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

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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.