放电等离子体高精度空间分辨光谱测量方法

High Accurate Spatially Resolved Measurements of Discharges Plasma Spectra

放电等离子体光谱技术及激光诱导击穿光谱技术由于实验系统相对简单、信号强,很早就被应用于组分测量领域。由于自由放电在时间和空间上都具有随机性,这使得放电等离子体空间分辨光谱的精确测量变得十分困难;而激光诱导击穿光谱技术又局限于点测量。介绍了一种基于飞秒激光诱导的放电等离子体一维空间分辨光谱的高精度测量方法。飞秒激光自聚焦可以形成一段丝状弱等离子体通道,将该等离子体通道靠近高压直流脉冲电极时,可作为高压电极放电的触发源。这种触发方式可在规定的时刻触发高压电并诱导其沿着等离子体通道的路径击穿气体。实验多次测量放电开始时刻与激光到达时刻的时间间隔的波动小于0.01μs,证实了使用本方法诱导高压放电具有很高的重复性。由此可知,利用飞秒激光自聚焦成丝产生的弱等离子体通道诱导高压放电,可实现对高压放电的空间和时间的精确控制,进而可以采集放电等离子体通道的一维空间分辨光谱。实验结果表明,在喷管结构主导的流场环境中,由于喷管中纯N2与喷管外空气的组分不同,在高空间分辨光谱中,可以清晰地看到一维等离子体通道上不同位置的组分浓度变化情况。在一维空间分辨光谱中将N+和O光谱信号强度与N2和O2的浓度进行关联,可实现流场组分的一维在线诊断。该方法不仅具有纳秒激光诱导击穿光谱技术的相同优点,还具有一维空间分辨能力,在组分一维精确测量方面极具优势。同时,该方法还有望实现高时间分辨测量,对研究放电等离子体的时空演化过程具有重要的意义。

Discharge plasma spectroscopy and laser-induced breakdown spectroscopy are widely used in the flow field diagnostics owing their simplicity and strong signal intensity.Due to the random nature of the discharge,the time and space of the discharge are hard to be controlled,so it is difficult to measure the discharge plasma spectra accurately,and the laser-induced breakdown spectroscopy is limited to point measurements.In this paper,we introduce a method of one-dimensional spatially resolved discharge plasma spectra measurements,based on femtosecond laser filamentation-guided discharge.Femtosecond lasers beam self-focusing could create a filamentous weak plasma channel,and this weak plasma channel can be used as a trigger source for the high voltage discharge when it is close to the pair of high voltage electrodes.This triggering way can trigger and guide the discharge to breakdown the air along the path of the plasma channel at a specified time.Under the experimental conditions,the fluctuation of the time interval between laser pulse arriving and the start of discharge is less than 0.01μs,which indicates that the laser filamentation-guided discharge is highly repeatable.Therefore,using the femtosecond laser can realize the precise control of the time and space of the discharge,and one-dimensional spatially resolved spectra of discharge plasma can be accurately collected.In a jet flow field environment,the mixture fraction in the jet is different from the ambient air.The experimental results show that the changes in the species concentration at different locations can be seen clearly from one-dimensional spatially resolved spectra.By correlating the concentrations of N2 and O2 with the signal intensity of N+and O atom,one can realize a one-dimensional measurements of the species concentration.Compared with nanosecond laser-induced breakdown spectroscopy,this method not only has the same advantages,but also has one-dimensional spatial resolution.Meanwhile,this method has the potential to achieve high temporally resolved measurement,which is of great significance for studying the spatiotemporal evolution process of the discharge plasma.