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

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 diagnostics methods. The optical emission spectra obtained under various discharge conditions (pressure, flow rate, and voltage frequency) indicated the major impurity species in the He DBD as 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 (LAS) was performed to measure the lifetime 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 on the voltage frequency. In addition, a model was proposed to estimate the H2O fraction under various He pressure and flow rate conditions from few reference data. The procedures to perform the optical plasma diagnostics and evaluate the fraction and behaviors of H2O impurity are expected to facilitate a better understanding and control of high-pressure plasmas.