Density of atoms in Ar*(3p54s) states and gas temperatures in an argon surfatron plasma measured by tunable laser spectroscopy

This study presents the absolute argon 1 s (in Paschens's notation) densities and the gas temperature, T-g, obtained in a surfatron plasma in the pressure range 0: 65 < p < 100 mbar. The absorption signals of 772.38, 772.42, 810.37, and 811.53 nm lines, absorbed by atoms in 1s(3), 1s(4), and 1s(5) states, were recorded with two tunable diode lasers. T-g is deduced from the absorption line shapes when scanning the laser wavelengths. The line profile, which is a Doppler broadening dominated Gaussian at gas pressures of p < 10 mbar, changes to a Voigt shape at p > 10 mbar, for which the pressure broadening can no more be neglected. T-g is in the range of 480-750 K, increasing with pressure and decreasing with the distance from the microwave launcher. Taking into account the line of sight effects of the absorption measurements, a good agreement is found with our previous measurements by Rayleigh scattering of T-g at the tube center. In the studied pressure range, the Ar(4 s) atom densities are in the order of 10(16) - 10(18) m(-3), increasing towards the end of the plasma column, decreasing with the pressure. In the low pressure side, a broad minimum is found around 10 < p < 20 mbar and hence the Ar(4 s) atom densities increase slightly with rising pressure. For the studied pressure range and all axial positions, the density ratio: 1s(5)/1s(4)/1s(3) is very close to a Boltzmann equilibrium by electron impact mixing at the local T-e, which was previously measured by Thomson scattering. The Ar(4 s) densities are successfully compared to a detailed Collisional Radiative Model. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4799152]