Density of atoms in Ar*(3p5 4s) 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, Tg, 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 1s3, 1s4, and 1s5 states, were recorded with two tunable diode lasers. Tg 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. Tg 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 Tg at the tube center. In the studied pressure range, the Ar(4 s) atom densities are in the order of 1016 1018 m3, 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: 1s5/1s4/1s3 is very close to a Boltzmann equilibrium by electron impact mixing at the local Te, which was previously measured by Thomson scattering. The Ar(4 s) densities are successfully compared to a detailed Collisional Radiative Model.