Uniform propagation of cathode-directed surface ionization waves at atmospheric pressure

The uniform propagation of positive-polarity surface plasmas in air at atmospheric pressure has been achieved using a multi-layer structure, consisting of a silicon wafer covered by a 1 mu m thick dielectric SiO2 layer as a propagation surface. Instead of the branched streamers observed in the same conditions when using conventional bulk dielectric surfaces, the plasma exhibits a homogenous ring-shaped structure with a high degree of reproducibility and stability. The plasma is generated by applying nanosecond positive voltage pulses to a tungsten wire touching the dielectric surface. The plasma has been imaged in single shot operation at high spatial resolution with an ultraviolet reflective microscope coupled with a fast intensified charge-coupled device camera. Time- and space-resolved optical emission spectroscopy shows that the homogenous ring corresponds to the propagation of an ionization front with a region of high N-2(+*) emission. We discuss the origin of the ring-shaped ionization wave, considering the role of the Si-SiO2 interface and the effect of illumination by an external light source. The ring ionization wave may result from branching inhibition, due to a photoelectric effect at the interface created by the photons emitted by the plasma. The generation of stable uniform surface ionization waves, in ambient air at atmospheric pressure, could be of interest for further advanced plasma-surface interaction studies or flow control applications.