激光诱导铝等离子体中原子和离子组分膨胀特性

为了了解等离子体中原子与离子组分的膨胀特性及背景气体存在状态下其运动状态的改变规律,设计了一系列实验,并进行了深入探究.采用波长为532 nm的纳秒激光剥蚀铝样品形成等离子体,并使用配有em ICCD检测器的C-T型三光栅单色仪对等离子体进行时序采集,同时使用2400 g·mm~(-1)的光栅替代窄带滤光片进行不同组分成像诊断,得到铝等离子体中Al Ⅰ (396.1 nm), Al Ⅱ (466.3 nm), Al Ⅲ (447.9 nm)的光谱分辨图像.在不同背景气压下采集了等离子体各组分光谱图像,探究背景气体对等离子体演化的影响.结果表明,在等离子体形成过程中,离子组分相对于原子组分分布在羽流前端,且角度分布较小.原子与离子组分的真空膨胀速度均处于10~4m·s~(-1)量级.等离子体中离子组分的运动速度较高,且其运动速度随着离子价态的增加而增大,但在本实验使用的能量密度范围下,随激光能量的变化波动不大.中性原子的运动速度较慢,但随能量的增加而增大.随着膨胀过程的进行,各组分羽流沿样品表面法线方向推进且发射强度逐渐降低,对应的羽流密度和温度也相应降低.环境气压逐渐增大时,各研究组分运动状态与在高真空度下时有明显区别.在气压大于1 Pa后,等离子体与环境气体发生相互渗透,膨胀前端出现的晕影,产生扰动,发生束缚缓速.且等离子羽因气压增大而收缩、与背景气体的碰撞概率增加,使得羽流发射强度加强,等离子体的寿命随之延长.提出的新颖诊断方法与实验所得结果可为等离子体组分动力学过程的研究提供参考.

Expansion characteristics of atom and ion component in laser-induced aluminum plasma

A series of experiments is designed in order to investigate the expansion and movement characteristics of atoms and ions of the plasma in the presence of ambient gas. To obtain two-dimensional spectral images of different components in the plasma, a nanosecond laser with a wavelength of 532 nm is used to ablate an aluminum sample, forming the plasma. A C-T type of tri-grating monochromator with an emICCD detector is used for diagnosing the plasma chronologically. At the same time, a 2400 g·mm^-1 grating is used to replace the narrowband filter for imaging diagnosis of different components in vacuum. The spectrally resolved images of Al I (396.1 nm), Al Ⅱ (466.3 nm), and Al Ⅲ (447.9 nm) in aluminum plasma are obtained. Besides, the spectral images of plasma components under different ambient pressures are collected to explore the influence of background gas on plasma evolution. The results show that in the plasma formation process, the ion component is distributed in the anterior segment of the plume relative to the atom component, and its angular distribution is smaller. The vacuum expansion rates of atoms and ions are all on the order of 10⁴ m·s^-1. The movement speed of the ion component in the plasma is higher than that of atom component, and its movement speed increases with the valence of the ion increasing. In the energy density range used in this experiment, the velocity varies slightly with the laser energy. For the neutral atom, the velocity increases obviously as the energy increases. With the expansion process progressing, each component of the plume advances along the direction normal to the sample surface, and the emission intensity gradually decreases, the corresponding plume density and its temperature also decrease. With the ambient pressure increasing, the movement characteristics of each component are obviously different from those under high vacuum. At a pressure higher than 1 Pa, the plasma and the ambient gas are infiltrated with each other, vignetting appears in the front of the plume, disturbance occurs, causing the expansion speed to decrease. In addition, the plasma plume shrinks due to the increase of pressure, and the probability of collision with the background gas increases, so that the plume emission intensity is strengthened and the plasma lifetime is prolonged. The results of the new diagnosis method and the experimental results demonstrated in this study can provide a reference for the study of plasma component dynamic process.