pressure gas that expands through a nozzle to create a strong pressure gradient across the window. The existing transverse supersonic jet aerodynamic window structure is subdivided into windows with a homogeneous flow on the nozzle section and free-vortex windows. In [15], the authors examined producing jets with a uniform velocity distribution on the nozzle section; however, a considerable air mass flow rate was required. The change in the momentum in the jet because of the pressure difference was accompanied by the appearance of a shock wave and a rarefaction-wave fan, which resulted in deteriorating the quality of the optical beam. With the use of nozzles, they generated a velocity distribution corresponding to the free vortex on the nozzle section, which could preserve the beam mass [16; 17]. The design of a supersonic free-vortex aerodynamic window ensuring optical quality and fluid-mechanical sealing of a simulated laser cavity for active controlling of the gas flow in the aerodynamic window by adjusting part of the diffuser has been discussed [18]. The mathematical models and computational tools for designing, analyzing, and predicting the supersonic jet and nozzle flow in the aerodynamic windows of high-power gas lasers have been considered. A nozzle contouring tool was developed, offering the possibility to solve the problem of the aerodynamic design of nozzle devices with specified characteristic flows [19; 20].