The anode region of electric arcs: a survey

Electric arc anodes frequently determine functional performance and lifetime of a number of arcing devices, ranging from discharge lamps to plasma spray torches. While there have been numerous studies of the anode region of electric arcs, our understanding of the detailed physical processes is still limited. The reason for this lack of detailed knowledge is that numerous factors influence the arc-anode interaction, and that the plasma-solid interface in high intensity arcs is in general not accessible to diagnostics and one has to rely on indirect measurements. Throughout this survey, the emphasis will be on high intensity arc anodes, i.e. on plasmas with temperatures of more than 10 000K and electron densities exceeding 10(22) m(-3) outside the boundary layer, and heat fluxes exceeding 10(7) W m(-2). The plasma parameters in the boundary layer as obtained with different techniques by a number of investigators for a variety of conditions are presented, and the effect of macroscopic flow conditions is discussed. Experimental and modelling results are presented. A brief comparison with low current arcs is also given, and the areas that need further research are highlighted.