A means of raising the threshold current for anode spot formation in metal-vapor arcs

Of the various processes occurring in an arc at high currents one of the most important with regard to application is the formation of an anode spot and the consequent melting associated with it. Vacuum arc devices depend for their operation on the formation of a high current transient metal-vapor arc. The limiting current in a particular design may be set by destructive melting at the anode. It is shown that it is possible to raise the threshold current for anode spot formation in a metal-vapor arc by suitably changing the electrode geometry. From a study of a few simple electrode geometries a set of guidelines has been evolved governing the choice of electrode geometry. As an illustration of the efficacy of these guidelines and the means of implementing them in practice it is shown how it is possible, with successive modifications of a coaxial-cylindrical electrode structure, to attain a peak current of 72 kA for a damped 60-Hz current wave without electrode melting.