Spatial Imaging of the Electron Density Depending on Cathode Material in Vacuum Arc Discharge Observed with a Mach-Zehnder Interferometer

Dense plasma jet near cathode spot region was investigated by a Mach-Zehnder interferometer with high spatial ( 0.2 μm per pixel) and temporal (9 ns) resolution. The discharge was initiated between two needle-type electrodes. Four electrode materials including Cu, Ag, Sn and graphite were used simultaneously as both electrodes. Nanosecond interferometry yielded spatial-temporal plasma density distributions in the arc phase. Measurements of three metal cathodes showed the similar interferometric data, which were reported previously as the spot fragments connected with the cathode surface. However, four dense particle blocks were recorded for the graphite cathode. Two blocks contained the spot fragments connected with the cathode surface and the plasma balls separated from surface, but two dense gas balls contained the one connected with and the one separated from surface. The maximum electron density for four electrode materials showed a weak dependence on melting temperature but similar tendency as the freezing value occured in the Saha freezing model. The interferogram revealed the small localized dense density blocks and the maximum electron density expanding from Cu spots decreased slower than a quadratic drop in density. Above results confirmed that the explosive emission mechanisms of spot operation were dominant in plasma generation mechanism for metal electrode, but the thermal evaporation and droplets were the main sources of plasma for graphite cathode.