Cathode spot observation in vacuum arc of zinc cathode under oxygen gas flow

Magnetically steered cathode spot(s) of a vacuum arc with a zinc (Zn) cathode was (were) observed using a high-speed video camera. The camera can take 4,500 frames per second of 256 by 256 pixels. The transverse magnetic flux density for steering the cathode spot was 1.0 and 5.5 mT at the bottom of the cathode (64 mm in diameter) shoulder. The arc was operated at an arc current of DC 30 A and O/sub 2/ flow rate of 40 ml/min. The pressure was varied from 0.1 Pa to 5.0 Pa. The principal results obtained in the present study were as follows. The cathode spots were driven in the retrograde direction. When a strong magnet was used, the number of the cathode spots was smaller, the cathode spots revolved at the outer region of the cathode surface more of the time, and the velocity of the cathode spots was higher than when a weak magnet was used. The velocity of the cathode spots increased as the pressure increased.