Study of gas flow dynamics of helical plumes in a radiofrequency atmospheric pressure plasma jet

In this work, we present a systematic experimental study on the physics of helical plume formation in a cross-field atmospheric pressure plasma jet (APPJ) by comparing plume dynamics under continuous and pulsed radio frequency (RF) excitation. While continuous RF discharge predominantly produces a stable conical plume, pulsed operation (500 Hz with 50% duty cycle, identified as the minimum threshold condition) results a self-organized helical plume with enhanced spatial reactivity. Schlieren imaging revealed a significant modification of background gas dynamics after plasma ignition, including accelerated mixing of argon with ambient air. A key finding is the appearance of swirl-like flow structures at the laminar-to-turbulent transition in the continuous case. This was unexpected, since a direct transition was anticipated. In the pulsed case, however, the swirling begins directly at the nozzle exit. It then couples strongly with the ionization wave front and produces a helical plume morphology. Dimensionless analysis using the Strouhal number confirmed the frequency scaling of flow instabilities, while the role of Kelvin–Helmholtz instability and baroclinic torque was identified as the dominant mechanism driving the swirling of the plasma plume. These results provide experimental evidence supporting our proposed physics-based theory for helical plume formation in pulsed RF APPJs. In addition, optical emission spectroscopy and molecular beam mass spectrometry reveal the enhanced reactivity of helical plumes, highlighting their superior mixing, longer interaction path, and broader application potential in plasma–surface processing and biomedical treatments.