Interaction between a helium atmospheric plasma jet and targets and dynamics of the interface

Abstract

This paper investigates the interaction of a helium atmospheric plasma jet impinging onto liquid and metal targets using experimental and numerical techniques. The primary motivation of this study is to experimentally understand the effect of liquid and metal targets on the propagation of a plasma jet, and to numerically characterize chemical and electrical behavior at the plasma-water interface. This study is relevant in the ongoing research of plasma self-organization for the development of medical devices capable of self-adaptation. Experimental measurements are made to obtain ionization wave (IW) propagation, average electron density, and optical emission spectrum of the plasma near the interface for cases with copper and water targets and a freely expanding jet. The induced acidity on the plasma-treated water is measured by pH balance. Results confirm that both liquid and metal targets alter the IW propagation velocity and electric potential. IW average propagation speed is highest with the copper target and lowest with the water target by a factor of two. Corresponding IW head potentials are highest for the metal target and lowest in the presence of the water target. Spatial average of electron density in the plasma column is of the same order of magnitude for all cases, with the case of the water target being the lowest. For the copper target, single IW contact areas on the plasma-target interface are observed to last well into the micro-second range after initial impact within each discharge period. A reflected IW is observed in the case of the metal but not the liquid target. The plasma jet delivers a small acid dose which changes the pH of the liquid. The numerical model uses parameters from plasma-liquid experiments to simulate reactive oxygen and nitrogen, acidic, and charged species in interacting gaseous and aqueous layers by employing a transient advection-diffusion-reaction-solvation equation. The interacting layers are connected by a two-way coupling governed by solvation through Henry’s law. Accumulation of charges in the aqueous layer results from varying diffusion and mobility time scales of charged species. The electric potential on the aqueous layer is characterized to be in the order of tens of volts, a small fraction of the kilo-Volt measurement of the IW head potential. Electric fields and Maxwell stresses on the aqueous layer are also simulated and are proposed to affect the movement of the plasma contact spot on the gaseous layer. The mechanism for the plasma contact spot movement on the gaseous layer is implemented using a level-set technique which uses charge-induced electric stresses to calculate local advection velocity. Energy equations are solved in both phases for neutrals, ions and electrons by considering the effect of the Joule heating term. Current density through the gaseous-aqueous interface is calculated to be in the order of micro-Amperes, which creates a small enough Joule heating term so that the temperature of heavy species remains unchanged.