Investigation of laser-induced plasma in SF 6 at different pressures using Thomson scattering

Thomson scattering was applied to measure the electron density and temperature in laser-induced SF 6 plasmas at various pressures (0.2-2atm). The plasma was induced by the Nd:YAG laser (1064nm, 200 mJ, and 7ns) focused into a chamber filled with SF 6. A second harmonic Nd:YAG laser (532nm, 50 mJ, and 6ns) 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 0.2atm decreases from 1.66 x 10 23 m - 3 at 2 mu s to 4.50 x 10 22 m - 3 at 6 mu s, and meanwhile, the electron temperature drops from 22050K to 15600K. At 2atm, from 2 mu s to 6 mu s, the electron density decreases from 3.00 x 10 23 m - 3 to 5.23 x 10 22 m - 3 and the temperature drops from 44000K to 14800K. 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 0.2atm to 2atm, indicating that the pressure increases the decaying rate of electron density. From 0.4atm to 2atm, 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 SF 6 are superior to air and argon.