Mode transition in 1D He plasma jet arrays dominated by hydrodynamic interaction

As an emerging technology for flexibly generating large-area atmospheric plasma, jet arrays are facing an urgent problem to control discharge modes caused by jet-to-jet interactions, apart from issues of stability and uniformity. Here, two modes in one-dimensional ns-pulsed He plasma jet arrays are reported, namely coupling and collimated modes. Discharge characteristics and transition rules of the two modes are explored by optical, electrical, spatial-temporal resolved and Schlieren methods. Results reveal that the coupling mode appears only if multiple conditions are satisfied, including low gas flow rate, high voltage frequency and small jet separation. Fluid dynamics and ICCD images prove that the mode transition depends on hydrodynamic interaction, which exceeds the effect of electrostatic repellence between ionization waves. Shrinkage of He flows is visualized to be enhanced at coupling mode, contributing to inward deflection of outmost plasma jets. Eddies on both sides of jet array and low-pressure regions near nozzles are generated by electric winds, promoting the transversal momentum transfer and gas fusion among adjacent He flows, thus tending to jet couplings. Fresh insights are provided into jet coupling and generation of controllable large-area plasma sources. The results could help to understand the importance of hydrodynamic interaction among proximal plasma jets, and plasma action on adjacent gas flows.