Non-Invasive Diagnostics of Molecular Gas Plasmas with Quantitative Optical Emission Spectroscopy

本项目旨在开发一种新的无创且易于实施的电离气体等离子体光学诊断方法，用于特殊类别的低压分子气体等离子体。涉及等离子体的工业过程带来了一长串的社会效益，这些效益继续以快速的速度增长。低压电离气体等离子体的独特特性使其能够以其他方法无法实现的方式操纵材料。具体来说，它们的复杂动力学导致了非平衡化学；高能离子、自由基（与衬底表面发生强烈反应的原子、分子或离子）和等离子体中通过与高能电子碰撞而产生的光子在材料表面蚀刻、修饰或沉积薄膜。因此，过程结果对不同能量的电子丰度和其他等离子体参数非常敏感。该项目还将庆祝等离子技术，并通过开发一个新的实践推广展览，重点介绍用于航天器推进的等离子推进器，提高公众的“等离子素养”。展览的中心将是一个在大气压下工作的等离子体推进器。该展览将与麦迪逊西高中的火箭俱乐部合作制作，并将成为火箭俱乐部受欢迎的巡回展览的一部分，每年有成千上万的人在科学节和学校参观。等离子体发射的光谱携带等离子体参数的编码映射。通过计算控制等离子体光发射的激发和去激发过程来预测发射光谱的模型，可以通过测量特征波长的等离子体光发射强度来探测等离子体的特性。这个项目的创新之处在于创建了分子气体的定量排放模型，最初的重点是氧气。通过仔细考虑已知的激发态速率，本研究将确定光谱特征以纳入模型，选择一些对电子能量分布函数有很强定量依赖性的特征，以及一些对解离分数（即分子分解成组成原子的比例）有很强定量依赖性的特征。这种方法将允许等离子体参数被确定为模型产生的最适合观测光谱强度的值，使用光学发射光谱（OES）测量。为了展示和探索分子OES诊断的能力，它将应用于选定的放电现象(i)表现出尚未完全理解的复杂等离子体动力学，（ii）新的分子OES可能提供新的见解，以及（iii）获得的新见解与工业过程的发展相关。

This project aims to develop a new non-invasive and easily-implemented optical diagnostic of an ionized gas, a plasma, for the particular class of low-pressure molecular gas plasmas. Industrial processes involving plasmas contribute to a long list of societal benefits that continues to grow at a rapid pace. The unique properties of low-pressure ionized gas plasmas enable the manipulation of materials in ways that cannot be achieved by other means. Specifically, their complex dynamics lead to non-equilibrium chemistry; energetic ions, radicals (atoms, molecules or ions that react strongly with substrate surfaces), and photons produced in the plasma through collisions with energetic electrons etch, modify or deposit thin films on materials surfaces. Process outcomes are thus critically sensitive to the abundances of electrons of different energies and other plasma parameters. This project will also celebrate plasma technology and enhance public 'plasma literacy' through the development of a new hands-on outreach exhibit highlighting plasma thrusters for spacecraft propulsion. The centerpiece of the display will be a working plasma thruster operating at atmospheric pressure. The display will be created in partnership with the Rocket Club at Madison West High School and will become part of the Rocket Club's popular traveling exhibit, viewed by thousands of people each year at science festivals and school visits.The spectrum of light emitted by the plasma carries an encoded mapping of plasma parameters. Probing plasma properties by measuring the intensity of plasma light emissions at characteristic wavelengths is made possible through a model that predicts emission spectra by accounting for the excitation and de-excitation processes that govern light emission by the plasma. This project's innovation is the creation of a quantitative emission model for molecular gases, with oxygen as an initial focus. Through careful consideration of known excitation rates, the study will identify spectral features for inclusion in the model, choosing some that show a strong quantitative dependence on the electron energy distribution function and others that are strong functions of the dissociation fraction, i.e., the fraction of molecules broken up into constituent atoms. This approach will allow plasma parameters to be determined as the values for which the model produces a best fit to the observed spectral intensities, measured using optical emission spectroscopy (OES). To demonstrate and explore the capabilities of the molecular OES diagnostic, it will be applied to selected discharge phenomena (i) that exhibit complex plasma dynamics not fully understood, (ii) for which new molecular OES is likely to contribute new insights and (iii) for which the new insights obtained have relevance for development of industrial processes.