Influence of Airflow on Nanosecond Pulsed Discharges

This contribution presents a head-on imaging study of an air-plasma flow reactor consisting of a dielectric barrier discharge operated by nanosecond repetitively pulsed voltage. The experiments explore the influence of varying flow rates and pulse repetition frequencies on the discharge dynamics and the transition between the uniform and micro-discharge regimes. The results from this work show the important role of the flow renewal rate, as compared to the pulsing rate, on the discharge behavior. A characteristic dimension for flow replenishment in between pulses is identified as a self-similar parameter to explain the behavior. The micro-discharge to uniform transition occurs for a critical replenishment distance around 200 micrometers. This value is of the order of typical streamer diameters, which are responsible for the early stages of development of the discharge. Replenishment distances above this critical value (corresponding to either higher flow speed or lower pulse repetition frequency) favor the uniform discharge. By using a diverging flow-field reactor, the transition can be spatially resolved, and the boundary moves downstream of the flow as this parameter increases: uniform conditions are encountered in the expanding upstream region. When micro-discharges are formed they are clearly seen to follow fluid streamlines: each pulse deposits the energy in an advected position downstream of the flow. Despite the strong influence on the spatial morphology, the total energy deposited remains comparable between tests, indicating that the micro-discharges present a much higher energy density, per micro-discharge, than the energy density of the uniform mode.