Implementation of Optical Emission Diagnostics for EEDFs on a Helicon Plasma

Optical emission spectroscopy (OES) is attractive as a non-invasive approach for measuring electron energy distribution functions (EEDF) in low-temperature plasmas. Adaptation of an approach successfully developed and implemented on an argon inductively-coupled plasma (ICP) to a helicon plasma system is the focus of this study. Trial EEDFs are an input to model calculations of excitation rates for select electronic transitions using known collision cross sections, and yielding the system EEDF as the trial that results in a best match to a set of measured emission intensities. Preliminary helicon OES data show regimes with much stronger argon ion emissions than observed in the ICP, where argon ion lines were so weak that they could not be productively included along with neutral lines in the OES analysis. Because of the high threshold energy for excitation of argon ion emissions, they signify the presence of energetic electrons, providing an opportunity to extend testing of the diagnostic to a new regime. Potential for improvements in the OES diagnostic for plasmas with a significant population of relatively high energy electrons, through the addition of argon ion lines to the emission model, will also be discussed.