Estimation of the Effects of Operating Pressure on the Degree of Non-Equilibrium in DC-RF Hybrid Plasma Jets Using the Boltzmann Plot Method

DC-RF hybrid plasma torches have attracted a great attention to their possible applications to materials synthesis and thermal plasma spray coating by taking advantage of their combined thermal plasma characteristics available from the respective DC and RF torches operating at either atmospheric or reduced pressure condition of ambient gas. However, this flexible range of operating pressure may cause the thermal plasma characteristics to deviate from the local thermodynamic equilibrium (LTE) state. Thus, the operating ambient pressures are expected to have an influence on the degree of non-equilibrium, and will be of great importance to not only practical applications of the hybrid torches but also the diagnostics and descriptions of thermal plasma properties. Therefore, it is essential to estimate the degree of non-equilibrium in the DC-RF hybrid plasma jets according to the operating pressures. To this purpose, optical emission spectroscopic measurements for plasma temperature in DC-RF hybrid plasma jets are carried out in this experimental work. The line intensities from the lower and higher excited states are measured and the emission coefficients are obtained using an Abel inversion. The excitation temperatures of the lower and higher excited states are calculated by the Boltzmann plot method, and the deviation from the Boltzmann equilibrium depending on the operating ambient pressures is qualitatively investigated based on comparisons between the different excitation temperatures.