Axial heterogeneity in low-frequency nonthermal plasma: Pump-induced flow asymmetries on electron-ion transport dynamics

The spatial distribution characteristics and large-aperture surface treatment capability of low-pressure non-thermal plasma have garnered significant attention. Consequently, there has been a concerted effort to enhance the performance of these systems with the aim of broadening their applicability across a range of fields. This study systematically investigates the spatial distribution of 50-60 Hz low-pressure non-thermal plasma using Langmuir probe diagnostics, with controlled power (50-200 W) and pressure (10-50 Pa) conditions. Key findings reveal that reducing chamber pressure and increasing discharge power synergistically enhance electron density, electron temperature, and plasma density, directly governing plasma reactivity and surface treatment efficacy. Axial profiles demonstrate a characteristic rise and fall in the electron density within the chamber, with electron current density scaling proportionally to electron density and inversely to electron temperature. Ion and ion current density exhibit distinct non-monotonic profiles, decreasing initially before rising toward the exhaust port (30 cm). The electron temperature at the exhaust port increases significantly and surpasses that at the front window (0 cm), indicating combined effects of boundary conditions and flow field acceleration. At 30 cm, ion density declines more rapidly than electron density due to ions' higher mass and lower velocity, making them more susceptible to flow perturbations. Moreover, the simultaneous reduction in the ion density and increase in the ion current density correlate with enhanced ion velocity. These observations conclusively demonstrate that flow field asymmetry drives plasma spatial heterogeneity.