Observation of vacuum arc cathode spot with high-speed framing camera

A copper vacuum arc cathode spot at current 10-100 A was imaged by three-frame camera K 0011 (BIFO Company, Russia) with frame exposures 30 ns and 6 ns and spatial resolution up to 7 μm. It was found that spot splitting into separate fragments occurs at currents higher than 50 A. The average fragment size was found to be 20 μm at current about 10 A (cathode spot consists of a single fragment). With the rise of the arc current the average fragment size rises too and reaches 50 μm at current 100 A (cathode spot consists of two or three fragments). The overall dimension of region occupied by cathode spot fragments rises from 20 μm at current 12 A to 120 μm at current 100 A. Observations with high temporal resolution (exposure time 6 ns) reveal the significant changes of cathode spot brightness occurring within about 10 ns time interval.