Surface-wave-sustained discharges (SWDs) form a particular class of high frequency (HF) discharges: their HF sustaining field is provided by a traveling wave that transfers energy as it propagates along the discharge column, yielding a plasma column with an axially decreasing electron density. SWDs have proved to be ideal for investigating experimentally and theoretically both the HF field and discharge aspects of HF plasma sources at reduced gas pressure. In this article, SWDs are utilized at atmospheric pressure to gain insight into the departure from thermodynamic equilibrium (TE) of HF sustained discharges. This departure is found to increase significantly as the electron density decreases along the plasma column whereas the gas temperature and the power absorbed per electron do not vary axially. The two-temperature plasma model provides an adequate description of this departure from TE.

In this paper the electron density and temperature of atmospheric helium and argon plasmas operated under similar experimental conditions are compared. The conditions are chosen such that both plasmas are ionizing. It is found that a helium plasma has a higher electron temperature and a lower electron density than an equi-operational argon plasma, i.e. an argon plasma that is operated at the same external conditions. This is mainly caused by the higher excitation potential of the first excited state and the lower mass of helium, respectively. Due to these differences in electron density and temperature the densities of the helium ground state and of the excited states are much larger than their corresponding Saha equilibrium values for a wide range of conditions. The consequence of this is that the spectroscopic methods, which are used to determine the electron density and temperature, have a very limited validity region in the case of helium. For argon the deviations are much smaller so that these methods can often be applied safely.

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This work presents an experimental study of argon microwave discharges maintained by a surface wave at 2.45 GHz, at atmospheric pressure and under low flow, in a capillary tube. Emission spectroscopy techniques are used to characterize these discharges. Special attention is paid to the determination of the electron temperature by using the line-to-continuum intensity ratio method. We show that, even in situations very close to local thermal equilibrium, simple substitution of the excitation temperature Texc by the electron temperature Te in the equation governing the line radiation emitted by a plasma should not be done. Causes for such an assertion are analysed and a parametric study is conducted to determine the sensitivity of the proposed method. This method is then applied to provide Te from experimental data corresponding to an atmospheric pressure argon plasma produced by a surface microwave.

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