An analysis of LTE effects in inductively coupled RF plasmas

Spectroscopic measurements are carried out on inductively coupled radio-frequency (RF) argon plasma over the pressure range 120 to 760 Torr. The electron number density is obtained by two methods: H_beta profile and the absolute intensity of the argon continuum. Number densities of excited argon levels are deduced from absolute line intensities. The electron density measured in the hottest zone of the plasma decreases from 10¹⁶ to 6*10¹⁵ cm^-3 with the decrease of pressure from 760 to 120 Torr. Comparison between the different number densities shows that the atmospheric plasma is in local thermodynamic equilibrium (LTE) whereas weak departures from equilibrium are observed at pressures below 300 Torr.