High-speed schlieren imaging of a highly-efficient atmospheric plasma jet*

This study presents an extensive flow visualization of a highly-efficient microwave plasma jet. This atmospheric jet is a helium plasma plume exiting from a glass capillary tube with an inner diameter of 0.9 mm which passes through a gap region of an evanescent-mode cavity resonator. The specific design of the cavity concentrates the electric field mainly over the critical gap, and thus breakdowns the gas molecules even at very low values of input microwave power. The plasma jet is generated at 2.45 GHz with an input power in the range 0.5 to 10 W. A Z-type high-speed schlieren imaging system is used to investigate the hydrodynamic characteristics of the jet within the steady gas flow rate of 1 to 7 SLPM, both with and without the presence of plasma. We characterize the jet flow in terms of its core size and spread size, shear layer unsteadiness, and mixing of the helium jet with the surrounding air. The use of a knife-edge in radial and axial orientations provides the density gradient field perpendicular to it. We also present two-point correlation maps of the density gradient fields in both radial and axial directions to quantify the spatial coherence of the jetting flow and its mixing with the ambient air. This benchmarking study provides detailed flow visualization of this efficient atmospheric plasma jet that can later be used to optimize its performance for various applications including plasma actuation for flow control and propulsion in fluid dynamics settings.