Student Excellence Award Finalist: Ion Dynamics and Pre-Ionization Effects in an Ar-O₂Modulated RF-Driven Atmospheric Pressure Plasma Jet

In this work, we report a characterization of ion fluxes impinging on substrates as produced by a modulated RF-driven atmospheric pressure plasma jet operating in a homogenous gas environment (Ar+1% O₂) using molecular beam mass spectrometry (MBMS). The influence of the RF modulation frequency (100 Hz-20 kHz) upon the ion fluxes was investigated by time-resolved measurements, and lifetimes of the dominant ions, O₂⁺, NO⁺, O^-, O₂^- and O₃^-, were found to be 28±2 μs, 117±8 μs, 7.3±0.4 μs, 17±1 μs, and 23±2 μs, respectively. The absolute ion densities in the near afterglow region were found to be on the order of 10¹¹ cm^-3. Significant differences in the dynamics of the positive and negative ions were found and explained by large electron densities in the afterglow produced by electron detachment reactions from negative ions due to the large concentrations of atomic oxygen and singlet delta oxygen. Transitions in ion flux dynamics for different modulation frequencies and at the startup of the plasma were analyzed together with ICCD images recording the plasma propagation, to assess the dynamics of plasma plume propagation and how it is impacted by “memory effects”. Quantitative measurements of the ion densities causing these memory effects are reported. The results highlight the tremendous impact of memory effects on plasma propagation and their corresponding pre-ionization densities.