Abstract: To study the instant flow process induced by high power pulsed laser ablation Al and energy change process of the plume flow field，the plume expansion and its evolution process were researched by shadowgraph technique. The sequence diagrams of the ablation plume evolution were obtained. The model of the laser ablation plume expanding in ambient gas was set up based on the momentum equation. According to the experimental results，the energy balance process among the kinetic energy of the plume，the kinetic energy of the background gas and the thermal energy of the plume and background gas in dimensionless units were analyzed. Results show that，at early times pressure gradients drive the ablation plume expansion as if it were expanding in vacuum，then the kinetic energy of the plume is gradually dissipated into kinetic and thermal energy of the background gas. The dimensionless kinetic energy of the plume is almost equal to the kinetic energy of the gas and the thermal energy is nearly equal to the half of the total energy when the dimensionless shock wave radius is 1. Eventually，the energy converts into the thermal energy of the plume and background gas.

Key words: Laser ablation；Shadowgraph；Plume flow field；Dimensionless

摘要： 为研究高能脉冲激光烧蚀铝靶材瞬态流动过程和形成的羽流流场能量转化过程，采用阴影法对纳秒激光烧蚀铝靶羽流膨胀及演化过程进行研究，得到烧蚀流场演化时序图；利用动量守恒方程，建立烧蚀羽流在环境气体中膨胀模型。由实验测量结果，通过对流场参数的无量纲化处理，分析得到烧蚀流场无量纲羽流动能、环境气体动能以及热能之间的能量转化过程。研究表明，初始时刻，由于烧蚀压要远大于环境气压，羽流膨胀与真空中的自由膨胀行为类似，此后，羽流动能逐渐转化为环境气体动能和热能；当无量纲激波半径为1时，羽流的无量纲动能等于被羽流推起的气体的动能，而热能约等于总能量的1/2，最终全部转化为羽流和环境气体热能。

关键词: 激光烧蚀；阴影法；羽流流场；无量纲