Investigation of small-fraction molecular impurities in high-pressure helium plasmas using optical plasma diagnostic methods

Abstract

In high-pressure plasmas using gases diluted via a rare gas, small-fraction impurities in the discharge space significantly impact the basic plasma parameters and excited-species generation processes. This study investigated the behaviors of molecular impurities in a dielectric barrier discharge (DBD) generated in a flow of high-purity He gas using optical plasma diagnostic methods. The optical emission spectra obtained under various discharge conditions (pressure, flow rate, and voltage frequency) indicated that the major impurity species in the He DBD was the H₂O molecule, and the DBD decomposed the H₂O before reaching the measurement spot. To quantitatively analyze the H₂O fraction, time-resolved laser absorption spectroscopy was performed to measure the exponential decay time of He metastable (Hem) atoms in the He-DBD. The H₂O fraction in the He gas flow was derived from the dependence of Hem lifetime decay on the voltage frequency. In addition, a model was proposed to estimate the H₂O fraction under various He pressure and flow rate conditions from reference data. The procedures to perform optical plasma diagnostics and evaluate the fraction and behavior of H₂O impurities are expected to facilitate a better understanding and control of high-pressure plasmas.