Towards launching a stable wide plasma jet from a single tube: I. The importance of controlling the gas dynamics

The success of plasma jets in broad fields is raising the demand to enlarge the jet size to be used in practical and industrial applications. However, conventional upscaling approaches that use multi-tubes have considerable drawbacks resulting from fluid-dynamics and electrical interactions among the jets. Therefore, developing a new upscaling approach to launch a wide plasma jet from a single tube is a great of interest. Thus, this study aims to unveil and address the challenges faced in obtaining a stable, uniform, and wide plasma jet from a single tube with a diameter of up to 30 mm. The focus is on the dynamic flow of the effluent gas, where its modification showed a significant influence on the jet formation in wide tubes. It was discovered that the presence of a sudden expansion flow at the inlet of wide tubes has a remarkable negative effect on the jet formation, owing to the increase in the air entrainment. Therefore, no discharge could be ignited at relatively low flow rates, and only an unstable jet with a brush-like shape could be launched at higher flow rates. Meanwhile, the mitigation of the sudden expansion influences by controlling the geometric parameters such as expansion ratio and length-diameter ratio at different Reynolds numbers indicated a significantly positive effect on the discharge ignition and stability of the formed jet. Furthermore, a new approach to address the challenges of upscaling the plasma jet is presented; in this approach, a porous material (a sponge) is used as a gas diffuser and embedded at the tube inlet. The gas diffuser greatly modified the gas flow inside the tube, thereby a stable, uniform plasma jet could be launched from a wide tube at relatively low flow rates. Finally, the interaction of the jet with different targets is presented to show the applicability of the launched jet. The treatment patterns from a wide single tube are similar to that observed using a narrow tube, where a wide diffused discharge spot found over a dielectric target, whilst intense discrete spots were created on a floating metallic target.