Two-Temperature Electron Energy Distribution Function and Its Device Dependence in Atmospheric-Pressure Nonequilibrium Argon Plasma

Atmospheric-pressure nonequilibrium argon plasmas are used in many applications, and their electron properties may depend on the device configuration. Here, the electron temperature and density of two devices of contrasting geometry, a closed flow-channel type dielectric barrier discharge (DBD) and an open-air tube-type plasma jet, were determined from continuum optical emission spectra. The electron energy distribution function was represented by a two-temperature model combining a Druyvesteynian bulk component, which contains the great majority of the electrons, and a Maxwellian tail component, which is a small high-energy minority, with all parameters obtained by an exhaustive grid search. Introducing the tail component reduced the fitting residual by more than two orders of magnitude, supporting a two-component parameterization of the time-averaged distribution. The bulk temperature remained within 0.35–0.42 eV for every condition in which the two-component model was applicable, whereas the electron density spanned six orders of magnitude, from 6.6×1016 cm−3 in the DBD to 5.5×1010 cm−3 in the jet plume; at the same monitored power of 12 W, the DBD was about ten times denser. The continuum constrains only about 1% of the electrons, those above the photon-energy threshold, so the total density is a model-dependent extrapolation. Two checks that do not use the continuum, a Stark-broadening estimate and a collisional-radiative analysis of the line spectra, are both incompatible with the extrapolated total: inserting it into the collisional-radiative model overestimates the measured line emission by five to seven orders of magnitude, while the line spectra themselves require a density of order 1014 cm−3. The two methods do not, however, reproduce each other’s dependence on the operating condition. Within the continuum analysis, the device configuration therefore governs the density, whereas the bulk temperature is comparatively insensitive to geometry.