Radial excitation temperatures and electron number densities in a 9, 27 and 50 MHz inductively-coupled argon plasma
This work is the result of experiments performed on a 9, 27 and 50 MHz atmospheric pressure argon ICP uing the same thermometric species and instrumentation. Radiation originating from plasma volume with 0.25 mm2 projected areas were measured. The lateral observed spatially integrated radiances were transformed to radial functions by using Abel integral equations. Variations of radiances from Ca atom and ion transitions as function of observation height were studied. Two ion transitions of Ca with sufficient excitation energy difference were chosen for two-line temperature determinations, while substitution of atom to ion radiance ratios into the Saha equation were used in calculating values for the electron number density. The cooling effect of the central penetrating nebulizer gas on the plasma centres was clearly observed through a decrease in the excitation temperatures at the lowest observation heights above the r.f. coils. Experimental evidence indicates a general decrease in excitation temperature with increasing generator frequency. The same tendency was observed for the electron number density. These phenomena are probably due to the variation in skin depth at the different frequencies. © 1983.