Time-resolved spectral characteristics of SF<sub>6</sub> plasma under femtosecond laser-guided high-voltage discharge
Authors: Zhang Yun-Gang , Liu Huang-Tao , Gao Qiang , Zhu Zhi-Feng , Li Bo , Wang Yong-Da ,   School of Electrical Engineering, Yanshan University, Qinhuangdao 066004, China,   State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China
Published: December 2020

SF<sub>6</sub> is widely used in gas insulated switchgear due to its excellent insulating and arcing performance. SF<sub>6</sub> arc plasma has been extensively studied, but time-resolved spectral characteristics of SF<sub>6</sub> arc plasma have not been reported. In this paper, the optical filament generated from focused femtosecond laser is used to guide the high-voltage discharge for generating SF<sub>6</sub> plasma in SF<sub>6</sub> environment. The SF<sub>6</sub> plasma spectrum is obtained in a wavelength range of 300–820 nm, and the identification and attribution of the spectral lines are investigated. The S and F lines are mainly in the 300–550 nm band and 600–800 nm band, respectively. The analysis shows that the S and F atoms are mainly directly or indirectly generated by the collision between SF<sub>6</sub> and high-energy electrons during the SF<sub>6</sub> decomposition caused by discharge. The S ions are generated by the collision of S atoms with high-energy electrons. The time-resolved spectrum of the SF<sub>6</sub> plasma superimposed by the continuous spectrum and the line spectrum is given, and its intensity increases and then decreases. The continuous spectrum is mainly generated by the combined effect of bremsstrahlung and recombination radiation. The recombination radiation is mainly generated by the collision of electron with ions and the recombination between molecular and atoms after SF<sub>6</sub> decomposition. The fluorescence lifetime of S ion at 409.91 nm is 57 ns, and the fluorescence lifetime of F atom at 685.60 nm is 341 ns. The evolution law of electron temperature and density with time are given. The electron temperature reaches 2047 K in the early stage of plasma formation. After that, the electron temperature quickly falls to about 1600 K within 300 ns due to the rapid expansion of the plasma and the increase in energy loss during electron movement. At the beginning of discharge, a large number of electrons are generated due to the rapid decomposition of SF<sub>6</sub>, and the electron density is highest (<inline-formula><tex-math id="M1">\begin{document}$ 10.1 \times {10^{17}}\;{\rm{c}}{{\rm{m}}^{ - {\rm{3}}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20200636_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20200636_M1.png"/></alternatives></inline-formula>). After that, the electron density drops rapidly within 200 ns because the recombination between electrons and ions decreases with delay time. Finally, it is proved that the SF<sub>6</sub> plasma is in local thermal equilibrium based on the Mc Whirter criterion. The results are of great significance for studying the decomposition mechanism of SF<sub>6</sub> and the on-line monitoring technique of high-voltage equipment.