Polarized laser-induced fluorescence spectroscopy for measuring electric field distribution in plasmas

我们开发了一种测量放电等离子体鞘层电场分布的光谱方法。电场可以由脉冲激光激发亚稳He原子通过禁止跃迁即斯塔克跃迁和电四极矩跃迁所引起的允许荧光的极化程度来确定。为了将该方法应用于具有较高电子密度和/或气体密度的等离子体，必须精确测量激光诱导荧光（LIF）脉冲上升部分的偏振度。我们开发了一种快速LIF检测系统（上升时间为1ns），该系统同时记录了Glan-Thompson偏振分束器分离的LIF脉冲的两个偏振分量（p和s）。利用该系统，观察了氦辉光放电等离子体鞘层区域的LIF信号。由于等离子体中激发态原子频繁地与基态原子碰撞，极化显著衰减。利用本研究建立的包含碰撞去极化过程的速率方程模型对这些数据进行分析，成功地得到了电场分布。结果表明，该方法可以应用于快速退极化速率为10^8s^<-1>的系统。从得到的LIF强度估计，如果亚稳原子的密度为-10^<10>cm^<-3>，可能的空间分辨率约为0.1mm。结果表明，该方法适用于有磁场作用的情况。

We have developed a spectroscopic method to measure sheath electric field distribution of discharge plasma. The electric field can be determined by the polarization degree of allowed fluorescence induced by pulsed laser excitation of metastable He atoms through forbidden transition, i.e.Stark and electric quadrupole moment transitions. To apply our method to a plasma with higher electron and/or gas density, it is indispensable to measure accurately the degree of polarization at the rising part of laser-induced fluorescence (LIF) pulse. We developed a fast LIF detection system (the rise time of 1 ns) which simultaneously recorded two polarization components (p and s) of LIF pulse separated by Glan-Thompson polarizing beam splitter. Using the system, LIF signals were observed in the sheath region of a He glow discharge plasma. The polarization decayd remarkably because the excited atoms frequently collided with ground state atoms in the plasma. A rate-equation model including the collisional depolarization process, which is also developed in the present study, was employed to analyze these data and then the electric field distribution was successfully obtained. It was demonstrated that our method can be applied to the system with fast depolarization rate of 10^8s^<-1>. From the obtained LIF intensity the possible spatial resolution was estimated to be about 0.1mm if the density of metastable atoms was -10^<10>cm^<-3>. It was also shown that the method was applicable to the case that magnetic field was applied.