In this study, the electron densities and temperatures in helium and argon nonthermal atmospheric-pressure plasmas are obtained by continuum spectral analysis, and a comparison is made to investigate the factors that cause differences in these two quantities. The helium and argon plasmas are irradiated onto a floating copper disk. In both plasmas, the plasma plume is formed with the same length between the edge of the powered electrode and the surface of the copper disk, and the flow rate of each neutral gas is controlled to obtain the same electron thermal pressure, which is the product of the electron density and temperature. The same charge is generated in both plasmas, and so the same potential and thermal energy are obtained in both plasmas by controlling the flow rate of each neutral gas. When the electron temperature of both plasmas is 1–2 eV, the electron collision cross section of helium gas is larger than that of argon gas. Because the collision frequency between electrons and neutral particles is high under atmospheric pressure, the electron density is affected by the electron collision cross section corresponding to the electron thermal energy. Consequently, the electron temperature of the helium plasma is lower than that of the argon plasma, while the electron density of the helium plasma is higher than that of the argon plasma for the same electron thermal pressure.
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