Anode Material Adhesion to Cathode during Spark Conditioning Process in Vacuum

Spark conditioning is an effective method to improve electrical insulation strength of vacuum circuit breakers. In the conditioning process, defects on cathode surface are removed by repetitive breakdowns (BDs). However, we have elucidated that at the same time, micro-protrusions can be generated on the cathode surface by the adhesion of melted particles of anode material. In this paper, we discuss the relationship between the anode material adhesion to cathode and the conditioning saturation from an experimental approach. We set rod-plane electrodes made of Cu and stainless steel (SUS) in a vacuum chamber. We conduct spark conditioning by repetitive impulse voltages for several pairs of Cu and SUS electrodes. To one pair, the voltage is applied until the conditioning saturation. To the others, the smaller number of voltages before conditioning saturation is applied. After that, we observe the surface of rod cathodes with a digital microscope. As a result, we observe protrusion-like adhesion of the anode material to the cathode surface after conditioning saturation. On the other hand, we confirm coat-like adhesion of the anode material to the cathode surface before conditioning saturation. From these results, it is considered that coat-like adhesion mainly occur up to the saturation voltage, and then protrusion-like adhesion occur due to BD at the saturation voltage level. In other words, the generation of micro-protrusions can be a dominant factor of conditioning saturation process in vacuum.