Investigation of laser-induced plasma in SF6 at different pressures using Thomson scattering

Thomson scattering was applied to measure the electron density and temperature in laser-induced SF6 plasmas at various pressures (0.2–2 atm). The plasma was induced by the Nd:YAG laser (1064 nm, 200 mJ, and 7 ns) focused into a chamber filled with SF6. A second harmonic Nd:YAG laser (532 nm, 50 mJ, and 6 ns) was used to probe the distributions of electron density and temperature. The images after breakdown indicate that higher pressure accelerates the plasma evolution and enhances the asymmetry of the plasma structure. Additionally, different from toroidal structures in other gases, a special vortex structure appeared in SF6. The electron density around the axis at 0.2 atm decreases from 1.66×1023 m−3 at 2 μs to 4.50×1022 m−3 at 6 μs, and meanwhile, the electron temperature drops from 22 050 K to 15 600 K. At 2 atm, from 2 μs to 6 μs, the electron density decreases from 3.00×1023 m−3 to 5.23×1022 m−3 and the temperature drops from 44 000 K to 14 800 K. The time exponent obtained by fitting the maximum electron density using the power law decreases from −1.206 to −1.669 in the pressure range from 0.2 atm to 2 atm, indicating that the pressure increases the decaying rate of electron density. From 0.4 atm to 2 atm, the time exponent of the decay of electron temperature decreases from −0.499 to −0.926. The comparisons among laser-induced plasmas in various gases show that the decaying rates of both electron density and temperature in SF6 are superior to air and argon.