Dynamics of colliding microplasma jets

Abstract

Because of their capabilities to generate plasmas that are not confined between electrodes, low-temperature plasma jets offer unique opportunities for applications such as material processing and biomedicine. The need to generate multiple jets in order to cover larger treatment areas has recently become desirable. However, the interaction between neighbouring jets is unavoidable. It is therefore crucial to elucidate the physical processes that occur between jets. In this paper we present the case of two counter-propagating jets generated by two DBD-based devices. We show that the plasma bullets emitted by the two jets interact with each other as soon as they leave their respective DBD device, resulting in a decrease in their velocities. The bullets do not actually meet but rather approach each other at a minimum approach distance. The location of the region of minimum approach is not midway between the nozzles of the jet devices but rather depends on the operating conditions. In addition, we discovered the emergence of a ‘secondary’ discharge exactly in the region of minimum approach. This discharge exhibits a pink glow, reminiscent of the pink afterglow occurring in some nitrogen discharges. Time-resolved spectroscopic measurements and current measurement analysis showed that the pink glow is a transient negative glow discharge that cannot be attributed to kinetic processes associated with re-excitation of nitrogen molecules. It is rather ignited by electrons accelerated from both jets towards the region of minimum approach. This process is found to be exactly timed with the measured current reversal.