Study on the Velocity Measurement and Dynamic Characteristics of Laser-Sustained Plasma Flow Field Based on Schlieren Method
Wang Pengyu1, Li Jie1, Ma Xiao1, Cheng Zhiwen2, Cheng Liang2, Xiong Qing1
1. School of Electrical Engineering Chongqing University Chongqing 400044 China; 2. Shenzhen Xinkailai Industrial Machinery Co. Ltd Shenzhen 518116 China
Abstract：Laser-sustained plasma (LSP) is an advanced plasma radiation source characterized by high brightness and a broad spectrum, offering significant application value in fields such as semiconductor wafer defect detection. However, its stability is affected by self-excited oscillations caused by thermal convection, making the dynamic behavior of its flow field a key point of interest. In this paper, the schlieren method is used to visualize the LSP flow field. Through image processing techniques such as the optical flow method, the two-dimensional velocity distribution of the flow field and the variation of the oscillation period with laser input power are obtained. Additionally, numerical simulations of the flow field are conducted using COMSOL, providing mutual validation with the experimental results. In this study, the LSP itself is utilized as a point light source, and by leveraging the advantages of a reflective schlieren system, a schlieren setup consisting of nine components is designed. After initiating the LSP, a high-speed camera is used to record 1 000 schlieren images within one second. The laser power is adjusted proportionally to obtain six sets of schlieren images under different power levels. The captured schlieren images clearly show periodic “heat bubble” rise near the plasma region, successfully visualizing the internal flow field of the LSP. Based on the schlieren images obtained at different power levels, Fourier transform is first applied to measure the LSP oscillation frequency. The results indicate that the oscillation frequency decreases as the laser power increases. Subsequently, the optical flow method is used to measure the two-dimensional velocity distribution of the schlieren images under different power levels. Based on the experimental results, a heat source was configured according to the LSP dimensions observed in the schlieren images, and a thermo-flow coupled model of the LSP was developed using COMSOL simulations. This model provided two-dimensional dynamic distributions of density, temperature, and velocity. Analysis of the simulation results revealed periodic oscillations in all parameters, consistent with experimental observations, and identified the toroidal vortex near the plasma region as a key factor likely responsible for the periodic LSP oscillations: when the flow near the LSP core is heated and rises, the resulting vortex draws surrounding gas into the central heating zone, establishing a self-sustained periodic cycle of heating and ascent. Furthermore, by varying input parameters such as the gas medium and pressure in the simulation model, the oscillation frequency of the LSP under different operational conditions was obtained. The following conclusions can be drawn from the experimental and simulation analysis: (1) The combination of the schlieren method and the optical flow method enables effective visualization of the LSP flow field oscillation and measurement of key parameters such as oscillation frequency and velocity. (2) The mechanism behind the periodic LSP oscillation is closely related to the formation and evolution of the vortex. The vortex periodically draws surrounding gas into the central heating area, creating a recurring cycle of heating and ascent. Furthermore, the oscillation frequency v can be estimated by an empirical formula related to the equilibrium radius r? of the plume bubble. (3) The oscillation frequency of the LSP can be reduced by increasing the input laser power, using gas media with lower molecular weight, or reducing the ambient pressure. These findings are of great significance for suppressing LSP oscillations and advancing the development of high-brightness, broadband laser plasma radiation sources based on LSP technology.
王澎钰, 李杰, 马骁, 程治文, 程梁, 熊青. 基于纹影法的激光维持等离子体流场测速及动态特性研究[J]. 电工技术学报, 2026, 41(15): 5296-5306.
Wang Pengyu, Li Jie, Ma Xiao, Cheng Zhiwen, Cheng Liang, Xiong Qing. Study on the Velocity Measurement and Dynamic Characteristics of Laser-Sustained Plasma Flow Field Based on Schlieren Method. Transactions of China Electrotechnical Society, 2026, 41(15): 5296-5306.
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2026, Vol. 41 
