Observation of fine-ordered patterns on electrode surfaces subjected to extensive erosion in a spark discharge

This note describes the observation of pattern formation on the surface of metal electrodes used in a spark discharge. The spark discharge used in the experiments was a spark discharge generator (SDG) designed for the generation of nanoparticles. The generator was run in atmospheric pressure nitrogen flow, with the electrode gap adjusted to 3 mm. The spark repetition rate was about 30 Hz, at an average breakdown voltage of 6.6 kV. It was observed, that after extended duration (several hours) of sparking, the front face of the cylindrical electrodes exhibited an erosion pattern, which had a characteristic appearance both in the macro and micro dimensions. Apart from the expected rounding-off of the circular edge of the electrodes, the erosion pattern most often found consisted of a closely packed ordered arrangement of protrusions on the surface. The ordered-protrusion pattern occurred predominantly on initially cathodic electrodes and was discernible on copper, gold, nickel, silver and copper-nickel alloy electrodes. The topology of the erosion pattern and its time evolution was studied in detail on nickel electrodes by using optical microscopy and confocal laser scanning microscopy.