Effect of the microstructure of electrode materials on arc cathode spot dynamics

The role of the microstructure of cathode materials in the geometrical parameters and dynamic processes of cathode spots is studied on the basis of investigation of the microscopic traces of vacuum arcs without an external magnetic field. The arc erosion pattern shows that spot craters locate on the phases with weak voltage withstanding, whose size, morphology and distribution determine those of the arc spots. The spot tracks over the cathode alloys with ultra-fine microstructure disperse to a greatly increased electrode area, suggesting that the spot motion is controlled by the microstructure of the electrode materials. A proposed spot dynamics model infers that the spot velocity may increase by two orders when the typical microstructure scale is less than a characteristic value, i.e. the spot fragment size. It is concluded that the microstructure, including the constituent phases and their size, is an important factor dominating the microscopic migration mechanism of arc cathode spots.