脉冲激光烧蚀材料等离子体反冲压力物理模型研究与应用

Research and application of plasma recoil pressure physical model for pulsed laser ablation material

分析了脉冲激光烧蚀材料等离子体等温膨胀阶段的物理特性,建立了脉冲激光烧蚀材料等离子体压力三维方程与动力学模型.应用所建模型,数值分析了单脉冲激光烧蚀青铜金刚石砂轮等离子体相关特性,得到等离子体的反冲压力最大值870 Pa出现在约25 ns后,距离砂轮表面距离约0.05 mm处.相关条件下开展脉冲激光烧蚀青铜金刚石砂轮试验,采用高速相机观测烧蚀砂轮过程中的飞溅现象;采用光栅光谱仪测量等离子体空间发射光谱,计算了等离子体电子温度、电子密度以及反冲压力.实验表明脉冲激光烧蚀青铜金刚石砂轮等离子体反冲压力可以不计,同时也验证了气体方程与动力学模型的正确性和可行性,对脉冲光纤激光烧蚀工艺优化具有启示意义.

In this paper, the physical properties of plasma in the isothermal expansion process when material is ablated by pulsed laser is analyzed. It is shown that the recoil pressure distribution of the plasma near the material surface indicates an exponential decrease as the distance from the material surface increases and the recoil pressure distribution exhibits the characteristics of a Poisson distribution in the X direction; the recoil pressure distribution is in accordance with Maxwell＇s velocity distribution law in the Y direction; the recoil pressure distribution conforms to a Gaussian distribution in the Z direction. A three-dimensional plasma recoil pressure equation and the plasma kinetic equation for laser-ablation materials are studied. These equations only require parameters to relate to plasma temperature, laser parameters and material properties, thus having a certain diversity.The equations are used for numerically analyzing the pulsed laser ablation of a bronze-bonded diamond grinding wheel. The numerical analysis shows that in the X and Y direction the plasma expansion dimension shows linear growth.After the pulse is ended, the plasma expansion dimension values reach their maxima. The plasma expansion velocity shows nonlinear growth. After the pulse is ended, the expansion velocity first increases and then decreases along the X direction and Y direction. Based on the analyses of the plasma expansion dimension and the plasma expansion velocity,the maximum plasma recoil pressure appears at a location approximately 0.05 mm away from the surface of the grinding wheel after approximately 25 ns. Through calculating the Saha equation, the degree of ionization is 0.0012 at 7506 K,and the maximum plasma recoil pressure value is approximately 870 Pa.The experiments on the pulsed laser ablation of a bronze-bonded diamond grinding wheel under the corresponding conditions are conducted. A high-speed camera is used to observe splash phenomenon in the laser ablation process. A grating spectrometer is used to measure the plasma emission spectrum. According to the Boltzmann plot method, the electron temperature value is calculated to be 7506 K; according to the Stark broadening method, the electron density values range from 7.645×10^（15）to 1.1608 ×10^（16）cm^（-3）and the recoil pressure values from 792 to 1203 Pa. The experiments show that the recoil pressure during the pulsed laser ablation of bronze-bonded diamond grinding wheel process can be ignored, and the correctness and feasibility of the plasma recoil pressure equation are also verified, which has heuristic significance for optimizing the laser ablation process.