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

Abstract We investigated the influence of humidity on development of helium sinusoidal-driven atmospheric-pressure plasma streamers using Intensified Charge-Coupled Device (ICCD) imaging, mass spectrometry (MS), and laser-induced fluorescence (LIF). A critical humidity threshold of ∼20 ppm was identified, below which streamers become diffuse and unstable. Importantly, the origin of water vapor (adsorbed in the gas line vs bubbler injection) was found to significantly affect streamer morphology. ICCD observations of streamer length correlate with changes in dominant ion species detected by MS, and with spatial OH distributions revealed by LIF. Although OH density was measured only at 250 ppm, the results confirm that humidity is the main source of reactive OH radicals. These findings highlight the crucial role of trace humidity in plasma jet behavior and demonstrate the need for precise humidity control in plasma diagnostics and applications.