## 1 Introduction  
The chemical laser was first used to realize population inversion using exothermic chemical reactions, and its main medium is gas. Chemical lasers are widely used in industrial fields due to their self-contained light source, minimal external dependence, and suitability for fieldwork [1, 2]. In chemical laser equipment, the optical cavity connected upstream of the supersonic diffuser has a large aspect ratio. The Reynolds (Re) number is several orders of magnitude lower than that of a general flow, an exothermic reaction exists, and the boundary layer is relatively thick. The thick boundary layer generated in the optical cavity is unfavorable for operating the chemical laser equipment. A thicker boundary layer reduces the area of the effective airflow channel, which can cause congestion in the incoming flow. However, because the incoming flow of the optical cavity is supersonic, a thicker boundary layer produces complex shock-boundary layer interference, resulting in severe separation of the flow [3, 4]. This situation destroys the flow field structure in the optical cavity and affects the working state of the chemical laser equipment. Therefore, controlling the boundary layer of the optical cavity, reducing its thickness, and improving the flow field structure in the optical cavity are effective means of improving the operating efficiency of chemical laser equipment. Many scholars have conducted extensive research on boundary layer control problems. Common boundary layer control technologies include boundary layer blowdown, vortex generator, flow channel, and microjet control [5; 6; 7; 8; 9]. [PERSON] et al. [10] introduced a fluid microjet into the boundary layer to increase fluid momentum and numerically analyzed the control of a separation bubble behind a ramp. [PERSON] et al. [11] experimentally considered the control of flow separation over a NACA-0025 airfoil using microjet actuators, and the experimental results are presented for a novel approach to nonlinear model predictive control. [PERSON] et al. [8] focused on the self-excited shock train flow in an isolator and its control by partial removal of the boundary layer to examine the variation in the inlet to outlet pressure ratio with and without suction flow control. Their findings show that suction control provides an advantage via a 50% reduction in the amplitude of the shock train movement with the self-excited oscillation frequency in the range of 14-15 Hz. The advantages of using suction for boundary layer control include effectively preventing the shock wave-boundary layer interaction, making the downstream boundary layer thinner, and reducing flow distortion; however, part of the mass flow is lost. The flow of the optical cavity and hyper expander has a low Re number and supersonic flow containing a chemical reaction. The flow state in this context is unique compared to that in a supersonic wind tunnel and supersonic inlet, distinguished by two factors: first, the more rapid development of the boundary layer owing to the lower Re number; second, the exothermic chemical reaction in the optical cavity, which raises the static pressure of the airflow and reduces the Mach number. These two factors affect each other, such that the flow in the optical cavity is easily choked. Therefore, controlling the development of the boundary layer in an optical cavity effectively improves the performance of chemical lasers. Downstream of the cavity in the chemical laser is a supersonic diffuser, whose main function is to realize the deceleration and pressurization of the supersonic airflow in the diffuser from the cavity and maintain the pressure of the cavity within a reasonable working range. The main index used to evaluate the supersonic diffuser is the pressure recovery performance of the airflow. Because the flow in the diffuser is complicated and there is complex shock wave boundary layer interference, the flow field may appear as a separation zone, exhibiting the characteristics of flow separation and a complex wave system structure.