Evidence and consequences of cathode-spot substructure in vacuum arcs

Time and spatial variations of vacuum arc spots have been studied by laser absorption and emission imaging techniques. It is shown experimentally that for t<10 ns there exist plasma structures with a size <5 /spl mu/m. Experimental estimates of fragment size and lifetime are 1-2 /spl mu/m and 1-2 ns, respectively. These estimates are supported by modelling the nonideal plasma state, based on the power input and the erosion rate. The model results in mean charge numbers of 2-3, electron temperatures of 8-9 eV, and plasma densities of about 10/sup 27/ m/sup -3/.