Electrohydrodynamic coupling and stochastic branching in a miniaturized ns-pulsed He plasma jet

Abstract

This study focuses on the complex coupling between discharge and flow properties in a ns-pulsed He micrometer scale atmospheric pressure plasma jet ($\mu$APPJ). This is investigated by integrating electrical measurements, schlieren photography, ICCD imaging, and space-resolved Optical Emission Spectroscopy (OES) with Computational Fluid Dynamics (CFD) simulations. In the flow rate range QV=0.1-1 slm, a critical threshold emerges at 0.3 slm, where the discharge consumes the highest energy overall, achieving maximum propagation length and remarkable collimation. Below 0.3 slm, insufficient momentum renders the jet susceptible to buoyancy and air entrainment, leading to shorter effluents, while higher flow rates enhance shear layer instabilities. CFD simulations reproduce the schlieren flow profiles to quantify the axial helium mass fraction (YHe) confirming a stable helium-rich core at 0.3 slm (YHe=90%), not seen in other flow rates. Furthermore, lower and higher flow rates promote stochastic branching which is more pronounced at QV>0.3 slm. Numerous lateral branches are clearly distinguished and quantified via single-shot ICCD imaging for the first time in a He $\mu$APPJ. The increase of voltage amplitude (VP) in the range 4-9.5 kV, amplifies their activity in the effluent tip at 0.3 slm. At VP=9.5 kV, their number increases for QV<0.3 slm compared to QV=0.3 slm, while for QV>0.3 slm they occur much closer to the nozzle exit and intensify farther downstream. Timeresolved imaging reveals a distinct peak in ionization wave velocities (up to $\approx$600 km/s at 0.3 slm/9.5 kV) just after the nozzle exit, followed by a progressive decay which becomes more abrupt at the higher flow rates. This correlates spatially with a surge and subsequent axial drop in N2 + (FNS) emission intensity, indicating Penning ionization as a key mechanism behind this acceleration. Average gas temperature estimations (TGas$\approx$350 K) suggest that localised thermal expansion could contribute to the instabilities observed, possibly combined with a sudden rise in the average electrohydrodynamic force at the nozzle exit for QV$\ne$0.3 slm. Finally, the device geometry also plays a decisive role in internal vortex formation, especially at higher flow rates, affecting effluent stability. These results provide a unique framework for optimizing $\mu$APPJs for high-precision applications such as in analytical chemistry and surface processing.