An experimental study is conducted to determine the property fields of 40 MHz argon radio frequency inductively coupled plasma using optical emission spectroscopy. The pure argon plasma was operated at the input power of 0.3 kW and under atmospheric pressure. 29 atomic argon lines with upper level energies ranging from 12.9 to 15.5 eV, continuum emission and line width are used to evaluate plasma parameters such as temperature and electron number density. Since 40 MHz plasma is in almost complete nonequilibrium, the validaty and accuracy of most usual spectroscopic methods are questioned. Analysis based on the Boltzmann diagram, line-to-continuum intensity ratio, population of continuum extrapolated level, and continuum intensity reveals the departure from thermodynamic equilibrium in the plasma. Among these methods, the Boltzmann diagram method is shown to provide reliable plasma excitation temperature as long as the Boltzmann plot is drawn based on enough spectra lines covering from infrared to ultraviolet regions. The continuum emission at wavelengths within visible region can give good estimation of the electron density by using excitation temperature in the continuum relation. The line-to-continuum is not a reliable method for the temperature measurement of nonequilibrim plasma. The electron density obtained from the Saha plot can provide rough estimation of the electron density. It is shown that the electron-atom interaction contribution to the continuum radiation is more important than being expected before for the argon plasma in our study. The non-axisymmetric distribution of the emission was found to exist within the coil zone of the plasma, which may affect the estimation of the local emission coefficient, and consequently the measured plasma fields.

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