Experimental verification of the Boltzmann relation in confined plasmas: Comparison of noble and molecule gases

Abstract

Experimental verification of the Boltzmann relation is performed in argon and oxygen gas inductively coupled plasmas from the measurements of both the spatial electron currents (as a fluid approach) and the electron energy probability functions (EEPFs, as a kinetic approach). At a low gas pressure of 10 mTorr, the measured electron currents are spatially uniform, and the EEPFs in the total electron energy scale are identical, which indicate that the Boltzmann relation is valid at both the argon and oxygen gases. As the gas pressure increases to 30-40 mTorr, however, the Boltzmann relation is broken in the oxygen gas discharge, while the Boltzmann relation is still valid in the argon gas discharge. This different variation in the oxygen gas discharge is mainly due to the presence of various inelastic collisions in the entire electron energy region, which causes the transition of the electron kinetics from a non-local to a local regime.