Characterization of Bright Spots and Erosion Structures Formed on a Grounded Electrode during Nanosecond Diffuse Discharges at Both Polarities

Low-temperature plasma is applied in various manufacturing processes. It is used to modify surface properties. While doing this, it is important to avoid damaging the surface being processed. The results of studies on the dynamics of low-temperature plasma formation during a diffuse discharge in air under atmospheric pressure in a sharply inhomogeneous electric field with positive and negative polarity are presented. This study focuses on its impact on electrode surfaces coated with soot with a thickness of approximately units of micrometers. The uniformity of the impact was assessed by the removal of the soot layer. Bright spots and corresponding erosion structures (discharge imprints) on the soot-coated electrode were observed. Analysis of time-resolved images and corresponding voltage and current waveforms shows that bright spots can appear both within 1 nanosecond after a breakdown and also tens of nanoseconds later, during repeated low-current breakdowns caused by the arrival of the voltage pulse reflected before the first breakdown. The diffuse discharge imprint consisted of craters with diameters ranging from 15 to 300 μm, depending on voltage pulse polarity, indicating the uneven impact of plasma on the surface. Large craters (50–300 μm) with complete removal of soot were observed at both polarities. We believe that an explosion of a micro-protrusion on the electrode surface initiates a shock wave that forms a crater. However, more than half a thousand small craters (15–25 μm) were sometimes observed in a 1.5 mm imprint with positive polarity. Sometimes diffuse discharge left no imprint at all at the same conditions. The results emphasize the need for further research to understand the mechanisms behind crater formation and the uniform impact of diffuse discharge on the electrode surface, whose properties are to be modified by plasma.