High-Speed Schlieren Analysis of Instability-Driven Helical Plasma Plumes in Radio Frequency Atmospheric Pressure Plasma Jets

Atmospheric Pressure Plasma Jets (APPJs) are widely used for generating reactive species under conditions suitable for heat-sensitive applications [1-2]. While helically structured plasma plumes have been observed to enhance air mixing and reactive species transport compared to conical plumes, the underlying physical mechanisms responsible for their formation remain less explored [3]. The APPJ features a pin-to-ring electrode inside a 2 mm thick Pyrex tube, with argon as the working gas. A 13.56 MHz RF power supply operates in both continuous and pulsed modes. A pulse frequency of 500 Hz with 50% duty cycle is chosen to promote thermal modulation. High-speed Schlieren imaging captures flow evolution under varying powers and flow rates.

High-speed Schlieren imaging reveals that plasma ignition significantly alters the flow structure of argon jets, reducing the laminar length beyond what Reynolds number scaling alone can explain. This suggests that plasma-induced heating modifies the flow stability in a more complex way. The observed early onset of vortices indicates the presence of Kelvin–Helmholtz (KH) instability, which becomes more prominent with increasing flow rate. Under pulsed RF operation, the rapid heating and cooling cycles generate steep pressure and density gradients at the nozzle itself. The resulting baroclinic torque, due to the misalignment of these gradients, introduces swirl into the KH vortices—twisting them into helical structures. This is visible in the Schlieren images as the formation of coherent helical plumes emerging directly from the nozzle. Interestingly, the location of vortex onset aligns well with theoretical KH growth lengths, and the estimated Strouhal number (~0.29) supports a shear-driven instability mechanism. These findings suggest that the formation of helical plasma plumes is rooted in a coupling between flow instabilities and plasma-induced thermal effects. Rather than treating helical plumes as a visual outcome, this work proposes a fluid-dynamic mechanism behind their formation raising important questions about how plasma and flow interact in space and time.

[1] Radhika T. P. & S. Kar, Scientific Reports, 13(1), 10665 (2023).

[2] Radhika T. P. & S. Kar, Physics of Fluids 36(8) (2024).

[3] Mahreen et. Al Journal of Applied Physics, 130(8), 083301 (2021).