Influence of humidity on formation of pulsed atmospheric-pressure plasma streamers

In this study, we investigate the influence of humidity on pulsed atmospheric-pressure plasma streamers (PAPS) in a helium plasma jet using time-resolved optical emission spectroscopy, molecular beam mass spectrometry, and laser-induced fluorescence (LIF). Our results reveal that PAPS, although appearing as a continuous plasma plume, actually consist of discrete, high-velocity ring-like structures that lose their shape as humidity decreases. A critical humidity threshold (~20 ppm H₂O) was identified, below which PAPS transition into a diffuse, irregular plume. Mass spectrometry measurements confirm that decreasing humidity alters the positive ion composition and reduces helium metastable production, impacting streamer propagation. LIF diagnostics provided spatially resolved measurements of OH radicals, revealing that their concentration strongly depends on humidity, with peak values occurring where the plasma interacts with ambient air. These findings highlight the crucial role of water vapor in plasma streamer formation and reactive species generation, offering insights for optimizing plasma-based applications in medicine, material processing, and environmental technologies.