Plasma columns sustained at high enough gas pressures undergo radial contraction as manifested by their glow not entirely filling the radial cross-section of the discharge tube. This phenomenon has been reported with direct current, radio frequency, and microwave discharges. However, its modeling is still incomplete, in particular for rf and microwave discharges, a situation attributed to a lack of experimental data. To fill this gap, we took advantage of the extreme flexibility in terms of field frequency, tube diameter and gas nature of surface-wave sustained discharges to achieve a parametric study of this phenomenon. Special attention was paid to filamentation, specific to rf and microwave discharges, which is the breaking of a single channel of plasma into two or more smaller filaments as a result of the skin effect. We used emission spectroscopy as the main diagnostic means. Electron density was obtained from Stark broadening of the Hβ line, while molecular-band spectra emitted by the OH radical and the N2+ molecule were employed to determine the discharge gas temperature, leading to its radial distribution upon performing Abel inversion. For a given tube radius, contraction is shown to increase with decreasing thermal conductivity of the discharge. As a result, He and N2 discharges are the least contracted, while contraction increases with increasing atomic mass of noble gases. Of all these discharges, the N2 discharge appears to be the closest to local thermodynamic equilibrium.

The development of an atomic emission detector for gas chromatography is described. An atmospheric pressure microwave helium plasma generated with an original device called Surfatron was used as an atomization and excitation source. The characteristics of operation are defined and the analytical performances for carbon, chlorine, bromine, and iodine analyses are measured. Detection limits are in the order of pg/s and the dynamic range of the detector covers three orders of magnitude.

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