Romadanov I, Raitses Y, Smolyakov A
Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, USA.
Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada.
Rev Sci Instrum. 2024 Jul 1;95(7). doi: 10.1063/5.0187687.
Laser-Induced Fluorescence (LIF) spectroscopy is an essential tool for probing ion and atom velocity distribution functions (VDFs) in complex plasmas. VDFs carry information about the kinetic properties of species that is critical for plasma characterization. Accurate interpretation of these functions is challenging due to factors such as multicomponent distributions, broadening effects, and background emissions. Our research investigates the use of Wavelength Modulation (WM) LIF to enhance the sensitivity of VDF measurements. Unlike standard Amplitude Modulation (AM) methods, WM-LIF measures the derivative of the LIF signal. This approach makes variations in VDF shape more pronounced. VDF measurements with WM-LIF were investigated with both numerical modeling and experimental measurements. The developed model enables the generation of both WM and AM signals, facilitating comparative analysis of fitting outcomes. Experiments were conducted in a weakly collisional argon plasma with magnetized electrons and non-magnetized ions. Measurements of the argon ion VDFs employed a narrow-band tunable diode laser, which scanned the 4p4D7/2-3d4F9/2 transition centered at 664.553 nm in vacuum. A lock-in amplifier detected the second harmonic WM signal, which was generated by modulating the laser wavelength with an externally controlled piezo-driven mirror of the diode laser. Our findings indicate that the WM-LIF signal is more sensitive to fitting parameters, allowing for better identification of VDF parameters such as the number of distribution components, their temperatures, and velocities. In addition, WM-LIF can serve as an independent method to verify AM measurements and is particularly beneficial in environments with substantial light noise or background emissions, such as those involving thermionic cathodes and reflective surfaces.
激光诱导荧光（LIF）光谱是探测复杂等离子体中离子和原子速度分布函数（VDF）的重要工具。VDF携带有关物种动力学特性的信息，这对于等离子体表征至关重要。由于多组分分布、展宽效应和背景发射等因素，准确解释这些函数具有挑战性。我们的研究调查了使用波长调制（WM）LIF来提高VDF测量的灵敏度。与标准幅度调制（AM）方法不同，WM-LIF测量LIF信号的导数。这种方法使VDF形状的变化更加明显。通过数值建模和实验测量对WM-LIF的VDF测量进行了研究。所开发的模型能够生成WM和AM信号，便于对拟合结果进行比较分析。实验在具有磁化电子和非磁化离子的弱碰撞氩等离子体中进行。氩离子VDF的测量采用窄带可调谐二极管激光器，该激光器在真空中扫描以664.553 nm为中心的4p4D7/2 - 3d4F9/2跃迁。锁相放大器检测由二极管激光器的外部控制压电驱动镜调制激光波长产生的二次谐波WM信号。我们的研究结果表明，WM-LIF信号对拟合参数更敏感，能够更好地识别VDF参数，如分布组分的数量、它们的温度和速度。此外，WM-LIF可以作为一种独立的方法来验证AM测量，并且在具有大量光噪声或背景发射的环境中特别有益，例如涉及热电子阴极和反射表面的环境。
