Mechanism of signal uncertainty generation for laser-induced breakdown spectroscopy
Mechanism of signal uncertainty generation for laser-induced breakdown spectroscopy
复制标题
激光诱导击穿光谱信号不确定性产生机制
DOI：
10.1007/s11467-020-1006-0
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发表时间：
2021-04-01
影响因子：
7.5
通讯作者：
Wang, Zhe
中科院分区：
文献类型：
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作者：
Fu, Yang-Ting;Gu, Wei-Lun;Wang, Zhe
较大的测量不确定度严重阻碍了激光感生击穿光谱（LIBS）的广泛应用，因此了解信号不确定度的产生机制，包括信号的产生和传播，具有重要意义。结果表明，等离子体形貌的起伏是导致信号不确定性的主要原因。然而，目前还不清楚是什么机制导致的激光诱导等离子体形态波动。在本工作中，我们采用了三个快速成像相机捕捉三个连续的等离子体图像从同一激光诱导的钛合金等离子体，这使我们能够更清楚地了解等离子体的演变过程，特别是对于早期的等离子体演化阶段，当等离子体和周围的气体强烈相互作用。从图像上可以看出，随着延迟时间的增加，等离子体的形态起伏越来越大，从延迟时间为100 ns之前的“稳定等离子体”转变为延迟时间为300 ns之后的“起伏等离子体”。值得注意的是，等离子体的前沿部分从延迟时间150 ns到200 ns表现出显著的向下运动，并与等离子体的下部碰撞，使等离子体变平坦，后来甚至将等离子体分成两部分，这被认为是“稳定等离子体”向“不稳定等离子体”转变的关键过程。通过计算连续延迟时间下等离子体图像对的相关系数，发现两个等离子体在早期相似性越高，在后期也越相似，这意味着在临界延迟时间（150-200 ns）之前的微小等离子体波动被放大，导致后期等离子体出现较大的波动，从而增加了LIBS测量的不确定度。由于等离子体-环境气体界面在早期阶段（50 ns之前）剧烈的材料互穿，轻微波动的开始与瑞利-泰勒不稳定性（RTI）有关。即LIBS的不确定性产生是：等离子体形态起伏在早期不可避免地被RTI所掩盖，而微小的起伏又被等离子体前沿物质的后压下过程放大，导致严重的形态起伏和LIBS信号的不确定性。
Relatively large measurement uncertainty severely hindered wide application for laser-induced breakdown spectroscopy (LIBS), therefore it is of great importance to understand the mechanism of signal uncertainty generation, including initiation and propagation. It has been found that the fluctuation of plasma morphology was the main reason for signal uncertainty. However, it still remains unclear what mechanism leads to laser-induced plasma morphology fluctuation. In the present work, we employed three fast-imaging cameras to capture three successive plasma images from a same laser-induced Titanium alloy plasma, which enables us to understand more clearly of the plasma evolution process especially for the early plasma evolution stage when plasma and surrounding gases interact drastically. Seen from the images, the plasma experienced an increasing morphological fluctuation as delay time increased, transforming from a “stable plasma” before the delay time of 100 ns to a “fluctuating plasma” after the delay time of 300 ns. Notably, the frontier part of plasma showed a significant downward motion from the delay time of 150 ns to 200 ns and crashed with the lower part of the plasma, making the plasma flatter and later even splitting the plasma into two parts, which was considered as a critical process for the transformation of “stable plasma” to “unstable plasma”. By calculating the correlation coefficient of plasma image pairs at successive delay times, it was found that the higher the similarity between two plasma at early stage, the more similar at later stage; this implied that the tiny plasma fluctuation earlier than the critical delay time (150–200 ns) was amplified, causing a large plasma fluctuation at the later stage as well as LIBS measurement uncertainty. The initiation of slight fluctuation was linked with Rayleigh-Taylor Instability (RTI) due to the drastic material interpenetration at the plasma-ambient gas interface at earlier stage (before 50 ns). That is, the uncertainty generation of LIBS was proposed as: plasma morphology fluctuation was inevitably trigged by RTI at the early stage and the tiny fluctuation was amplified by the back pressed downward process of plasma frontier material, leading to severe morphology fluctuation as well as LIBS signal uncertainty.