Multiple Breakdown Induced by Previous Breakdown in Higher Frequency AC Conditioning

We have confirmed that the multiple breakdowns (BDs) occur in a half cycle of AC voltage application in the AC conditioning process in vacuum. In previous research, it has been experimentally confirmed that the higher frequency AC conditioning suppresses the multiple BDs and improves the conditioning effect. In this paper, we discuss how the multiple BDs are induced. By the BD light emission images during AC conditioning, we found that the multiple BDs occurred in close to the previous BD position. This suggests that the multiple BDs were induced by the previous BD. Two possible causes of the multiple BDs are investigated: (a) metal vapor generated by the previous BD and (b) metal particles generated by the previous BD. The diffusion time of metal vapor is much shorter than the time interval for the multiple BDs. Therefore, the multiple BDs may not be induced by metal vapor. On the other hand, for particles of 10 μm order in diameter, the time of collision to the counter electrode matches the time interval of the multiple BDs. Particles of the order of 10 μm in diameter were found on the electrode surface after AC conditioning. These results suggest that the multiple BDs were initiated by metal particles generated by the previous BD and do not contribute to protrusion melting, but damage the electrode surface. Therefore, the higher frequency AC conditioning to suppress the multiple or harmful BDs can be promising for AC conditioning process in vacuum.