Electron temperature of an RF discharge in argon up to atmospheric pressure

A cold argon plasma is generated inside a dielectric tube (inner diameter of 2 mm) using two long linear electrodes painted on diametrically opposed sides of the tube. The optical emission spectra from Ar 4p-to-4s transitions were compared to the predictions of a collisional-radiative model using the electron temperature T$_e$ (assuming a Maxwellian EEDF) and the Ar 1s$_2$ level number density as the only adjustable parameters. T$_e$ was deduced from the best fit between measured and simulated line emission intensities. At 760 Torr, the best fit is obtained for T$_e$=1.28 eV. When the power density increases from 4.2 to 7.0 W cm$^{-3}$, T$_e$ remains constant while $n_e$ (estimated from electrical measurements) increases from 3.7 to $8.8\times10^{11}$ cm$^{-3}$. In this range of power density, the discharge remains in the $\Omega$ mode with a maximum of the light emission (dominated by a continuum in the UV-VIS range) at the center of the tube. By reducing the pressure to 90 Torr, the best fit is achieved for a higher T$_e$ of 1.50 eV. On the other hand, the power density, $n_e$ and the continuum intensity decrease with decreasing pressure. In contrast with a helium discharge in the same range of discharge current, the argon discharge does not switch to the $\gamma$ mode.